Lathe Compound Feed Nut

[thestelster]

Lathe: Standard Modern Utilathe 1654

I decided that I want to address the slop that is in the compound feed mechanism. And, yes, I'm currently using a solid tool post setup, so what better time than now to fix the problem.

I have 0.020" backlash. Taking apart the compound, it looks like it has been worked on before. I think the original crossfeed nut was integral with the base of the compound. It looks like it was replaced with a fabricated unit from cold rolled steel, and attached by a screw from the bottom. The threads of the feed screw, 1/2"-10RH acme, are, I think, acceptable. I measured the threads in 4 different places with the 3-wire method, and the pitch diameters were consistent within 0.0015", of each other.

So I will only make a replacement nut from C95400 aluminum bronze. And I will use a tandem tap to do the threads.

 

[PeterT]

I wish my lathe had a nice logical layout like that. You could almost accommodate an anti-backlash slit/adjustment. OTOH you have a nice stick of bronze, make 3 while you have the setup! LOL

Is the the going price for the tap? (and USD, gasp)
Have you ever looked into internal threading inserts? Can 'metric' trapezoidal cut IMP acme or different? This might be in my not so distant future

 

[thestelster]

I wish my lathe had a nice logical layout like that. You could almost accommodate an anti-backlash slit/adjustment. OTOH you have a nice stick of bronze, make 3 while you have the setup! LOL

Is the the going price for the tap? (and USD, gasp)
Have you ever looked into internal threading inserts? Can 'metric' trapezoidal cut IMP acme or different? This might be in my not so distant future

View attachment 30805

Hi Peter, yup, that's the cost. I was actually planning on using a Micro100 solid carbide threading tool, but it wouldn't fit into the bore. If it was 5/8" acme, it would have worked. And I do have several pitches of acme and trapezoid carbide inserts for internal and external threading.

Acme threads have a 29° included angle, while trapezoid threads are 30°.

 

[Mcgyver]

I've replaced nuts a few times. A few thoughts for you, use or discard as you like.

- 20 thou is not bad, I wouldn't worry about. Backlash doesn't really doesn't matter on a lathe and its far from excessive. There has to be some or the thread wouldn't turn.

- That tap is 2G, the loosest fit with the widest tolerances for acme threads. You might not end up much better than 20 thou.

- If you do so, I suggest bore for the new nut in situ. Set it up on the mill with the mating slide, gibs tight, and indicate in the hole for the feedscrew. With it centred, take off the mating part and without disturbing the work in the vise, bore the nut. This assures perfect alignment beween the nut thread and feedscrew

- Consider buying a nut, cheaper than the tap and taping acme isn't always easy. Turn the nut down, then loctite into the bore you made in the piece that will hold the threaded nut - $25 in bronze. https://www.mcmaster.com/products/t...screws-and-nuts/component~nut/nut-type~round/

 

[thestelster]

I've replaced nuts a few times. A few thoughts for you, use or discard as you like.

- 20 thou is not bad, I wouldn't worry about. Backlash doesn't really doesn't matter on a lathe and its far from excessive. There has to be some or the thread wouldn't turn.

- That tap is 2G, the loosest fit with the widest tolerances for acme threads. You might not end up much better than 20 thou.

- If you do so, I suggest bore for the new nut in situ. Set it up on the mill with the mating slide, gibs tight, and indicate in the hole for the feedscrew. With it centred, take off the mating part and without disturbing the work in the vise, bore the nut. This assures perfect alignment beween the nut thread and feedscrew

- Consider buying a nut, cheaper than the tap and taping acme isn't always easy. Turn the nut down, then loctite into the bore you made in the piece that will hold the threaded nut - $25 in bronze. https://www.mcmaster.com/products/t...screws-and-nuts/component~nut/nut-type~round/

Hi @Mcgyver you've read my mind. Pictures on their way shortly. I agree that the 0.020" isn't bad, I've lived with it for 30+years, and has never really bothered me. But this is actually a little practice before I tackle the crossfeed screw, which I want to convert to a telescoping unit for the taper turning attachment.

 

[PeterT]

Acme threads have a 29° included angle, while trapezoid threads are 30°.

Ah yes, that's right. Drats, because 30-deg inserts are spit cheap. But if you have to match 29-deg its probably has to be a N-Am sourced thing.

@Mcgyver that's a very good point about the tap thread class. And their threaded nut price is cheaper than some other places.

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[thestelster]

So I parted the required length of bronze, and because I'm going to drill and tap the bronze in situ, I wanted to make sure that the end piece, which supports the feed screw shaft, would be aligned properly, and consistently, so I installed alignment pins.

 

[thestelster]

I assembled the whole compound and tightened down the gib to make the whole unit one solid piece. Mounted it on the milling machine, and indicated everything necessary, and proceeded to drill to size. For 1/2"-10 acme need to drill to 0.405" for a 95% thread fit. The final drill was a 'Y', at 0.404". This C95400 aluminum bronze is tough stuff!!

 

[thestelster]

Did I mention this C95400 bronze is tough?!! Half turn at a time, blowing out chips with high pressure air, flood the tap and hole with dark cutting fluid, in between turns. I could feel and see some flexing happening. I don't think it was the tap, but probably the crappy tap handle. I haven't been this nervous in a long time.

Cleaned up everything and went to try the feed screw, and it is tight. Way too tight. I measured the newly tapped nut, and the tap raised some material in the bore, that instead of being 0.404" it is 0.398". So I will use some emery paper wrapped around a dowel to open it up a bit.

 

[Proxule]

Yup alu bronze is hard stuff, Perhaps phosphor bronze next time?
Nice work!

 

[thestelster]

@Proxule thank you. But it ain't over 'til the fat lady sings!

 

[Mcgyver]

Nice work!

 

[PeterT]

Yes, coming along nicely. Not saying this isn't the way to go, you have the tools. But for myself, its those nail biting eech-eech squeaking, friction, flexing tapping stories that made me ask about threading it a lathe. Its a straight through hole, short length. Take a couple thou off at a time in multiple light passes, check fit so its exactly how you like = less drama? I think I read about someone making an acme threading tool like D-bit style from O1 & it worked well. But the 'Buy-and-Glue' approach seems like what this lazy man will likely pursue. Now this really has me wondering, is my Taiwan 14x40 an ACME or Trapezoid? It has blended family IMP/MET fasteners, roll pins, shaft sizes, bearings... throughout. Nothing would surprise me.

 

[Proxule]

Yes, coming along nicely. Not saying this isn't the way to go, you have the tools. But for myself, its those nail biting eech-eech squeaking, friction, flexing tapping stories that made me ask about threading it a lathe. Its a straight through hole, short length. Take a couple thou off at a time in multiple light passes, check fit so its exactly how you like = less drama? I think I read about someone making an acme threading tool like D-bit style from O1 & it worked well. But the 'Buy-and-Glue' approach seems like what this lazy man will likely pursue. Now this really has me wondering, is my Taiwan 14x40 an ACME or Trapezoid? It has blended family IMP/MET fasteners, roll pins, shaft sizes, bearings... throughout. Nothing would surprise me.

I never was able to master the D bit reamer. Always bigger or smaller then what I aim for. Lol

 

[thestelster]

So I wrapped some emery paper around a rod and opened up the minor diameter
to what it should be, 0.404", but it was still to tight. The feed screw would go about 3/4's of the way into the nut then get real tight. So I ran the tap through the nut several times, and each time it was taking off the slightest of material, but not enough. So I used my old trick of jamming a small dowel into the flute of the tap, forcing the tap to cut on the opposite side. In this case , the shaft of a Q-Tip worked perfect.

So now the screw threads with some resistance, but virtually no play. Hopefully everything will align up proper and there won't be any binding.

 

[Former Member]

Now that your nut is tapped, slit it about a 1/3 and add to small set set screws in the thinner side. Apply a little pressure and bingo bango bongo preloaded nut. Beauty is you can always dial in a little more as it wears.

 

[thestelster]

Now that your nut is tapped, slit it about a 1/3 and add to small set set screws in the thinner side. Apply a little pressure and bingo bango bongo preloaded nut. Beauty is you can always dial in a little more as it wears.

I might utilize an anti-backlash mechanism when I address the cross feed screw. But this is the compound, which I'm not even using, so it should easily last my lifetime. But thank you for suggesting the set screw idea.

 

[Susquatch]

@thestelster - I've never seen a tap that looks like that! What is that double taper all about? Two fits in one tap? Seems crazy on first blush. Must be a reason.

Btw, nice to hear the fat lady singing......

 

[thestelster]

@thestelster - I've never seen a tap that looks like that! What is that double taper all about? Two fits in one tap? Seems crazy on first blush. Must be a reason.

Btw, nice to hear the fat lady singing......

They're called tandem taps. The first section is a rougher, and the second section finishes the thread profile. Only on acme and trapezoid threads.

She's just warming up. The performance is tomorrow!

 

[thestelster]

Well, the fat lady sang, and it was a pretty good performance!

I checked back-lash at front, center, and rear positions at I'm geeting between 3.5-4 thousanths play. So not so bad. Was it worth all the effort? Probably not, but at least I can say I made one and gained experience and knowledge.

Now to start figuring out how to make a telescoping cross-feed screw and associated parts for my taper turning attachment.