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View Full Version : Collet blocks - a different approach



Michael G
24th Jul 2017, 08:57 AM
Joe explained his approach to being able to do 4 and 6 sides using a collet block here http://metalworkforums.com/f303/t200771-joe-peter-collet-blocks

My approach was a from a different direction. For me one of the annoying things (apart from the 4/ 6 side thing) was the lack of repeatability that would result when rotating the block, so leaving the collet block itself essentially unchanged, I worked out that if I made a base with sections like a 12 point spanner then I could cut 6 or 4 sided parts with few problems. Making the holder was 'relatively' simple, done this way to keep things as concentric as possible. The block through hole was not bored all the way through but about 25mm was left and then reamed to stock size to provide support. I did grind the 6 sides of the block afterwards but only for final sizing/ clean up
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Making the base required the use of the large sine plate (confession - the small angle plate is still not finished). The first photo shows how to get angles over 45 degrees from a plate like this. The last photo is a set up the next day to fine tune the flats that the hex nests in. Once this was done I was able to rotate the block through all 12 positions and measuring the height of the CRS stock in the collet, got a result within 0.1mm. Probably better than the stated accuracy of the collet...
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There are several features that I have incorporated that I think enhance usability. Firstly, I have key'd the base and combined with a recessed hole to allow bolting to a T nut, this means the block itself does not have to be held in a vice but can be used directly on a mill table. In use, the base is secured to the mill table and then a standard clamp kit is used to hold the collet block into the base that cradles it.

The other is the incorporation of a 2 position end stop. In the first position it just acts on the end of the block, but in the second, a piece from a clamp kit can be secured to it so that it acts on the end of the stock in the block. For longer pieces a stop could be bolted to the mill table if that was required. I've used this for milling hex flats that effectively run into a shoulder. As you don't move the material in the collet mid operation then either will work, but the internal stop will allow flats to be staggered if for some reason you want to do that. The bolts securing the stop to the block are overly large for the job but all holes are common so they are 3/8" too. They may be better as button head cap screws. The alternative is a smaller size (say M6), but this then means more loose pieces.
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Michael

simonl
24th Jul 2017, 04:53 PM
Nice work Michael. I see real use for these collet blocks beit this design or Joe's. I may use these as a first project in the new mill, when I finally put it together.

Cheers,

Simon