[Nelson_2008]

Hello,

I'm hoping someone can help me figure out a problem.

I recently bought an ER40 MT3 collet chuck from a seller on Ebay, for my Micromark 7x14 mini-lathe.

Shortly after that, I bought a set of 23 ER40 collets from another seller on Ebay.

I installed the collet chuck into the spindle - and played around with it a little bit until I measured a runout of about 0.0003" (on the inside tapered surface of the collet chuck).

Then I installed the 3/8" collet from the set, and into that I inserted a 3/8" drill rod that I know to be reasonably straight.

The runout was about 0.0009" (measured right in front of the chuck), but the 3/8" rod was canted; IOW, when I tightened down the nut, the 3/8" rod tilted a little bit, and at a distance of about 4" from the chuck, it's off (radially) by about 0.005".

With the collet nut tightened down, I can see that the front face of the collet is not parallel to the front face of the nut.

How can I tell if the problem is the collet chuck body, the nut, or the collet?

[Nelson_2008]

I tried just putting the 3/8" collet (with the 3/8" rod inserted into it) into the chuck - without using the nut - hoping that friction would keep it there while I rotate it slowly and look at it. And when I did that, it looked like it was spinning true, but I couldn't take any measurements because just touching it with an indicator made it come loose.

Then I tried using the next larger size collet in the set, which is 13/32", with the same 3/8" rod (which requires that the nut be tightened down quite a bit), and it seems to work much better; i.e., at 4" from the collet I'm reading about 0.002", instead of the 0.005" that I was getting with the 3/8" collet.

Does this imply that the collet nut is machined incorrectly?

[pippin88]

New ER nuts are available quite cheaply

[Fastest1]

I dont meant to be rude but do you have the collet installed into the nut correctly (prior to insertion in the taper)? The collet will snap into the nut.

If the collet is installed in the bore and then the nut tigthened down it will never sit right nor get enough thread engagement to be secure.

[ninefinger]

I dont meant to be rude but do you have the collet installed into the nut correctly (prior to insertion in the taper)? The collet will snap into the nut.

If the collet is installed in the bore and then the nut tigthened down it will never sit right nor get enough thread engagement to be secure.

+1 on this suggestion - especially based on this comment from your original post "With the collet nut tightened down, I can see that the front face of the collet is not parallel to the front face of the nut. " When the collet is inserted in the nut correctly it should be very close to flush with the face of the nut.
Mike