MACHINING PLASTICS Machinery's Handbook, 31st Edition
603
Diameter of Drill
α Helix Angle
Flute Margin
Relief Angle
β
(b)
Specially Formed Twist Drill
(c) Specially Formed Twist Drill
Point Angle θ
(a)Twist Drill
(d)Modified Point Angle Drill
(e) Flat Drill (f) Specially Formed Flat Drill
(h) Hollow Drill
(g) Chit Drill
(Three-Point Drill)
Fig. 26. Drill Designs Used for Drilling Plastics Clearance angles of 9 to 15 degrees are usually sufficient to prevent the drill flanks from rubbing in the bottom of the hole, but acrylic materials require angles of 12–20 degrees. Tests may be needed to determine the drill diameter for accurately sized holes, allowing for thermal expansion and elastic recovery. Reaming may be used to size holes accurately, but even those diameters produced may also be affected by thermal expansion of the plastics. Close-fitting bushings in drill jigs may increase friction on the drill and cause swarf to plug up the drill flutes. For positioning accuracy, removable templates may be used to spot the hole position, then set aside for the drilling to be completed. Pilot holes are not necessary, except when the hole is to be reamed or counterbored. Frequent pull-out (peck drilling) to remove chips and compressed air cooling may be needed, especially for deep holes. Drilling and reaming speed and feed suggestions for various materials are shown in Table 9. These speeds and feeds are approximate, and can be increased where there is no melting, burning, discoloration, or poor surface finish. Drilling is best done with commer - cially available drills designed for plastics ( Fig. 26 ), usually having large helix angles, narrow lands, and highly polished or chromium-plated flutes to expel swarf rapidly and minimize frictional heating. Circle cutters are often preferred for holes in thin materials. Drills must be kept sharp and cool, and carbide tools may be needed in high production, especially with glass-reinforced materials. They must be cooled with clean compressed air to avoid contam - ination. Aqueous solutions are used for deep drilling because metal cutting fluids and oils may degrade or attack the plastics and may cause a cleaning problem. Plastics parts must be held firmly during drilling to counter the tendency for the tooling to grab and spin the work.
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