ard at p850ug1.demon.co.uk
Fri Jun 2 18:41:22 CDT 2006
> I bought a CGP-115 new about 20 years ago and had the same gear problem.
> I went to a TRS "computer store" and tried to order a gear but they
> insisted that I had to have the printer checked out by their technician
> before they would sell me one. After a bit of intense questioning they
That's odd... In the UK, when we had Radio Shack/Tandy stores, I never
had any such problems. I used to order the service manual (if there
wasn't a Technical manual available, as there was for the computers
themselves). The way to do that was to ask them to get MS260-xxxx (where
xxxx was the last 4 digts of the unit's stock number) from National
Parts. Then I'd go into the shop again with the manual and ask them to
get me '2 of those'. And they'd come. No problems.
> finally admitted that there was a one hour minimum charge at something like
> $40/hour! I told them to stuff it and I've seldom darkened the door of an
> Trash Shack since. A year or two later I meet the manager of another TRS
> store and I told him the story. He ordered 3 or 4 gears for me and they
> only cost about $1. So the gears WERE available.
> 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. The tap will cut longitudinal (actually nearly longitudinal) threads
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
There was some discussion in one of the UK model engineering magazines
about making a hobbing machine where the blank was driven by a stepper
motor, there was an optical encoder (n pulses per revolution) on the
cutter spindle, and you used electronic dividers to set up the ratio. I
must have a go at that.
> Another way to cut the gear is to turn a shaft of the correct od and
> mount it in a lathe. Lock the headstock so that the lathe spindle, chuck
> and shaft can't turn. Make a cutter that matches the shape and size of one
> of the spaces between the gear teeth. (1) Mount the cutter in the tool post
> holder and move it back and forth taking light cuts on the shaft until you
> get the desired depth of cut. (2) Unlock the head stock and turn it exactly
> the angle between the gear teeth and then lock it again. Reapeat steps 1
> and 2 until you've cut all the teeth. The advantage of this method is that
> you can cut a gear of just about any length. You can then cut them off to
> the desired size and that will yield a number of individual gears.
The more usueal way to do this is to mount the blank on some kind of
dividing head -- either a stand-alone one, or the lathe mandrel with some
kind of division plate (even a detent engaging in the last gear of the
backgear train has been used). And use a rotating milling cutter --
either a proper gear cutter (See above) or a homemade fly cutter. In the
former case you can use most small lathes as a milling machine to drive
the cutter, but they generally won't run fast enoung to use a fly cutter.
Of course if you have a milling machine, use that.
> FWIW THE standard reference for gear cutting is 'Gear Cutting Practice'
> by Colvin and Stanley. My copy is dated 1937.
There's a handy book in the UK 'Workshop Practive Series' on gear cutting
that covers the second method in some detail.
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