Recently, I found that the servo motor on my GM811G declination head was loose - that I can move it by hand against its gearbox.? Then I found?that I couldn't tighten the motor's 4-40 cap head mounting screw snugly anymore.? The threads in the screw hole had become worn.? Being made of?plastic, I suppose they wear with repeated removal and adjustment of the motor despite care not to over-tightening the cap screws.
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I read about the McLennan replacement but decided there was too much work involved.? Besides, my mount is just two years old and the gears?inside should still be okay.
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So I decided to see about repairing the stripped screw threads.? I considered two options: 1) completely fill the screw hole with epoxy then tap?new 4-40 threads, or 2) partially fill the screw hole with epoxy, and quickly insert the 4-40 cap screw, then remove the cap screw after the?epoxy has set by undoing the screw thus leaving threads behind.
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I decided to go with the second option but the challenge was how to be able to remove the cap screw after the epoxy has set.? One solution I?found was to coat the cap screw thread with Vaseline petroleum jelly (I read cooking oil could also work).? So here is what I did.
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1.? Enlarge the screw hole to allow enough epoxy to line the walls of the hole and be able to form threads along it.? The hole can only be?enlarged very slightly because the screw hole is actually a U-shaped cavity inside the gearbox casing separating the screw threads from the area?where the gears are, and the walls of this cavity must not be breached when enlarging the screw hole.
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I have attached a drawing (not to scale) with a cut-away showing approximately the original screw hole (yellow shaded area), and the enlarged?hole (red shaded area).
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2.? Coat one of the 4-40 cap screw completely with Vaseline petroleum jelly, threads and cap head, then melt the petroleum jelly (for example?with a hair dryer) to make the petroleum jelly more fluid and flow into the screw threads.? Then shake off the excess so only a thin film of the?liquid coats the threads.
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3.? I used Gorilla brand "GorillaWeld" epoxy that is sold in two tubes (hardener and resin) and is black in colour when mixed, instead of the?clear epoxy in an injection tube.? The epoxy starts to cure in 10 minutes so the steps described below must be done quickly.
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4.? If the gearbox's other mounting screw hole is still good, thread the other cap screw in.? This can serve as a guide to how deeply the screw?should go into the screw hole.
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5.? Mix thoroughly a pea-sized amount each of epoxy hardener and resin.
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6.? Apply an amount of epoxy into the screw hole and, using a toothpick or similar implement, spread the epoxy into the hole to coat the bottom?and sides of the hole.
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7.? Apply a thin coat of epoxy to the cap screw which is coated with petroleum jelly.
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8.? Insert the cap screw coated with epoxy into the screw hole using the other screw as guide to know how far this screw should go in.? Ensure?that this screw is centred into the hole and is upright like the other screw.? If excess epoxy is squeezed out of the hole, don't clean it up?at this stage.? Just ensure the cap head and the Allen key hole is free of epoxy to ease the screw's removal after the epoxy has set.
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9.? Allow the epoxy to set, about 10 minutes.? Then using an Allen key, carefully undo the screw.? The petroleum jelly should keep the epoxy?from binding firmly to the screw thread.? The screw should turn with little pressure on the Allen key.? Remove the screw completely and?allow the epoxy to completely cure overnight.
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10.? Using a box cutter or thin blade, shave off excess epoxy so the screw hole is flush with the gearbox casing.
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11.? Install the motor and tighten the 4-40 cap screws only enough so the motor is snug on the gearbox.
I hope this helps.
JohnS
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Richard Paul
David C. Partridge
Michael Herman