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Thread: How to check gear teeth
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22nd Sep 2018, 08:32 PM #16Senior Member
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22nd Sep 2018, 10:17 PM #17Most Valued Member
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The whole depth is 2.157x 1.25.
The 2.157 is constant across the module range.
The whole depth is the same across the full set of involute cutters for the desired module.
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23rd Sep 2018, 03:23 PM #18Senior Member
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One thing I don't have is an indexing head, but then I don't have a milling machine yet either.
I made a plate for my lathe chuck ages ago which has been handy but for this job I think I've come up with a cleverer idea.
In this lot, I've got 26,30,32,34 or 36 places I can drop a pin into.
P1030809.jpg
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23rd Sep 2018, 03:26 PM #19Golden Member
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Firstly, I'll answer your question about what that label tells you; you probably know it's the screw-cutting chart for your lathe. It's in two halves, metric on the left, imperial on the right. It tells you two things you need to know to set up the lathe to cut a thread; (a) how the gear train needs to be arranged, (b) where you need to set the levers on the screwcutting gearbox.
Say you want to cut a metric thread with a pitch of 0.75mm. Find that on the chart. Go across to the left in the first column you'll see you need to set the gears up as in the middle diagram, the second column with the small letters tells you that you need to set the ABCDE lever to D. The numbers across the top tell you to set the numbered lever on the gearbox to position 4 (0.75 is in column 4). The large letters ABCDE across the bottom of the left-hand half of the chart are confusing- are they directly above the actual lever? If not, they are there in error.
The right-hand (imperial) half of the chart is simpler; e.g. to cut a 10tpi thread, set the levers to B and 4. It doesn't tell you how to set up the gear train- perhaps it's obvious on this lathe? One thing for sure is that you don't need the 127-tooth gear for imperial threads as it is an imperial machine. 127, being the first whole multiple of 25.4, is the transposing gear to enable metric cutting on an imperial machine.
Secondly (and this follows on from the above), as the lathe appears to be an imperial machine, I reckon it is far more likely that your gears are imperial and use the DP system rather than the metric 'module' system. The dimensions of the gears you list above work out almost exactly to a 20DP gear. In which case you need a 20DP, number 4, involute gear cutter. (You're lucky in that both the gears you want to cut- being so close in number- use the same cutter. I need to buy the full set of 8 cutters for the missing gears on my universal dividing head.) The depth of cut will be engraved on the cutter. Assuming you don't have a milling machine and dividing head, you'll need a way to hold the gear blank and index it to cut each tooth gap. Ivan Law's book shows some ways to do this.
Edit- posted simultaneously- I see you don't have a mill but you've maybe got a way to index your blank.
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23rd Sep 2018, 04:57 PM #20Most Valued Member
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What method of calculating the DP are you using ?
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23rd Sep 2018, 05:33 PM #21Golden Member
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23rd Sep 2018, 07:00 PM #22Most Valued Member
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I think you will find it is much closer to M1.25 than DP20.
Quite a few machines run an Imperial leadscrew with Module gearing.
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23rd Sep 2018, 11:22 PM #23Senior Member
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I'm guessing a photo of the 120t might help decide if it's imperial or metric.
P1030811.jpg
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24th Sep 2018, 07:46 AM #24Philomath in training
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Not really. As was mentioned before,
Really the only way you will be able to tell is to measure the pressure angle. In a home environment, the usual way to try to to that is rolling a gear on a strip of Plasticine or similar to effectively define a gear rack and then measure the angle of the teeth. Having tried it, it can be tricky but can be done.
Michael
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24th Sep 2018, 03:59 PM #25Senior Member
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Well I used this photo and drew these lines. I dunno if it's the pressure angle you're talking about but it comes in at 40deg, half of that is 20.
gear angle.jpg
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24th Sep 2018, 05:42 PM #26Philomath in training
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If it is 20 then it is most likely M1.25. With a larger gear like that you are relatively safe, but the curvature on the teeth of smaller gears is enough to make measuring difficult.
Michael
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27th May 2019, 01:58 PM #27Senior Member
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I finally ordered a cutter. Should be here any time soon.
I've got some 6061T6 aluminium bar stock. Do you think it's suitable to make the gears from it ?
https://en.wikipedia.org/wiki/6061_aluminium_alloy
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27th May 2019, 06:27 PM #28Gear expert in training
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I wouldn't. Ally is fine for timing belt pulleys, but if it's meshing with a steel gear/rack and/or under load I reckon the teeth will either be worn out of shape or sheared off completely in a short time.
Stick with steel, doesn't have to be fancy, just mild steel will do.
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27th May 2019, 08:01 PM #29Senior Member
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Yeah that's what I imagined too.
I saw elsewhere they reckon aluminium will work fine but I wasn't sure to trust it in this case. I think they were referring to using cast aluminium.
I'd prob use brass or bronze if I had some so it would be quieter but I'll go and get a piece of steel I think.
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28th May 2019, 02:45 AM #30Golden Member
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FWIW I made 5 gears including a 37/47 metric conversion pair from 75x10 aluminium flat for a 9x18 low powered lathe which does not get a lot of use, for cutting odd unusual threads - don't know what the alloy is. They usually run with cast iron gears, but occasionally I need to run two aluminium gears together. So far they have been completely successful, showing no sign of galling or excessive wear. I wouldn't suggest using them in an industrial situation, but for my occasional use they are OK. Some of them were cut with fly cutters and were much quicker and easier to cut than steel gears using this method.
It might be worth making one as a proof of concept guinea pig from aluminium if you already have it in stock to check how your indexing system works and keep it for a spare if you decide not to use it.
Frank.
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