Metric thread on imperial lathe, without reverse.

[Stustoys]

Hi Ewan,
I'd rather your head hurts than mine
No. cutting metric on imp screw, the pick up distance gets shorter so the time remains the same. But cutting imp on metric screw the pick up distance seems to stay the same and the time increases as pitches get finer. 300 pitch for 20tpi, 600 pitches for 40tpi!

Sorry I should have labeled

The first is 1.5mm pitch thread on an 8tpi leadscrew
then 0.5mm on 8 tpi
0.25mm on 8tpi
40 tpi on 3mm lead screw
20 tpi on 3mm

Stuart

[cba_melbourne]

Thanks for the tip of cutting from a shoulder in reverse, i'll remember that one!

Ew

For outside threads it is easy, you can just reverse the cutting tool upside down in the standard toolholder and reverse the spindle. One would intuitively think this cuts a left hand thread; but you get indeed a normal right hand thread with the tool moving towards the tailstock. If you have a rear toolpost, you can of course also clamp the tool in there, not upside down, and run spindle in reverse. You need to make a relief at the shoulder though, as you have to somehow advance the tool before the cut starts. If you had a threaded spindle nose, the chuck could spin off under a heavy cut. I do not think the forces when cutting smaller threads say M12 and smaller would undo the chuck. But larger threads maybe yes. There are two methods, one cheap and nasty is to add a grub screw to the back plate, that locks it to the spindle thread. But the grub screw may over time damage the precious spindle nose, especially so if the spindle is not hardened. The better way is to make a new backplate with a locking collar, but its much more work - and if the type of spindle relies entierly on the threads to center the chuck, not on a tight fitting register, the locking collar may actually affect chuck runout.

But the greater problem when cutting to a shoulder are actually the internal threads. The angst factor is greater, because not only is there a shoulder, you additionally cannot see the shoulder. Just flipping the tool upside down and run the spindle in reverse as for external threads will not work here. You really need an internal threading tool that is a mirror image of the normal tool, a left handed threading tool. Chris

[Stustoys]

Here you go Ewan. Have been showing my working.
I think the maths to get the leadscrew turn is
1/(3/(25.4/14))

[Ueee]

I see where i stuffed up....my head is now unexploded.....

So to work out the turns of the spindle you need 127/(pitch in metric/pitch of leadscrew) divide that by the tpi to get the saddle reset distance. For 20 tpi its 127 pitches of the leadscrew or 15 inches. Actually for a 3mm screw it seems to be always 15"/127 pitches....works for odds....for half threads it needs to be 254/, so the reset distance is double.
For different leadscrew pitches it is still 127 (or 254) pitches, so the distance for reset gets bigger. 6mm pitch (whats a standardish metric leadscrew pitch? The Mars is 3.5mm) and you need 30" to reset......

The number of spindle turns for metric on an imp lathe is 127 too. So although it seemed like ages it really only took roughly 40 sec at 200rpm, minus some for moving the saddle back.

Chris, All the internal threading i have done with a HSS bit in a boring bar, so reversing it to cut in reverse would not be a problem. This only works if threading into a shoulder and not into a completely blind hole as the end of the bar would get in the way.

Cheers,
Ew

[Michael G]

But the greater problem when cutting to a shoulder are actually the internal threads. The angst factor is greater, because not only is there a shoulder, you additionally cannot see the shoulder.

I cut them 'normally' but put a piece of masking tape on the bar holding/ forming the cutter. I then know that as soon as there is no metal visible between the tape and the end of the job I need to flick the cutter in and disengage feed. (Having a lathe with the single tooth clutch I cut threads with the half nuts always engaged anyway). It takes some getting used to but the tape is a surprisingly good depth indicator.

Michael

[Stustoys]

Hi Ewan,
Thanks for that.
So cutting metric pitches on an imp leadscrew. Pick ups are 127 turns of spindle apart so pick up distance changes with pitch but the time between pick ups stays the same.(38 seconds at 200rpm)

Cutting imp threads on a metric leadscrew. Pick ups are 127 turns of the leadscrew apart so pick distance stays the same(15") and the time between picks changes. (about 1 minute for 14 tpi at 200rpm which depending, one might be able to live with. 3 minutes for 40tpi not so much)

3mm is standard for us little guys. certainly 6mm on larger lathes but I have no idea what others are used (I would never have guessed 3.5mm)


Stuart