1. CNC Machining Centers
Unit 78

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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
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10. Primary Components of a Machining Center
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11. 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

12. 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

13. 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

14. 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

15. 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

16. face milling cutters two-flute end mill four-flute end mill
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17. Stub Drills high-helix drill core drill oil hole drill
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18. Taps gun stub flute spiral flute fluteless
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19. 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
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20. 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
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21. 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
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22. 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

23. Common
Toolholder
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24. Variety of Toolholders
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25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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
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