Acme threads...

[AdeV]

This is the business part of my lathe's tailstock:



The problem is, between the brass nut & the steel screw, there's around 0.1" play. As a result, particularly when drilling something "grabby", the drill has a tendancy to lunge forwards into the work, until the screw catches up. Not to mention the fact there's bugger all actual metal left in the nut's thread...

Making a replacement should be fairly easy, with the exception of internally threading the brass nut... I could, at a push, probably grind up an external threading tool, but I've no chance doing an internal one. I'd like to make both the shaft & nut new, so there's absolutely minimal play. Plus, it'll be good practice making a left-hand screw thread.

Therefore.... does anyone happen to have a 3/4" 5-tpi left-hand Acme tap I could borrow? I know RDG tools do one for 25 quid, but that seems to be a tad wasteful when I only expect to use the thing once...

Also, should I cut the nut in brass, or phosphor bronze? Based on the fact the one I've got is likely to be original, and between 70 & 80 years old, is there any point making it in anything harder than brass?

[BillTodd]

That looks suspiciously like a square thread  - have a good look at the unworn end and check the sides of the threads are 29°.

Bill

[Rob.Wilson]

Hi Ade


What about here ,, http://www.roton.com/mating_components.aspx?family=7159433  scroll down a  tad


Rob


    $60 min order

[AdeV]

Bill,

It's definitely not square; here's a real close-up:



Although that's from the central (worn) part, the un-worn ends are the same form.

Rob - good link (8tpi, but I found the 5tpi stuff). 26 bucks + p&p though, just for the bronze bit... probably cheaper than buying the tap, but not by a huge margin. Besides, I fancy making one... what's the point of having all this machinery if I just end up buying everything?

[Rob.Wilson]

OOOOOOOOOOOPS    posted wrong link


Rob

[BillTodd]

Bill,

It's definitely not square; here's a real close-up:

Ah that's good

Why can you not single point it on the lathe?  (My Hardinge won't go down to 5tpi or I'd offer to do it for you)

Bill

[AdeV]

Bill,

I can single-point the shaft no problem; but the internal threading I'm not so sure about. I know it's possible to buy single-point internal acme threading tools... but TBH I may as well buy the tap from RDG if I'm going to do that... I was just hoping someone had already shelled out on the tap & was willing to share 

Maybe I'll try grinding up an internal threading tool... that'll be an interesting job in its own right.

[andyf]

Bill,

I can single-point the shaft no problem; but the internal threading I'm not so sure about. I know it's possible to buy single-point internal acme threading tools... but TBH I may as well buy the tap from RDG if I'm going to do that... I was just hoping someone had already shelled out on the tap & was willing to share 

Maybe I'll try grinding up an internal threading tool... that'll be an interesting job in its own right.

Hmm - you may need the leverage of a tap wrench with long handles for 5 tpi Acme, and it might be hard getting a good enough grip on that round nut. Might it be better to single-point it most of the way, so the tap doesn't have to shift as much metal per turn?

Andy

[Bogstandard]

Sorry Ade, I have a couple, but nothing near the size you require. I used mine when I worked on the top end of an Atlas.

C'est la vie


John

[Powder Keg]

On a side note...

When drilling anything Grabby. You need to grind your drill bit so that it doesn't grab. Hard to explain. Let me look for a picture.

[Powder Keg]

Near the bottom.

Linky thingy

[Stilldrillin]

Wes,
I knew about the grabby carry on, and it's cure, too many years ago....... But that link made very interesting reading. 

It's been bookmarked, to save me a lot of fumbling for words of explanation, in the future! 

David D

[Ned Ludd]

Hi Ade,
Have you tried FeePay for your tap, or if you buy from RDG perhaps you could sell it after. I suspect that if you do buy, you will keep the tap, just in case.
Re the actual making of the nut, might I state the obvious and say you should make it from square or Hex stock, cut the thread then turn the outside. Even if you screw cut most of the thread then use a tap, the forces are likely to be too large to hold firmly a round piece, but either Sq or Hex will hold better in the right chuck, without having to strain anything.
Ned

[doubleboost]

Hi
It should not matter what profile the thrwad is as long as you make a nut to fit 
http://madmodder.net/index.php?topic=3445.msg37217#msg37217

[NickG]

Ade,

I was going to do the same to make a nut for my milling vice, but then I bodged it instead. I was going to taper turn a bit of silver steel to the right angle then mill in exactly half and grind a relief on the front to use as a cutting tool, like a D bit then harden it. I think this would have worked.

[AdeV]

I've just remembered that, somewhere on here (or maybe HMEM) is a description of a technique for making a double-ended threading tool out of round bar silver steel, which will cut both internal & external threads, and requires no grinding (other than a touch with a stone to get a sharp edge.... I can easily make the Acme form in S/S, and having both the internal & external tool means I don't need to worry about getting them totally accurate & identical (they'll automatically be identical, and should fit together nicely.

Time to use the search engine.... I'll post the link back when I find it...


...here it is: http://madmodder.net/index.php?topic=3523.0

Thanks fatal-exception 

[andyf]

Ade, I think this is it: http://madmodder.net/index.php?topic=3523.0

How about making your screw out of silver steel, but longer than you need. Cut off the excess, gash flutes in it, square off the end and harden/temper it to make a tap. Use the tap to finish off a single-pointed internal thread in the nut.
In theory, that should produce a pretty good fit.

Andy
 

[AdeV]

Andy,

That's certainly an interesting idea... it should work, in theory. I don't actually have any silver steel of the correct size for this job, whereas I do have some standard leaded mild bar: I guess I should check the price of s/s...

[Jasonb]

While you are set up for threading the screw also thread a bit of silver steel this can be made into a tap quite easily. I would screwcut most of the nut ont the lathe and then use your homemade tap to chase the thread to size. I used this methof for the square thread on my traction engine brake. The tap is bottom centre



Jason

[Bogstandard]

Unless you do that initial single point threading to get rid of most of the material, you will really be putting the tap at risk of breakage.

If you notice Jason's tap, it has a very long taper lead in on it.

A while ago, before I bought mine from RDG, I went searching on the web for Acme taps, and found a few of them actually came as a complete unit for cutting the the required threads.

They consisted of a drill bit on the end, followed by a tap very similar but about twice as long as Jason's version, with a very long lead in and gradually increasing thread up to full size. Supposedly, you can use them on lathe or mill for power tapping the Acme thread.


Bogs

[AdeV]

Well, I spent the evening making a single-point tool like fatal-exception showed us. It seems to work very well, except that I lined it up wrong on the toolpost, so I started to run out of clearance... An attempted adjustment just wrecked the threads I'd cut. Lucky I was trying it out in aluminium...

I'll have another go tomorrow, & will report back with pictures... I have a feeling my cutter may be slightly too wide, which will cause it to cut sloppy threads... I will investigate that tomorrow as well... if necessary, I can fix it with a very careful trip to the grinder.

[Jasonb]

It does not matter what the cutter shape is (within reason) as if you also use it to cut the tap then the exact shape will transfer to the nut.

Jason

[AdeV]

It does not matter what the cutter shape is (within reason) as if you also use it to cut the tap then the exact shape will transfer to the nut.

You're right about the shape (since I'm making both screw & nut), but the width of the cutter is critical. Unlike fatal-standard's (which was a standard 60^o cutter, and was therefore simply cut until a sharp point was formed, I have to get the flat bit¹ the exact right width (0.1"). Any bigger, and the cutter will remove more metal than it leaves, and the fit will be sloppy (built-in backlash); any smaller, and it won't remove quite enough metal & the nut simply won't fit on the bolt. Smaller is potentially better than bigger, as one can take a few extra thousandths using the compound slide if it turns out not to fit, whereas bigger will always cut too much.

¹ it occurs to me, I have to make the _root_ of the cutter 0.1" (as that forms the top of the tooth)... good job that occurred to me before I started grinding!

[BillTodd]

If it were mine, I'd be looking to see if I could add some backlash compensation. Any chance you could cut the nut in two and add an adjustable threaded part to separate them?  Or, perhaps an outer nut to pull them apart the threads a little like the Hardinge?


Bill

[AdeV]

Bill,

I'd wondered about anti-backlash, having read up a bit on how it's done, I'd wondered about modifying the bolt I had y'see. The only problem I could foretell is, it's a bit of a tight fit in the holder (like, it needed bashing in with the Giant Plastic Hammer of Fine Adjustments), which wouldn't do a sprung anti-backlash nut any good at all... I could maybe use a couple of set screws to push the back half of the nut away from the front half; the only problem is it wouldn't be adjustable in situ, so the balance between backlash & "can't screw the bugger back together" would be awful close... but, worth thinking about for sure. I might try mocking something up in aluminium to give me a feel for it.