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meincer
10-17-2006, 10:28 AM
I am very new with metal machining and need help with a very simple task that I can't seem to master. I want to make a collet to mount an 1/8" engraving bit into the 1/4" collet of my hand held high speed router.

Every time I try to drill a hole in the center of a 1/2" rod (using a center drill mounted in a drill chuck) it ends up be close to - BUT NOT EXACTLY - in the center of the rod. I need the resulting collet to have very little runout. What am I doing wrong? I am using an imported 7X12 lathe (I also have a mini mill, if that might help?).

I have even thought of drilling a hole, putting an exactly fitted rod into it, welding it on one end so it won't turn, and chucking this small rod into the lathe chuck to then turning down the bigger rod concentric to the small hole.

Is there a trick to getting a very accurate hole location?

dp
10-17-2006, 10:53 AM
You can drill the piece and then turn it between centers using a lathe dog. This will be only as accurate as your tail stock alignment, but that should be perfect :) The final hole should be sized and cleaned with a properly sized reamer.

http://en.wikipedia.org/wiki/Lathe_center

SGW
10-17-2006, 10:56 AM
Drilling (as you have found out) is at best an approximate way to get a hole where you want it.

You might try checking to see if your tailstock center is properly aligned with the headstock spindle; chuck a dial indicator in the headstock and (with power off, of course!) sweep the indicator around the inside of the tailstock socket. You may be able to improve things there, assuming the tailstock alignment is adjustable. That might help some.

Othewise, the sure way to get a hole where you want it is to bore it with a boring bar, but boring a fairly deep 1/8" dia. hole with a boring bar would be some trick.

Another question: are you sure the headstock chuck is holding the rod you're drilling concentrically? If you're using a 3-jaw chuck, that is likely only accurate to a few thousandths for concentricity, so that could be another source of your error.

Your idea of mouting the part on an arbor (rod) and turning down the o.d. is not bad...assuming you can hold the arbor truly concentric while you're doing it.

One other thing you could try is to drill a 3/32" hole, then follow that with a 7/64" end mill (which will not tend to wander the way a drill will), and follow that with a 1/8" reamer to finish.

Don't feel bad that you're having trouble with this. It's hard to get a hole EXACTLY where you want it!

lynnl
10-17-2006, 11:59 AM
Maybe I'm missing something here. But if it t'were me I'd just drill, then ream, the 1/8" hole in a short piece of oversize stock, then turn the outside dia. down to the desired size, both in one chucking. That would insure concentricity, regardless of runout in the 3-jaw or misalignment of the tailstock.

SGW
10-17-2006, 12:03 PM
The problem is though, as I understand it, is that the hole isn't getting drilled concentrically. At least not concentrically enough.

Frank Ford
10-17-2006, 12:17 PM
Yep, boring is the way to go because even short drills are bendy and don't follow a nice straight track. Reamers tend to follow an off center or crooked hole, too. If you have a stiff little 1/8" end mill you might be able to use it in your tailstock drill chuck to make a good enough concentric hole in a piece of brass after drilling it just a trifle undersize.

I bought a teeny little carbide boring bar for just this job, and have made a bunch of collets for various tools. Once you get the hang of it, it's good fun. If I were using a chuck, I'd do what lynnl suggested, and turn the entire thing without taking it out to ensure concentricity.

Here's a piece I wrote for my Web site that contains more of the process:

http://www.frets.com/FRETSPages/Machining/Projects/BridgePinLathe/bridgepinlathe.html

dp
10-18-2006, 03:48 PM
Here's a piece I wrote for my Web site that contains more of the process:

http://www.frets.com/FRETSPages/Machining/Projects/BridgePinLathe/bridgepinlathe.html

I always find something of interest when I visit your site, Frank. I just came into possession of a lap steel guitar and have decided to attempt to make a steel for it from your example. I'm going to start with brass and if that works out I'll try it with SS. I haven't played a steel guitar in nearly 50 years, so I probably won't notice if the brass slide deaden's the sound.

Perhaps you could tell me why some slides have a flat machined into the length of them. I don't recall that from my yoot.

BillH
10-18-2006, 10:55 PM
Frank Ford, Wholy Cow, nice web page!!!!
I book marked it, One day when I grow up, I want a sharp lathe just like yours!