Special Milling Operations Unit 63

Objectives Set up and use the rotary table to mill a circular slot Set up and mill internal and external dovetails Jig bore holes on a vertical mill

Vertical Milling Machine Versatility increased using specially shaped cutters and accessories Rotary table accessory permits milling of radii and circular slots Boring head in spindle and measuring rods on tale allow accurate jig boring of holes

The Rotary Table Circular milling attachment Used on plain universal vertical milling machines and slotters Provide rotary motion to workpiece Cut radii, circular grooves and circular sections Two types hand feed and power feed

Rotary Table Construction Base bolted to milling machine table Rotary table fits into base Worm gear on bottom of rotary table Worm shaft mounted in base and meshes with and drives worm gear Handwheel mounted on outer end of worm shaft Vernier scale on collar permits setting to within 2 minutes of a degree

Rotary Table Construction T-slots cut into top surface to permit clamping of work Hole in center of table accommodates test plugs for easy centering of table with machine spindle Some have indexing attachment instead of handwheel

Indexing Attachment Serves same purpose as handwheel Permits indexing of work with dividing head accuracy Worm and wormwheel ratio not necessarily 40:1 Other common ratios: 72:1, 80:1, 90:1, 120:1 Example: Calculate indexing for 5 equally spaced holes on a circular plate using a rotary table with an 80:1 ration

Rotary Table with Powerfeed Worm shaft connected to milling machine lead screw drive gear by special shaft and end gear train Rate of rotation controlled by feed mechanism of milling machine Operator control by automatic feed lever or handwheel Suited for production work and continuous milling of numerous small, identical parts

To Center Rotary Table with Vertical Mill Spindle Square vertical head with machine table Mount rotary table on milling machine Place test plug in center hole Mount indicator with grasshopper leg in machine spindle With indicator just clearing top of test plug, rotate machine spindle by hand align plug with spindle

To Center Rotary Table with Vertical Mill Spindle Bring indicator into contact with diameter of plug and rotate spindle by hand Adjust machine table by longitudinal and crossfeed handles until dial indicator registers no movement Lock machine table and saddle, recheck alignment Readjust if necessary

Using Special Plugs to Center Problem: work on several identical workpieces that have machined hole in center - need to align Special plug made to fit center hold of workpiece and hole in rotary table Once machine spindle aligned with table, each succeeding piece can be aligned quickly and accurately by placing over plug

Method to Center Workpiece on Rotary Table (no plug) Align rotary table with vertical head spindle Lightly clamp workpiece to rotary table in approximate center Do not move crossfeed or longitudinal feed Disengage rotary table worm mechanism Mount indicator in machine spindle or on milling machine table

Clamp workpiece tightly and recheck accuracy of setup Bring indicator into contact with surface to be indicated and revolve rotary table by hand With soft metal bar, tap work until no movement registered on indicator in complete revolution of table Tap away from indicator movement Clamp workpiece tightly and recheck accuracy of setup Note: If center-punch mark must be aligned, wiggler mounted in milling machine spindle instead of indicator.

Radius Milling Procedure Align vertical milling machine spindle at 90º to table Mount circular milling attachment Center rotary table with machine spindle Use test plug in table and dial indicator in spindle Set longitudinal feed dials and crossfeed dial to zero

Mount work on rotary table Align center of radial cuts with table center Can use special arbor or wiggler Move either crossfeed or longitudinal feed an amount equal to radius required Lock both table and saddle Mount proper end mill Rotate work (using handwheel) to starting point of cut Set depth of cut and machine slot to size

Milling a T-Slot Procedure Consult handbook for T-slot dimensions Lay out position of T-slot Square vertical milling machine spindle with machine table Mount work on milling machine Align vise or slot if clamped with table

Machine center slot to proper depth of T-slot, using end mill Mount an end mill having diameter slightly larger than diameter of bolt body Machine center slot to proper depth of T-slot, using end mill Remove end mill, mount proper T-slot cutter Set T-slot cutter depth to bottom of slot Machine lower part of slot  

Milling Dovetails Used to permit reciprocating motion between two elements of a machine Composed of external or internal part Adjusted by means of a gib Machines on vertical milling machine or horizontal mill with vertical mill attachment Dovetail cutter Special single-angle end mill type of cutter ground to angle of dovetail required

Procedure for Milling an Internal Dovetail Refer to handbook for method Check measurements of workpiece in which dovetail is to be cut Remove all burrs Lay out position of dovetail Mount workpiece in vise and clamp on rotary table (long work – bolt to table)

Mount end mill of a diameter narrower than center section of dovetail Indicate side of workpiece or slot layout to see that it is parallel to line of table travel Mount end mill of a diameter narrower than center section of dovetail Start end mill and touch up to side of work, after checking cutter rotation Set crossfeed dial to zero Move work over until end mill in center of dovetail Distance from side of workpiece to center of dovetail plus half diameter of cutter

Lock saddle in position and set crossfeed dial to zero Touch edge of cutter to top of work Move work clear of cutter and set depth of cut Lock knee in position Depth of slot should be .030 to 0.50 in. deeper than bottom of dovetail Mill channel to width of cutter

Move work over amount equal to half difference between machined slot size and size of dovetail at top Take finish cut along one side of work Check width of slot Move work over to finished width of top of dovetail (Check for backlash) Cut second side and check width of slot Return saddle to zero

Set depth for roughing cut Mount dovetail cutter Set depth for roughing cut Should be about .005 to .010 in. less than finish depth Calculate width of dovetail at bottom Move work over .010 in. less than finished size of this side Note readings on crossfeed dial Rough out angle on first side

Move work over to other side the same amount from centerline and rough-cut other side Set cutter to proper depth Machine bottom surface (both sides) of dovetail to finished depth Using two rods, measure dovetail for size

Take finish cut on one side Move table over half the difference between rough dovetail and finished size Check for backlash Take finish cut on one side Move table over required amount, and finish other side Check finish size of dovetail Note: If work mounted centrally on rotary table, possible to rotate work a half-turn (180º) after 18 and take same cuts on each side of block for each successive step.

Procedure to Mill an External Dovetail Center cutter over dovetail position Remove as much material as possible from each side of external dovetail Cut to largest size of dovetail Mount dovetail cutter, and center it with workpiece Move work over ½ width of dovetail plus half diameter of cutter Allow 0.010 in. for finish cut

Take roughing cut Move work over equal amount to other side of centerline, taking up backlash Rough-cut second side Adjust work over, and take finish cut on one side Measure dovetail using two rods Adjust for finish cut on second side and take this cut Measure width of finished dovetail

Jig Boring on a Vertical Milling Machine Vertical milling machine does not have same lead screw accuracy as jig borer Must have some external measuring system Measuring rods and dial indicators, vernier scale, optical measure devices Measuring rods come in sets Both inch precision and metric precision

Measuring Rod Sets Inch precision end-measuring rods Supplied in sets of 11 rods hardened Two micrometer heads capable of measuring 4-5 in to accuracy of .0001 in. One for longitudinal settings and other for transverse Lengths of rods: 1, 2, 6 and 12 in rods Metric precision end-measuring rods Same set up as inch rods only in mm Accuracy of 0.002 mm

To Position Milling Machine Table Using Measuring Rods Set spindle to edge of work Clean trough and ends of stop rods and indicator rods Check indicator for free operation Place required number of rods in trough to take up space between stop rod and indicator rod

Adjust micrometer until extended enough to cause indicator needle to move half-turn Lock table Set indicator bezel to zero Increase rod and micrometer buildup by length of measurement between side of workpiece and hole location Move table along more than this required distance

Insert rods and micrometer head Move table back until needle moves half-turn and registers zero Lock table Recheck setting and adjust if necessary

Digital Readout Boxes Electronically controlled measuring devices Mounted on table and saddle Indicate table travel to accuracy of .005 or .0001 in. Digital readout display Indicates distance traveled through series of numbers visible Permits quick and accurate setting of machine table in both X and Y axes

Vernier Scale May be used with milling machine Mounted adjacent to feed screw collars Permits reading in .0001 in or 0.002 mm. Setting must be made in one direction only in order to remove backlash Note: Can position holes without scales by using the graduated feed collars. This method not as accurate.

Vertical Milling Machine Attachments Rack milling attachment Permits machining of racks and broaches on vertical mill Slotting attachment Fitted to back end of overarm (180º rotation) Operates independently of machine drive Transmits reciprocating motion to single-pointed tool by means of motor-driven eccentric Used for cutting keyways and slotting out small blanking dies

Die Sinking One important application of vertical milling machine Dies used in drop forging and die casting have impressions or cavities cut into them by die sinking Generally done by hand control of machine Followed by considerable filing and polishing Machine equipped with tracer head

Tracer-Controlled Machine Form of master transferred to workpiece by hydraulic tracer unit actuated by stylus Contacts master and moves cutter up or down with vertical or side travel of stylus Work and master fastened to table and travel at same rate Some machines equipped with ratio devices