With the jaws machined to take the disk and the disk fitted then this, and the end
of the jaws, can then be machined simultaneously to the required diameter, Photograph
2. The reason for machining the disk in this way is that there only being three jaws
it will not be possible to measure the resulting diameter directly, at least at larger
diameters. You can though measure the bore in the disk knowing that this will be
the same as that being machined on the ends of the jaws.
It is essential that the jaws are machined accurately to the tube's diameter, certainly
within + 0.0 - 0.05mm, oversize and you will end up with a three point grip, something
you are trying to avoid.
If an existing outer diameter requires a concentric bore to be made, or a smaller
concentric outer diameter turned over part of its length then the most frequently
used method is to use the four jaw and set the part to run true. This though can
be time consuming if a number of identical parts are to be machined, and especially
so if a high degree of precision is required. In this situation therefore, machining
the end of the soft jaws, as was suggested for holding thin walled tube, is worth
considering. With a chuck in good condition accuracy should be almost spot on, even
for batch production, and is obviously a better option than using the four jaw chuck.
Providing you are dealing with a robust component then wrap around of the end of
the jaws onto the workpiece will be unimportant and because of this, a plug of just
0.5mm to 1mm less than the part being made will minimise the amount being machined
off the end of the jaws.
Having machined the jaws as was seen in photograph 2 I entered a short workpiece
in the bored jaws and measured the run out present. To my delight there was only
just perceptible movement of the pointer on a dial test indicator with calibrations
of 0.01mm. However, after testing with longer workpieces I returned to the shorter
piece only to find that an error of 0.01mm total indicator reading (TIR) had appeared.
As this was the very first time I had used my shop made jaws I considered that there
would have been some minor irregularities with the teeth on the jaws and some bedding
in was taking place. The results though were still satisfying as my relatively new
100mm chuck of reputable make showed a TIR of between 0.04mm and 0.06mm at the same
diameter. Photograph 3 shows the test with the soft jaws taking place.