Manufacturing Processes

1. Manufacturing Processes Chap. 23 - Machining of Other Shapes

2. Milling • Definition • Generation of a surface by progressive chip removal. • Workpiece is fed into rotating cutter rotating along various axes with respect to workpiece. • Material removal rates are high as cutter has multiple teeth. • One of the most versatile machining processes available. Can achieve great surface finish.

3. Milling Categories • Peripheral or Slab Milling: • Teeth located on the outside of the cutter body create surface. • Surface is parallel to cutter axis. (See Fig. 23.2) • Can make both flat and contoured surfaces (a function depending on cutter shape). • Cutting of horizontal surfaces on horizontal millling machine is called SLAB milling.

4. Peripheral Milling • Parameters • Feed per tooth ft (in inches per tooth) • Cutting Speed V (in feet per minute) • Both selected by operator (function of material, tool, process). • See Table 25-1. • Cutter diameter D • Width of cut W = cutter width • Length of Cut L • Depth of cut t

5. Peripheral Milling

6. Peripheral Milling • RPM • determined from cutting speed. • N = 12 V / P D • Table Feed Rate • fm =ft N n - n = number of cutter teeth • Cutting Time CT = L + LA / fm • LA = ( t ( D – t ) )1/2 • MRR = volume removed /time • MRR = L W t / CT = W t fm(in3/min)

7. Milling Categories • Face Milling: • Surface is generated at right angles wrt the cutter axis. (See Fig. 23.2 b) • Most cutting is done by peripheral areas of teeth. • Some finishing is done by the bottom of teeth. • Can do on both horizontal & vertical mills.

8. Face Milling

9. Face Milling • Parameters • Feed per tooth ft (in inches per tooth) • Cutting Speed V (in feet per minute) • Both selected by operator (function of material, tool, process). • See Table 25-1. • Cutter diameter D • Width of cut W = cutter width • Length of Cut L • Depth of cut t

10. Types of Milling Processes • Slab (Peripheral) Milling • Cutter axis is parallel to workpiece surface to be machined. • Cutter has teeth along its circumference. • Teeth can be straight or helical.

11. Conventional vs. Climb Milling • Conventional Milling (or up milling) • Most common method of milling. • Work moves in direction opposite to cutter (at normal cutting point). • Good when cutting contaminated or scaled-surface materials- will not reduce the tool life. • Tool will have tendency to chatter. • Part requires proper clamping because tool tends to pull work upwards.

12. Conventional vs. Climb Milling • Climb Milling (or down milling) • Work moves in direction same direction as cutter (at normal cutting point). • Downward component of cutting force holds work in place. • High forces occur when teeth engage: must have a rigid setup. • Not suitable for parts having surface scale (hot worked, forged or cast pieces); scaling can damage teeth. • Use with CNC machining.

13. Face Milling • Cutter is mounted on spindle with axis of rotation perpendicular to work surface. • Can have conventional and climb milling as well. • Cutter must be selected so it doesn’t interfere with other setup elements. • Rule of thumb: Cutter Diameter D to width of cut w should be 3 : 2 min.

14. End & Straddle Milling • End Milling (fig. 23.2 c) • Cutter typically rotates on axis perpendicular to surface. • Ball nose endmills are used for curved surface production. • Straddle Milling • Two or more cutters are used to machine various surfaces. • 2+ cutters are mounted on arbor to machine parallel surfaces.

15. Other Milling Operations • Form Milling • Produces a specific curved profile. Used for gear cutting. • Circular cutters • Used for slotting or slitting. • Slotting produces a blind groove where slitting splits the material. • T-Slot cutters are used to mill T-slots.

16. Other Milling Operations • Shell mills • Have hollow inside, allow for mounting on a shank or arbor. • Fly-cutting • Milling with a single cutting tooth mounted on a high speed spindle.

17. Some Design Guides for Milling • Use standard milling cutters. Avoid special tooling. • Design features should consider tool shape, size, depth, width and corner radii. • Avoid internal cavities, pockets with sharp corners.

18. General Troubleshooting Guide

19. Milling Machine Components • Work Table - where work is secured using T slots, moves longitudinally. • Saddle – supports table, moves transversely. • Knee – supports saddle, gives vertical movement to table to adjust d.o.c. • Overarm – (horizontal machines) to accommodate arbor lengths. • Head – contains spindle and cutter holders.

20. Broaching • Used to machine internal external surfaces of various cross sections. • Is a linear cutting motion. • Usually hydraulically activated. • Broaching Machines can be horizontal or vertical, push or pull type. • Broach is a long multitooth cutting tool.

21. Broaching • Can remove as much as the depth of cut of each tooth. • Material thickness removed per stroke can be up to 1.5”. • Good dimensional accuracy and surface finish. • Can be expensive but justified in large production runs. • See figure 23.21 for sample products made.

22. Broaching • Broach parameters • Rake angle depends on material being cut. Ranges from 0 to 20 deg. • Tooth depth and pitch must be large enough to accommodate chips produced in broaching. • At least two teeth should be in contact at all times. Pitch = k (l)1/2 • Where k = 1.76 (metric) or 0.35 when l is in inches. (l is the length of surface to be cut).

23. Sawing • Cutting Tool Blade has small teeth . • Can produce multiple shapes. • Effective bulk material removal process. • Width of cut is called kerf. • At least two or three teeth should always be engaged in the work to prevent snagging. • The thinner the stock, the finer the saw teeth should be and # of teeth / unit length.

24. Saw Types • Hacksaw • Has straight blades and reciprocating motion. • Used to cut off bars, rods, etc. • Not as efficient as bandsaws because cutting takes place during only one of the reciprocating strokes. • Circular Saw • For high production rate cutting. • Blade is circular, produces relative smooth surfaces & good accuracy.

25. Saw Types • Band Saw • Has long continuous flexible blade and cutting action. • Vertical saws are used for contouring. • Horizontal saws are used for cutoff.