Using gears for odd size threads

Hi

I just bought a Warco Super Mini Lathe and I am pretty pleased with it - it looks and works a lot like its big brothers. I am sure that its inadequacies can be pointed out by experienced machinists but it is what it is.

My question is about thread cutting. I need a 0.75mm pitch but the table on the gear cover doesn't include it as an option. I have a good selection of gears supplied and I was working out how to use what I have to modify the threads shown on the table.

Can it be this easy? A 1.5mm thread uses A40 B60 C60 D40 and that seems to imply the lead screw is 1.5mm pitch (?). To get a 0.75mm thread, it looks like I could replace C with a 30 wheel. From the diagram it seems that B and C are keyed together or a single wheel can be used on that shaft.

Any comments welcome.

Cheers

Useful online gear calculator for the mini-lathe here.

Unlike my faulty memory it confirms the mini-lathe has a 1.5mm pitch leadscrew. To get 0.75mm you only have to set up the gearbox to divide by 2.

The calculator suggests:

• A20 : B-any : C-any : D40 or
• A40 : B-any : C-any : D80

So A40 : B60 : B60 : D80 should be fine (assuming there's room on the banjo!)

Always worth double-checking a gear set-up before actually cutting metal. All you have to do is engage the half-nuts and turn the chuck a few times bt hand to confirm the tool moves the right distance. (ie With the gears set up as per calculator for 0.75mm pitch, turning the chuck 10 times should move the tool 7.5mm towards the headstock.)

Dave

Edited By SillyOldDuffer on 25/08/2018 18:23:25

A 0.75mm pitch thread is half that of the Leadscrew, so the Saddle needs to travel half the pitch of the Leadscrew, per revolution of the Spindle.

That suggests a reduction gear ratio of two to one. Brian Wood, in his book, "Gearing of Lathes for Screwcutting", suggests 30 - Idler - 60, but any 2:1 ratio should do equally well, (20 - Idler - 40, 25 - Idler - 50, 35 - Idler - 70, or 40 - Idler - 80) as long as the gears mesh correctly. (Set the backlash, by running a piece of paper through each mesh)

The Idler is merely there to ensure that the drive passes from the Spindle to the Leadscrew, so that both rotate in the same direction, so no need to have two the same on the same shaft, unless it is impossible to mesh all three in the same plane.

Beware of using two idlers, unless you want to produce a Left Hand thread, because the Leadscrew then rotates in the opposite direction to the Spindle!

The calculation for any pitch, relative to the Leadscrew, is: Drivers divided by Driven.

(In this case 30/60 = 0.5 x 1.5mm = 0.75mm)

Several other books will give similar advice, (Martin Cleeve's "Screwcutting in the Lathe" No 3 in the Workshop Practice Series, covers the same ground, but in a more theoretical way.

Sparey's "The Amateur's Lathe" gives similar information. The gear set up for 16 tpi with a 8tpi Leadscrew would produce a 0.75mm pitch thread with a 1.5mm Leadscrew. It is the ratio between Spindle and Leadscrew that counts!

It may be worth buying a copy of Brian's book for future reference?

NO he is not paying me commission.

Howard

Edited By Howard Lewis on 25/08/2018 18:58:50

Edited By Howard Lewis on 25/08/2018 19:05:48

Watch out! You need an idler in the middle, you can't add a random compound gear!

You can download my change wheel tables for mini lathes here:

www.stubmandrel.co.uk/change-wheel-tables-for-mini-lathes

Neil

Howard,

Thank you for the endorsement, even I don't get commission! It was a flat fee from the publishers and they get any benefits. Fair enough I suppose, they took the risk to publish

And yes, as you say; for the 1.5 mm leadscrew, any 1:2 reduction from A to D will do the job with a suitably sized idler between them

Regards

Brian

You have the maths right. But if you try that, I doubt you will able to get C and D to mesh. The shaft holding D will collide with B.

IF your lathe is like mine, you can remove the B/C key and reduce the system to three gears, all in the same plane. Then it is as others have described - A needs to have twice the number of teeth as D. The simplest solution would be that with a single (any that fits) gear between

Pitch tables cover the most common/popular options. 0.75, while easy to achive, is less common (M4.5 or fine).

Other more complicated solutions would work.

Thanks so much guys. That was just the info I needed. I can do arithmetic with the best of them but 'you never know' about what goes on inside. I tried counting the gear teeth in the box but I went into Ratio Overdrive. The numbers in the Table gave me a strong message and you confirmed it.

@Alan Vos I appreciated the caveat about the actual choice of wheels and the fact that some combinations may not fit

"Pitch tables cover the most common/popular options. 0.75, while easy to achive, is less common (M4.5 or fine)." This an odd requirement for a thread on a 2" telescope filter barrel.

I was looking for a suitable handbook so I may go for that recommendation.

Cheers

Brian 's book is a real mine of information and it looks good too. I leave it out on the coffee table to confuse visitors!

Seriously it is money well worth spending, get a copy, you wont regret it.

Andrew.

Yes indeed!!!

There's two ways of connecting 4 gears:

Neil's right to comment because I was ambiguous! In the first case the two idlers (B & C) can both have any number of teeth.

The second case is more likely on a lathe banjo, it's a compound. B and C can still be any size for a simple ratio between A and D provided they're both the same. If they're not the same all the ratios have to be taken into account.

Maybe worth mentioning that idlers are often used to reverse direction of rotation between shafts. On a lathe you have to get the ratio right and finish the train with the lead-screw turning the right way. Normally the compound set-up on a lathe banjo gets this right for you.

Dave