Whenever holes in Plexiglas® acrylic sheet penetrate the stock, standard twist drills should be modified, as previously described, up to a 1.00 inch diameter. A 60° tip angle allows the drill to emerge from the second surface without fracturing the sheet.
When holes are to be drilled that do not penetrate the Plexiglas® acrylic sheet, the rake and lip clearance angles of the drill should be modified as described, but tip angles larger than 60° clear chips from the hole better.
Shallow holes having depth to diameter ratios less than 3:1 should be made with drills ground to a tip angle of 90°. Large deep holes with depth to diameter ratios greater than 3:1 should be made with drills having a tip angle of 118°.
Coolant is seldom required when drilling Plexiglas® acrylic sheet, but better surfaces result if coolant is used when drilling large deep holes. Water or a 10 percent solution of a compatible oil in water or kerosene makes a satisfactory coolant and is best fed into the hole by filling a pilot hole nearly as deep as the finished hole. If masked sheet is drilled using coolant, the wet masking paper should be removed promptly. If the masking paper dries in place, a residue may be left behind when the sheet is finally unmasked.
Unusually clear and smooth deep holes can be made in Plexiglas® acrylic sheet by filling a pilot hole with a wax stick, then redrilling to the final diameter. The wax lubricates the cut and supports and expels chips during drilling. A mixture of tallow and kerosene, mixed to the consistency of a slurry, also lubricates well and acts as a coolant.
Large-diameter holes can be cut with hollow end mills, circle cutters or trepanning tools. The cutters of the latter should be ground to a 0° rake angle and adequate back clearance, just as lathe tools are ground.
Plexiglas® acrylic sheet may be drilled using any of the conventional tools: portable electric drills or flexible shafts, drill presses or lathes. The drill should always run true since wobble will affect the finish of the hole. When drilling holes that penetrate the second surface, it is desirable to back up the sheet with wood and slow the feed as the drill point breaks through. For accuracy and safety, Plexiglas® acrylic sheet should be clamped during drilling.
Tapping and Threading
Standard machine shop tools and procedures may be used for tapping and threading. Wherever possible, the “national coarse thread” system should be applied. On diameters over 1 inch, Acme threads are preferred to V-threads. The root should be radiused.
When transparency is a requirement for the tapped threads, a wax stick should be inserted in the drilled holes before tapping. As in drilling, the wax lubricates the cuts and supports and expels the chips, resulting in cleaner, more-transparent threads.
Taps should be backed out often to clear the chips. The wax stick is particularly useful for blind holes, since it tends to clean out the chips as they are formed.