getting atlas lathe to cut straight

[rlfervan]

Hi all:

Starting to build a PM Research #5. (Coke bottle) Thought I would check alignment with Rollie Dad method and I ended up putting almost 1/8" under front of tailstock end foot. that seams like a lot. I checked carefully and i am within 2 thou 16 inches from chuck, about .0005 at 6". i am ok with that but it seams like a lot of bending to get there. Lathe is mounted on a board about 1-1/2" thick.
built a large dial for the cross slide and a steady rest and was very happy with the accuracy i was getting. I prb should have not f@#ed with it.

Thanks in advance.
Robert

 

[Blogwitch]

Have you checked the nose wear on the tailstock ram?

When I had an Atlas 10F, 70 years of wear in the nose of the casting had caused the nose to droop considerably, so I had to do a rescue job on it before it went to it's new owner.

I did a post about how I did it somewhere. It wasn't a difficult job, but it had to be done very accurately, I got it to within 0.0005".

John

 

[Hopper]

That's a heck of a lot of packing under one foot there. Don't stand too close the thing might go off like a mousetrap any second.
Or it might be the wooden board you have it mounted on is warping and bending up instead of the lathe pulling down. Wood is not very rigid compared with cast iron.

Have you tried doing an actual turning test? If you can get a one inch bar sticking out of the chuck six inches to turn to within a thou on an old lathe, you are doing quite well enough. All the rest is theory. Measuring with dial indicators etc is a good starting point but does not take into account real world, real time cutting forces and wear on older machines.

Have you checked your headstock bearings for tightness? They should have half a thou or so measureable movement at the spindle behind the chuck when yanking up and down on a one-foot length of bar held in the chuck. Too much slack affects alignment.

It may also be the headstock is out of alignment with the bed. Twisting the bed to make up for an out of line headstock is not a good thing to do. Does your lathe have ajustment on the headtsock alignment?

But my money would be on your wooden board flexing.

PS, no point in aligning the tailstock until you have the spindle and bed aligned first. Totally different issue.

 

[Blogwitch]

The head is aligned to the bed side to side by using a cold chisel to deform the edge of the headstock base to kick it one way or another.

I dropped a second hand bearing head onto mine to replaced the plain bearing head, and it fitted exactly except it required kicking over slightly with the cold chisel method to get it to turn parallel, so I don't think the nose will be that far out, it most probably hasn't been off any time during it's life to get the base worn, and if it is the bearings, that would soon show up as mentioned above.

I remember reading how to set up a Southbend lathe in a manual about 80 or 90 years old, and I think that suggested it needed 3" to 4" levelled thick boards to clamp it to, to stop any twist or bowing. My stand was made up from 3" thick wall steel square tubing, no twisting encountered.

My suspicion would still be wear in the tailstock casting.


John

 

[rlfervan]

Wow thanks for the quick reply guys.
Blogwitch I don't think it us a wear issue, the lathe ( a 618 by the way ) is overall in very good condition. I have not checked it with a test bar cut yet, but that is next. Unfortunately I don't a machinist level to check the ways but I think I could borrow one. Maybe that should be the next step. Hopper you make a very good point and it very well could be the board bending and not the lathe bed. I should probably be thinking about building a good bench, I think the level would help determine what is going on. Dose that sound logical?

 

[Gordon]

The mounting base is indeed important but the lathe bed is pretty rigid and you would have to be torqued down quite a bit in order to twist it to the point that you needed 1/8" of shim. I would tend to believe that wear in the tailstock is much more likely. Try extending the ram in the tailstock to different lengths. Are you getting different results with the ram extended 1" and 3"? If you shimmed the front of the tailstock it would make the ram extend in a non horizontal plane.

 

[rlfervan]

Thanks Gordon. I understand what you are saying but the but the Rolie dad method of checking is only for headstock alignment. it dose not use the tailstock at all.when I an reasonably sure I have the bed aligned I will move on to the tailstock. I did put a dial test indicator in the chuck and indicate off a center in the tailstock and t is very close. the more i think about it the board has to be moving.

 

[petertha]

I just completed an alignment checking mission on my lathe cutting a slight taper. I chased down the levelling avenue, only to discover my headstock was slightly out & the principle culprit. If your Atlas headstock can become out of alignment, I would check into that first.

There are some related YouTube resources to watch
- Tom Lipton (Oxtool) Leveling and Setup of the metalworking lathe P1 & P2
- Keith Fenner Let's Look Under The Old Girls Apron Part Eleven
- This Old Tony has a similar demo but he only talks about the how's & whys of lathe levelling, not the headstock

To my mind the proper sequence goes:
- lathe level as accurate as you can get it in both fore-aft & cross-bed axis. Machinist level is the weapon of choice.
- adjust headstock angle (assuming it is adjustable). Use the test bar cut method (aka Dumbell or Rollie Dad Method). Or DTI off a precision test bar held in headstock spindle socket (not a chuck or collet)
- then (and only then) with headstock true, introduce tailstock concentricity alignment into the mix because it references off headstock rotation

I've seen a lot videos & articles where people start jacking the lathe feet right off the bat to correct taper. This is called 'lathe levelling', but what they really mean is 'lathe bed twisting' to dial out a taper cut. I can only assume this is the recommended go-to method if your headstock is fixed (unable to adjust in top view plane). Worded another way, if your headstock is out & you haven't dealt with it properly & now you twist your lathe by jacking the feet up or down to compensate, you now have 2 separate issues that might be somewhat masking one another & this is not ideal.

I bought this 330mm MT3 precision ground test bar from ebay at reasonable cost. Using an MT5/MT3 socket I was able to replicate the headstock alignment by running DTI down the length vs. cutting method. To my mind, the test bar is WAY easier than the cutting method. But its an expense & then sits on teh shelf until you want to check again so personal choice. Cutting is the final proof but can introduce other variables IMO.

 

[petertha]

With headstock aligned & lathe twist dealt with, now you can introduce tailstock alignment. I splurged on this gadget which has 2 precision ground rings separated by a longish bar length. You set between centers, zero DTI on headstock ring, then compare that to tailstock ring. Tweak the tailstock until it reads zero. Pretty straightforward. You can also read any vertical misalignment in same manner but luckily that isn't quite as important for taper issues.

You could probably make something like this yourself, I was lazy & impatient. I used to do this by rotating a DTI in my chuck against the end of an MT dead center shaft set in the socket. The longer length of the Edge type tool is what's helping here because its exaggerates the offset distance. I guess you could also use this to set a tailstock offset for taper turning.