I have one more problem to solve before my CNC engraver is operational. It is that end mills come loose in the spindle.
This is the spindle, it is 1.2 inch diameter and uses a “WW” collet (0.125 inch bore). WW collets were originally designed for watchmaker’s lathes. When an end mill comes loose, it can migrate either up or down from the desired height.
From hex stock, I made a test part to fit the chuck. With a small end wrench, I can feel the grip of the collet.
I am sorry to find that the grip is not very good. I do not think that I have a bad collet, as I bought another and the result is the same. It is not a problem of grease in the wrong place; I washed the collet and brass hex piece in denatured alcohol.
This reminds me that I had a solid carbide end mill (3/8 inch diameter shank) once slip in an R8 collet.
I have a lot of 3/8 end mills, so I also bought a 3/8 inch “end mill holder”. It has a set screw and so is suitable for HSS and cobalt steel end mills that have a flat. It does not help to hold a carbide end mill that has no flat.
If anyone knows how to improve collet grip, I would be interested to hear.
Update 10 April 2016: I am adding a photo here to illustrate engraving gone bad.
Here I have used a .018 inch diameter square end mill and have aimed for a .005 inch groove depth. The decorative pattern was done first and all was satisfactory to that point. The lettering followed, and that is where the collet grip on the end mill failed. The letters went on clockwise; you can see that N on MICHIGAN looks OK, but after that the groove becomes deeper. By the time it got to the N on NORTH, the groove was nearly .06 deep. I am amazed that a .018 end mill would survive. The material is c544 bronze, a free machining alloy.
I am thinking that a WW collet is the wrong thing for this application. I have set my brass test piece (1/8 shank and 1/4 hex) in my Dremel tool collet and found that it has a better grip than the WW. Then I put it in the ER16 collet chuck of my lathe and the grip was great; I am sure there would be no slip. So the next thing is to harangue the spindle maker to offer an ER8 chuck.
I have sought help from the forum at practicalmachininst.com, and have received many responses. Two that may have the quickest benefit: reduce friction a) at the conical interface of the collet and spindle, and b) at the interface of the spindle and the draw tube knob. Here is the photo that one responder requested:
The area at the top of the (aluminum) spindle is 0.8 inch OD and 0.5 inch ID. Maybe a Teflon washer there would help.
Update 11 April 2016: Yesterday I made a Delrin AF washer to go between the pull tube knob and the spindle. By my test with the brass part and the small end wrench, this was a big improvement in collet clamping. Then I put a little grease on the conical surface of the collet, and I believe that there was further improvement. So I will try running this way for a while and see if there are any further problems.