atari printers - test-drb@ccmp.vtda.org

3 Jun 2006

On Jun 3 2006,  0:41, Tony Duell wrote:
...
  On Jun 2 2006,  9:32, Joe R. wrote: 
...
  > I've seen a crude but effective method of
cutting gears by
 > turning a shaft of the desired od and then mounting it horzontally
 > on a verticle milling machine so that it can turn freely. Then
 > mount a TAP vertiaclly in the headstock and spin it slowly and
 > then moving it slowly in against the shaft. 
...
  THis is related to a gear cutting method called
'hobbing'. A real
 hobbign machine has the blank mounted on a spindle that's geared to
 the cutter spindle by the appropriate ratio. And you use a hob, not
 a tap -- the differenece being that the cutting edges are a
 different shape.

 The advantage of this method is that you only need one cutter for a
 particular pitch of gear, no matter how many teeth you want to cut.

 If you use the other method you've suggested, you, in theory, need a
 different milling cutter for each number of teeth, since you're actually
...
  cutting the spaces between the teeth (not the teeth
themselves), and the
...
  shape of the space varies with the number of teeth on
the gear. In
 practice, there are typically 8 cutters to cover the range from 10 teeth
...
  to a rack (the last one typically goes from 135 teeth
upwards, all the
...
  way to a rack which is an 'infinite number of
teeth' if you think about
...
  it). But those cutters are not cheap -- I've been
quoted \pounds 45.00
...
  each (not for a set of 8, for one cutter) for the sort
of size you'd need
...
  here. 
There's a third method which gives reasonably-shaped gear teeth, but
only needs one cutter.  It's based on the idea that the teeth in a rack
are straight-sided.

The cutter is like a short length of a rack (think of the side view)
rotated around its long axis to make a cylindrical cutter.  It's easy
to make such a cutter by turning a bar in a lathe, using a V-point tool
to cut the valleys so you end up with something like a ribbed cylinder.
 Then mount the cylinder in a dividing head, and mill or cut narrow
slots along the length to form the front faces of the teeth, and
finally apply some relief to these teeth by milling or grinding away
part of the back of each.

      _   _   _   _   _   _
     / \_/ \_/ \_/ \_/ \_/ \               side view
    |                       |__
    | _ _ _ _ _ _ _ _ _ _ _ _ _|
      _ _ _ _ _ _ _ _ _ _ _ _ _   <-- hole through centre for arbor
    |                        __|
    |   _   _   _   _   _   |
     \_/ \_/ \_/ \_/ \_/ \_/

You mount the cutter on an arbor (easiest to make it that way to start
with), and put your gear blank under it in a dividing head, such that
it will cut both sides of what will be the uppermost tooth (with a
normal cutter, you form one gap at a time between two teeth, with this
you form the two sides of one tooth between two gaps).  You'll find
that for gears with a reasonable number of teeth, it cuts one tooth in
the centre to full deptth, but cuts part of the teeth to either side,
and for large gears, two more either side of those.  The point is that
as you cut one tooth, the ones either side get their profiles modifed
to something much more like an involute form than a straight form.

It works well, and I've cut replacement gears for a few things like
that, mostly out of nylon, brass, or aluminium.  It doesn't give a
perfect involute tooth form, of course, but it's close enough for most
purposes.  If you want a better shape, you can smooth out the curves
with a swiss file.

-- 
Pete						Peter Turnbull
						Network Manager
						University of York