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Harold Hall

Workshop Projects

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ER 32 Collet Chuck for the Lathe,   Harold Hall

ER collets

Before making comments regarding the collet chuck, some viewers may benefit from a description of the ER collet system. Photograph 1 show's two of the collets in close up illustrating that they have a shallower taper and more slits than many other collet forms.

 

Less obvious, is that alternate slits are made from either end allowing it to close at both ends, as a result, having two major advantages. One, the bore remains parallel as the collet closes and two, the collet can close over a wider range than most others. Because of this, an individual collet can close onto diameters within a range of 1 mm and as sizes are made in increments of 1 mm, every diameter can be held within the range covered by a given series. The same collets can therefor be used for both metric and imperial diameters. However, at the smaller diameters, closing the collet 1mm needs more torque than most would be comfortable with. Because of this imperial diameters are available for smaller sizes.

 

Available series

Collets are available in seven series, ER11, 16, 20, 25, 32, 40 and 50, where the number is nominally the diameter of the collet, in millimetres, at the larger end. Maximum sizes griped range from 7 mm for the smallest up to 34 for the largest,  20 mm for the ER32, the subject of the chuck in this project. Smallest size gripped is in the order of 1 to 2 mm for sizes up to ER 32.

 

Internal Angle

The collets having a relatively small internal angle, 16°, are capable of a substantial holding force making them suitable for holding milling cutters, though not quite with the certainty of the threaded shank systems. The closing force is applied to the angled end of the collet (60° internal). However, to ensure that the collet can be removed from the holder the collet has a groove as seen in photograph 1. This engages with a flange in the closing ring and is used to jack the collet out of the holder when being undone. This flange though adds some unexpected complication to the closing ring, more about that later.

 

The closing ring

Whilst still in the design stage, I became aware that closing rings could be purchased separately from some sources and as it is more complex to make than it would outwardly appear, felt that purchasing one was the best option. However, prices vary widely so do search out the cheaper ones as at the upper price limit you may be deterred from making the chuck. A pair of closing rings can be seen in Photograph 2.

ER Collets
ER Collets, Closing Rings
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 1

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 2

All pictures can be clicked on to provide a larger view