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
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.
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.
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.