Hi im trying to figure out how people cut drive worm gears (screws) on a lathe. I know how to figure out the Dp, pressure angle..... but as far as a index or the correct spacing on the lathe im at a loss. I dont believe you use the same technique for thread cutting as for gear screw cutting? Hope some one can help thanks.
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cutting a drive worm gear on a lathe
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There's an easy method in Ivan Law's book, and I believe a more complicated but same in Machinery's Handbook. Spec your lead and away you go.
I made a worm. Learned a bunch, mostly not to try again. You have to make both the worm and wheel because of OE manufacturing (the cutter diameter), and making the wheel requires you to make a hob. That's where I failed  keyway broach went wonko and ruined the hob. It was for my lawnmower, found out I didn't need the power drive anyway....
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You do not show your location so I do not know if you are more likely to be talking about a metric or an English worm. I will talk about an English one, but it applies equally to metric, just with different arithmetic.
Worms are cut exactly like threads, except the pitch of the worm is not going to be as easy to set up. Why is that?
DP is given in terms of the number of teeth on a gear with a one inch pitch diameter. So a 16 DP gear will have 16 teeth in a distance of 1 X pi inches or 3.141... inches. Stated another way, a 16 DP gear has 16/pi teeth per inch. And that is an irrational number. That means that you will never be able to find an exact solution for the gears needed on your lathe to cut that pitch EXACTLY. Real gears always provide a rational number for their ratio. They can never provide an irrational one. Never. But really good approximations are possible and that is how it is done. But this generally requires several pairs of gears combined as compound gears (two gears on one shaft that are locked to turn together). So, if you are using a standard DP for your worm wheel, there is very little chance that a quick change gear box would provide that pitch and generally individual change gears are used. And yes, reference works like Law's book and others do provide ways of calculating these gear combinations. You are going to wind up with at least four gears on the banjo arms, probably six.
That being said, you can produce a worm and a worm wheel with a non standard pitch by just cutting the worm with a standard thread pitch and also making a hob using that same thread pitch. Then you can use that shop made hob to cut the worm wheel. I suspect that many worm drives are made this way.
Considering the difficulty of making the hob (relieving the teeth) I am not sure which method is more difficult. Your choice, I guess.Paul A.
Make it fit.
You can't win and there is a penalty for trying!
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Its the same process as for threading....but if you're making a standard one, you need a diametrical or modular pitch instead of standard inch or metric pitches. My DSG does this off the gearbox  the manual is out there, grab a copy of it and and I believe it explains the gears and ratios necessary. Be prepared for your head to hurt..
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Tell us what kind of lathe you have, what thread pitches it will cut natively, what end gears are on it, what the leadscrew pitch is, and what DP or Module worm you want to cut. I have some code I can use to bruteforce solve for possible geartrains.
For instance, see this thread at PM: http://www.practicalmachinist.com/vb...ehobs245474/
allanLast edited by kitno455; 01162016, 09:52 PM.
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How many starts to that worm? (look at the end)
Ratios are the teeth on the wheel divided by the starts on the worm.
It's yet another problem to solve if there's more than one start. And go slow  the lead will probably require new changegears and really yanks the saddle across. A lot of strain on the gear train.
Some guys use the jaws of a chuck to index the starts (2, 3 or 4). Do the work between centers and shift the dog to the next jaw to get another start going. Work within .00000001" and have fun.
Acme allthread doesn't have the same profile. Study worm manufacturing and you'll see that it is very dependent on the particular OE process used. A large grinding wheel does not produce the same profile as a single point tool. That's why you need to make both. We don't know what they did to make the original.
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I agree with Sunset Machine, a 2 start 4TPI worm means you would be cutting a 2 TPI thread. This is beyond the capabilities of a light lathe, damage to the leadscrew gear train would probably occur. Old machinery textbooks refer to the problem and say that if it MUST be done, fit a handle to the leadscrew and cut the thread manually with the motor off, set up as for normal screwcutting but the leadscrew drives the chuck. Later books recommend driving the leadscrew manually but cutting the thread with a powered milling spindle held in the toolpost.
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we've been told its multi start and a largish DP?
For all I know the OP wants a single start 80 DP worm....his lathe will do that if he can figure out the gears to a DP....(or he makes a non standard thing using more common lathe pitches (ie tpi))Last edited by Mcgyver; 01202016, 05:13 PM..
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