29 Jan
2002
29 Jan
'02
12:57 p.m.
but the mould would be very hard to make (cutting
internal teeth on a mould that size)
Wire EDM
Tony Duell wrote:
I've just been trying to locate replacement
gears for my 1520 - So
As is everyone who has this type of plotter mechanism. I have a 1520,
Tandy CGP115, a couple of Sharp PC1500 printers, a Sharp MZ700 with
built-in printer, an ORIC printer, and at least one other. At least half
of them have this problem...
far, my measurements have yielded the
following...
14 teeth
0.175" tall
~0.144" diameter (hard to get an exact measurement with my micrometer)
I do not know the pitch or the depth. Not sure how to accurately
measure them except with a fine-pitch measuring stick and a magnifying
device (I have neither).
There are various ways using rods of known diamater. You insert them into
the teeth, and then measure the overall size with a micrometer. Rather
like measuring screw threads.
Or you can make a few good estimates using the known overall diameter and
number of teeth.
I do not know the shaft diameter yet, but that's only because I haven't
gotten the micrometer to where the plotter is.
There are two of these gears in the plotter, one on the X and one on
the Y gear trains. They are the last step in the reduction. I have
I would call it the _first_ step in the reduction. It's tbe pinion on the
motor spindle.
Unfortunately the service manual I have for the mechanism doesn't give a
separate part number or description for the pinion. You're expected to
change the complete stepper motor :-(. And don't ask me where to get
those from either....
an Atari plotter with the same innards as the C=
1520 and both of the
gears in there are split along the bottom of a groove - thin material
there, plus a bit of stress and who knows what kind of thermal expansion.
It's a very common problem.
The local hobby stores do not carry gears with such a fine pitch. For
that diameter, they have 10 and 12 toothed gears, not 14. There are
gear specialty companies, but I don't have enough measurements to search
the catalogs or place an order.
I've not found them ready-made either.
There seem to be several possibilites :
1) Make a mould and injection-mould them yourself. I think the David
Gingery injection moulding machine could easily do it, but the mould
would be very hard to make (cutting internal teeth on a mould that size)
2) Use tranditional gear cutting techniques (dividing head and involute
cutter) to make a replacvement from scratch.
3) Make a lantern pinion of the appropriate size. This actually looks
very possible. The meshing gear's teeth won't be the right shape for the
trundles of a lantern pinion, but that may not matter.
4) Kludge it. One suggestion is to make a groove in the teeth away from
the meshing part and to bind it with fine wire. I've heard this can work,
but I've not tried it.
-tony