# Can I get away with using 20 TPI for a 1.25 mm metric thread



## woodfarmer (12 Nov 2014)

pretty much all in the header.

just need about 1/2 inch or less 10-12mm of thread to fix a nut to.
My lathe only does Imperial threads and I need to make something using 12mm steel rod.. should have though more about it and ordered 1/2 inch  Of all the 8 mtres of rod I only need to thread two ends, rest is welded.


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## flh801978 (12 Nov 2014)

Your pitch will be 1.27 so over 12 mm you will be fine with that
maybee a fraction tight so plenty of oil when fitting nuts

Ian


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## Inspector (12 Nov 2014)

If you are welding rods together couldn't you cut the heads of some bolts and weld them on too? 

Pete


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## twothumbs (12 Nov 2014)

If threads are very tight, some cutting/grinding paste will ease them, but wash it clean afterwards. You will only need a dab on your finger so perhaps a local garage workshop would oblige,or someone who does cars or bikes will have it.

Best wishes.


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## woodfarmer (12 Nov 2014)

Thanks, guess best thing to do istry a test piece.


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## RogerP (12 Nov 2014)

Can't you used a die? Am I missing something?


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## Myfordman (12 Nov 2014)

I'd turn to 12mm OD, rough out on the lathe screwcutting at 20tpi and then take the last few thou off with the 1.25mm pitch die (M12 iso fine I think)

MM


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## woodfarmer (12 Nov 2014)

Myfordman":228eg55c said:


> I'd turn to 12mm OD, rough out on the lathe screwcutting at 20tpi and then take the last few thou off with the 1.25mm pitch die (M12 iso fine I think)
> 
> MM



Trying to avoid spending £20+ for just two threads

may just weld the stainless 12mm rod to a SS flange and use a couple of bought stainless /nuts & bolts.


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## DTR (13 Nov 2014)

Is there a particular reason why you can't screwcut the thread on your lathe? Metric approximations can be made on an imperial lathe using the standard gears. The only issue is you have to leave the leadscrew engaged and reverse the lathe back to the starting point.


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## woodfarmer (13 Nov 2014)

DTR":ifwmescs said:


> Is there a particular reason why you can't screwcut the thread on your lathe? Metric approximations can be made on an imperial lathe using the standard gears. The only issue is you have to leave the leadscrew engaged and reverse the lathe back to the starting point.



On my ninety year old lathe screwcutting is from a gearbox. just move the lever to the TPI you require from 4 to 28 TPI .

Metric threads are somewhat difficult . Normally I buy imperial bar or if bigger just make whatever size suits. but cant sensibly cut 12mm internal thread. If I had thought about it more thoroughly I should have bought 1/2" bar and not 12mm. Then I could have threaded the bar and just bought a couple of stainless steel nuts. Just sort of painted myself into a corner 

Just this week made a toolpost mount for my woodcut bowlsaver (1 inch bar with a 1/2" UNF threas on the end).


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## DTR (14 Nov 2014)

woodfarmer":oncfmdcr said:


> DTR":oncfmdcr said:
> 
> 
> > Is there a particular reason why you can't screwcut the thread on your lathe? Metric approximations can be made on an imperial lathe using the standard gears. The only issue is you have to leave the leadscrew engaged and reverse the lathe back to the starting point.



On my ninety year old lathe screwcutting is from a gearbox. just move the lever to the TPI you require from 4 to 28 TPI ./quote]

A gearbox! #-o That would make sense then


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## woodfarmer (15 Nov 2014)

DTR":29zdevrd said:


> woodfarmer":29zdevrd said:
> 
> 
> > DTR":29zdevrd said:
> ...



Like this...


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## bugbear (15 Nov 2014)

woodfarmer":7n33i2l3 said:


> On my ninety year old lathe screwcutting is from a gearbox. just move the lever to the TPI you require from 4 to 28 TPI



Can you add new gears on the outside (left hand of your photo) ?

BugBear


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## woodfarmer (15 Nov 2014)

bugbear":ngrypfo4 said:


> woodfarmer":ngrypfo4 said:
> 
> 
> > On my ninety year old lathe screwcutting is from a gearbox. just move the lever to the TPI you require from 4 to 28 TPI
> ...



It might be possible, but many are on fixed centres so they would have to be made in pairs.


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## bugbear (16 Nov 2014)

woodfarmer":2l6q87fq said:


> Can you add new gears on the outside (left hand of your photo) ?
> 
> BugBear



It might be possible, but many are on fixed centres so they would have to be made in pairs.[/quote]

Tricky. And an extra jockey wheel would reverse the motion :-(

I was thnking (of course) of a 127, or 65 (introduces a very small error) gear.

Maths

BugBear


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## woodfarmer (16 Nov 2014)

bugbear said:


> woodfarmer said:
> 
> 
> > Can you add new gears on the outside (left hand of your photo) ?
> ...



Tricky. And an extra jockey wheel would reverse the motion :-(

I was thnking (of course) of a 127, or 65 (introduces a very small error) gear.

Maths

BugBear[/quote]

I can reverse direction with a lever  (for cutting LH threads and reverse feeds.

One day i will try to count the teeth on the last two gearwheels.

But back to my original question..12mm metric fine is 1.25 mm 25mm of thread uses 20 turns, and 20 tpi does 20 turns in 25.45mm so 1/2 a mm runout over and inch.. about 0.2 mm over the length of a nut. Think I will buy a couple of staonless nuts and just cut the 20tpi threads until the nut will go for its length and see what it looks like.


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## chipmunk (17 Nov 2014)

In answer to your original question, it will work if you cut a 20 TPI thread to fit a 1.25mm threaded nut but the bottom line is it won't be very strong as the thread mesh will only effectively be in one place.

The other option would be to buy a cheap carbon steel imperial tap to make a nut and thread-cut your rod to match that. You can pick up a 3/8" or 1/2" UNC carbon tap for £3 from Chronos. Or with a short piece of silver steel you can even make your own tap.

HTH
Jon


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