With one exception, all rules applying to drilling in metals apply equally well to drilling cast resin plastics. Ordinary metal drills of any type, as well as reamers and countersinks, mounted in either hand-drills, drill-presses or lathes may be used.

Proper Ancle For Drill Bits

Fig. 3. Proper ancle for drill-bits need on plasties note that the customary angle is ground off, leaving the face of the point at 90 degrees to the work. At the left is shown the corresponding lathe-tool angle.

The only exception to the usual rules is that in drilling holes from diameter upwards, it is best to "back off" the cutting edges of the drill-bit, in the same manner as a drill is treated for drilling in brass, i.e., giving it a "negative rake". This operation consists merely in a light touch of the cutting edge of the bit to an emery wheel, removing the angle of the drill as it approaches the work. This is illustrated in Figure 3. Only a very small amount of metal need be removed.

Various other cautions that might be born in mind are as follows. Never drill deep holes at top speed without frequent backing out for the purpose of clearing out the chips, as otherwise the material might pack and prevent the free escape of these gasses formed when the material is machined at high speed. The pressure of these gases occasionally results in a small explosion in a deep hole, causing the splitting of the work being drilled.

If a quantity of taper-reaming or countersinking is being done, it might also be wise to grind the angle of these tools so that they form a right angle when approaching the work, as otherwise chatter and digging-in are possible.

The screw-point type of wood-augur can never be used on plastics, as it would immediately split the material, and the sharp- knife-shaped angles of the cutting edges would only chew up the work. The straight-flute type of pin-drills however can be used, as well as fish-tails and fly-cutters, provided only that the angle of the cutting-edges is properly ground. As in lathe and other tool bits, it will be found that hard bronze will probably stand up as long as tool steel or longer, and that the very hard special steels will save time on quantity work by holding their edges much longer.

One of the most frequent drilling operations in plastics is the drilling of very small holes (No. 55) for drive-screws. These holes are the size of the shank of the drive-screw, leaving its flutes or fins to cut their own threads as the screw is driven into the material. It can easily be seen that if the hole is drilled the least bit oversize, or the drill allowed to wobble at all, the drive-screw will pull out easily, especially since many of them are only J4" long or less. It is therefore necessary to use care in drilling such holes, to use a good bit with an accurately-centered hole and mount it in an accurately-centered chuck, and to leave as little of the bit projecting from the chuck as possible, to reduce wobble and give it strength.

Because of the heat generated in working cast resins, and the consequent expansion of the material, the cool finished hole is smaller than the actual size of the bit used.

In the case of fly-cutters depending on a center-hole for the accuracy of the circle being cut, it is advisable to drill this guide-hole clear thru the material and into the surface on which it is lying, as the plastics itself makes a poor bearing easily enlarged when carrying a load of this sort.

As to speeds, it will be found that in all machining operations on this material, higher speeds are preferable if the equipment being used is powerful enough.

Routing with a highspeed fluted cutter mounted in the drill-press is another common operation, as is also the method of carving on a drill-press by which a jig-sawed pattern of the design to be carved is glued to the bottom of the sheet of plastic to be carved, and the sheet is guided by a pin mounted in the drill-press table directly under the point of the carving cutter. This method of course only provides for carving of uniform depth, but it is very useful when effects of this type are desired.