In two earlier blog posts (4 Jan 2013 and 6 July 2012), I have discussed ER16 collets. My concern here is holding small round parts “on center”, and for the discussion here, consider “on center” to be a TIR of 0.0002 inch or less. This is my limit of resolution when using antique “Last Word” indicators.

After some time, I found that the MT1 mount collet holder would itself run on center if I properly “clocked” it (that is, selected an optimum angular location). A gauge pin would then also run on center if I properly clocked the collet inside the holder.

I bought a second collet holder, one that installed on the spindle nose thread. But this holder by itself had a runout of 0.0055 inch, and was therefore useless. Since that time, I investigated the centering of Sherline chucks (see blog post immediately preceding this one) and found that the chuck mounting system provided accurate centering. This makes me think that the second collet holder probably had a defect in its threads that engage the spindle. The advantage of a holder like the second one (but running on center) is that long stock can be chucked through the center of the hollow spindle.

Then I found an improved solution, an ER16 collet holder with a flat back.

This is it, screwed to a homemade faceplate. The bolt joint allows a little adjustment of the holder so that it can be made to run on center.

So now I have a holder that automatically clocks itself to run on center whenever it is installed. Also, it accomodates long stock. Only 12 collets are needed to cover the range of zero to 3/8 inch diameter. I still have to mark the individual collets for optimum clocking. Of the collets that I bought as the original set, some run on center at all clockings and others have a preferred angular position, but all will run on center. I bought a second 5/16 collet (from another source) that will not run on center at any clocking; TIR varies from 0.0014 to 0.0026 inch.

So there are good and bad ER16 parts available. I wish that I knew how to tell what was good before purchase. The original collet set was from Micro-Mark. The flat back collet holder came from the Ebay store xs-tooling.

Additional note: The small TIRs reported above are for gauge pins chucked in collets whose bore is equal. What happens if the collet must be drawn down? I just chucked a 5/16 gauge pin in a collet with 11/32 bore. The best TIR that I could find (various clockings) was 0.0004 inch. This is not quite “on center” according to my definition, but still quite good.

Update 10 Feb 2013: I discovered a minor limitation of ER collets, that they cannot hold short pieces of material. I wanted to make a cylindrical spacer of 0.219 diameter and 0.231 length. I had the last 1/2 inch of 1/4 inch diameter rod left over, and figured that it would be enough (Scottish heritage). But this short piece of rod wiggled around in the ER collet. A Sherline “milling collet” has no trouble holding a part like this.
I do not consider this limitation serious; I have milling collets of 1/8, 3/16, and 1/4 inch diameter (i.e., stock rod sizes), and I can use these when working on short stock.

Update 1 July 2013: For someone who works exclusively with collets (say in making rod ferrules), Sherline offers headstocks that accept ER16 collets rather than M1. Look at the sherline.com web site, “Industrial Products Division”, “Industrial Headstocks and Spindles”.
Now reader Robert has advised me of another company that makes larger ER headstocks to fit the Sherline lathe, see http://www.sherlinecollet.com/collet%20spindles.htm.