Custom Thread Sizes

[ALLAN QURASHIParticipant @allanqurashi51750]

I'm new to single point thread cutting, and dropped myself in at the deep end on my first ever thread; making a 2" and a bit diameter x very fine thread securing nut for an unknown brand of 60 year old pressure regulator which didn't fit any pitch or diameter standards. But it worked….it only had to tighten up and not strip, so not much to ask.

I've since practiced a few male threads sizes on a bit of 1 1/8" dia bar (on sections which would be swarf anyway on reduction down to required dimensions), so have had a little bit of 'free' practice.

But I want to make a replacement leadscrew for a lovely little Rollo Elf roundbed lathe because its current one is 1/2" x 12 TPI, BSW which means 83 and a third thou per turn…Yuk. Can't really go bigger than 1/2" dia, so I was thinking 10 TPI would be nice….so I can make a 100 division resettable dial for it.

The thing is I can't find any thread standard with this combination. I found this very helpful chart online https://www.haguefasteners.co.uk/guide-screw-threads-fastener-data-pdf/ and the 10 TPI offerings are only on 3/4" diameter and up.

So my puzzlement is whether there's any must-have limits relating to OD, threadform, and TPI? Obviously the helix angle gets silly if choosing low TPI on small diameters, but I was hoping to use a 60 degree thread, get the threadform right so male & female fit well, so I can make the leadscrew a couple of inches overlength for an off-cut to make & harden a tap for the carriage nut. But I'm thinking if there is no 1/2" x 10 TPI combination already in existance…..maybe there's good reason why not !

Any ideas anyone please?

[ALLAN QURASHIParticipant @allanqurashi51750]

Any Reason Why Not 1/2″ dia x 10 TPI

[JasonBModerator @jasonb]

You could cut an Acme thread form as taps are available for that in 1/2" x 10tpi which would save making the tap.

[pgk pgkParticipant @pgkpgk17461]

According to lathes.co.uk the Rollo Elf had an 8TPI square thread lead-screw. If it currently has a 12tpi thread form then going 10tpi shouldn't be a problem, makes the sums easier and as Jason points out a suitable tap is available.

pgk

[not done it yetParticipant @notdoneityet]

I see there is also a split die available from RDG (possibly from elsewhere, too).

1/2” ACME Die

Ad says ‘quality’ and pic shows a 3/8 die, so down to you to decide on what ‘grade’ of quality.

However, in combination with a tapped nut, a roughed-out single pointed thread could be ‘massaged’ to get a very close finished fit between the two?

[colin vercoeParticipant @colinvercoe57719]

Lengths of standard size Acme threaded bar are available to buy also bronze nuts to fit

[egaParticipant @ega]

If you screwcut the leadscrew you will need to deploy a travelling steady.

[Pete RimmerParticipant @peterimmer30576]

1/2" dia x 10tpi (left hand) is a standard size for Denford cross-slide screws. Oddly enough the metric screws are 1/2" dia x 2.5mm pitch.

[old martParticipant @oldmart]

You have picked a fortunate size of thread to practice on, at least you can easily see what is going on. If you can produce both male and female threads yourself, they can be whatever you like. When I modified a Tom Senior mill spindle to change it from MT2 to R8, using a bottom half from Arc Eurotrade and the original top half from the mill, I screwed them together with a thread that was no particular size, but the dimensions were worked out from the thread depth according to the pitch selected.

[ALLAN QURASHIParticipant @allanqurashi51750]

Thanks for the advice everyone. It gives me wider options to consider. I'd like to go for a LH thread so direction of travel matches my Myford ML10. Not for fussiness, but so my inherent forgetfulness doesn't cause a whoopsie!

I have seen leadscrews and nuts for sale, thanks Colin, but only metric in the UK. Seems like if we want to stick with imperial, it might pay to live in the USA.

I'm thinking an ACME die might be heavy going, especially over the length of a leadscrew, but NDIY's idea for roughing the worst out by single pointing seems a good idea.

ega, your right about a travelling steady, especially on a twangy 1ft length of mere 1/2" dia, and worse still if cutting an acme thread I suspect. I was going to try and make one, but hopefully will be getting a real one soon.

Pete & Old Mart, thanks for confirming 1/2" x 10 TPI isn't a no-no.

I'm thinking pitch directly governs thread depth, and; OD (major dia) along with pitch, determines helix angle, (which is accomodated for on cutter relief) and apart from that, I'm inspired by Old Mart's comment 'they can be whatever you like' ( within physical limits of course).

Thanks all. Just got to work on my threads not ending up looking like a round wood rasp next!

[old martParticipant @oldmart]

Left hand threads are nicer to produce as you are threading away from the chuck, and that is safer. You have plenty of time to stop things going to the right. A saddle stop would be a great advantage with lefthand threads as your starting point can be fixed.

[Pete RimmerParticipant @peterimmer30576]

Whether a cross slide thread is LH or RH is determined by whether the handle moves with the slide or not. Handle moves with slide = RH thread. Handle stays fixed when slide is moving = LH thread.

You are correct that the thread depth is governed by the pitch for standard leadscrew threads (pitch=depth) but it absolutely does not have to be. If you are making the thread you can decide the depth in fact stub form threads are typically half depth and if a thread has multiple starts then typically the depth will be equal to lead/starts (which is equal to the thread pitch).

[John PParticipant @johnp77052]

Posted by ALLAN QURASHI 06/08/2021 14:35:45

ega, your right about a travelling steady, especially on a twangy 1ft length of mere 1/2" dia, and
worse still if cutting an acme thread I suspect. I was going to try and make one, but hopefully
will be getting a real one soon.

To avoid all of this have a look at the articles in MEW 241 to 243 Milling lead screws.
Cut to full depth in a single pass, left or right hand makes no difference,
3 off lead screws here all 10 tpi longest is 20 inches of thread 1/2" dia in EN 8.

John

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