drilling

DRILLING

When drilling Chemcast® GP, best results will be obtained using standard twist drills which have been modified as described below. High speed steel drills Is should be used.

Specially ground drills for Chemcast® GP are available from Chemcast® GP Distributors.

MODIFICATION OF DRILL

Instructions for Sharpening Bits for Use on Chemcast® GP

Chuck the bit in an electric drill. Insert the bit through a wood or metal bushing clamped to the support rest of an abrasive wheel. The bushing must be so clamped that an approximate 60 degree total included angle is ground on the bit. With the electric drill operating, insert the bit through the bushing so that it contacts the face of the rotating abrasive wheel, thus grinding a conical point on the bit. Remove the bit from the chuck.

Using the side of the rotating abrasive wheel, grind a zero rake angle on each of the cutting edges. This provides a scraping. rather than a cutting action.

Then using the face of the rotating abrasive wheel, grind a clearance behind each of the cutting edges of about 8 to 12 degrees. Do not touch the cutting edges!

The resulting bit will produce good quality surfaced holes in Chemcast® GP, and will eliminate the problem of breaking out of the bottom surface when the holes are drilled through. Pickup of masking paper adhesive will also be eliminated.

DRILLING TECHNIQUES

Whenever holes in Chemcast® GP penetrate the stock, standard twist drills should be modified as described above, regardless of depth or diameter. A 60° tip angle allows the drill to emerge from the second surface without fracturing the Chemcast® GP.

When holes are to be drilled which do not penetrate the Chemcast® GP, the rake and lip clearance angles of the drill should be modified as described above 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 Chemcast® GP but better surfaces result if coolant is used when drilling large deep holes. Water or a 10% solution of soluble oil in water make satisfactory coolants and are best fed into the hole by filling a pilot hole nearly as deep as the finished hole. If masked Chemcast® GP 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 Chemcast® GP by filling a pilot hole with a wax stick, then re-drilling 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, fly cutters or trepanning tools. The cutters of the latter should be ground to zero rake angle and adequate back clearance, just as lathe tools are ground.

Chemcast® GP 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 which penetrate the second surface, it is desirable to back up the Chemcast® GP with wood and slow the feed as the drill point breaks through. For accuracy and safety, Chemcast® GP should be clamped during drilling.

Machining Acrylic Sheet