The Jig Grinder.

The Jig Grinder

Objectives Select wheels and grinding methods required for jig-grinding holes Set up work and jig-grind straight hole to within tolerance of in. (0.005 mm)

Jig Grinder Uses Need for accurate hole locations in hardened material led to development in 1940 Other uses Grinding of contour forms Radii Tangents Angles Flats

Advantages of Jig Grinding
Holes distorted during hardening process can be accurately brought to correct size and position Holes and contours requiring taper or draft may be ground Because more accurate fits and better surface finishes are possible. service life of part greatly prolonged Many parts requiring contours can be made in solid form

Jig Grinder Similar to jig borer
Capable of positioning table within in. accuracy Both vertical spindle machines Main difference is in spindles Equipped with high-speed pneumatic turbine grinding spindle Permits outfeed grinding and grinding of tapered holes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Jig Grinder Grinding Spindle
May be offset from main spindle Planetary path of rotation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Grinding Head Outfeed Horizontal dovetail slide connects grinding head to main spindle of jig grinder Grinding head offset from center of main spindle to grind various-size holes Amount of offset can be accurately controlled by internally threaded outfeed dial, mounted on nonrotating yoke Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Three Depth Measuring Devices
Adjustable positive stop Located on left-hand end of pinion shaft Microadjustments made by limiting screw Graduated dial Located on downfeed handwheel Indicates travel of quill Set to zero at any position and reads .001 in. steps Micrometer stop Fastened to column of grinder

Diamond Dressing Arm Jig grinders must rapidly dress grinding wheels without disturbing setup May be quickly swung into approximate grinding wheel location and locked into position Final approach done by fine-adjusting knurled screw Advances diamond through dressing arm

Grinding Two methods each with its own advantages
Outfeed grinding Plunge grinding Small holes (less than ¼ in. in diameter Ground using diamond-charged mandrels Holes larger in diameter than normal range Use extension plate Up to 9 in. in diameter

Outfeed Grinding Similar to internal grinding where wheel fed radially into work Passes as fine as in. at a time Cutting action takes place with periphery of grinding wheel Generally used to remove small amounts of stock when high finish and accurate hole size required

Plunge Grinding Compares to cutting action of boring tool
Wheel fed radially to desired diameter and then into work Cutting done with bottom corner of wheel Keeps work cooler than outfeed grinding Rapid method of removing excess stock Wheel properly dressed produces satisfactory finishes for some jobs

Diamond-Charged Mandrels
Used instead of conventional grinding wheels for grinding holes < .250 in. diameter Made of cold-rolled steel turned to correct size and shape Grinding end placed in diamond dust and tapped sharply to embed dust in surface Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Advantages Over Conventional Grinding Wheel
Mandrels have maximum strength and rigidity Mandrels made for ideal diameter and length for each hole Velocity required for efficient grinding ¼ of that for wheel Cost per hole is less due to greater efficiency

General Principles in Selecting Grinding Wheels
Shank or mandrel of mounted wheels be short as possible Grinding wheel diameter should be approximately ¾ diameter of hole to grind Widely spaced abrasive grains in bond increase penetrating power of wheel Hard abrasive grain with strong or hard bond used for soft, low-tensil-strength materials Hard abrasive grain in soft or weak bond recommended for high-alloy hardened steels

Wheel Speed Majority operated most efficiently at 6000 surface feet per minute (sf/min) Diamond-charged mandrels at 1500 sf/min Spindle speed varied for types and diameters of wheels used Three grinding heads for Moore jig grinder Speed varied by adjustment of pressure regulator

Conditions That Indicate Improperly Dressed Wheel
Poor surface finish on hole Surface burns Out-of-round holes Taper or bell-mouth holes Locational error

Techniques for Dressing a Grinding Wheel
While wheel running at reduced rate, dress top and bottom face with abrasive stick held in hand Dress diameter of wheel with sharp diamond Repeat steps 1 and 2 with wheel at proper operating speed

Relieve upper portion of diameter so approximately. 250 in
Relieve upper portion of diameter so approximately .250 in. of cutting face remains Bottom face of wheel should be concaved slightly with abrasive stick for grinding to shoulder on bottom of hole In outfeed grinding, only diameter of wheel should be dressed when required. When plunge grinding, dress bottom face of wheel with abrasive stick. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Factors Influencing Grinding Allowances
Type of surface finish in bored hole Size of hole Material of workpiece Distortions that occur during hardening process

General Rules For Amount of Material Left For Grinding
Holes of up to .500 in. diameter should be .005 to .008 in. undersize for grinding Holes of over .500 in. diameter should be .010 to .015 in. undersize for grinding

Setting Up Work When bolts or strap clamps used, keep bolts close to work Strap clamps placed exactly over parallels supporting work Bolts should not be tightened any more than required to hold workpiece Do not clamp work too tightly in precision vise clamp Set up work on parallels high enough to allow bottom of hole being ground to be measured

To Locate the Workpiece
Workpiece may be set up parallel to table travel by three methods: Indicate edge of workpiece Set work against table straightedge; then check alignment with indicator On heat-treated piece, indicate two or more holes and set up work to suit average location of group of holes

Grinding Sequence Rough-grind all holes first
Finish-grind all holes that can be ground with same grinding head Holes whose relationship to others is most important should be ground in one continuous period of time Grind holes with shoulders or steps only once

To Grind a Tapered Hole Steps when an extremely accurate angular setting required: Convert angle into thousandths taper per inch by mathematical calculations Mount indicator in machine spindle Set angle plate or master square on machine table Move indicator through 1 in. of vertical movement as read on downfeed dial Set adjusting screws until desired taper attained

Tapered Holes Most also have a straight section
Taper ground to certain distance from top Difficult to see where taper begins Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Two Methods To Show Where Tapered Section Begins
Apply layout dye to top portion of hole with pipe cleaner Dye removed from taper portion during grinding operation allowing length of straight hole to be measured On holes too small or difficult to see and measure, taper ground first to dimension X. Use formula to calculate size hole would be at dimension X Once tapered hole ground, straight hole ground to proper diameter

Suggestions for Grinding Shouldered Holes
Select proper-size grinding wheel for hole size Make bottom of wheel slightly concave with an abrasive stick Set depth stop so that wheel just touches bottom or shoulder of hole Rough-grind sides and shoulder of hole at same time Dress wheel and proceed to finish-grind hole Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

CBN Wheels Perform best when jig grinding tool and die, carbon, and alloy steels hardened to Rc >50; hard abrasive cast iron; superalloys Rc >35 Advantages over aluminum oxide wheels Long wheel life Less wheel maintenance Positive size control Consistent surface finishes

Wheel Selection Guidelines
Abrasive type Bond types Resin- and vitrified-bond wheels Metal-bond wheels Electroplated wheels Grit sizes Choose largest abrasive size Choose finer abrasive size

Mounting Have shank extending as little as possible to avoid overhang
Results in chatter, vibration and spindle deflection if large overhang Use indicator on wheel shank and rotate spindle slowly by hand to check runout of spindle Should not be more than .001 in.

Truing Resin-, vitrified-, and metal-bond jig-grinding wheels must be trued Never true or condition electroplated wheels Always use 150-grit-size diamond-impregnated nibs for truing Never use single-point diamond

Truing Mount diamond-impregnated truing nib in sturdy holder
Position diamond nib close to CBN wheel Take light infeed truing increments of in. or less Vertically feed full wheel length past diamond truing nib at feed rate of in./min Continue truing until wheel is true

Dressing Operation removes some of wheel bond and exposes sharp edges of CBN crystal Truing glazes CBN wheels and leaves abrasive crystals and bond on same plane Use 220-grit, medium hardness aluminum oxide dressing stick Soak dressing stick in water-soluble oil so slurry created while dressing

Again force dressing stick in to depth of wheel's abrasive section
Start wheel and force dressing stick in horizontally for depth of SBN abrasive section Never use an up-and-down motion for dressing Withdraw dressing stick and move it up a distance equal to wheel length Again force dressing stick in to depth of wheel's abrasive section Continue this procedure until wheel seems to draw dressing stick in with ease

Jig-Grinding Guidelines
For best results, grind wet Use proper wheel speeds, outfeeds, and reciprocal and planetary speeds Never use wheel at higher speeds that those recommended by manufacturers Outfeeds depend on spindle r/min, wheel diameter, wheel bond, and workpiece material Reciprocal and planetary speeds should be fairly fast at continuous light outfeed rate

Use proper grinding mode to suit operation required.
Wipe grinding Hole or outfeed grinding Chop grinding Shoulder grinding Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Summary CBN jig-grinding wheels and pins twice as hard and more abrasion-resistant than aluminum oxide wheels Expensive, but cost-effective due to longer wheel life, shorter grinding time, cool cutting action, and less wheel maintenance Remove 30% to 50% faster resulting in higher productivity

Jig-Grinding Hints Calculate all coordinate hole locations first
Clamp work just enough to hold it in place Select grinding wheel three-quarters diameter of hole to be ground Wheel with widely spaced grains should be selected for rough grinding

More Hints Relieve wheel diameters so that only .250 in. (6 mm) of cutting face remains Never use glazed wheel for grinding Rough-grind all holes by plunge grinding Allow work to cool before finish grinding Finish-grind holes with freshly dressed wheel by outfeed grinding

Finishing Processes - Reaming, Broaching, and Lapping

Objectives Identify and explain the purpose of several types of hand reamers Ream a hole accurately with a hand reamer Cut a keyway in a workpiece using a broach and arbor press Lap a hole or an external diameter of a workpiece to size and finish

Hand Cutting Tools Reamers Broaches Lapping
Used to bring hole to size and produce good finish Broaches Used with arbor press to produce special shapes in workpiece Multi-tooth tool forced through hole Lapping Where very fine abrasive powder, embedded in tool is used to remove minute amounts of material from surface

Solid Hand Reamer Made of carbon steel or high-speed steel
Available in inch sizes from in. Metric from 1 – 26 mm in diameter Not adjustable and may have straight or helical flutes Should not be used on work with keyway or any other interruption (chatter and poor finish) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Expansion Hand Reamer Designed to permit adjustment of approximately .006 in. above nominal diameter Hollow and has slots along length of cutting section Tapered threaded plug fitted into end of reamer provides for limited expansion Cutting end of reamer ground to slight taper Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Adjustable Hand Reamer
Has tapered slots along entire length of body Inner edges of cutting blades have corresponding taper so blades remain parallel for any settings Adjusted to size by upper and lower adjusting nuts Blades have adjustment range of 132 in. on smaller reamers to almost 516 in. on larger ones Manufactured in sizes ¼ to 3 in. in diameter Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Taper Reamer Used to finish tapered holes accurately and smoothly
Made with either spiral or straight teeth Spiral-flute superior to straight due to shearing action and reduced chatter Roughing reamer Nicks ground at intervals along teeth Used for more rapid removal of surplus metal Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Finishing Taper Reamer
Used after roughing reamer to finish hole smoothly and to size Either straight or left-hand spiral flutes Designed to remove only small amount of metal (about .010 in from hole) Do not clear themselves readily Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Reaming Precautions Never turn reamer backward (counterclockwise), it will dull teeth Use cutting lubricant where required Always use helical-fluted reamer in hole that has keyway or oil groove cut in it Never attempt to remove too much material (maximum = .010 in.) Frequently clear taper reamer and hole of chips

To Ream Hole With a Straight Hand Reamer
Check size of drilled hole ( in. smaller than finished hole size) Place end of reamer in hole and place tap wrench on square end of reamer Rotate reamer clockwise to align with hole Check reamer for squareness with work Brush cutting fluid over end of reamer Rotate reamer slowly clockwise and apply downward pressure

Broaching Process in which special tapered multitoothed cutter forced through an opening or along outside piece of work to enlarge or change shape of hole First used for internal shapes (keyways, splines) Cutting action performed by series of successive teeth Each protrude .003 in. farther than preceding tooth Last three teeth same depth and provide finish cut Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Advantages of Broaching
Machining almost any irregular shape is possible (providing it is parallel to axis) Rapid: entire process usually in one pass Roughing and finishing cuts combined Variety of forms, internal or external, may be cut simultaneously and entire width of surface may be machined in one pass

Cutting a Keyway With a Broach
Keyways may be cut by hand quickly and accurately by means of broach set and arbor press Broach set covers wide range of keyways Equipment necessary to cut keyway Bushing to suit hole size Broach size of keyway to be cut Shims to increase depth of cut of broach

Procedure for Cutting a Keyway With a Broach
Determine keyway size required Select proper broach, bushing and shims Place workpiece on arbor press Use an opening on base smaller than opening in work so bushing properly supported Insert bushing and broach into opening Apply cutting fluid if workpiece is steel

Check broach to be sure that it has started squarely in hole
Press broach through workpiece Maintain constant pressure on arbor-press handle Remove broach, insert one shim and press broach through hole Insert second shim, if required, and press broach through again This cuts keyway to proper depth Remove bushing, broach, and shims 5. Check the broach to be sure that it has started squarely in the hole.

Lapping Abrading process used to remove minute amounts of metal from surface Reasons for lapping Increase wear life of part Improve accuracy and surface finish Improve surface flatness Provide better seals and eliminate need for gaskets Intended to remove only about in.

Lapping Abrasives Both natural and artificial abrasives used
Flour of emery and fine powders made of silicon carbide or aluminum oxide used extensively Used for rough lapping should be no coarser than 150 grit Fine powders run up to 600 grit Fine work uses diamond dust in paste form

Flat Laps Close-grained cast iron laps used for flat surfaces
Roughing operation (blocking down) Lapping plate scored with narrow grooves .500 in. apart both lengthwise and crosswise to form square or diamond pattern Finish lapping done on smooth cast-iron plate

Charging the Flat Lapping Plate
Spread thin coating of abrasive powder over surface of plate Press particles into surface of lap with hardened steel block or roll When surface charged, clean surface with varsol and examine for bright spots Until entire surface assumes gray appearance after it has been cleaned Charging the Flat Lapping Plate

Lapping a Flat Surface Place a little varsol on finish-lapping plate that has been charged Place work on top of plate and gently push it back and forth over full surface of lap using irregular movement Do not stay in one spot! Continue this movement with light pressure until desired surface finish obtained

Lapping: Precautions to Be Observed
Do not stay in one area; cover full surface of the lap Never add fresh supply of loose abrasive Never press too hard on work because lap will become stripped in places Always keep lap moist

Internal Laps Holes accurately finished to size and smoothness by lapping Made of brass, copper, or lead Three types Lead Internal Adjustable

Lead Lap Made by pouring lead around tapered mandrel that has groove along length Turned to running fit into hole Sometimes slit on outside to trap loose abrasive Adjust by lightly tapping large end of mandrel with soft block Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Internal Lap May be made of copper, brass or cast iron
Threaded-taper plug fits into end of lap Slit for almost its entire length Lap diameter may be adjusted by threaded-taper plug Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Adjustable Lap May be made from copper or brass
Split for almost full length, but both ends remain solid Slight adjustment provided by means of two setscrew in center section Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Charging and Using an Internal Lap
Note: Before charging, lap should be running fit in hole. Sprinkle some lapping powder evenly on flat plate Roll lap over powder, to embed abrasive into surface of lap Remove excess powder Mount lathe dog on end of lap

Fit workpiece over end of lap
Lab should not be wringing fit in hole of work and about 2.5 times length of work Place some oil or varsol on lap Mount lap and work between lathe centers Set machine to run at slow speed, 150 to 200 r/min for 1 in. diameter Hold work securely and start machine Run work back and forth entire length 11. Remove the work and rinse it in varsol to remove the abrasive and to bring it to room temperature. 12. Gage the hole for size. Note: Always keep the lap moist and never add loose abrasive to the lap. Loose abrasive will cause the work to become bell-mouthed at the ends. If more abrasive is necessary, recharge the lap and adjust as required.

Remove work and rinse it in varsol to remove abrasive and to bring to room temperature
Gage hole for size Note: Keep lap moist and never add loose abrasive to lap. Loose abrasive will cause work to become bell-mouthed at ends. If more abrasive necessary, recharge lap and adjust as required.

External Laps Used to finish outside of cylindrical workpieces
May be several forms, however, basic design same Made of cast iron or may have split brass bushing mounted inside by setscrew Must be some provision for adjusting lap

Charging and Using an External Lap
Mount workpiece in three-jaw chuck on lathe or drill press Adjust lap until it is running fit on work Grip end of lap in vise Sprinkle abrasive powder in hole With hardened steel pin, roll abrasive evenly around inside surface of lap Remove excess lapping powder

Place lap on workpiece. It should now be wringing fit.
Set machine to run at slow speed (150 to 200 r/min for 1 in. diameter) Add some varsol to workpiece and lap Hold lap securely and start machine Move lap back and forth along work Always keep lap moist To gage work, remove lap and clean workpiece with varsol

CNC Turning Center

Objectives State the purpose and functions of chucking, turning, and turning/milling centers Identify the applications of computer numerical control (CNC) for turning centers Name the machining operations that may be performed simultaneously

CNC Turning Center In mid-1960, 40% all metal-cutting operations performed on lathes Not very efficient Research led to development of numerically controlled turning centers and chucking lathes Could produce round work almost any contour automatically and efficiently

Three Main Types of Turning Centers
CNC chucking center Holds part in some form of jaw chuck Some have dual spindles (work both ends) CNC universal turning center Can use continuous bar feed system to machine and cut off parts from bar Some have dual tool turrets Combination turning/milling center Uses combination of turning tools

CNC Chucking Center Designed to machine work held in chuck
Variety of sizes from 8 to 36 in. in diameter Four-axis chucking center has two turrets Separate sides; each machine work at same time Seven-tool upper turret Seven-tool lower turret Two-axis model has one or two turrets Will drive only one turret at a time Type of chucking center discussed in rest of slides

Construction Main operative parts of all turning centers basically same Framework components and CNC components Bed and machine frame must be rugged Heavy, one-piece cast-iron casting or polymer cast base Slanted 40º from vertical plane

Turning Center Parts Framework Components CNC Bed Head- stock Carriage
MCU Servos Cross slide Turret Housing

Tooling Toolholders for machining
Outside diameters located in lower turret and are preset Inside diameter mounted in dovetailed block and preset off machine by tool-setting gage Mounted on upper turret Automatic tool-setting probe used for presetting tools Available on some machines

Computer Numerical Control
Microprocessor controls logic calculations, mechanism control and input-output control Video display Visual output of data, processes, and diagnostics Input unit Keyboard and/or diskettes used to communicate with system, enter setup and data Program storage

CNC Turning Center Designed mainly for machining shaft-type workpieces supported by chuck and heavy-duty tailstock center On four-axis machines, two opposed turrets, mounted on separate cross-slides One above and one below center line of work Balance cutting forces so extremely heavy cuts can be taken on workpiece

Other Operations Can Be Performed by Dual Turrets
Roughing and finishing cuts in one pass Machining different diameters on shaft simultaneously Finish-turning and threading simultaneously Cutting two different sections of shaft at same time

Other Turning Center Equipment
Steadyrest Allow facing and threading on end of shafts Follower rest Used to support long, thin shafts Bar-feeding mechanism Permits machining of shafts and parts from bar stock smaller than spindle through-hole Production part loader Can complete part changeover when individual precut shafts machined

Combination Turning/Milling Center
Allows operations such as drilling, milling, and tapping to be performed on part while still in machine Special tool turret contains pockets that have own drive for live tools Drilling and tapping can be performed if machine has contouring spindle Can be indexed to exact locations around circumference of workpiece

Programming Considerations
Programmer must be able to analyze part print and decide on sequence of machining operations Good practice to develop habit of labeling start and end points for both roughing and finishing operations Be certain the programming format suits your equipment before machining parts

Typical Tooling System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Inserts Made from many types of material
Great variety to suit any workpiece material or machining operation Include carbide, coated carbide, ceramic, cermet, cubic boron nitride, and diamond coating Standardized so most inserts fit in same holders

Tool Nose Radius Compensation
Wide variety of tool nose radii Starting with sharp point and increase in 1/64 in. increments from 1/64 to 1/8 in. Theoretical sharp point of tool is programmed Does not position tool at correct location G41 or G42 turns on tool nose radius for finish cuts Radius of each insert stored in numbered tool list of control tool management system

Tool Offsets Programmer must provide tool setup sheet for setup operator MCU will calculate correct position at which tool should be located to accurately machine part

Diameter Versus Radius Programming
Method used determined by preset parameters within machine control unit or by correct G-code Diameter (default) Part print drawn complete with both sides of centerline and full diameter dimensions Radius Part print drawn on just one side of centerline

Establish Part Zero Programmer's choice to place part zero at most convenient location Location of X axis usually centerline of part Z axis either: Right-hand (tailstock) end of part Movements into part will be negative numbers (-Z) Left-hand (chuck) end of part Movements into part will be positive numbers (Z)

Codes Function of some G-codes and M-codes may differ from function of those on machining center Many of common turning center G-codes and M-codes that conform to EIA standards shown in tables 77.1 and 77.2 in textbook

Programming Procedures
CNC control units can vary from manufacturer to manufacturer Important to follow programming manual supplied for each machine This textbook concentrates on two classes of CNC machines: Bench-top teaching model Standard turning center

Bench-Top Teaching Machines
Very easy to program and ideal for teaching Perform turning operations similar to larger machines Except smaller workpieces and lighter cuts Relatively inexpensive Most of B- and M- codes apply to both bench-top CNC turning lathes and standard-size turning centers Few variations

Simple Programming Example of notes and code to machine a sample part (radius programming) Program Notes Program in absolute mode (G90) All programming begins a zero point, centerline and right-hand face of part Carbide tool will be used for all operations Use radius programming Position established to right front corner for safety Material aluminum; cutting speed 600 sf/min, feed rate at .010 in.

Programming Sequence (sample code)
% Rewind stop code/parity check. N10 G24 . N20 G92 X.690 Z.1 N30 M03 . N40 G00 X.590 Z.050 N50 G84 X.500 Z F.010 H.050 N60 G00 X.500 Z.050 N70 G84 X.400 Z-.750 F.010 H.050 N80 G00 X.400 Z.050 Number of instruction Command to MCU Information needed to carry out command :

Programming Sequence (sample code)
% Rewind stop code/parity check. N10 G24 N20 G92 X.690 Z.1 N30 M03 N40 G00 X.590 Z.050 N50 G84 X.500 Z F.010 H.050 N60 G00 X.500 Z.050 N70 G84 X.400 Z-.750 F.010 H.050 N80 G00 X.400 Z.050 G Reference point offset X Tool located .100 in. off the outside finish diameter/.690 in from part centerline (X0) (point a) Z Tool located .100 in. to right of part face (Z0) G Radius programming M Spindle ON clockwise G Rapid traverse rate X Tool located .590 in from part centerline (point b) Z Tool located .050 in from part face Full code with descriptions in text – Follow through for better understanding! :

Standard-Size Turning Center
To introduce additional machining and use of diameter programming, a full sample program is given in the text Complete with program notes and code with explanations Similar to previous example

Turning Center Setup Before setup, become familiar with control panel and operational procedures Power on to machine: Turn on servos and zero out/align all axes so control knows location of machine home position Load program if not already in memory Check manuscript, and prepare tools listed by programmer

Program Test Run Part never machined without test running program first Some controls have control screen which allows visual progression through program without cutting part Dry run program without part Use step/single block mode and feedrate override to slow programmed rate Finger on hold button in case of error in program Good idea to know where emergency stop button located

CNC Machining Centers Unit 78

Objectives Describe the development of the machining center
Identify the types and construction of machining centers Explain the operation of the machining center Understand a basic CNC program for a machining center

CNC Machining Centers Industrial surveys in 1960's showed smaller machine components requiring several operations tool long time to complete Part sent to several machines before finished There was much "operator intervention" during machining process In late 1960s and early 70s, begin to design machine that would perform several operations and do 90% of machining on one machine

Types of Machining Centers
Three types: horizontal, vertical and universal Factors to determine type and size Size and weight of largest piece machined Maximum travel of three primary axes Maximum speeds and feeds available Horsepower of spindle Number of tools automatic tool changer can hold

Two Types of Horizontal Machining Centers
Traveling-column One or usually two tables where work mounted Column and cutter move toward work on one table while operator changes workpiece on other table Fixed-column Equipped with pallet (removable table) After workpiece machined, pallet and workpiece moved off receiver onto shuttle; shuttle rotated, bringing new pallet into position for shuttle and finished work pallet into position for unloading

Vertical Machining Center
Saddle-type construction with sliding bedways that use a sliding vertical head instead of quill movement Generally used to machine flat parts held in vise or simple fixture Versatility increased by addition of rotary accessories

Universal Machining Center
Combines features of vertical and horizontal machining centers Spindle can be programmed in both vertical and horizontal positions Allows for machining all side of a part in one setup Useful for small and medium batch parts Has additional accessories such as indexible pallets and rotary-tilt tables

Advantages of Universal Machining Centers
Eliminate handling and waiting time between machines Reduced number of fixtures and setups Reduced programming time Improved product quality Less work-in-process (WIP) inventory Faster product delivery to customers Lower manufacturing costs

Main Operative Parts Y axis Main operative parts of both vertical and horizontal centers basically same. Position of machining spindle determines whether it is classified as vertical or horizontal. Column Saddle X axis Z axis Bed Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Primary Components of a Machining Center

Machining Center Accessories
Number of accessories available Two types Those that improve efficiency or operation of machine tool Those that involve holding or machining workpiece

Torque Control Machining
Calculates torque from measurements at spindle drive motor Increases productivity by preventing and sensing damage to cutting tool Torque measured when machine turning, not cutting and value stored in memory As cutting begins, stored value subtracted from reading at motor giving net cutting torque Goes higher, computer reduces feedrate, turns on coolant or even stops cycle

Automatic Tool Changers: Large Capacity Horizontal-Type
Hold up to 200 tools Identified by either tool number or storage pocket number Held in storage chain Process: (~ 11 seconds) When one operation being performed, tool required for next moved to pick-up position Tool change arm removes and holds it; exchanges when operation complete; returns tool to storage

Automatic Tool Changers: Smaller-Capacity Vertical, Disk-Type
Holds from 12 to 24 tools Next tool selected upon completion of machining operation (~ 2.5 to 6 seconds) Tool carriage mounted on shuttle that slides carriage next to tool spindle Tool pocket aligned, spindle orients toolholder and tool lock releases Tool changer rotates to number called, tool lock energized and carriage slides out of way

Tools and Toolholders Wide variety of cutting tools
Conventional milling machines, cutting tool cuts 20% of time Studies show machining center time 20% milling, 10% boring, and 70% hole-making in average machine cycle Cutting time can be as high as 75% Large consumption of disposable tools caused by increased tool use

face milling cutters two-flute end mill four-flute end mill

Stub Drills high-helix drill core drill oil hole drill

Taps gun stub flute spiral flute fluteless

Single-point boring tools are used to enlarge a hole
rose reamer Single-point boring tools are used to enlarge a hole and bring it to location. fluted reamer carbide-tipped reamer Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Combination Tools If machining center has helical interpolation capability, one tool can perform drilling, chamfering, and threading operations in one cycle Solid-carbide combination drill/thread tool with drill tip on end, chamfer located at correct length for selected application Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1 3 2 4 Sequence of operations for combination tool, the Thriller®
1. Drill point can produce through hole or a blind hole no deeper than two times tool diameter On completion of the cycle, the tool is retracted out of the hole Tool fed radially into wall of hole to full thread depth during ½ of a turn (180º, while moving ½ of thread pitch in –Z axis Tool is brought out radially from wall, to center of hole during ½ of a turn (180º) while moving ½ of a thread pitch in the –Z axis. 3. Next, thread is formed by helical interpolation cycle during one full turn (360º), while moving one thread pitch in –Z axis. 2. Chamfer is cut, and tool is retracted approximately ½ thread pitches from the bottom of the hole 1 2 3 4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Toolholders Must have compatibility in toolholders in order for wide variety of cutting tools to be inserted into machine spindle quickly and accurately Most common toolholder has V-flange and self-releasing taper shank Size (range from No. 30 to 60) determined by machine capacity and designed horsepower

Variety of Toolholders

Work-Holding Devices Standard step clamp Table plate
Used to hold down flat, large parts Quick-release clamp good when clamps have to be temporarily moved to machine edge Table plate Flat aluminum plate bolted to machine table Dowel pin and tapped holes machined into plate to permit fastening vises or clamps More flexible than limit of table T-slots

Plain-style precision vises
Keyed directly to table slots Make positioning and clamping accurate and simple When machining multiple identical parts, matched set of qualified vises can be used Qualified vises used when long part requires support on both end to maintain parallelism When using double-station cluster vises; total of up to 20 parts held for machining operation

Vise jaw systems CNC fixtures
Set of master jaws placed in vise and items snapped into position Parallels, modular workstops, angle plates, V-jaws, and machinable soft jaws Add versatility and increase flexibility of a precision vise Can be used in both single-station and double-station vises CNC fixtures Used to accurately locate many similar parts and hold them securely for machining

Programming Procedures
Programming can vary slightly from machine to machine so important to follow manual supplied with machine Two classes concentrated on in text: Bench-top teaching model Inexpensive and easy to operate for students Standard machine model

Bench-Top Teaching Machines
Simple programming example explained in detail in text as was done in Unit 75 Program notes plus full program sequence with explanations to help understand code Refer to G-code and M-code charts in Unit 75

Machining Center Setup
Before using machining center, operator needs to become familiar with control panel and operational procedures Different modes and how to use menus, how to establish machine zero, set tool length offsets and test run program When machine powered up, need to zero out all axes so control know location of machine home position

Setting Part Zero Each part has established part zero
Not same as machine zero Using jog mode and edge finder or dial indicator, locate part zero position in X and Y axes Work offset distance (position shift offset) is distance traveled from machine home Entered on control's work coordinate page Distances traveled for X and Y entered, while Z axis distance left at zero

Setting Tool Length Offset
Start with empty automatic tool changer Load tool #1 by indexing to proper location of tool carriage Tool placed directly into spindle and locked Use jog mode to touch off tool to Z0 of part Distance traveled is Z tool offset and listed on control offset page under offset for tool #1 Process repeated with each additional tool

Program Test Run Never machine a part without test running program first Equipped with graphics display Allow operator to see steps on control screen without cutting part Without graphics display Dry run program without part in machine Use step/single block mode and feedrate override

Standard-Size Machining Center
Another full example of a new part that introduces additional machining cycles Circular and fixed drilling cycles Program notes and full programming sequence shown in text with explanation of programming steps Refer to G- and M-code charts in Unit 75 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Grinding

Grinding Characteristics of an abrasive must be:
Harder than material being ground Strong enough to withstand grinding pressures Heat-resistant so that it does not become dull at grinding temperatures Friable (capable of fracturing) so when cutting edges become dull, they will break off and present new sharp surfaces to material being ground

Types of Abrasives Unit 80

Objectives Describe the manufacture of aluminum oxide and silicon carbide abrasives Select the proper grinding wheel for each type of work material Discuss the applications of grinding wheels and abrasive products

Abrasive Classes Natural abrasives Manufactured abrasives
Sandstone, garnet, flint, emery, quartz, corundum Used prior to early part of 20th century Almost totally replaced by manufactured abrasives Best natural abrasives is diamond (high cost) Manufactured abrasives Used because grain size, shape and purity can be closely controlled Aluminum oxide, silicon, carbide, boron carbide, cubic boron nitride and manufactured diamond

Aluminum Oxide Most important abrasive Make up 75% of grinding wheels
Used for high-tensile-strength materials Manufactured with various degrees of purity Hardness and brittleness increase as purity increases

Aluminum Oxide Purities
Regular aluminum oxide (Al2O3) at 94.5% Tough abrasive capable of withstanding abuse Grayish in color Used for grinding steel, tough bronzes, etc. Aluminum oxide at 97.5% Not as tough as regular but still gray in color Used in manufacture of grinding wheels for centerless, cylindrical, and internal grinding of steel and cast iron Purest form of aluminum oxide White material that produces sharp cutting edge Used for grinding hardest steels and stellite

Manufacture of Aluminum Oxide
Made from bauxite ore Mined by open-pit method in Arkansas and Guyana, Suriname, and French Guiana Calcined (powered form) in large furnace Mixed with coke screenings, iron borings Coke used to reduce impurities Electrodes lowered, coke heated and fusion of bauxite starts then more bauxite and coke added When furnace full, shut off, let cool Material broken up and fed into crushers

Silicon Carbide Suited for grinding materials that have low tensile strength and high density Harder and tougher than aluminum oxide Color varies from green to black Green used mainly for grinding cemented carbides and other hard materials Black used for grinding cast iron and soft nonferrous metals (also ceramics)

Manufacture of Silicon Carbide
Mixture of silica sand and high-purity coke heated in electric resistance furnace Sawdust added to produce porosity and to permit gas to escape during operation Salt added to assist in removing impurities Time required for operation is 36 hours Cool for 12 hours, remove sidewalls where unfused mixture falls to floor leaving silicon carbide ingot Ingot crushed; resultant silicon carbide treated, screened and graded

Zirconia-Aluminum Oxide
First alloy abrasive produced Made by fusing zirconium oxide and aluminum oxide at extremely high temperatures Contains about 40% zirconia Used for heavy-duty rough and finish grinding in steel mills, for snagging in foundries and for rapid rough and finish grinding of welds Performance superior to standard aluminum oxide (last 2 to 5 times longer)

Advantages of Zirconia-alumina Over Standard Abrasives
Higher grain strength Higher impact strength Longer grain life Maintains its shape and cutting ability under high pressure and temperature Higher production per wheel or disk Less operator time spent changing wheels or disks

Boron Carbide Hardest material manufactured with exception of diamond
Not suitable for use in grinding wheels Used only as loose abrasive and as cheap substitute for diamond dust Manufacture of precision gages and sand blast nozzles Used in ultrasonic machining applications

Manufacture of Boron Carbide
Produced by dehydrated boric acid being mixed with high-quality coke Mixture heated in horizontal steel closed cylinder, hole for graphic electrode and hole for escaping gases To remove air, dampen mixture with kerosene to volatize and expel air High current at low voltage applied for about 24 hours, then cooled Resultant product is hard, black lustrous material

Cubic Boron Nitride (CBN)
Synthetic abrasive has hardness properties between silicon carbide and diamond Developed by General Electric Company in 1969 Capable of withstanding grinding temperatures up to 2500ºF Cool-cutting and chemically resistant to all inorganic salts and organic compounds Capable of maintaining very close tolerances

Manufacture of CBN Synthesized in crystal form from hexagonal boron nitride with aid of catalyst, extreme heat (2725ºF) and tremendous pressure Strong, hard, blocky crystalline structures with sharp corners Two types Borozon CBN: uncoated abrasive used for general-purpose grinding Boraxon Type II CBN: nickel-plated grains used in resin bonds for general-purpose dry and wet grinding

Manufactured Diamonds
1954, General Electric Company produced Man-Madey diamonds in laboratory 1957, General Electric Company began commercial production of diamonds First success involved carbon and iron sulfide in granite tube closed with tantalum disks were subjected to pressure of 66,536,750 psi and temperatures between 2550ºF Temperatures must be high enough to melt metal saturated with carbon and start diamond growth Industrial diamonds referred to as bort

Diamond Types Type RVG Diamond
Elongated, friable crystal with rough edges Letters indicate it can be used with resinoid or vitrified bond and used for grinding ultrahard materials Tungsten carbide Silicon carbide Space-age alloys Used for wet or dry grinding

Type MBG-II Diamond Type MBS Diamond Tough and block-shaped crystal
Not as friable as RVG type Used in metal-bonded grinding wheels Used for grinding cemented carbides, sapphires, and ceramics as well as electrolytic grinding Type MBS Diamond Blocky, extremely tough crystal with smooth, regular surface and not friable Used in metal-bonded saws to cut concrete, marble, tile, granite, stone, and masonry

Ceramic Aluminum Oxide
Known as SG abrasive, introduced by Norton Company in 1988 Outperforms conventional aluminum oxide Made by nonfused process Thousands of submicron-sized particles are sintered to provide single abrasive grain of uniform shape and size with more cutting edges that remain sharp SG abrasive well suited to CNC grinding Fewer wheel changes, less wheel dressing, higher productivity and therefore lower labor costs

SG and CBN Abrasives Combination of technologies of CBN and SG abrasives used to produce vitrified CVSG abrasive grinding wheel Provides most of high material-removal rates and low wheel wear of CBN yet can be trued with single point diamond tools Free-cutting, allow increased depths of cuts and feedrates, reduce burning, and lower grinding costs

Advantages of SG Abrasives Over Conventional Abrasives
Last 5 to 10 times longer than conventional wheels Metal-removal rates are doubled Heat damage to surface of very thin workpieces reduced Grinding cycle time reduced Dressing time reduced as much as 80%

Abrasive Products After abrasives produced, formed into products
Grinding wheel Most important Abrasive material held together with suitable bond Material components are abrasive grain and bond; other physical characteristics such as grade and structure Coated abrasives Polishing and lapping powders Abrasive sticks

Basic Functions of Grinding Wheels
Generation of cylindrical, flat and curved surfaces Removal of stock Production of highly finished surfaces Cutting-off operations Production of sharp edges and points

Abrasive Grain Aluminum oxide or silicon carbide abrasive used in most grinding wheels Each grain on working surface of grinding wheel acts as separate cutting tool Removes small metal chip as passes over surface of work As grain becomes dull, fractures and presents new sharp cutting edge to material Fracturing action reduces heat of friction, producing relatively cool cutting action

Grain Size Abrasive ingot (pig) removed from electric furnace, crushed, grains cleaned and then sized by passing them through screens Contain certain number of meshes or openings per inch 8-grain 24-grain 60-grain Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Commercial Grain size Classification
Very Coarse Coarse Medium Fine Very Fine Flour Size

Grain Sizes General applications for various grain sizes
8 to 54 for rough grinding operations 54 to 400 for precision grinding processes 320 to 2000 for ultra precision processes to produce 2 to 4 µ (micron) finish or fine

Factors Affecting Selection of Grain Sizes
Type of finish desire Type of material being ground Amount of material to be removed Area of contact between wheel and workpiece

Bond Types Function of bond is to hold abrasive grains together in form of wheel Six common bond types used in grinding wheel manufacture: Vitrified Resinoid Rubber Shellac Silicate Metal

Vitrified Bond Used on most grinding wheels Made of clay or feldspar
Fuses at high temperature and when cooled forms glassy bond around each grain Strong but break down readily on wheel surface to expose new grains during grinding Bond suited for rapid removal of metal Not affected by water, oil, or acid

Resinoid Bond Synthetic resins used as bonding agents
Generally operate at 9500 sf/min Wheels are cool-cutting and remove stock rapidly Used for cutting-off operations, snagging, and rough grinding, as well as for roll grinding

Rubber Bond Produce high finishes
Ball bearing races Used for thin cutoff wheels because of its strength and flexibility Used also as regulating wheels on centerless grinders

Shellac Bond Used for producing high finishes on parts such as cutlery, cam shafts, and paper-mill rolls Not suitable for rough or heavy grinding

Silicate Bond Not used to any extent in industry
Used principally for large wheels and for small wheels where necessary to keep heat generation to minimum Bond (silicate of soda) releases abrasive grains more rapidly than does vitrified bond

Metal Bond Generally nonferrous
Used on diamond wheels and for electrolytic grinding operations where current must pass through wheel

Grade Defined as degree of strength with which bond holds abrasive particles in bond setting Hard grade When bond posts very strong (retain abrasive grains during grinding operation) Soft grade Grains released rapidly during grinding operation Wheel grade symbols indicated alphabetically, from A (softest) to Z (hardest)

Factors that Determine the Grade Selected for Particular Job
Hardness of material Area of contact Condition of machine Speed of grinding wheel and workpiece Rate of feed Operator characteristics

Structure Space relationship of grain and bonding material to the voids that separate them Density of wheel Dense structure has close grain spacing Open structure has relatively wide spacing Selection of wheel structure depends on type of work required Indicated by numbers ranging from 1 (dense) to 15 (open)

Factors Affecting the Selection of the Proper Wheel Structure
Type of material being ground Soft material require greater chip clearance, therefore open wheel Area of contact Greater area of contact, more open structure Finish required Dense wheels give better, accurate finish Method of cooling Open-structure wheels provide better supply of coolant

Grinding Wheel Manufacture
Most grinding wheels used for machine shop operations are manufactured with vitrified bonds Main operations in manufacture of vitrified grinding wheels: Mixing Molding Shaving Firing (Burning) Truing Bushing Balancing Speed Testing

Mixing Correct proportions of abrasive grain and bond carefully weighed and thoroughly mixed in rotary power mixing machine Certain percentage of water added to moisten mix Molding Proper amount of mixture placed in steel mold of desired wheel shape and compressed in hydraulic press to form wheel slightly larger than finished size Amount of pressure used varies with size of wheel and structure required

Shaving Firing (Burning)
Some machines requires special wheel shapes and recesses Shaped or shaved to size in green, or unburned, state on shaving machine which resembles potter's wheel Firing (Burning) Green wheels carefully stacked on cars and moved slowly through long kiln 250 to 300 ft long with temperature held at ~2300ºF Takes about 5 days Causes bond to melt and form glassy case around each grain producing hard wheel

Truing Bushing Cured wheels mounted in special lathe
Turned to required size and shape by hardened-steel conical cutters, diamond tools, or special grinding wheels Bushing Arbor hole in grinding wheel fitted with lead or plastic-type bushing to fit specific spindle size Edges of bushing are trimmed to thickness of wheel

Balancing Speed Testing
Remove vibration that may occur while wheel revolving Small, shallow holes drilled in "light" side of wheel and filled with lead to ensure proper balance Speed Testing Wheels rotated in heavy, enclosed case and revolved at speeds at least 50% above normal operating speeds Ensures wheel will not break under normal operating speeds and conditions

Standard Grinding Wheel Shapes
Nine standard grinding wheel shapes established by: United States Department of Commerce Grinding Wheel Manufacturers Grinding Machine Manufacturers Dimensional sizes for each of the shapes have also been standardized Table 80.3 in textbook

Mounted Grinding Wheels
Driven by steel shank mounted in wheel Produced in variety of shapes for use with jig grinders, internal grinders, portable grinders, toolpost grinders, and flexible shafts Manufactured in both aluminum oxide and silicon carbide types

Grinding Wheel Markings
Standard marking system chart used by manufacturers to identify grinding wheels Information found on blotter of all small and medium-size grinding wheels Stenciled on side of larger wheels Six positions in standard sequence Prefix is manufacturer's symbol and not always used by all grinding wheel producers Marking system used only for aluminum oxide and silicon carbide wheels, not diamond wheels

Standard Marking System
opt 51 A 36 L 5 V 23 Grade Soft Medium Hard A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Grain size Manufacturers private marking to identify wheel (use optional) Record Bond type V – Vitrified S – Silicate R – Rubber RF – Rubber reinforced B – Resinoid BF – Resinoid reinforced E – Shellac O - Oxychloride Manufacturer's symbol indicating exact kind of abrasive Abrasive Type A – Aluminum Oxide S – Silicon Carbide Structure Dense to Open …..etc.

Selecting a Grinding Wheel for a Specific Job: Example 1
It is required to rough surface grind a piece of SAE 1045 steel using a straight wheel. Coolant is to be used. Type of abrasive: steel to be ground, use aluminum oxide Size of grain: surface not precision-finished, medium grain about 46 grit Grade: medium-grade wheel will break down reasonably well, use grade J Structure: steel of medium hardness, wheel should be medium density: about 7 Bond type: operation standard surface grinding and since coolant is to be used, a vitrified bond should be selected After all factors have been considered, an A46-J7-V wheel should be selected to rough-grind SAE 1045 steel.

Selecting a Grinding Wheel for a Specific Job: Example 2
It is required to finish-grind a high-speed steel milling cutter on the cutter and tool grinder. Type of abrasive: cutter is steel, an aluminum oxide wheel should be used Size of grain: must have a smooth finish, a medium to fine grain (60 grit) Grade: cool-cutting wheel be used to prevent burning, medium-soft grade J Structure: smooth cut, medium-dense wheel should be used, use a #6 Bond type: most cutter and tool grinders designed for standard speeds, vitrified bond should be used; when speed excessive for wheel size, a resinoid bond should be used The wheel selected for this job (disregarding the manufacturer’s prefix and records) should be A60-J6-V.

Precautions to Observe When Handling and Storing Grinding Wheels
Never handle wheels carelessly Treat them as precision instruments Dry at a reasonable temperature Store wheels properly Straight or tapered wheels best stored on edge in individual racks to prevent rolling Thin, organic bonded wheels laid on flat horizontal surface to prevent warping Small cup and internal wheels put separately into boxes, bins, or drawers Large cup and cylindrical wheels should be stored on flat sides with packing between wheels

Wheel Grade Faults Grade of grinding wheel most important and difficult to select to suit workpiece material and grinding operation Wise to start medium-grade wheel such a J Note performance, adjust grade until best grinding conditions are reached Know characteristic of wheels that are too soft or too hard

Characteristics That Indicate Wheel Too Soft
Breaks down too fast Poor surface finish Cuts freely Sparks out quickly Difficult to maintain size Scratches (fishtails)

Characteristics That Indicate Wheel Too Hard
Wheel glazes quickly Loading (material ground fills voids) Burned work surface Squealing noise Doesn't cut freely Inaccurate work dimensions Surface finish get progressively better Won't spark out Heat checks

Inspection of Wheels Inspect wheels after they have been received
Damage might occur during transit Suspend and tap lightly with screwdriver handle for small wheels or with wooden mallet for larger wheels Vitrified or silicate wheels give clear, metallic ring when sound Organic-bonded wheels give duller ring Cracked wheels do not produce ring

Diamond Wheels Used for grinding cemented carbides and hard vitreous materials Manufactured in variety of shapes Straight, cup, dish, and thin cutoff wheels Wheels ½ in. diameter or less have diamond particles throughout wheel, larger than ½ in. made with diamond surface on grinding face only

Diamond Grain Sizes Grain sizes range from 100 to 400
Proportions of diamond and bond mixture vary with application Diamond concentration identified by letter A, B, or C C concentration contains four times number of diamonds in an A concentration Mixture coated on grinding face of wheel in thickness ranging from 1/32 to 1/4 in.

Three Types of Bonds for Diamond Wheels
Resinoid-bonded Give maximum cutting rate and require little dressing Remain sharp for long time and well suited to grinding carbides New development has been to coat diamond particles with nickel plating before mixed with resin Reduces tendency to chip and results in cooler-grinding, longer lasting wheels

Vitrified-bonded wheels
Metal bonds Generally nonferrous Particularly suited to offhand grinding and cutting-off operations Holds form extremely well and does not wear on radius work on small areas of contact Vitrified-bonded wheels Remove stock rapidly but require frequent cleaning with boron carbide abrasive stick to prevent loading Suited for offhand and surface grinding of cemented carbides

Diamond Wheel Identification
Method differs from that used for other grinding wheels ASD 100 R 75 B99 1/8* Grade Res. Metal Vit. H R L Q H P J N R J R L O T L T N P N Grit Size S S S S S 100S S Depth of Diamond Section 1/ / /4 Bond Modification Numeral to designate special bond modification. Example: Resinoild – 6 and 22 This symbol may be sometimes omitted. Abrasive Diamond D = Mined SD = Human-made ASD = Armored diamond Note: No grade shown for hand hones * Manufacturer's identification symbol Concentration Low = 25 50 75 High = 100 Bond Type B = Resinoid M = Metal V = Vitrified

Cubic Boron Nitride Wheels
Recognized as superior cutting tools for grinding difficult-to-machine metals Have more than twice hardness of conventional abrasives Also toughness to match so cutting edges stay sharp longer with much slower wear rates Prolonged cutting capacity and high thermal conductivity help prevent uncontrolled heat buildup (reduce chances of glazing) Thermally and chemically stable at temperatures above 1832ºF

Hardness of Various Metals and Abrasives
Knoop Hardness Scale 7000 6000 5000 4000 3000 2000 1000 Diamond CBN Vanadium AL2O3 Tungsten High Steel Soft Steel Carbide Carbide HRC HRB 85

Properties of Cubic Boron Nitride Wheels
Contain four main properties necessary to grind extremely hard or abrasive materials at high metal-removal rates: Hardness Abrasion resistance Compressive strength Thermal conductivity

Wheel Selection Type of wheel selected and how used will affect metal-removal rate (MRR) and life of grinding wheel Generally affected by: Type of grinding operation Grinding conditions Surface finish requirements Shape and size of workpiece Type of workpiece material

Wheel Selection Guidelines
CBN grinding wheels available in complete range of shapes and sizes Individually engineered wheels available to suit specific systems Constructed with precision, preformed core with abrasive portion on grinding face of wheel Usually between 1/16 to 1/4 in. in depth Generally one readily available to suit any grinding operation

General Guidelines for Using CBN Grinding Wheels
Select bond Specify normal wheel diameters and widths Choose largest abrasive mesh size that produces desired finish Choose wheels with optimum abrasive concentration

Coated Abrasives Consist of flexible backing (cloth or paper) to which abrasive grains have been bonded Two purposes Metal grinding and polishing Coarse-grit used for rapid removal of metal; fine grits used for polishing Two types Natural: garnet, flint, and emery Manufactured: aluminum oxide, silicon carbide .

Selection of Coated Abrasives
Emery (natural abrasive) Black in appearance Used to manufacture emery cloth and emery paper Grains not as sharp as artificial abrasives Generally used for polishing metal by hand

Aluminum Oxide (manufactured abrasive)
Gray in appearance Used for high-tensile-strength materials Characterized by long life of cutting edges Hand operations use grit for roughing and 120 to 180 grit for finishing Machine operations use grit for roughing and grit for finishing operations Silicon Carbide (manufactured abrasive) Bluish-black in appearance Used for low-tensile-strength materials Selection of grit size same as aluminum oxide

Coated Abrasive Machining
Over past few years, coated abrasive machining has become widely used in industry Improved abrasives and bonding material, better grain structure, more uniform belt splicing, and new polyester belt backing, abrasive belt machining Now capable of grinding to less than .001in tolerance and surface finish of to to 20 µin.

The Jig Grinder.

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