metric thread on my imperial lathe

[chris mcnicollParticipant @chrismcnicoll83540]

ive put 60 on top,50 in middle and 127 on bottom with gearbox C thats not a 1.5 thread.

i will try gearbox in another gear

[HopperParticipant @hopper]

Posted by chris mcnicoll on 29/04/2017 10:12:35:

ive put 60 on top,50 in middle and 127 on bottom with gearbox C thats not a 1.5 thread.

i will try gearbox in another gear

What pitch did this combination give you? You can Measure this precisely with a dial indicator against the carriage and rotate the chuck exactly one turn.

Did you try it in position A to give you the 50 per cent reduction?

 

Edited By Hopper on 29/04/2017 10:22:05

Edited By Hopper on 29/04/2017 10:25:08

[chris mcnicollParticipant @chrismcnicoll83540]

just going to check this.

reading elsewhere someone has said that i need to have the carriage on an angle?

i have had my 60 degree tool set square to the chuck??

chris

[chris mcnicollParticipant @chrismcnicoll83540]

[chris mcnicollParticipant @chrismcnicoll83540]

stuck it in A and im getting 1.5

thank you everyone for your help.

[KWILParticipant @kwil]

The tool set square to the work as shown in your pic is correct. The angle of the topslide (compound) can be set at 1/2 the thread angle (ie 30 degrees in your case) to enable an easier cut as you go deeper, the cut is put on by the topslide. calculate the correct angle depth of cut.

[chris mcnicollParticipant @chrismcnicoll83540]

Right then I had a few practice runs and checked the thread is 1.5 I've done rh and lh passes on the scrap material, but now I need to get the threads cut correctly, my motor doesn't run backwards and there is no reverse gear on the box.

What is the easiest way for me to make sure I get back into the correct start of the thread? Tipex on the gears to align them maybe?

[HopperParticipant @hopper]

Good you got a result on the 1.5mm pitch. Good start.

Not sure how you will go cutting a thread with that tool bit. It looks like a general turning tool with too large a radius on teh tip for cutting a thread like a 1.5mm. But you can give it a try. Otherwise you might need to grind up a 60 degree HSS tool bit, or buy a specific threading tool bit.

Also, looks like you need to get the job in closer to the chuck, or use a tailstock centre to support the work.

The most reliable but most laborious way to keep the metric thread co-ordinated with your imperial leadscrew is going to be to never undo the halfnuts after the first cut. To move the carriage back to the starting position at the end of each cut, with the motor switched off, undo the back gear so the chuck can be easily turned by hand and rotate the chuck backwards until the tool reaches the starting point again. Re-engage back gear, set tool depth, start motor and take next cut. Repeat.

As far as using the thread indicator dial, try this method, but I make no guarantee so test it on a trial piece first.

Set the tool bit at the beginning of the cut or within a millimeter or so. Start the lathe and engage the halfnut when the threading dial reads 1. Make the cut. Disengage halfnuts at the end of the cut and stop lathe immediately. Wind the carriage back to the starting position and start the lathe up again. Engage half nuts immediately when dial reads 1 again. I have had this work sometimes over the years, and not others. I think it will work if the length of the thread being cut is less than the distance the carriage travels in one rotation of the threading dial, which varies from lathe to lathe depending on leadscrew pitch and number of teeth on the gear on the dial. But can be seen readily by winding the carriage along by the handwheel and measuring with a ruler how far it travels in one revolution of the threading dial.

Usually I use the first method described, because on my little Drummond it is easy to disengage the back gear and wind the lathe backwards via the leadscrew handle which then turns the spindle through the change gears. Thankfully I don't often turn metric threads so I just put up with the slowness of it.

I think Ady1 mentioned a method he uses by setting the tailstock as a back stop and putting the carriage back against it every time to locate it then engaging the halfnuts as a mark on the leadscrew gear comes to 12 o'clock.. But I think that is subject to the same rule of thread length also. Good chance for you to do some experimenting and see which works best for you.

Edited By Hopper on 30/04/2017 07:59:19

[Martin ConnellyParticipant @martinconnelly55370]

As you cannot reverse the motor the safest option until you have used the lathe for a while is to do as Hopper says and disconnect the drive to reduce the force required to manually wind the tool back to the start position. As an alternative to turning the chuck if you can devise a suitable hand crank for the left hand end of the spindle it would be an easier job. If you Google lathe hand crank there are lots of hits that will show the idea and lots of reasons to make one. Just remember that it should not be in place when the motor is engaged and started.

Martin C

[HopperParticipant @hopper]

Yes, hand crank is probably a good way to go in this case.

[Howard LewisParticipant @howardlewis46836]

As usual, one job spawns a load more!

Having got the change gear set up worked out for the 1.5mm pitch thread, Chris needs to make a mandrel handle so that he can rotate the Chuck in reverse to wind the saddle back, ready for the next cut.

At least with the tumbler gears set for a L H thread, the tool will be moving away from the Chuck whilst cutting, and turning the Mandrel backwards between cuts will move the saddle towards the Chuck again.

No wonder, with a dose of the 'flu Hopper became confused! (Get well soon)

Howard

[chris mcnicollParticipant @chrismcnicoll83540]

That handle sounds like hard work

[Geoff Garrett 2Participant @geoffgarrett2]

I am used to imperial threading on my imperial L5. I have the necessary gears for metric threading and understand what is necessary but what about the clutch? Should I leave it engaged all the time too?

I have no reversing switch on my motor so winding the carriage back is the issue. Will I be able to hand-wind the gears backwards with or without disengaging the back gear? Could I stop the motor and reverse the feed direction in order to use the motor for the rewind and still pick up the thread?

Any suggestion please!

If metric threading becomes a regular need I will fit a reversing switch at some point.

[Howard LewisParticipant @howardlewis46836]

The mandrel handle may sound like hard work, but if that is the only way to do the job, you have no alternative.

Gives an incentive not to be too brave with depth of cut!

Do you have tumbler reverse on your machine? If YES, it can be used to ensure that the saddle moves AWAY from the Headstock while the chuck rotates in the normal direction, to produce the required L H thread..

BUT you do not touch it once you start cutting the thread! The relationship between spindle ( mandrel ) and leadscrew MUST remain the same until the thread cutting is finished.

My advice would be not to open the half nuts, but to withdraw the tool between cuts, and wind back the saddle by hand, if the motor cannot be reversed. Wind it back a little too far, to ensure that any backlash has been taken out before the tool starts cutting again.

Keep us posted!

Howard

[chris mcnicollParticipant @chrismcnicoll83540]

morning folks, sorry for dragging up my old thread but i need to cut a 1.25 pitch thread now and ive lost the screwcutting book somewhere.

any help would be great, thanks

chris

[Nick HulmeParticipant @nickhulme30114]

Posted by not done it yet on 25/04/2017 16:54:42:

Whatever you do, do not open the lead-screw half-nuts under any circumstances between passes.

While good advice for a beginner, that is not entirely true. There are several vids on utoob which clearly demonstrate that the half buts can be opened – but must be closed again before reversing beyond the end of the cut.

Unnecessary for braked lathes or where plenty of undercut is available after the thread, but useful for instances where things are 'limited' in some way.

Cut away from the shoulder and outwards from the bottom of a bore and these issues are non-issues.
From personal experience, and not from watching YouTube videos, it's easier not to open the half nuts when cutting Metric threads on an Imperial lathe

[JohnFParticipant @johnf59703]

Posted by chris mcnicoll on 24/02/2020 09:16:32:

morning folks, sorry for dragging up my old thread but i need to cut a 1.25 pitch thread now and ive lost the screwcutting book somewhere.

any help would be great, thanks

chris

Chris, more info needed — what make/model is your lathe, is it imperial or metric, or whats the lead screw pitch, what change wheels do you have etc.

John

[chris mcnicollParticipant @chrismcnicoll83540]

Hi john, it’s all above but here it is my friend

harrison l2

imperial lathe

4tpi leadscrew

20, 40, 50, 55, 60, 60,80,127

[Howard LewisParticipant @howardlewis46836]

First thought that comes to mind is:

If you have a note of the train for a 1.5 mm pitch, and it contains a 50T , (unless it is an Idler ) take it out and replace it with a 60T.

That should rotate the Leadscrew at 6/5 the speed of the 1.5 mm set up, and so produce a 1.25 mm pitch.

Howard

Edited By Howard Lewis on 29/02/2020 15:02:52

[BazyleParticipant @bazyle]

50/80/40/127

[Pete RimmerParticipant @peterimmer30576]

Edited – poor information.

Edited By Pete Rimmer on 29/02/2020 21:20:53

[chris mcnicollParticipant @chrismcnicoll83540]

Posted by Bazyle on 29/02/2020 21:14:34:

50/80/40/127

There are only 3 shafts for gears to go on, or do I double gears up on the middle?

[SillyOldDufferModerator @sillyoldduffer]

Posted by chris mcnicoll on 29/02/2020 21:27:53:

Posted by Bazyle on 29/02/2020 21:14:34:

50/80/40/127

There are only 3 shafts for gears to go on, or do I double gears up on the middle?

I god a code in duh hed dis morning, so haven't checked the maths, but that's how I read Bazyle's instructions:

50 drives 80
80 and 40 on same shaft
40 drives 127

Although checking screw-cutting ratios on paper isn't rocket-science, I normally make a mess of it. Safer for me to set the gears up and check on the machine. Turn the lathe by hand to scratch a short spiral on a length of rod and measure the distance between turns in mm with a caliper. Or compare the scratched spiral with a known bolt or thread gauge.

Dave

[HopperParticipant @hopper]

If you go to this online calculator and put in "Other Lathes", your leadscrew and collection of gears, it gives you half a dozen alteratives including the one posted above . A handy resource to bookmark.https://littlemachineshop.com/reference/change_gears.php

Edited By Hopper on 01/03/2020 10:46:18

Edited By Hopper on 01/03/2020 10:48:55

Edited By Hopper on 01/03/2020 10:49:31

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