Change lead screw to metric?

[liveaboard]

My lead screw and half nuts are pretty bad.
The old screw is acme 6tpi, although the rest of the lathe is metric.
Except the tailstock screw.

Anyway, a new acme lead screw + nut costs around $500, but a metric 5mm trapezoidal screw + nut only costs around $100.
So it comes down to the change gears I think?
Is there anything else to consider?

Probably the gears I have would do. Does anyone have a change gear chart for a 5mm leadscrew?

[pete]

I can't help with your conversion questions. And my lathes lead screw is 8 tpi so no help there. But how much single point threading do you do, or any real long decent accuracy threads being done? While your current lead screw and half nuts are as you say pretty bad, there overall pitch spacing might still be quite good. I've spent a bit of time on ACME feed screw manufacturers web sites after reading about Moore Tools efforts to manufacturer "perfect" feed screws. Surprisingly you've got to spend some pretty healthy money to get even a few thou per foot of lead or lag errors in the pitch spacing of the screws. And that's so even with ground threads. Better accuracy than that and they start getting really really expensive. At that point you almost need ground ball screws. And simple non ground cut threads are much lower accuracy. For most short single point threading jobs the errors in our lead screws are probably small enough to mostly average out and not matter in a home shop. So about the only thing I can think of that might be worth mentioning is how are the guaranteed allowable + - pitch error numbers on that trapezoidal screw and nut?

I've seen a few web sites in the past but haven't rechecked for any today that allowed you to to use there free program to input your lead screw pitch, available gear tooth counts and thread pitch being cut and the program would make recommendations for the gearing set ups if you get stuck. Some of them would also give the inaccuracy's if the gearing combinations you have isn't a perfect match. Obviously a copy of a threading chart from any lathe with that 5 mm pitch lead screw would be a whole lot faster and better if you can find one. I don't see a threading dial on your lathe, but if you've got one the gear would need to be changed to match the new pitch of the lead screw. Sorry I can't be of more help.

[spro]

Yes and Hi liveaboard. I think you already made new half nuts and maybe turned the lead screw around, front to back. I don't have the answer to 5mm screw but somebody else does. I hope it works out. That is one stout lathe.

[liveaboard]

I do a fair amount of single point threading but all rough work; Almost all metric. I'm not making supper fine or super accurate things.
Nothing else on this lathe is quite accurate either...
My half nut threads look like round knives and the lead screw threads are thin and rounded at the area that does the most work.
Some fine threads I made I had to do at the rear of the lathe.
It could be that it's not worth fixing; so far, it does my work one way or another.
I bought a big lead screw cheap a few years ago for another job, and replaced the cross slide screw on the lathe for a few bucks; but that company isn't trading anymore.


Spro; making the half nuts from scratch would be a stretch for me; I could turn the screw around easily enough.

[spro]

Hi. I thought it was you that did the half nut repair. Man, sometimes they poured metal and sometimes silver soldered. Get you down the road further. I was reluctant to mention turning the lead screw around because with long shafts, as you deal with, maybe you have to turn threads at the tail stock. By turning the lead screw around you get fresher threads which will mate correctly to the better half nuts and you don't need the lead screw for that.
It may not be easy but generally a person gets a better acme screw near where the work happens. In the meantime, Members here and far away are looking out for 6 TPI Acme before they are destroyed. We need to know the diameter and I expect the length at least 7'. Well no. Doesn't have to be that long but it will still fit your charts/ ratios.
Until someone else confirms the alternative by metric lead screw.

[spro]

Oh pete. I have to reread but you are a fountain of knowledge about these topics. Thanks.

[pete]

No far from a fountain Spro, mostly repeating bits and pieces I've managed to pick up from random places. But thanks for that anyway.

[liveaboard]

My leadscrew is 1"x36", + a few for the bushings.

Thinking about it, turning the screw around doesn't really make sense; sometimes I thread things right at the end [anyway, I did that once].
If I'm going to get into it and do the work, I might as well do the work. Anyway, the nut is the expensive part.

Looking for 5mm leadscrew gear change charts is hopeless; in English searches 95% are imperial and few show the leadscrew the chart is intended for.
When I did find metric leadscrew charts, the gears used are not what I have.

So I'll just have to get into it with paper and calculator and work out whether it would work out.
There may be other possibilities, like 4mm. or 6.
A neighbor told me he knows a shop up in Lisbon who sells gears and things, they might have the leadscrew + a nut.
I need a pinion gear too. Maybe the rack.
This machine is very used.

[spro]

Maybe the fellow in Lisbon has what you need. I'll keep an eye open for 1" X 6 TPI too.

[liveaboard]

Of course I'll ask; but in this part of the world, I might as well be asking for moon rocks.
Portugal adopted the metric system in 1875.

[tornitore45]

It appears, looking at the levers, you have a full Norton gearbox between the change gears and the lead screw.

I assume you already have a chart or a table for your old 6TPI lead screw, you can use the same with some conversion factor.

Let's see

Since you do most metric threads with an imperial screw your change gears must have the 120/127 ratio (or whatever approximation of that you happen to have) ratio installed which needs to be removed.
By virtue of the 120/127, your old leads-crew was actually "transformed" into a 10/3 ~= 3.333mm
so by replacing the lead-screw with a 5mm and removing the 120/127 ratio you have a 1.5 time faster screw generating metric pitches 1.5 time smaller (finer)

Use your old thread table but set up for your metric pitch 1.5 time coarser

Same thing appears for TPI. Now the 127/120 gear ratio need to be installed reversed (127 Driving a 120) making your 5mm lead screw look like a 1/4". As a result you are now cutting screw with 2/3 of the TPI listed on the original table. Set up for 1.5 x TPI you want.

[liveaboard]

No, there is no gearbox; this old lathe has change gears.
To make metric treads, it has a 42 [or 84] to 89 tooth gear set; so it seems to me that to make imperial from metric, I should just turn that around, and use the 89 to drive the 84.
But I can't seem to get the numbers to come out right.
gears I have; 25, 30, 38, 40, 42, 50, 55, 60, 60, 70, 80, 84, 89, 100.
Gears are available, not too expensive. The minimum postage cost is around $30 but the gears are only $5-$15 each.
25 tooth is a minimum diameter for the shaft, and bigger that 100 might cause trouble too.