By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Forum House Ad Zone

Module and DP

Please explain

All Topics | Latest Posts

Search for:  in Thread Title in  
Steve Pavey17/11/2015 12:43:41
369 forum posts
41 photos

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?

Ajohnw17/11/2015 13:10:31
3631 forum posts
160 photos

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

Andrew Johnston17/11/2015 13:17:04
avatar
7061 forum posts
719 photos

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

Brian Wood17/11/2015 14:54:29
2742 forum posts
39 photos

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

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Sign up to our Newsletter

Sign up to our newsletter and get a free digital issue.

You can unsubscribe at anytime. View our privacy policy at www.mortons.co.uk/privacy

Latest Forum Posts
Support Our Partners
cowells
Sarik
MERIDIENNE EXHIBITIONS LTD
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest

 

Donate

donate