It is likely that owners of the very small milling machines that are available, or
use a milling head attached to a small lathe, will find it difficult to purchase
a boring head of a size comparable to the machine being used. The boring head, Photograph
1, in this project seeks to overcome the problem.
Whilst small, and having only five main components, the head would appear a complex
item to make if one just studies the part drawings individually. This is definitely
a case where the method of manufacture is all important as I believe there are few
if any alternative methods, certainly in the average home workshop. Do not though
be put off by the apparent complication as by working to the methods described you
should find the process relatively easy and definitely rewarding.
The head as drawn uses an M6 thread to provide the feed to the tool holder and having
a pitch of 1mm its dial is calibrated with 40 divisions of 0.025mm. If however you
prefer it to be calibrated to suit imperial measurements, then 1/4" Whitworth having
20 TPI would be a good choice. In this case, 25 divisions would result in increments
of 0.002". Fifty I think would be too fine for easy reading. You could as an alternative,
use a 1/4" x 40 TPI ME thread as this could be calibrated with 25 divisions of 0.001".
However, the shallower thread would have less scope for coping with errors in the
distance between the feed screw (5) and the cutter holder (3).
If you use milling cutters with imperial sized shanks you will also require to change
the diameter of the shank on the body. Do note though, that the 20 TPI Whitworth
thread form is used for both metric and imperial shanks and for all diameters.
You will require two pieces of 20mm steel, one 32mm long for the Cutter holder (3)
and the other 28mm long for the Gib strip (4). Both should have their outer diameter
skimmed and to precisely the same diameter. At the same time as skimming the longer
length, centre drill one end with a small centre drill, thereby ensuring that it
is concentric with the outer diameter.