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Post by alanstepney on Jan 23, 2008 23:44:50 GMT
Leveque? Sorry, cant help there.
My guess is that the original gear would have been paxolin. Apart from providing a weak point, it would also quieten the gear train.
Moulding one as has been mentioned seems the obvious way, although paxolin is still available so you could replicate the existing one.
In that case, the way I would do it would be to make a single point cutter to fit the profile of an existing tooth, and fly-cut the new wheel. Although the form may not turn out exact, it would probably wear in.
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Post by petercolman on Jan 27, 2008 20:35:07 GMT
If you give me the data that you mention in your first post, OI would be happy to the calculations and make the gear if needs be. Peter
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Post by steammadman on Jan 28, 2008 18:30:46 GMT
pity you not nearer to me havoc, i think we,between us could defiately make a new gear i have made quite a few for diferrnt things.
I too am a "tinkerer" , i dont buy anything if i can make it i learnt that at doncaster plant in 1951, all the guys there spent any spare time making diferent "bits", you couldn't buy em then.
HAVE A GO MATE ITS ONLY A BIT OF TUFNOL or whatever
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Post by havoc on Jan 28, 2008 19:23:10 GMT
Well thanks for the offer. If the moulding doesn't work out I'll let you know. Right now the gear (or what's left of it) is covered under a large blob of silicone to make a mould of it.
Went to a shop specialised in that sort of stuff and returned with the necessary vials of magical compounds. Some waxy stuff to put the gear on while casting the first part of the mould, 2 component silicone to cast a mold and 2 part urethane to cast a new gear. Well I'll first have to make a few copies of the broken one, then make a new gear out of the broken ones and then cast from that a new gear in 1 piece.
Only positive side so far: it doesn't smell. I'll post some pics if you want.
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Post by havoc on Jan 28, 2008 19:30:38 GMT
Chris, I must have missed your post. I know there is a method to make gears where you use another gear to cut the new one. Is that the method you refer to? Because I don't see how I could use a tap if you mean the thing you normally use to make internal thread. And yes, you can still find film! That's the reason I go this lenght. Otherwise I would just clean it and put it on display. www.kahlfilm.de/ (color, b&w, 8mm, 2x8mm, super8, 16mm whatever)
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Post by chris vine on Jan 29, 2008 0:30:16 GMT
Hi Havoc,
No, the method I am talking about does not use another gear.
In the lathe chuck you put a tap (the int threading type). The pitch off the tap needs to be the same as the pitch of the teeth on the gear.
You machine the gear blank and set it on a mandrel held in the tool post so that the axis is vertical. you then wind the cross slide in so that the edge of the blank is beginning to touch the side of the tap which is rotating. Like the worm in a worm and wheel, only a cutting one.
the rotating tap should start to rotate the blank and then as you feed in the teeth will get cut roughly right.
I have never done this but locked it away for just such a moment as you have. I have also read that it may help to mark out the blank and file the teeth in just as notches in about the right places so as to help the rotation get started and to avoid skidding.......
Chris.
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Post by jgb7573 on Jan 29, 2008 8:56:52 GMT
The technique, I beieve, is called hobbing and is used commercially for some gear cutting.
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Post by chris vine on Jan 29, 2008 13:41:44 GMT
Yes but the proper hobbing method involves rotating the blank under power from the machine using change gears or a gearbox.
This is very much the poor man's method but I suppose it is still called hobbing after a fashion.
As long as the tap has a thread form with straight sides to the flanks of the threads, then this crude method should still produce the involute tooth from on the gear.
BUT I have never done it!!!!
Chris.
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Post by havoc on Jan 29, 2008 20:34:03 GMT
OK, the first try at casting has ended...but not well. I made the first part of the mould with silicone, and it went very well. After harding a day it looked like this: So I poured another batch of silicone on top of this in order to make a 2 part mould. Thinking that as that silicone is made for casting and the casting material doesn't stick to it, then another layer of silicone will also not stick to it. Well, it sticked. And not a little bit either! I got a single blob of silicone with the gear embedded in it. Couldn't even see where the first and second layer met. Took a knife to it, but I was a bit to hasty and damaged it at the side of the teeth. Second try is poured right now. Lucky they sell this in 1kg packages ;D
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Post by baggo on Jan 29, 2008 21:00:18 GMT
Chris,
in my telescope making days the 'tap' method was often used to cut the large worm wheels (6 inch diameter for example) for the axis drives if you wanted to make your own. I think the hard bit is getting the gear blank the right size so that the first tooth cut matches up with the last one! Commercially made worm drives were (and probably still are) very expensive, hence the DIY ones. I am sure the method has been described in ME over the years.
John
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Post by chris vine on Jan 29, 2008 22:33:47 GMT
Hi John,
I can imagine that quite a lot of experimenting is required for this! Unless one is very good with calculating geometry......
C
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Post by Tel on Jan 30, 2008 8:24:48 GMT
In practice, as long as you can get the blank pretty close to size it's good enough, as the tap will carve away any slight excess. Made a couple of worm wheels that way over the years - not a heavily used procedure, but a useful one.
Think I first ran across it in Len Mason's book.
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abby
Statesman
Posts: 927
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Post by abby on Jan 30, 2008 12:42:46 GMT
Hi Havoc , can't see why you have made a mould of a stripped gear , and if you read my post I said clearly that the silicone rubber bonds to itself . The idea is to make a mould of the good part of the gear by embedding the damaged section in modelling clay. After curing of the rubber the gear is then removed by slicing the mould with a scalpel , the gear is rotated until the damaged section is inside the mould and a second pour of rubber is added, this will bond as one with the first part. Again slice the mould to release the gear and you will now have a mould for the complete undamaged gear. Silicone rubber is best vacuum de-gassed before pouring , and there are 2 types , addition cure and condensation , they have different curing and physical properties. Here is a picture of how to cut the mould so as to release the casting, but leaving enough mould material still joined so the mould goes back together accurately, sorry but it is the only picture I can find. The cut runs from top to bottom and extends halfway across the base , the holes are just air vents.
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Post by havoc on Jan 30, 2008 17:48:47 GMT
Looks like a chocolate figure, hmmmm.
More serious now, I tried that as well, but aligning the parts to make the teeth match isn't easy. I mean, I tried to make aligned imprints of the gear in a kind of wax. The idea now is first to make several copies of the broken gear and out of those contruct a complete one. Then make a mould of that fabrication.
Another problem I encountered is slicing along the plane of the gear without damaging the imprints of the teeth. They are rather small.
But I could have poured it in 4 times, using a kind of barrier.
Learning money I guess.
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