View Full Version : collet problem

loose nut
02-19-2008, 10:30 PM
A few weeks a go I scored a set of MT3 draw-in collets 1/8" to 3/4" by 1/16th's. I made a draw bar and tightening handle, stuck in a piece of 3/8" rod in the appropriate collet and put an indicator on it and was surprised to see a radial run out of .000075", not to bad for cheap off shore collets. Now for the bad news, they won't tighten up. No matter how tight I screw down the handle as soon as I try and take a cut, the work piece either just slides into the collet or grabs the tool bit and stops spinning. Shouldn't a piece of 1/4" rod hold tight in a 1/4" collet.

02-19-2008, 11:49 PM
I would expect that a 1/4 inch collet would hold 1/4 inch rod. Try drawing in the collet without anything in it- not too much, just enough to see if it closes smaller than the rod you're putting in it. If it doesn't it isn't going to hold- obviously. Could it be bottoming out on something before it closes to the marked size?

Are they properly made? It's possible that the entire length of the collet is the proper taper, including the threaded end. If the threaded end doesn't pass the small diameter of the taper in your bore, it will draw in tight, but there won't be any clamping force on the rod. If that's what's happening, it will hold something larger than the 1/4 inch, but not 1/4 inch itself or anything less. The slots cut in the collet have to pass the end of the taper in the spindle before the collet can be compressed by drawing it in. Normally the collet is reduced to a smaller diameter about halfway along it so there's no chance of this happening.

I'm assuming you tried the .375 collet with .375 rod and it wouldn't hold- maybe try some .001 shim around the rod and see if that does it. If it does, my best guess is that there's a manufacturing defect or design error.

02-19-2008, 11:57 PM
So you have a dial indicator that reads to 6 places.

Can you attach a photo of these MT3 collets or the site to view them? I am curious what they look like.

02-20-2008, 12:49 AM
surprised to see a radial run out of .000075", not to bad for cheap off shore collets. .

Yeah, right!!! Any other fairy tales?

02-20-2008, 01:35 AM
Total Indicated Radial Run-out (TIRR) of 0.000075" (3/4 of a "tenth") is equal to 0.00015" (1 1/2 "tenths") Total Indicated Run-out (TIR) which is quite achievable with a good test indicator.

So it seems quite reasonable and credible to me and not unlike a lot of similar figures I've seen quoted in this forum.

A.K. Boomer
02-20-2008, 09:02 AM
Drawbar does not go all the way in to properly tighten, it just bottom its threads out,OR Drawbar too long, and physically gets involved with inside of collets holding area? OR, collet does not go all the way into bore, top of collet is too long and bottoms before tightening (just what I would verify first before going any further.)

02-20-2008, 10:32 AM
You can only get a reading like that IF the dial indicator reads to 6 places. Other wise you are only guessing. With an indicator reading 3 places you can guess a half thou. and so on up the line. A 4 place indicator may allow you to guess at a half a tenth of a thou. anything farther is using your imagination and wishfull thinking.

If a person says it has .000075" runout then the indicator would have to read to at least 6 places to be a valid value.

02-20-2008, 10:38 AM
I think that measurement may be in error. I say that because your saying that the collet doesnt hold tight. So, if it aint held tight how can you get an accurate measurement... FWIW

02-20-2008, 01:23 PM
Well. If you were missing the draw bar, I have to wonder what else is missing. Do you have the tapered sleeve and nose cap on the spindle?

02-20-2008, 04:18 PM
Add a washer to the top of the draw bar. like an 1/8" thick..

loose nut
02-20-2008, 06:31 PM
Sorry, typo their, put an extra 0 in and didn't double check it. Took the TIR and divided in half to get the radial error, so it was a typo guesstimate but the movement on the indicator was negligible. Doesn't matter what the run out is if they don't work properly. The collet doesn't seem to be hitting anything, and the hand wheel is threaded right through so the drawbar can be threaded all the way into the collet and it can be taken right up tight by the hand wheel. It will tighten down so that you can't move it with your hand but as soon as a cutting load is put on it shifts positions. How much force is necessary to tighten a collet, possibly the hand wheel is to small to get enough leverage.

As for the indicator it doesn't measure to 6 places, but it does measure to .00005 It's from the Dorsey Gage Co. Hyde park N.Y. couldn't find a model number on it, it's a bit old but still works great, although I can't swear to it's absolute accuracy it hasn't been calibrated in a long time. I rarely use it because for model work that kind of accuracy is usually necessary or on my equipment even possible , I just wanted to see how accurate the collets might be.

I'll see about pictures if I can get time tomorrow.

02-20-2008, 06:39 PM
Where did you get these collets? Can you post a site that has a photo of them?

A picture is necessary here. Take a picture of your set up.