Metric threads on an Imperial lathe

[WobblyHand]

It's not super hard to pick up a thread, but it's not something you'd want to do every pass. By far, the easiest way is to leave the half nuts engaged when single pointing metric threads on an imperial lathe. Try picking up the threads on an M8x1 internal thread, then you will experience some fun. It's really hard to see what you are doing!

 

[WobblyHand]

As for an ELS implementation, of auto pickup of metric threads on an imperial leaf screw, one needs to exactly understand it all, before programming it! I'm not there yet. I'll figure it out eventually. Just have to work through all the details.

 

[SLK001]

I know that there are different ways to pick up a metric thread, but by far the easiest is to just leave the half-nuts engaged throughout the threading operation. I'm not a production shop, so speed isn't more important that accuracy. I take my time and hopefully, I get it right the first time.

South Bend made a thread dial for metric threads, but I have never seen one, or used one.

 

[woodchucker]

Yes, some kind of compound gear with a 127 tooth gear in it will allow you to cut perfect metric threads on an Imperial machine. The lowest factor between metric and Imperial is 127 (127 x 2 = 254). The compound gear with the 127 will bring the difference to easy fractions to gear like 1/10, 1/8, 1/6, etc, instead of fractions like 1/11.1334278128399445060133231.



No, the 100/127 is exact.

I don't think so. And it is only good on an 8tpi lead screw I believe.
The 100/127 is close
I should have looked at the chart b4 I spoke. it's deviation is 0.00 so yes, for an 8tpi lead screw it's exact.

 

[Jake M]

It's not super hard to pick up a thread, but it's not something you'd want to do every pass. By far, the easiest way is to leave the half nuts engaged when single pointing metric threads on an imperial lathe. Try picking up the threads on an M8x1 internal thread, then you will experience some fun. It's really hard to see what you are doing!

Agreed, whole heartedly. I didn't mean to imply that that was a "solution", only that it could be done. More in that the pressure on the operator to "not screw up" is greatly reduced, since "loosing the position" does not mean a scrapped part. It is a pain in the (toe), but it CAN be recovered. Recovering every time is not really feasible. But knowing that it's possible makes it plausible to release the half nuts at the end of a cut IF IT'S REQUIRED, which in many cases it isn't. Not releasing the half nuts at all is far and away the easiest way. And frankly, even with inch threads on an inch threading lathe, where the dial DOES work... Unless it's a long thread, it's just as fast to reverse the lathe as it is to use the thread dial anyhow.

As for an ELS implementation, of auto pickup of metric threads on an imperial leaf screw, one needs to exactly understand it all, before programming it! I'm not there yet. I'll figure it out eventually. Just have to work through all the details.

I guess I was "thinking out loud" there. I'm not saying it's not possible or not realistic, as I think it's probably very doable. As I said, I just recently acquired (and used) a transposing gear set, and the ins and outs of that have fascinated me since, and I'm wrapping my head around it one step at a time. So simple, yet on the other hand, not so simple... Stuff like that keeps me entertained. I'm not sure why.

 

[SLK001]

I don't think so. And it is only good on an 8tpi lead screw I believe.
The 100/127 is close
I should have looked at the chart b4 I spoke. it's deviation is 0.00 so yes, for an 8tpi lead screw it's exact.

Even for other TPI lead screws, it's just a matter of simple gearing.

 

[homebrewed]

if you release the half nuts, stop the lathe, and the dial advances a quarter turn, you've got to back out of the cut, reverse the lathe, and drop the half nuts on the EXACT same INSTANCE of the exact same mark you had it on before. (It's usually a small amount, quite easy to back it up by hand).

That's pretty much what Joe Pie was doing. I haven't tried that method yet but it looks like it will work OK.

 

[Jake M]

so yes, for an 8tpi lead screw it's exact.

After the 127 tooth and -some other- gear compounded with it, every gear and every quick change gear, everything rotating downstream of that is "thinking" in metric. The 127 tooth "input" side of the gear is making the assembly, the compound gear, rotate a "metric number of revolutions". It'll be compounded with some gear that has some good factors for downstream gear changes to select different thread pitches. They'll all be metric pitches, even if you dial in an unusable thread pitch and get something silly like a 0.82mm thread, it'll be wrong in metric. Every ratio after that transposing gear is multiplying and/or dividing IN METRIC.
The way to wrap your head around it is not necessarily mathimaticallly correct, but it makes the lead screw "act" like it is metric. So if you make your leadscrew ACT like a 6mm pitch lead screw for example, you'd select gears (or tumblers) to get a 4 to 1 reduction, you're going to cut a 6mm/4 or 1.5mm thread.

 

[Jim F]

After the 127 tooth and -some other- gear compounded with it, every gear and every quick change gear, everything rotating downstream of that is "thinking" in metric. The 127 tooth "input" side of the gear is making the assembly, the compound gear, rotate a "metric number of revolutions". It'll be compounded with some gear that has some good factors for downstream gear changes to select different thread pitches. They'll all be metric pitches, even if you dial in an unusable thread pitch and get something silly like a 0.82mm thread, it'll be wrong in metric. Every ratio after that transposing gear is multiplying and/or dividing IN METRIC.
The way to wrap your head around it is not necessarily mathimaticallly correct, but it makes the lead screw "act" like it is metric. So if you make your leadscrew ACT like a 6mm pitch lead screw for example, you'd select gears (or tumblers) to get a 4 to 1 reduction, you're going to cut a 6mm/4 or 1.5mm thread.

The only gears that count are stud and screw, everything else are idlers........

 

[Jake M]

The only gears that count are stud and screw, everything else are idlers........

I don't think I said different?