Three or four start thread ?

[blacksmith]

Beginning to think you may be right .JC, might have a look at the Indian or Chinese stuff. Can buy a flypress from there but I like making my own machines.

[PDW]

It is 1/2 TPI isn't it? 2" lead ?

You would want a BIG lathe to cut a thread that coarse on especially since it would be a 6mm wide cut... My Purcell lathe has 1/2 TPI on the thread plate but I fear something would break cutting a thread like that.... You would want something like a 25"+ swing lathe to do it... With a big leadscrew...

I recently cut some 2 tpi LH single start thread for a cast iron nut using my Monarch CY lathe. Screw was 2" OD. That was a 6mm wide cut. Not a problem at all in cast iron, nice big hole so stiff boring bar.

Issue with the 0.5 tpi thread is more the tool angles leading to weakening of the edge and swapping some gearing around to get the lead. You can tilt the cutter bit to reduce the acute angle though. The CY goes down to 13 rpm and with the auto-disengage single tooth clutch in the drivetrain the actual cutting is pretty simple. I'm pretty sure it could be done.

The screw is dead easy on any vertical mill. You take 1 handwheel off and put a sprocket in its place. Make up a set of centres with one being driven by taking another sprocket of the desired size/tooth count to translate the X axis screw pitch to the desired TPI. Insert end mill/slot drill of desired size & cut away. If you want to get fancy use a gear train instead of sprockets.

PDW

[Stustoys]

Issue with the 0.5 tpi thread is more the tool angles leading to weakening of the edge and swapping some gearing around to get the lead. You can tilt the cutter bit to reduce the acute angle though. The CY goes down to 13 rpm and with the auto-disengage single tooth clutch in the drivetrain the actual cutting is pretty simple. I'm pretty sure it could be done.

Sounds like a nice piece of kit you have there PDW.

I wouldnt like to try it on my lathe, My math says with the spindle at 75rpm the leadscrew would be doing about 1200rpm. Even if I slowed the spindle to about 10rpm with a VSD to get the leadscrew rpm back to "normal" the power going to the gearbox would be the same but the torque on the first gear would be about 8 times "normal".

Am I worring about nothing again?

There are no warnings about thread pitchs v spindle rpm on these lathes.

Stuart

[RayG]

Hi PDW,

Nice idea for a vertical mill, gearing the axes to cut threads... That Monarch sounds like a handy thing to have.

Hi Stuart,

Just for the sake of interest, following your train of thought, 75 rpm x 50 mm pitch = 3750 mm/min... 62.5mm/sec Cutting a thread 300 mm long, that's under 5 seconds.. you'd want good reflexes. (or an autostop.)

Regards
Ray

[Stustoys]

Hi Stuart,

Just for the sake of interest, following your train of thought, 75 rpm x 50 mm pitch = 3750 mm/min... 62.5mm/sec Cutting a thread 300 mm long, that's under 5 seconds.. you'd want good reflexes. (or an autostop.)

lol I hadn't thought about that. Imagine trying pick up the dial and the noise if you got it wrong.

Stuart

[AndrewOC]

Hi blacksmith and all!
In about 1998 i was an apprentice in Botany. The ol' toolmaker there was asked to replicate a bronze/brass nut for a two post car hoist. It was a 4 start coarse thread jobbie. The mechanic that owned it was going to scrap the hoist as the nut was that buggered.
So a scrapped old two post hoist might be something to look out for...
cheers,
AndrewOC
-another blacksmith.
PS my fly press has a 63mm pitch, 4 start, 46mm dia

[PDW]

Sounds like a nice piece of kit you have there PDW.

I wouldnt like to try it on my lathe, My math says with the spindle at 75rpm the leadscrew would be doing about 1200rpm. Even if I slowed the spindle to about 10rpm with a VSD to get the leadscrew rpm back to "normal" the power going to the gearbox would be the same but the torque on the first gear would be about 8 times "normal".

Am I worring about nothing again?

There are no warnings about thread pitchs v spindle rpm on these lathes.

Stuart

A lot of the old Monarch lathes and other big American iron had single tooth dog clutches in the drive train. You set stops on a rod going through the apron and the clutch gets cammed out of engagement at the same spot each time. Always picks up the lead again too because there's only 1 place it can engage. Just reverse the feed using the apron lever.

I would worry about the torque in a light/medium lathe, yes. A square thread is very forgiving to cut though esp if it's in cast iron so you could easily cut it to depth using a 1/8" tool then widen out the groove. Slow but would keep the cutting forces down.

PDW

[Stustoys]

A lot of the old Monarch lathes and other big American iron had single tooth dog clutches in the drive train. You set stops on a rod going through the apron and the clutch gets cammed out of engagement at the same spot each time. Always picks up the lead again too because there's only 1 place it can engage. Just reverse the feed using the apron lever.

One day if I ever run out of other things to make I'm going to look at making one for my lathe. I have plans around here somewhere. The gearing inside the headstock would have to be correct, I haven't checked that yet.
Stuart

[Ropetangler]

:

It is 1/2 TPI isn't it? 2" lead ?

You would want a BIG lathe to cut a thread that coarse on especially since it would be a 6mm wide cut... My Purcell lathe has 1/2 TPI on the thread plate but I fear something would break cutting a thread like that.... You would want something like a 25"+ swing lathe to do it... With a big leadscrew...

As someone who is a "wanna- be " machinist, with very little machining experience, (in other words this idea may be impractical at best) but what if the thread was cut in sections, with tools of smaller section than the complete thread profile, maybe even using a v thread tool to start, and roughing down one side of the thread to near full depth. Lets say that the resulting thread is 3/16" wide at the top, when you get to depth, which would enable you to come across 1/16", and take another series of cuts, to end up at the same depth as the first series, - say at 0.245", leaving a little for a finishing cut to full depth.
At this point my idea would be to use tools ground for either acme or square thread profile, depending what is required, and then cut the rest of the profile, one side at a time, and also clean up the thread root with the same tool(s).
Granted, it would be fiddly, and a bit of a trial indexing up on the previously cut v thread, with the final tool/s when squaring the profile up,(and it would have to be done 4 times for a 4 start thread), but I imagine that the best result for this job would be a pretty loose fit anyway, because you would want the big cannon ball/s to really spin to drive the punch.
O.K. fellers, shoot me down in flames, cannon fire maybe, but might this work?
Rob

I must be the worlds slowest typist, I just realised that another 8 posts were made as I was typing mine,- maybe I'm just too long winded.