Craigslist Seduces me Again

I have a bad habit of looking at tools on Craigslist, even when I’m determined not to buy anything. Most of the time, I’m fine, but every so often, I just have to reply.

Last week I found something really cool: a Dumore sensitive drill press. It’s like any other drill press, but it needs constant reassurance and has a poor body image. I think I’ll name it Caitlyn.

That was humor. It may not have been obvious.

How about “Dudley Dumore”?

I guess not.

Sometimes it’s interesting to go into the reasons why an unusual tool exists. This may not be one of those times, but I will do it all the same.

A sensitive drill press is a small machine that drills really small holes at very high rotational speeds. It’s a nice thing to have if you drill holes in circuit boards for electronics projects. Circuit board material is very hard on bits, so the best move is carbide, which stands up well to abrasion. Small bits require high speeds and low runout, and because carbide is very hard, it permits speeds that are even higher than small HSS bits.

Why do small bits require high speeds? I will ‘splain.

When you use a metal cutting tool such as a drill bit, what you have is a sharp metal edge being forced across whatever it is you’re cutting. As the edge cuts the work, heat is generated. If the bit gets too hot, the edge gets soft, and then the work rubs the edge off. Then you have an aggravating tool that just slides on the work.

This is why you keep getting stuck and ruining drill bits when you floor your hand drill on a 1/4″ hole. You’re supposed to limit your speed and add a little lubricant to reduce heat. About 0.001% of American men really know how to use a $20 drill.

Consider a rotating cutting tool. The speed at the outer edge will be higher than the speed farther in. The linear speed of a point on a rotating object equals the radius times the frequency of rotation (omega times r, as we former physicists like to say), so as the radius gets small, the speed drops off.

Say you have a half-inch bit, moving at x RPM. If you reduce the size to 1/8″, the speed at the outside of the bit is quartered, so to get the same cutting speed at the outer edge, you have to multiply the RPM by four. You can go slower, but you will spend a very long time drilling every hole, and besides, in addition to tolerating high speeds, carbide actually requires high speeds to cut well, for reasons I do not know.

One interesting thing about all this is that every drill bit has a cutting speed of 0 at the center, so it’s not really cutting until you move farther out. You’re cutting the metal surrounding the center and sort of pushing the metal in the center out of the way. This is why it can be helpful to start big holes with small bits. You can drill a 1/8″ pilot hole for a 1/2″ hole, running at high speed, and then when the 1/2″ bit follows up, it doesn’t have to worry about the 1/8″ of metal in the center of the hole that isn’t being cut very fast.

Sensitive drill presses work with really tiny bits. One example people have mentioned to me is the #80 bit, which is 13.5 thousandths of an inch in diameter. That’s a little over four human hair widths. You can’t just cram that in your Harbor Freight drill press and expect good things to happen.

Let’s say you have a #80 bit, and you’re cutting mild steel, which means you want the outer edge of the bit to move at around 100 feet per minute. You will want the drill to turn at about 28,000 RPM. This is around 10 times what your Harbor Freight special will do. You need a sensitive drill press.

The drill press I bought turns at 17,000 RPM. That’s not 28,000, but it beats a big drill press turning at 3000.

If speed is the main thing, why not call them “really fast drill presses” instead of “sensitive”? I don’t know. These machines have little tables you can move up and down with the work (instead of lowering the spindle), and that gives you a delicate feel for what you’re doing. I guess Dumore thinks that’s more exciting than the speed.

The drill press I found on Craigslist sells for over 900 dollars new. Wait till you see it. You’ll wonder where the money went. I’ll post a photo.

Here’s what I’m told about the expense: it goes into the chuck and motor. They have to turn the drill bit very precisely, with very little of the wobble machinists call “runout.” When you use a very expensive large drill press which is tuned perfectly, you can expect the bit to wobble about 0.003″ on each turn. This is not a problem when you’re drilling big holes in a toilet seat mount. When your drill bit is 13.5 thousandths wide, and your three-thousandths runout is almost a third of that, the drill bit will break.

The chuck on the Dumore is tiny, but it costs over a hundred dollars. A new motor retails for around $900 (like buying a new machine). Eliminating runout is not cheap.

I saw the drill press on Craigslist, and I knew exactly what it was. The price was $120. Come on. I was buying that.

I checked Ebay, and it seemed like they generally ran around $225. Low for a $900 tool. Some tools are like that. Crazy expensive new but merely expensive used.

The machine was 30 miles away, and my first chance to get there was on a Friday, during rush traffic. I can’t say “rush hour,” since the rush is pretty much a five-hour ordeal here. It took about an hour and a half each way. Ordinarily, I wouldn’t make that drive to buy air if I was drowning, but it was a miracle that the press was still available, and I had been burned by catching a similar $45 machine too late.

I drove to the seller’s house, had him turn the machine on, and handed him my money. I asked him where he got the drill press. This is the horrible part of the story. It came from his job. They had three Dumores, and he found out they were THROWING THEM OUT. He snagged one before it got to the dumpster.

So right now, two more drill presses are sitting in the landfill.

Someone needs a punch in the mouth.

The press runs fine. I haven’t checked the runout, but I would have taken it even if the chuck had needed work. Even if it can’t drill a 0.0135″ hole, it can drill a #31 hole much better than my big drill press.

Now you know what a sensitive drill press is and why they cost so much. Are you not entertained?

I think I’ll go turn it on and listen to it hum.