Acme leadscrew supplier? DIY anti-backlash nuts...?


Hi, can any denizen of the group recommend an UK supplier for Imperial
leadscrews? I'm after a replacement for my cross-slide screw (5/8" x 8tpi
Acme, left-hand thread, very worn!) that won't be too expensive nor so
inaccurate there's no point swapping it...
I appreciate that I'll need to make a new bronze nut to fit, interesting
bit of screwcutting to try there! Were I to try making an anti-backlash nut,
is the simplest way going to be making part of the nut a separate item that
screws into the main nut via a *slightly* different pitch thread joining the
two parts? i.e make an insert with the 5/8" - 8tpi L/H Acme on the inside
and a (for instance) 7/8" - 9Ttpi Whitworth (also L/H) on the outside,
locknut to suit? The main body of the leadscrew nut would have most of its
length threaded for the Acme screw, the rest for the insert (approx. 1-3/4"
of Acme, 3/4" of Whitworth so that more of the wear would occur on the
adjuster Acme thread).
This I think would mean a full clockwise turn of the "adjuster" would
withdraw it by 1/9" and move 1/8" along the leadscrew taking up 1/72" of
slack (roughly 14 thou") as it wears - I'd hope it wouldn't *need* a full
turn though! I'm aware that at a couple of points in the insert there would
be "minimal" thickness between the threads (about 50 thou") but this should
only be for a fraction of a thread turn...
Why Whitworth? I'm trying to stick with Imperial and contemporary threads
if possible!
Any pitfalls, pratfalls, practical difficulties I should be aware of?
Cheers all,
Dave H.
Reply to
Dave H.
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For anti-backlash, you use the same pitch, two pieces. Cheap and cheesy just slices an existing nut mostly through perpendicular to the hole and squeezes/expands the slit by whatever means you like to take out the slop. Have seen some very elaborate adjustments, one consisted of another threaded part insert into the main nut and finely threaded on the inserted portion. The rim protruding had indexing notches with a stop dog. Need less backlash, undo the dog and move the thing a notch or two, ala Winchester 1897 takedown. Just depends on how much space you've got to mess with things, this had to have been a couple of inches long. There's a lot of these types of things in those old "xxx Mechanical Mechanisms and Movements" books, www.archive.o= rg would have some downloads. Henley published a bunch.
Stan
Reply to
stans4
Hello Dave, I used to use Halifax Rack & Screw for quality leadscrews, I'm happy to see they are still going:
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but I have no idea of how 'cost effective' they will be.
I have made an anti backlash nut arrangement on my lathe. I made the two halves with a flange/face between the parts and a pair of axial 4BA cap heads to lock the adjusting part relative to the fixed part. Simply rotating one part relative to the other and locking the screws does the job. I did mill some arc slots for the screws.
I have also used the differential thread arrangement you mention for other adjustment purposes and I suspect you would find it very tedious and possibly impossible to use depending on the relative position of the threads. If you do take that route, make the adjsuter threads first and assemble the two parts and only then cut the Acme thread through both parts together. You may need to use shims between the two parts so that you can screw them hard together or some other means of locking one relative to the other.
Richard
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Reply to
Richard Shute
"Dave H." wrote in message news:VIyco.144858$ex1.16941@hurricane...
Imperial
interesting
anti-backlash nut,
outside,
1/72" of
there would
men" -
The later Colchester Students and Masters use two sections of the same pitch, with a Vee shape where they meet, with a wedge pushed into the Vee on the cross slide, by an allen headed grub screw.
I have seen a dealer squash a bronze nut in a vice onto the lead screw, but even if you pull my nails out I'm not naming names !!!!!!
AWEM
Reply to
Andrew Mawson
Buy a second-hand early Myford ML7 leadscrew and cut the unused bit off at the tailstock end, they are 5/8" x 8 tpi LH.
Try Jim Marshall over at
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he's always breaking Myfords for spares. John S.
Reply to
John S
A UK distributor
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this company.
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Or get someone to order a piece from McMaster-Carr and ship it to you.
98940A343 4140 Alloy Steel Precision Acme Threaded Rod 5/8"-8 Size, 1/8" Travel Distance/Turn, Lh Thread $11.73/foot
Reply to
Ned Simmons
Buy a second-hand early Myford ML7 leadscrew and cut the unused bit off at the tailstock end, they are 5/8" x 8 tpi LH.
Try Jim Marshall over at
formatting link
he's always breaking Myfords for spares. John S.
Why not make a tap from the remaining piece then use that to thread some plastic for a cross-slide nut?
Reply to
Pete
I had thought about a plastic nut, but due to a lack of physical space and the way it'd be mounted (about 1-1/8" square section, a single 5/8" BSF and a 1/8" dowel pin through the cross-slide) was concerned that it wouldn't be a rigid enough assembly, hence the choice of bronze as per the original or if not, some decent brass?
I think I'd rather spend a few extra quid to get a new piece of precision leadscrew rather than take a chance with an used one out of a Myford heading for the scrapper! I'll need upwards of 18" of screw by the time I've turned ends down etc, possibly 24", which might well get me into the already-worn bit of a scrapper's leadscrew, as my cross-slide has about 11" of travel (which should come in handy when I want to mill things on the carriage, but makes for a bit more expense!).
Tapping with a leftover hadn't occured to me though, might be better than trying to screwcut an internal L/H Acme through two separate parts in one go (erk), I suppose it should be easy enough to grind some flutes and relieve the threads a touch[1] (another skill to learn - I'd need plenty of required practice on some junk bolts first!) I have just spotted the Right Taps on Ebay for not too much money, though! Presumably I'd have to pilot drill and bore/ream slightly oversize to get about a 75-80% engagement depth in the nut? Would I need to get as close as poss' to 100% to ensure accuracy?
Cheers all for the info, very much appreciated :)
Dave H.
Reply to
Dave H.
Dave.
I am just about to make a cross-slide nut from plastic, and had the same concerns about rigidity of the mounting (though I think we are both under-estimating the structural rigidity of the material). My nut has a 5/8" stud machined in it that plugs into the bottom of the slide. What I'm thinking of doing, is boring the quite substantial nut that I have and in-setting it with a slug of delrin or ptfe, then threading that plastic slug.
Just for fun my cross-slide screw has an imperial OD and a metric pitch, so the only way to replace a nut is to make it. I figure that making a plastic nut will reduce wear on the odd-ball screw.
I'll keep the group up-dated with the results.
Reply to
Pete
Dave, A couple of points. When I made a (leadscrew) nut I initially tried tapping it. I found it quite impossible. It's not like a 'normal' tap, it shifts a LOT more material. In the end I thread-cut the thread and cleaned it out/finished it with the tap. The tap I bought did have very little lead-in and it is possible that my life was also a little harder because I was using some gun-metal which I happened to have, lovely bearing material, but it seemed quite 'sticky' to machine.
As part of the process I discovered that it is usual for an Acme tap to be significantly oversize, 030" I think on a half inch (ie 0.530" OD), further, I found that the cheap RDG tap I had bought was an additional 025-odd in excess of that, so that's around 055" over the nominal size. You do need some oversize to give clearance to the thread tips, but it plays havoc with thread depth infeed until you end up reading the book (Machinery's) to find what went wrong!
Machining a left hand thread is in many ways less stressful than RH as you are coming out, away from the chuck so there's no worry about running into the chuck or other important bits inside while you are concentrating on the thread. Give it a go, fear of the unknown is usually far worse than the reality.
Depth of engagement should not have any significant affect on accuracy, just longevity and ease of manufacture.
Richard
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Reply to
Richard Shute
I machined a right hand thread (for a mop handle adapter) by placing the boring bar / homemade threading tool on the back side, running the lathe in reverse. That makes the bit come out of the hole, very handy at 5 tpi.
Pete Keillor
Reply to
Pete Keillor
Thanks for that useful info', Richard!
Right, in that case I won't try making my own tap! The 8tpi should be reasonably easy to cut on the lathe (assuming I can get the tool geometry right), low backgear goes down to 22rpm (perhaps a bit *too* slow), imperial thread-dial...
I assume it'll be a case of using a slightly narrow flat-topped tool tip (so it cuts on one angled face and the flat, allowing some room for chip clearance) in a boring bar and then advancing it axially a few thou" at a time to get a good fit to the leadscrew? Fortunately the cross-slide is fitted with a magic quick-withdraw handle that works consistently to much less than a thou" in either direction so I won't have to memorise top-slide settings! Does one do the half-angle-less-a-half-degree bit on Acme screws? I've only ever cut 55* and 60* threads, and that was Some Time Ago... Perhaps it'd be simpler to keep the compound along the lathe, cut with the cross-slide and a smaller amount of compound feed for the thread angle then put on some lengthwise compound feed to widen the thread for a good fit? (If you see what I mean...) Considering the price of a decent bit of bronze (or even brass), I can see practice and experimentation on a chunk or three of cheap ally ahead of me!
It sounds like the oversize on the taps might prove a bit excessive if I'm going with the inserted nut (won't leave much room for minor diameter on the 7/8" x 9tpi Whitworth outer thread), so I may try grinding a thread chaser from a fragment of the Acme screw rather than cleaning up with a tap? That should leave the working surfaces tidy, even if the crest? (major diameter) of the thread is a bit of a mess...
Dave H.
Reply to
Dave H.
That is the accepted method and if I were to do it again it's what I would do. As I said, I had the tap so I 'roughted out' the thread and finished it with the tap.
Sounds like a nice lathe, given that comment and the bottom speed I'm guessing a CVA???
Yes, I can, and yes, that's how I would do it. As you feed into the thread with the cross slide, you can put a small feed on the top slide to get the same effect as angling the compound. For a 29deg thread if you apply 4 thou on the cross slide then 1 thou on the compound puts all the cut on the front face (tan 14.5 = 0.26). Personally I haven't found that it makes a great deal of difference, but some people consider it mandatory.
Wise move, that aspect did rather focus my mind on the unfolding mess while I wrestled with a tap that didn't fit.....
A thread chaser from a fragment of leadscrew should be possible, but you will need _some_ clearance at the tip of the thread especially if you get a new feedscrew whcih will not be worn as the old one. To get that you will inevitably get an over-width thread groove. Not sure how to avoid that unless you use just a single 'point' and reduce the width slightly. Say 008 clearance is still only a notional 002 backlash so I suppose given that you are making the nut adjustable it should be easy enough to adjust that out in the finished assembly.
Richard
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Reply to
Richard Shute
Lots of stop-and-try-for-fit then!
Close, but no cigar! It's a Holbrook C13 (currently undergoing quite a bit of renovation...) that originally went to the ARDE at Fort Halstead (which became the RARDE and then the AWRE) some time in the mid-50's, has passed through a few hands (including some slightly careless ones) before mine! Strangely it's not listed on the lathes.co.uk website so I may submit some pics and info once the renovation's finished? At 2 tons despite its small size, 6-3/4" centre height x 28" b.c., it may be a bit of overkill for the model engineer's or practical motorcyclist's home shop and is definitely pushing the boundaries as far as space in the shed goes, but it fell in love and followed me home! The fellow who sold it to me has three other Holbrooks (and is looking for a fourth), and won't use anything else :)
I'm glad I'm not intending to cut a new leadscrew though, not yet having travelling steady etc. - accessories are as rare as rocking-horse droppings so will have to be made or adapted... I'm even considering making the travelling steady as a "toolholder" for the Dickson QCTP as hanging it there may be the easiest way and would ensure that the tool-to-steady geometry was consistent? That does, of course, imply making the toolholder with all the vees and slots to fit the 'post... Erk.
My thought was that the underwidth tool tip could establish a bit of clearance (maybe 10thou"?) on the major diameter, the chaser (from a piece of the *new* leadscrew) finish the flanks and crest of the thread although not quite touching the minor diameter - not perfect, but seemed a sensible compromise?
Thanks for the feedback, Dave H.
Reply to
Dave H.
Nice machine. There were a couple of B8's on fleabay recently. One with a heap of accessories and not at all expensive - I was quite tempted, but I've got enough t be gettng on with. The S&B Sabel is my everyday machine and (one of) my others is a S&B 1024 so explaining yet more ironware to the management could have been tricky.
Are you local there? I grew up in Bromley and used to go for Sunday lunch in Seal so I know Ft Halstead well enough. I'm near Folkestone now.
I can't see how that would work because you want to move the tool in and out relative to the work while the steady remains fixed relative to the work.
I'll be interested to hear you progress.
Cheers Richard
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Reply to
Richard Shute
In full pedant mode lol
ARDE > RARDE > DERA > DRA > dstl (or maybe DERA and DRA were the other way round) AWRE are based at Aldermaston AFAIK
Bob
Reply to
Bob Minchin

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