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Setting up for intermediate threads

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David Paterson 415/05/2017 11:56:35
83 forum posts
8 photos

Hi,

I need to cut a very small thread for an old American watch. Best I can discover it is 0.6mm and 200 tpi.

I have a Hercus lathe and an 8mm wolf Jahn (no lead screw).

The gearbox on the Hercus can set to 192 or 208 TPI, My initial thoughts were to put the gearbox at a smaller number such as 50 or 100 and use change gears to hit the 200. But the gearbox neatly brackets all of the obvious numbers.

I know this is going to be a real fiddle and is very small so will take a few gouges, but can anyone give me a start on the pitch please.

David

Hopper15/05/2017 12:09:09
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3651 forum posts
72 photos

Good luck screwcutting something 0.6mm diameter. With a core diamter of under half a mm there will be a lot of flex there. Can you not buy a die from watchmaking suppliers?

Back to change gears though, for our US/UK readers, the Hercus is an Aussie clone of the South Bend, so would probably be same gears also as the Boxford?

You may have to sit down with an online change gear calculator such as the one at littlemachineshop.com and the list of gears you do have and see if it can be done. You may have to use compound gearing though, which can get complicated.

David Paterson 415/05/2017 12:23:31
83 forum posts
8 photos

After 3 tries I managed to make a couple of screws the right diameter with a screw plate that I do have on the 8mm lathe. However, the pitch did not match. The big trick seems to be to make the OD with a hand graver so no stress 😀

A close look at a book on threads indicates that all of the makers seem to have their own 'standard', with the Waltham being the target here. I have not been able to locate a Waltham screw plate yet, and don't want to re-tap the watch plates to match a metric thread which I am sure I can do.

There is no real rush, I may have sourced a couple of screws but they have round heads so are not quite right. Would like to get a range of plates, but suspect that will be pretty hot and miss. Any help on suppliers in that direction would be appreciated also.

Cheers

David

Speedy Builder515/05/2017 13:31:02
1790 forum posts
127 photos

Boxford - 8tpi leadcrew and the 100/127 metric conversion gears:-

Pitch Mm /Screw gear/ Gearbox select /Stud gear
1.25 /56 /16 /20
0.125mm /56 /160 /20 which is close enough to 0.127mm pitch?
(Sorry,  all my spaces have gone proportional, tabs didn't work,  so I substituted the slash mark ///, must learn more)

BobH

 

 

Edited By Speedy Builder5 on 15/05/2017 13:34:20

David Paterson 415/05/2017 23:53:51
83 forum posts
8 photos

Bob,

I knew there would be someone here to help, thanks.

I have the metric pair, so just need to check the rest - lucky I have a night in the shed today !! (with the heater turned on)

David

Neil Wyatt17/05/2017 08:49:43
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Moderator
16257 forum posts
679 photos
74 articles

As a screw plate only has a couple of threads, one solution might be to mount a screw plate on the toolpost and drive the toolpost with appropriate gearing.

This would minimise flexing of the workpiece and ensure accurate pitch and alignment of the thread.

Neil

Ady117/05/2017 09:13:15
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3463 forum posts
513 photos

As a screw plate only has a couple of threads, one solution might be to mount a screw plate on the toolpost and drive the toolpost with appropriate gearing.

I was thinking along the "use the thread to make the thread" line as well

The unimat SL has a moveable in the horizontal headstock which can be screw thread driven from the back while the toolpost stays stationary

David Paterson 417/05/2017 10:57:59
83 forum posts
8 photos

Thanks all,

Went through the box of stuf I have never used, never done a metric thread before, and doesn't look like the 100/127 set has ever been used. The setup needs a spacer with key way behind the screw wheel that I don't have, so need to make that first, otherwise, going well.

I used the screw plate I have with a tailstock runner to keep it square. The only problem was trying to get the right starting diameter and a nice square shoulder with the graver. The screw has afinished length of only about 2mm, so I started with a bit over 3 to get it running well. It's not that the screw plate did not produce a good thread, just the wrong pitch and I have been unable to source a Waltham plate.

Brian Wood17/05/2017 11:11:20
1930 forum posts
37 photos

Hello David,

​If you have it, fit a 24 T gear as the stud wheel with 56 T gear as the screw gear with 127/100 as translation gears and set the gearbox for 40 tpi. That will give you a pitch of 0.6mm exactly

​The set up is 24----127
​                            100-----56 The gearbox ratio is 0.56

​However, I'm afraid I don't agree with you on the conversion of 200 tpi to metric. 200 tpi has a pitch of 0.005 inches and in metric measure that is 0.127 mm, not 0.6 mm.

I would be inclined to use the 208 tpi setting, after all the error is only 4% which on such a small screw with short threads may not be a problem

​Regards
​Brian

 

 

Edited By Brian Wood on 17/05/2017 11:12:39

Edited By Brian Wood on 17/05/2017 11:13:08

Edited By Brian Wood on 17/05/2017 11:13:35

JasonB17/05/2017 11:46:04
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Moderator
15750 forum posts
1647 photos
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Brian, pitch is 200tpi, screw diameter 0.6mm

mark smith 2017/05/2017 11:46:14
577 forum posts
297 photos

Isnt 0.6 or according to this chart 0.65mm the diameter of the thread not the pitch??? Sorry if im wrong as i dont know about so fine a screw threads.

waltham.jpg

mark smith 2017/05/2017 11:49:24
577 forum posts
297 photos

smileyJason beat me to it.

Edited By mark smith 20 on 17/05/2017 11:49:57

Brian Wood17/05/2017 12:34:01
1930 forum posts
37 photos

Jason and Mark,

​Thank you for the correction, it was the way it was presented with mixed dimensions that threw me. I still stand by using the 208 gearbox setting though, would that not work on such a tiny short screw?

​I was not aware of the Waltham chart for these fine pitches having never needed it.

Regards
Brian

David Paterson 417/05/2017 12:48:02
83 forum posts
8 photos

Thanks,

That chart is the same data I used, just a different source. I should have been more precise. The one I am shooting for is size 17. With the minor diameter determined by turning a short length until it just fit into the tapped hole in the plate. I got the best result from the screw plate with an OD of 0.6, But wrong TPI, assume because of the same deformation experienced with larger dies. I expect to start with 0.65mm when on the lathe.

I have the facility to grind a single point cutter under a microscope, not really sure or the thread angle but will aim for 60deg in a piece of 3mm HSS.

I am finding this small stuff not too hard - just a matter of being careful and mistakes only cost time.

Brian Wood17/05/2017 14:04:07
1930 forum posts
37 photos

Perhaps I can redeem myself from my previous effort with some new suggestions. I don't know what change wheels David has available, but the following arrangements are looking promising

As before, using the 100/127 translation wheels with 100 as driver, a screw gear of 80 T and 32 T as the stud wheel, gearbox set for 176 tpi, the pitch produced will be 0.00501 inches [ 199.53 tpi]

Another arrangement using this set up with 20 T as the stud wheel and the gearbox set for 112 tpi, the pitch produced will be 0.00492 inches [203.2 tpi]

​And finally, with a stud gear of 25 T coupled directly to an 80 T screw gear via an idler, gearbox set to 176 tpi the pitch produced will be 0.00497 inches [201.1tpi]

I can't get an exact match, but these are rather more realistic I think

Regards

Brian

Brian Wood17/05/2017 18:24:30
1930 forum posts
37 photos

David,

​I can be something of a terrier worrying at a problem I checked to see what the standard set of change wheels are for Southbend copies, you should have a wheel of 16 teeth

​Using that as the stud gear, directly coupled to 80 T on the leadscrew [via a suitable idler] and the gearbox set to 112 tpi, you will get the magic pitch of 0.005 inches---200 tpi. Bingo! idea

Regards
Brian

​When I have a bit of spare time I'll see if the pitches for the whole Waltham table can be set up

David Paterson 421/05/2017 23:16:56
83 forum posts
8 photos

Thanks to all, success.

The OD is a little undersize (.63mm vs .65), and I could have come closer to the shoulder if the LHS of the tool was finer, but this will work when I finish off the head. Saw wont touch it, so will need a screw head file.

img_1242.jpg

img_1246.jpg

img_1249.jpg

David Paterson 422/05/2017 01:46:00
83 forum posts
8 photos

PS

this was all done on the Hercus with HSS tooling honed using fine Arkansas

Speedy Builder522/05/2017 06:42:58
1790 forum posts
127 photos

looks like a good job David. I wouldn't have thought you could do something that fine on a Boxford, Hercus etc.
BobH

Brian Wood22/05/2017 08:56:53
1930 forum posts
37 photos

Hello David,

​Nice to know you got there in the end. I have since worked out the gearing to produce these threads on a Southbend lathe for the rest of the Waltham series, but because of incompatibility problems with Microsoft Edge, I am unable to put them in my albums.

If you or anyone else would care to have a copy of the table please send me a PM with your email address as that seems to be the only way I can get round the problem

Regards
​brian

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