Module and DP

I've done plenty of screw cutting on all sorts of lathes, but there has always been a big gap in my knowledge - I have never learnt anything about module or DP screwcutting other than seeing a plate on the end of the lathe. I have a vague idea they are something to do with cutting worm gears but that's as far as it goes. So can someone please explain what they are, what they're for and when would I use them?

They are used to cut worms to drive worm wheels or ordinary gears if the worm is angled to suit it's helix angle.

John

-

Edited By John W1 on 17/11/2015 13:10:59

It's based on the premise that it is simplest to cut gears using an integer value of DP. Given that the circumference of a circle involves pi, which is irrational, and that useful gears have an integer number of teeth, it follows that the circular pitch of a gear with integer DP will be irrational. For a worm to mate with an integer DP gear the pitch of the 'thread' on the worm, to match the circular pitch of the gear, will also be irrational. So in essence you need to screwcut a thread with an irrational value for the TPI.

Of course since the 'TPI' will involve pi as a factor it is impossible to cut an exact thread, no matter how long the gear train. However, worms are fairly short, so an approximation is fine. My lathe uses 88/56, which is pi/2 to within 0.04%.

Andrew

Edited By Andrew Johnston on 17/11/2015 13:18:09

Edited By Andrew Johnston on 17/11/2015 13:18:56

Hello Steve,

Martin Cleeve's book, Screwcutting in the Lathe, Workshop Practice Series #3 goes into the topic in sufficient detail to hold the interest without the risk of glazing over.

I think you might find it money well spent, it is not the only topic he explains with great clarity. If you have a screwcutting gearbox on your lathe, by including the factor pi or pi/2 in the changewheel gearing, it turns into a variable circular pitch generator so that TPI values give DP values directly. Cleeve didn't spot that, but it is a fact nevertheless.

Regards

Brian