Mini lathe threading problems

[T Bredehoft]

One more time.

In your photograph of your work, it appears that the compound is parallel the clamping arm of your "Aloris type" tool block. If this is true, your compound is at the WRONG angle. You are feeding the tool at 60º, not 30º to the work. Lathes made in the US of A and lathes made in Europe/Asia start numbering at different points, resulting in different locations of the 30º mark.

Your comment about feeding less than half of the number of thousands suggested your error, (30º feeds more than half, nearer 80% of the distance). Chasing 20 pitch threads on 1/4 in rod, .001 is about all you can expect to get per pass, the load on the tool WILL deflect the work.

 

[WobblyHand]

Re: die threading, I found the picture.

@T Bredehoft is right, 0.005" cut on 1/4" diameter rod will cause deflection. Really need to have a very light touch for threading on small diameters. Try 0.002" in the beginning, and down to 0.001" per pass to get to final dimension. For long pieces, even with a center, you will get deflection. What that means in practice is that the threads in the middle section will be tight, since they are not at proper depth. That's because the workpiece moved away from the cutter, rather than being cut. A way to fix this is to use a follow rest (which is a pain), take extremely light cuts, or follow up with a die afterwards. Having a cut yourself sharp tool bit is really important! If you can't cut yourself on the edges, the bit isn't sharp enough. Dull bits cause more deflection.

Here is a picture of the correct angle for the compound for threading. Taken way back when my mini-lathe was neat and tidy. Yes, the "DRO's" can foul each other at the correct angle, if not careful.

 

[addertooth]

WobblyHand,

Yep, my 7350 no longer looks like a show-room queen either. But that is the nature of tools which see real use.

p.s. great pictures. I have considered making a smaller handles to reduce fowling when the compound is at an angle. This was partially corrected by putting the extended saddle on it, which moved the cross-feed handle and DRO back an inch.

 

[WobblyHand]

I had since put on the extended saddle, which helps some. But I did end up busting my saddle DRO, which I repaired. The repair lasted a while, but the brass screw insert holding the display down has come free from the ABS housing. Need to take it apart, plastic weld the hole shut, redrill it and press in the insert. My epoxy fix did not hold. The fact that I broke it to begin with, annoys the daylights out of me. Like I need another project in the queue...

 

[jschmidling]

One more time.

In your photograph of your work, it appears that the compound is parallel the clamping arm of your "Aloris type" tool block. If this is true, your compound is at the WRONG angle. You are feeding the tool at 60º, not 30º to the work. Lathes made in the US of A and lathes made in Europe/Asia start numbering at different points, resulting in different locations of the 30º mark.

Wow! That's news to me. Never noticed that when downgrading from South Bend to Mini. Thanks for catching that.

js

 

[jschmidling]

There's a simple fix for die threading on a mini lathe. Let me go find a picture for you. You need a die holder, a die, a drill chuck mounted on the appropriate taper, and your tailstock.

That is exactly how I used to do it with my South Bend but now the problem is that the chuck on the mini won't hold the piece. Someone suggested flats but that is not an option.

Someone also suggested that it could be the type of steel but this is the only piece I have on hand. I think it's cold rolled but not sure.

js

 

[Lo-Fi]

The headstock bearings in those mini lathes are often total junk. Replacing them is about the best upgrade you can do and actually turns them into quite usable machines. My mates had 5 thou radial play in the spindle, it was absolutely appalling. It would turn mostly ok, but parting you could forget, along with most any form tool operation. Replacing with some decent quality name brand bearings and setting up the preload correctly transformed it into quite a usable machine.

Worth putting an indicator on it and seeing how much you can push the spindle around in the bearings. I'd chuck up a long piece of stock, put the indicator on the chuck and move the end of the stock around by hand. Check both up/down and left/right.

 

[WobblyHand]

That is exactly how I used to do it with my South Bend but now the problem is that the chuck on the mini won't hold the piece. Someone suggested flats but that is not an option.

I didn't go look at your other thread. My main computer keeps dying on me, so I am on a Raspberry Pi. Is the piece too large to be chucked? Or rectangular and you have a 3 jaw? Something else?

You do need to put on an indicator on the work piece next to the chuck to check for runout. If that is bad (high TIR) then you probably should remove the chuck and measure your spindle runout. It could be that the chuck was mounted with a piece of crud between it and the spindle, which gives you more runout that you should have. Always look for the simplest things first. No need to tear into bearings, if they aren't bad.

 

[Jim F]

That is exactly how I used to do it with my South Bend but now the problem is that the chuck on the mini won't hold the piece. Someone suggested flats but that is not an option.

Someone also suggested that it could be the type of steel but this is the only piece I have on hand. I think it's cold rolled but not sure.

js

Hold the chuck still with the chuck key.

 

[homebrewed]

There are plenty of things to look at before you replace the bearings. The OEM bearings on my lathe were good enough to (eventually) bore bearing pockets accurately enough to achieve an interference fit. The biggest improvement for me in that regard was getting a decent boring bar, it had nothing to do with the headstock bearings.

Other bits on your lathe to examine/refine include: all the gib strips, the saddle slide plates (some call them gibs too), and the lead screw drive chain. On these machines, poor alignment of the lead screw relative to the half nuts is an often-found problem. The bearing blocks that hold the lead screw also can be problematic, causing binding problems along with lead screw alignment problems. You need to check the bearing preload for the headstock bearings if it turns out your problems really are related to the bearings. But keep in mind that deep groove bearings, the ones that come with these lathes, can't tolerate as much as angular contact or tapered roller bearings. The nuts just need to be tight enough to remove excess runout, no tighter.