Screwcutting Acme threads

Perhaps unwisely I've offered to help someone out by cutting an acme thread in a half nut for a woodturning lathe restoration project. Options would be to make a tap (the leadscrew thread is apparently 5/8 x 4tpi) or screwcut a new nut. The nut is Delrin, so I'm leaning towards the latter. I've never cut an acme thread, so any advice would be welcome. My lathe will do 4tpi - though at the minimum speed of 65 rpm it's going to be exciting....

Rob

PS have Googled obviously, but conflicting advice....

Edited By Robin Graham on 15/10/2016 23:28:30

Edited By Robin Graham on 15/10/2016 23:34:23

Robin - before you commit to machining have a look at this link - there are easier ways

I'm not sure what all the common trepidation about machining Acme threads is due to. No more to it than turning any other thread. And in Delrin it should be a doddle. Grind up your tool bit to a suitable gauge and proceed as with any other thread.

If it's a lathe halfnut, check whether the thread is lefthand or not. If it is, you simply set up the gear train to run the correct way then start your cut at the chuck end of of the job and work towards the tailstock.

65rpm should be ok. If lefthand, you have all the space out to the tailstock to pull up in. If going the conventional way, simply set the job up and leave sufficient boring bar overhang so you have a bit of leeway as to where exactly you stop the tool. Put a felt pen mark on the boring bar shank so when it gets to the mouth of the hole, that's where you disengage the halfnuts. If you are really worried you can run your lathe belt a bit loose so in the unlikely event of a crash the chuck will simply stop turning.

BTW, 4tpi x 5/8" sounds pretty wild. That would leave a core diameter of somewhere around 1/8" which does not sound right. You might want to check carefully what is going on there, measure twice cut once and all that...

Edited By Hopper on 16/10/2016 02:27:07

Or if its right hand mount the tool upside down and run lathe in reverse then like a L/H thread you have the whole length of the lathe to stop in.

As hopper says 4tpi is odd for that dia, run it with a sharpie pen first to be sure.

As that lathe uses a square sided lead screw half nut, a larger piece of delrin could be used, suitably supported, and then the whole block machined to size on a milling machine. See the thread, on here., where Richard was looking for a 4tpi tap.

Standard Acme thread pitch for 5/8" is 8 TPI, I would definitely double check the thread count.  4 TPI is used on 1 3/8", and up to 2", got a friend who has an old Pratt & ? Whitney lathe with a 4 TPI lead screw 1 1/2" dia, some of the chucks for it are 14", and others 16", they are Bl**#y heavy, I think the bed is a bit over 6ft long.

Ian S C

Edited By Ian S C on 16/10/2016 09:51:57

It IS 4tpi. It is a wood lathe, so the cutter is separately powered (a router on a frame). It is the frame which is traversed as the item turns veerry slowly. Def not an insurmountable problem. I suggested casting lquid acetal, but Richard had already been advised of the crush and melt option. I am now going to follow the same 'squeeze and melt' for my lead screw nut as that seems easier than melting and casting.

Thanks to all for advice - as Not Done it Yet says the guy (Richard) for whom I've offered to do this is sure that it is indeed 4TPI, strange as it seems. Richard has measured the thread depth as 0.09" thought, which doesn't seem right for either standard or stub acme though, so further investigation is clearly necessary.

Hopper, thanks for the reassurance - I couldn't see why it should a massive problem, but when I googled acme threading there seemed to be much tooth-sucking on the subject - hence my asking here. I don't understand how you get a core diameter of around 1/8" though - the calculator on the engineersedge site gives around 0.34".

JasonB - thanks for the tip about running backwards. It may not be relevant here as my lathe has a metric leadscrew, so I'll have to keep the leadscrew engaged and run back under power between passes, but a good tip nonetheless.

Regards, Robin

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Robin,

Is it perhaps a multi-start thread ?

... that would make sense of the 4tpi and 0.09" depth.

MichaelG.

Robin - Michael G has just beaten me to it - my thoughts exactly - careful checking is certainly called for.

Michael and Keith, yes that may be possible - the only definite info I have is that one revolution advances the carriage 1/4 inch, but as ndiy seems to know the lathe (it's a Legacy ornamental wood turning machine) and says it's definitely 1/4" I'll wait to see the bits before cutting anything. There was a suggestion on the woodworking site where this question arose that it may be a deliberately non-standard thread - it happens. I've had cars which require 'special tools only available to main dealers' to fix, but there's always been a way round.

Robin

Brainfade (again.) My apologies. Was thinking on square/Acme thead depth equals pitch but it is of course half the pitch. Roughly. Get the leadscrew in hand and make the nut to fit the screw, whatever it is.

Robin,

I grabbed an image from the video [at 5min 21sec] **LINK**

http://legacywoodworking.com/demoVideo.cfm

Assuming this is the right machine ... it looks like the thread is a special.

MichaelG.

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Edited By Michael Gilligan on 17/10/2016 14:54:49

That's it. Like I said earlier (fourth reply), easy enough to make one by 'squeeze and heat', then machine to size afterwards.

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Curiously:

(a) no-one else but you seems to have noticed Richard's original query

http://www.model-engineer.co.uk/forums/postings.asp?th=121161

(b) from what I can see on the video, that could not strictly be called an Acme thread.

Given what it looks like ... I agree that heated and squeezed Delrin is the obvious answer

MichaelG.

Edited By Michael Gilligan on 17/10/2016 17:05:47

Curiously

Hardly my fault (that no one else remembers or didn't read it), and I have never said it is ACME. II've not actually measured the pitch myself, but I trust that Richard has measured it correctly, so it is 4tpi. He has done some smart work on, from what I have seen.

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I wasn't suggesting that any of this was your [or indeed anyone's] "fault" ... merely pointing out [mainly for posterity] that the title of both threads turns out to have been unfortunate.

MichaelG.

Not so for this thread Michael. What I asked for was advice on cutting ACME threads, and that's what I got - together with a pointer to the useful technique of hot-forming, of which I was unaware. As I have indicated in previous posts (I think!), I knew before starting this topic that the particular thread might be non-standard, but my thought was that if I could cut a standard ACME at this pitch and diameter, I would probably be able to adapt the technique. It was a general enquiry reduced to specific terms because my experience is that specific questions elicit focussed and useful responses, which can be pursued further if necessary. So rests the defence of my topic title!

As it turned out I also got some useful info about this particular project as the topic developed - looking at Michael's video capture (thanks Michael) it does indeed look like a non-standard thread, but I'm now sure it can be sorted one one way or another. And I will be able to add making ACME - erm - random power transmitting - threads to the list of Things I Can Do.

Thanks for all the advice, Robin.

Edited By Robin Graham on 17/10/2016 22:19:01

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Robin; Please accept my apologies for being presumptuous.

MichaelG.

The biggest issue I see with single pointing a 5/8-4 internal thread relates to having enough clearance to withdraw the tool after each pass. You might have to reverse the spindle and wind the tool out of the bore.