Monday, September 24, 2007

Machining Lessons: Forstner Bits vs. Drill Bits

I've been using Forstner bits to hog out cylindrical holes in Delrin for some time. Usually I need a flat bottom hole that is 1.715" diameter and 2.05" deep. My 1 5/8" Forstner bit in the drill press makes a 1.620" diameter hole--and with a little care, the drill press is accurate enough on depth that if I making three pieces, I end up with one that is 2.049" deep, another that is 2.056", and the third might be 2.051".

Then I put the workpiece onto the lathe, and use a boring tool to expand the hole diameter from 1.620" to 1.715". A boring tool is a very slow but very precise method of accomplishing this end--you certainly wouldn't want to use a boring tool to make the entire hole!

One of the difficulties with using the drill press is that getting the Forstner bit exactly centered in the workpiece isn't easy. Consequently, I start out by putting the Forstner bit in the lathe to make a perfectly centered hole 3/10" deep. (A lathe is good for making perfectly centered holes; you have to work at it to make one that isn't perfectly centered.) Now, when I put the workpiece in the drill press, I just have to lower the Forstner bit into that hole, and adjust the mill vise to match.

Now, here's the learning part. It turns out that Forstner bits, while they make a very precise circular hole, do most of their cutting at the edges of the bit, and very little at the front of the bit. (There is some cutting going on, but the close you to get to the center, apparently, the less force the blade exerts.) When you are using Forstner bits on wood, it doesn't much matter, because nearly all woods are so soft that the bit just goes right through.

When you are using Forstner bits on Delrin, however, which is far, far harder than wood, you discover that a large diameter Forstner bit takes a long time to cut through a solid workpiece. On the drill press, I can make a 1 5/8" diameter by 2" deep hole in Delrin, but it takes several minutes. If I tried to do this on the lathe, where there is much less power, I am not even sure that I could do it.

The trick to using a large diameter Forstner bit on Delrin is to start with a much smaller diameter bit. If I start with a 1/2" Forstner bit, it cuts down through Delrin very quickly. Then you move up to a 7/8" Forstner bit. This is mostly cutting at the edge of the hole--but there's a 1/2" void under the part of the bit that isn't doing much cutting anyway. This gradual enlargement of the diameter with progressively larger Forstner bits works well--but it is definitely a bit slow!

So I started experimenting yesterday with using twist drills. (If you didn't know that there are different kinds of drill bits, a twist drill is probably the only kind of drill bit that you know about. To my surprise, the twist drill is very recent--patented in 1861.) Twist drills do effectively all their cutting on the front of the bit. Compared to a Forstner bit of the same diameter, a twist drill is much faster.

There are some downsides to twist drills. They tend to be a bit less precise in where the hole ends up in your workpiece. This is why you always start with a pilot hole produced with a center drill.

Because most twist drills are pointed, they don't produce a completely flat bottom--so you may want to follow them up with a Forstner bit.

On the plus side, I discovered that where the lathe, because of its lack of power, required me to start with a 1/2" Forstner bit to have any hope of cutting a hole in a reasonable time in Delrin, I could put an 11/16" twist drill in the lathe, and cut through in about the same time.

What this means is that in the future, instead of using Forstner bits sized 1/2", then 7/8", then 1 5/8" on the lathe, then use the 1 5/8" Forstner bit on the drill press, I will be able to use some of my twist drills so large that they look like props from the old TV series Land of the Giants on the drill press to hog out the hole, then switch to the 1 5/8" Forstner bit to finish the excavation.

No comments:

Post a Comment