Commonly available drill bits work well for drilling in metal. However, when drilling a hole in plastic, especially thin plastic, regular drill bits do not work well at all. They tend to grab the work piece – especially when nearing the bottom of the hole in thin material.
Unless the work is securely clamped down, the work usually ends up catching and spinning in a dangerous manner. Also, even if the work is securely clamped down, more often than not the end result is cracking or other damage to the work. The reason for this is the sharp edge on the drill, while necessary for drilling in metal, it is not needed for plastic and causes the drill to grab and the plastic work piece to spiral up the drill flutes.
Although you can buy special drill bits for use in plastic, there is a simple modification that can be made to a regular drill bit that allows it to work well in plastic.
All that is necessary is to intentionally dull the cutting edge of the drill bit. This is accomplished by carefully grinding or honing a small flat surface on both cutting edges of the drill bit. This can best be done using a small slip stone and the resulting flat surface need not be very wide – it is not critical as to the exact width of the flat.
This modification changes the cutting action of the drill edge into more of a scraping action.
Important Note: The flat surface must be ground across the entire cutting edge and parallel to the axis of the drill bit. Also, this modification pretty much ruins the drill for use in steel, although it may still work well enough in softer metals such as brass or aluminum.
The example modified drill bit shown in the photograph below is actually ground more than necessary in order for it to be easier to see – it still works just as well. (The red circle in the photo encloses one of the ground drill edges.)
I have confirmed that this drill modification works and works very well. I drilled some test holes in some 0.050 inch sheet styrene. This material is quite brittle and I found it impossible to drill a hole in it without cracking and other damage occurring using an unmodified drill bit.
As shown in the other photograph below, when using a modified drill bit, I was able to drill the same ½ inch diameter holes without any cracking or grabbing of the plastic. The result was clean round holes.
It was actually quite amazing to see the difference in the drilling action and the excellent end result.
I also used the modified drill to drill a ½ inch diameter through hole, ¾ inch deep in some nylon rod – also with excellent results.