Small Gear Troubles

Greetings all,

I need to replace a mangled plastic (likely nylon) gear that has been removed from a power antenna motor shown here:

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I would appreciate some suggestions on how I might do this. I have explored various options such as investment casting and machining, but the setup for a small part like this eludes me.

I have a 3-in-one Mill/Drill/Lathe, as well as crude facilities to do casting of aluminum (which I was considering using for the new gear) but I don't know what the best (and cheapest!) way to create this dual gear is.

Another option is to simply find a source for a replacement gear and buy it from there. I'd pay nicely for a replacement, as the cost to replace the entire motor is over $400 CAD.

Thanks for the suggestions...


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Unless you're really anal about keeping the car that antenna goes on "Original Equipment Correct", I'd consider picking up an aftermarket complete power antenna and spending your "engineering and grunt time" learning how to mount it neatly and hook it up electrically. See for example:

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$sessionid$V4UWYBAAAAMSWQSNDV0SFFCCJUOXKIV0?CATID=3241&_requestid=22162 (You may have to cut and paste that URL into your browser if it's too long to click on. Or, just go to
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and search for "antenna".)

But if you're really insistant on gaining the satisfaction of "beating the system" by doing it yourself (and considering that G-d does not count the hours spent on such hobby projects.) you could:

Find two stock gears and join them together with a pressed in or soft soldered inner sleeve, but the helical teeth on the larger section of what you showed us would probably make that gear a bitch to find.

Or, if you could "borrow" an intact gear, you could probably make a mold from it and cast your own metal gear by using the heat resistant RTV silicone rubber mold material and the low melting point alloys the "toy lead soldier" hobbyists use these days.

I posted here about a month ago looking for tips on where to find some of the stiff nylon "cord" which pushes and pulls the power antenna mast on SWMBO's car as the original stuff had hardened and snapped. Several clever people suggested weed whacker line, and by gum it worked out for me, but, in hindsight, and considering the time and milage I had to spend to find some close to the right diameter, I probably should have just replaced the whole damn antenna and not been so stubborn about the project.


-- Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can keep smiling when things go wrong, you've thought of someone to place the blame on."

Reply to
Jeff Wisnia

You can cut it on your machine, but you will need a few things. The main one being a dividing head.

I would cut this as two gears and pin them together. This is not a two hour project. Before I tackled this, I would see what a replacement gear costs, and why it failed.

Gear cutting is very rewarding in the "I did that!" category.

Reply to
Ron Thompson

They're gonna carve that exact saying into my tombstone.


================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================

Reply to
jim rozen

You might find something you could 'bash' into a suitable gear at

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Cheers, Fred McClellan the dash plumber at mindspring dot com

Reply to
Fred McClellan

Unless it's out of some unobtanium vehicle, go over to your local auto junkyard and see what you can dig up there. Even if you get one that doesn't work, you can rob the parts and make one out of two. My cost for a Buick one was $10, my Dad said dealer list was something like $200. That's going to be your parts source, those probably aren't standard parts and unless your time is worth nothing, it probably isn't going to be worth tooling up to cut gears just for that. it took like maybe 10 minutes to get the one I bought out of the wreck.


Reply to
Stan Schaefer

This "reward" is part of the reason that I was considering machining it in the first place. I suppose it's very easy to oversimplify something that we see and use every every day. When it comes down to the processes that make these mass-produced units, it's really quite amazing. Unfortunately, there are still some things that are *just* within the reach of the hobbyist such that it hurts (usually in the pocketbook) to attain that goal.

I'm sure that I'll attempt this some day, but I think for now I'll focus on honing my skills with simpler projects.


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Yes, I suppose that this is the most viable option... Although I'm not happy with it.... :(


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As already mentioned, you will need a dividing head to do this (to rotate the gear the precise amount between teeth.

Secondly, you will need cutters to produce the precise gear tooth shape. The smaller one is the easier part -- you really should make them as two pieces and join them together, as cutting the smaller gear to the shoulder which is formed by the larger gear will be a major pain. I would attempt that with a form cutter in a shaper, but it would be very difficult to do, requiring stopping the shaper and breaking the chip after each stroke.

The normal way to make these with a milling machine (after turning the gear blank to the proper diameter on the lathe) is with a gear cutter on an arbor. There are typically eight cutters for each size of tooth, each covering a restricted range of tooth counts, as the shape of the tooth has to change with the tooth count. The #8 handles

134 teeth to a straight rack gear. As you get fewer teeth, a given cutter spans a smaller range. Certainly 14 teeth and 27 teeth will not be the same cutter -- even assuming that the tooth pitch is the same for both -- and a glance at one of the other photos on your web page shows that it is not.

You have to deal with the style of gear (involute, Diametric pitch, module (metric gears), etc, and with the PA (Pressure Angle) of the mated teeth. For diametric pitch gears, the common PAs are 14-1/2 and 20. I'm not at all sure what the tooth form is on the smaller gear, just from looking at the end-on photo. But you could make your own form tool, using a lathe bit ground to fit and mounted in a hole passing diametrically through an arbor as a form of fly cutter. The smaller gear looks as though it would be possible to get the shape from there farely easily. For the larger gear, I'm not as sure -- though perhaps at the far edge of the gear, where it seems a bit less distorted. You'll have to adjust the shape for cutting that to correspond to the dividing head being rotated to produce the proper angle. It also looks as though it was used with a worm gear. I think that you will want a fairly soft aluminum to allow it to bed in with the worm in service, since it will be rather hard to come up with a precise match.

I'm willing to bet that the gear was made specifically for this antenna assembly, and not too many more than the total count of antenna assemblies were ever moulded.

If you *really* want to be able to say you've made it, you *can* do it, but it will not be a trivial task.

If you can find two corresponding aluminum or steel gears (with the smaller one providing the ID bearing, and a boss onto which you press the larger), that might be a good way to go.

Or -- as someone else suggested, buy a much cheaper universal replacement antenna assembly, and mount it in place of the failed unit.

Casting the gear (one of your suggestions) has to deal with at least two problems:

1) Most metals and alloys which ware strong enough for the job shrink as they are cooling, so you will need a master somewhat oversized. 2) You will probably not get a good enough finish with the casting to make a directly serviceable. gear -- so you will need to machine it after casting -- and the problems of cutting the teeth up to the shoulder returns.

Just some points.

Best of luck, DoN.

Reply to
DoN. Nichols

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