OT metric threads

[duckman903]

I just learned a cool trick we have a reasonably new Knuth lathe which cuts metric but you can't open the half nuts once started, cut, retract tool, reverse, repeat, when doing internal threads and you're checking thread fit it sucks when the test piece is 10" long, so the trick is reverse to clear the part, stop the chuck, reset the DRO to -0- open half nuts crank carriage away from work check fit with test piece return to -0- reengage half nuts tada done works great "JUST DON'T MOVE THE CHUCK" .

[Magicniner]

Totally logical but although I have an imperial lathe and cut metric threads on it I don't think I'd have thought of that!
Good on you for sharing!

- Nick

[liveaboard]

My lathe is metric except for the lead screw, which is imperial.
How weird is that?
I mean literally, is that weird or is it a common thing?
Even the fasteners and the headstock are metric.
Except the cross slide gib screws, which are inexplicably 1/4" while the compound slide ones are 6mm.

[Peter.]

You think that's odd - my cross-slide screw is 1/2" diameter 2.5mm pitch

[tornitore45]

Cars come with LH drive and RH drive made by the same manufacturer.
A manufacture of machinery must offer Imperial and Metric version regardless of the native design.
If you have a metric design machine with metric fasteners, gears and basic stock dimensions and you customer want an imperial machine you would change the smaller number of parts and make a 30mm 5 TPI lead screw, get a 60Hz motor and relabel the speed chart 20% higher.

[liveaboard]

Yes, but the machine is all metric EXCEPT for the lead screw.
Cross + compound, metric. Fasteners, headstock, metric.
Made in Poland, Metric I assume.

[tornitore45]

I missed the metric cross slide.
That makes it really weird.

[liveaboard]

The only other imperial thread I've found is the gib screws in the cross slide, which are 1/4" ; the ones in the compound slide are 6mm metric.
It could be that it's made out of 2 machines; but it's not a common model and I bought it in Holland. There are no imperial threads there.
The lead screw is visibly worn near the chuck end, this little machine must have done a lot of thread cutting. The ways have been re-ground, and after that allowed to rust deeply before being restored.
It's old.

[pete]

A trick I'll have to remember Duckman. Using a zeroed out dial indicator should work just as well if you need to move the carriage to check the fit.

[dly31]

You could just as well make a pencil mark on the ways before disengaging the half nuts. Your re-positioning only has to be accurate enough to hit the right thread on the leadscrew (+/- 0.125" on an 8 TPI screw).

[tornitore45]

You could just as well make a pencil mark on the ways before disengaging the half nuts. Your re-positioning only has to be accurate enough to hit the right thread on the leadscrew (+/- 0.125" on an 8 TPI screw).

While that works on principle, pencil mark on oily ways?
Some lathes (mine) have some slop in the half nut position, at 16TPI is safer to use a more accurate reference such as a travel indicator or a lightly clamped carriage stop.

[BadDog]

I don't cut LOT of metric threads, but I have needed to turn a fair few. That includes a 30mm left hand thread stub shaft on a steel hub to adapt an electric fan clutch from a MD application onto a Cummins 6BT, try buying a die for that. Anyway, most threads aren't all that long, and I don't really even notice the impact of power reversing back to start the next pass. And I'm not by any means one of the hobby machinist guys that approaches this sort of thing with the "I don't work production so I take things slow" mind set. In fact, in that respect I'm probably one of the more impatient folks on this board, and I've had fairly experienced (hobby) machinists nearly go into cardiac arrest upon seeing the speeds I single point at. So when I say it doesn't really register as a problem that needs solving, to me that seems significant. I don't keep records and my memory is terrible, but as I recall the longest metric thread I've ever turned was maybe 3" long and something like 10 mm diameter on a tube for a lighting fixture I was "adapting". It registered enough that I remember having to do it, but that was a complete aberration in being a longer length. Just my take on it, and it's an interesting thought exercise, but not something I'm likely to ever use. However, if you live in a place where metric threads are pervasive, I can definitely see it being worth adding to your "tool kit".

Edit: It just occurred to me that I may be thinking about this backward. I was thinking mostly in terms of me being impatient, and thinking back over how I've never been particularly bothered by reversing with out disengaging the half nuts. My biggest problem is just avoiding the automatic disengage without thinking about it (but it can be picked up again with a little diligence). Anyway, it may be precisely BECAUSE of my impatience that it doesn't register. If I were going slow and waiting on the thread, then going slow waiting even longer for it to return <repeat>, THAT would drive me bonkers. But when your running single point at typical form tool speeds, flicking the cross feed out a tiny moment before power reversing back to start, hit brake, and then set in-feed/compound for next pass; I'm guessing maybe that's why it's not really that big of a burden in my mind.