Thread: Imperial Threading on Metric Leadscrew

No, I mean't one gear for metric another for imperial. I may be way off base, but I vaguely remember reading something to that effect many years ago.

Why doesn't a thread dial work with metric?

Has a long discussion about why this can't be done.

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Originally Posted by sacc51

No, I mean't one gear for metric another for imperial. I may be way off base, but I vaguely remember reading something to that effect many years ago.

So you mean one thread dial and different gears so you can use the same thread dial for metric and imperial threading? There may have been some convoluted contraption invented, almost certainly in fact, but I can't say I've ever seen one personally, certainly not on any standard sort of lathe such as you'd get from Hercus/H&F/etc. The normal procedure when cutting a non-native thread is to leave the half-nuts engaged and reverse the spindle. That will wind the carriage back and away you go again. Indeed some people choose to do that when threading normally. It certainly saves any possibility of closing the half-nuts at the wrong time, but it's normally a slower way to thread, especially if it's a long thread.

Thanks for all the replies fellas
Hoping to give it a show over the Easter break

Originally Posted by Pete F

So you mean one thread dial and different gears so you can use the same thread dial for metric and imperial threading? There may have been some convoluted contraption invented, almost certainly in fact, but I can't say I've ever seen one personally, certainly not on any standard sort of lathe such as you'd get from Hercus/H&F/etc. The normal procedure when cutting a non-native thread is to leave the half-nuts engaged and reverse the spindle. That will wind the carriage back and away you go again. Indeed some people choose to do that when threading normally. It certainly saves any possibility of closing the half-nuts at the wrong time, but it's normally a slower way to thread, especially if it's a long thread.

I think from memory, the drop in location is not just the same place on one revolution of the lead screw, but a precise location on the lead screw over multiple turns.

What about hooking up a cheap encoder to the lead screw. ( maybe mount it in place of the existing thread dial?) Keeping track of the number of whole revolutions as well as the rotation angle, then lighting up a led or buzzer when you hit the zone.

Sort of like a smart electronic thread dial. I wonder if someone has already done anything along those lines? Good app for an arduino?

As for me, I keep the half nuts engaged, and reverse.

There was an article in MEW a few years a go I think about this. The author did the maths and discovered that the only combination that really worked was to have a wheel with 127 teeth (5x25.4 - the first integer that both inches and metric will divide into). If the lathe uses a 63 tooth combination as an approximate combination of a metric thread then I suppose you could use a 63 tooth worm.

Michael

Here is a solution to the problem, these (or similar) were manufactured but were very expensive, i have never seen
one myself or even seen one advertised.
There is limited info on them after a quick goog......
Patent US4218942 - Inch/metric thread dial - Google Patents

Here is a seller metradial

Here is a post about metraldial Metric tread chasing "dial" for imperial leadscrew - new design

Originally Posted by RayG

I think from memory, the drop in location is not just the same place on one revolution of the lead screw, but a precise location on the lead screw over multiple turns.

What about hooking up a cheap encoder to the lead screw. ( maybe mount it in place of the existing thread dial?) Keeping track of the number of whole revolutions as well as the rotation angle, then lighting up a led or buzzer when you hit the zone.

Sort of like a smart electronic thread dial. I wonder if someone has already done anything along those lines? Good app for an arduino?

As for me, I keep the half nuts engaged, and reverse.

Yes I believe that's right. You wind the carriage back to a specific point where the half-nuts can drop in. More theoretical that practical in my opinion, as I can imagine all manner of woes, but I have heard of it being done.

An ESL is definitely on the cards Ray. I think by the time a manual version was developed it would be just as easy and more versatile to just put a stepper on the leadscrew.

Oh yes I thought I'd seen those dials before, as mentioned, not something you're going to find on the average lathe.

Originally Posted by Pete F

Yes I believe that's right. You wind the carriage back to a specific point where the half-nuts can drop in. More theoretical that practical in my opinion, as I can imagine all manner of woes, but I have heard of it being done.

They showed us that trick at TAFE many years ago. I forget the exact details, it did work but struck me as a PITA.

I just cut metric threads on a metric leadscrew and inch threads on an inch leadscrew. Problem solved. You could do the same if you've still got the second Hercus, making a set of metric half nuts isn't all that hard.

Bigger toolroom lathes with single tooth dog clutches, problem more or less goes away. On the Monarch I don't disengage the half nuts regardless unless I'm cutting a really long thread as moving one lever on the apron reverses the carriage drive/leadscrew as selected.

Incidentally this is where a variable speed spindle drive really saves time when cutting long lengths of thread. Reverse the carriage, dial up the speed, get near the end, ramp down the speed, reverse direction, ramp up again etc.

PDW

Originally Posted by PDW

They showed us that trick at TAFE many years ago. I forget the exact details, it did work but struck me as a PITA.

I just cut metric threads on a metric leadscrew and inch threads on an inch leadscrew. Problem solved. You could do the same if you've still got the second Hercus, making a set of metric half nuts isn't all that hard.

Bigger toolroom lathes with single tooth dog clutches, problem more or less goes away. On the Monarch I don't disengage the half nuts regardless unless I'm cutting a really long thread as moving one lever on the apron reverses the carriage drive/leadscrew as selected.

Incidentally this is where a variable speed spindle drive really saves time when cutting long lengths of thread. Reverse the carriage, dial up the speed, get near the end, ramp down the speed, reverse direction, ramp up again etc.

PDW

If I can remember all what we learned in those years ago days Pete , the repeat distance had nothing to do with a magic bullet for tracking a metric on imperial or vv. The repeat distance was the minimum distance a number of work leads matched a number of leadscrew leads so that the half nuts could be closed again hence we had it drummed into us: lead of work/lead of leadscrew = tracking ratio. The resulting fraction top number is the turns of the leadscrew and the bottom is the turns of work where they both line up. The same formula for both metric on metric and imperial on imperial leadscrews. It doesn't matter if we stop the lathe, disengage the half nuts and move back the repeat distance or multiples of and then close the half nuts and start rotation for next cut....................... bloody slow!! or whether we leave the lathe running, disengage the half nuts, move back to start and engage on the required number on the chasing dial or star wheel. Both ways is only measuring the repeat distance.

Those old metricated Nuttalls had 24 and 35 tooth starwheels which in a way were better than the chasing dials on the Colchester etc. For short threads reversing the spindle is just as quick as the disengaging way but can be butt clenching with single phase reversing as we all know! However we still have to stop the spindle whilst resetting the tool otherwise we'll push the tailstock off the end (or the centre out of the work)!!

All this makes my head spin. I just accept that doing imperial threads on a metric lathe requires me to leave the half nuts engaged. There is a section in the workshop practice book on thread cutting that shows you how to disengage by going back 254mm or something like that. I remember reading it and sort of half understanding the concept. I assume it's along the same lines as what is mentioned above by others here.

The simplest (and sexiest) method I have seen is by Joe Hovel with his electronic leadscrew. It includes a micro switch on his saddle stop which, with a rheostat resistor, allows him to thread at full speed to a shoulder and the lathe stops. The leadscrew is driven by a stepper motor which is indexed and it picks up the thread each and every time a new pass is done, metric or imperial. Also, because it's electronically driven from a signal generated from the spindle, it allows for tapered threads etc without the need for a taper turning attachment. No need to change gears or anything to choose different pitches, just select the pitch from a menu.....

Cheers,

Simon

Originally Posted by RayG

I think from memory, the drop in location is not just the same place on one revolution of the lead screw, but a precise location on the lead screw over multiple turns.

What about hooking up a cheap encoder to the lead screw. ( maybe mount it in place of the existing thread dial?) Keeping track of the number of whole revolutions as well as the rotation angle, then lighting up a led or buzzer when you hit the zone.

Sort of like a smart electronic thread dial. I wonder if someone has already done anything along those lines? Good app for an arduino?

As for me, I keep the half nuts engaged, and reverse.

yah Ray thats funny and a great idea. it could have a sinc meter

Originally Posted by RayG

What about hooking up a cheap encoder to the lead screw... then lighting up a led or buzzer when you hit the zone.

The relationship that you want is the rotational position of the spindle to the tool (and the tool is 'fixed' to the leadscrew). A standard thread indicator just shows when the spindle position coincides with a spot where the half nuts can drop into the leadscrew. This is why a single tooth dog clutch works so well - leaving the half nuts engaged, you are always going to pick up the right point on the spindle.

Metric on an imperial lathe (or the other way around) is a problem because that first integer coincidence is a big number. To get an electronic indicator you would need a position encoder on the spindle, one on the leadscrew and a linear encoder on the carriage so the positions are all known. (if you could input a gear ratio the encoder on the leadscrew becomes redundant).

To simplify further, it could done with a sensor on the spindle (so once per revolution you know where the spindle is), a stop on the bed as a fixed starting point for the carriage, and a sensor on the leadscrew so that again, once per rev you know where the leadscrew is. The method would be to set the sensors (one would need to be adjustable) so that when both switches are closed a light comes on to engage the half nuts. Reaction time then becomes critical.

Originally Posted by RayG

As for me, I keep the half nuts engaged, and reverse.

Much simplier

Michael