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adapted from a tip by Morgan Hall on the Metalworking FAQ
How a simple twist drill manages to create a non-circular hole:
You can model the working end of a drill bit as a single straight line of finite length. If you fix one end and try to rotate it, the opposite end of the line sweeps out an arc. (the drill flexes) After about 1/3 revolution, the stuck end breaks free and sweeps out another arc while the formerly free end sticks. With alternate ends sticking, then breaking free, the arcs will form a kind of polygon with arcs of radius equal to the drill's diameter. After the first cuts, the "corners" of the polygon tend to stop the sweeping cut for each drill flute. The most common I've seen is the triangular hole, but other polygons are definitely possible. I suspect that this occurrence is related to some sort of resonance in the drilling setup.
The suggestions that followed may be useful to anyone trying to drill holes. Some of them may qualify as 'obvious' but they're still worth bearing in mind...
Ensure the drill is sharp.
Make sure the work is firmly clamped
Don't try and run the bit too fast for the drill size and work material.
Don't force the feed rate; as with *any* cutting process, let the cutter do the cutting.
Keep as much of the drill in the chuck as possible. The more flexibility there is in the drill, the more likely you are to have problems.
When drilling thin material, it is often useful to provide some form of backing clamped to the work. This has the added advantage of keeping the burrs to a minimum.
The drill tip may need to be ground to a different angle, depending on the material being worked.
An undersize pilot hole is often a good idea. If you are drilling using a mark made with a centre punch and the tip of the drill is larger than the mark, you are unlikely to get accurate placement.
Don't forget to use a cutting lubricant
The quality of the hole is only going to be as good as the machine you are using will allow. If the drill spindle is sloppy, there may be nothing you can do about it.
As a final comment, if you really want a round, accurately sized hole, you are unlikely to get it with a twist drill. Drill undersize and use a reamer if it's important.
Another alternative to very finely finished holes is to force a hard
polished sphere through a slightly undersized hole.
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