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SB Fourteen can't hold tolerance with finishing cuts

Glenn Brooks

Aluminum
Joined
Nov 16, 2014
Location
Woodinville, Wa
Hi all,

Just discovered my new to me 1985 Southbend Fourteen can't hold decent tolerance on the X axis when making finishing cuts, e.g. Turning down the radius of a shaft. Spent the day making mucho test cuts. Short of it is:

- dial in .020" depth of cut on the cross slide, lathe cuts .024"

- dial in .010" depth of cut on the cross slide, lathe cuts anywhere from .014" to .019"

- dial in .005" depth of cut, lathe cuts .008" up to .015"

It seems to cut .004" to .006" over my selected depth consistently, with smaller cuts showing the greatest variation. SOmetimes twice what I dial in.

All of this even with the gibs tightened up and the carriage locked down to the taper attachment to prevent movement. Work supported by the tailstock. And also when making cuts with both HSS and Carbide tooling.

This looked like a nice tight machine. Good original paint. Gears show little wear. Bed is near perfect. The tool post is a stout QCTP. The spindle has around .001" of play, maybe a bit less.

I could understand dialing in a cut, and coming up oversize due to tool pressure. But consistently and widely turning over the dialed in amount has me baffled.

What could I look at to tighten this thing up? Iam sort of stumped.

Thanks
Glenn
 
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where in Washington are you? might be several folks near by.

not to make a bad joke about current levels of; quite frankly, terrifying politics; but honestly the numbers you're reporting are within the range of see something.. say something kinds of numbers.

meaning you should be able to see something moving around. tool pressure kinds of problems should be proportional to how far you are from the chuck.

how can you report a near doubling of your depth of cut without also seeing the tool get sucked into the work? and if that is happening, i'm surprised something else isn't going wrong. like the tool post ripped off the compound.
 
Hi Johansen,

Iam in the Woodinville area.

Yes, thought the same thing. I looked closely at the tool post and bit, and can't see any significant (visual) movement. The only thing that seems to be moving is a little oil running out of the rear of the compound where it sits on the cross slide, while making the cut.

Glenn
 
I had the same problems with my heavy 10. I narrowed it done to looseness and backlash to the cross slide and compound. Also need to make sure the tool is sharp and on center. To verify the cross slide, I mounted that threading stop, a dovetailed bar that clamps to the saddle and tightened the cross feed to take out any play. Made repeated cuts always bring the cross feed dial back to the same setting. It nails it to within a few tenths.

Tom
 
So you say Z, do you mean Z? AS in parallel to the spindle axis?

You say compound? Do you mean compound? or cross slide?

No matter the play on a clapped out lathe it can usually repeat enough to be useful

IOW:

dial in clockwise to a known number make a cut, measure the diameter, do not move the cross slide[at all]

dial in the cross slide .01,[do not back up at all do not if you go past, well, that is your new number, .012 or .015 who cares] the part diameter should reduce by .01, or .02 depending if the lathe is direct or indirect

this assumes you mean 'x' not 'z' and cross slide not compound

if that is not what you mean I don't know what you are saying, be more clear
 
So you say Z, do you mean Z? AS in parallel to the spindle axis?

My mistake, iam actually describing the x axis, the axis that controls radius of the work. E.g. Turning down the diameter of a shaft...

You say compound? Do you mean compound? or cross slide?

Actually meant, cross slide! Thanks for catching...

No matter the play on a clapped out lathe it can usually repeat enough to be useful

IOW:

dial in clockwise to a known number make a cut, measure the diameter, do not move the cross slide[at all]

dial in the cross slide .01,[do not back up at all do not if you go past, well, that is your new number, .012 or .015 who cares] the part diameter should reduce by .01, or .02 depending if the lathe is direct or indirect

this assumes you mean 'x' not 'z' and cross slide not compound

if that is not what you mean I don't know what you are saying, be more clear

Thanks I will try the method you suggest.

Glenn
 
What could I look at to tighten this thing up? Iam sort of stumped.

Tool-tip height vs center + a tad and whether it was hung too far out would be my first suspect.

Symptom being repeating sez under-centre and being pulled-in, if not off a locked cross & compound, then any other slack, compound to cross, even carriage to bed ways & lift.

Very light lathe, and not as if it had a coupla hundred lbs avoir of 4-way and inch-plus tooling on it after all.
 
I'm a bit confused also (on terminology here. Also kinda concerned this tightening things up. A well tuned machine tool (especially this size) isn't "tight".

If things are right there is little more effort to move the saddle, cross slide or compound than to roll a softball between two hands over a table. It "should" feel that good.

Seen a lotta good iron ruined by tightening things down when something remedial is the real problem.

Good luck
Matt
 
Thanks I will try the method you suggest.

Glenn

What he said.

'Nuther trick on seriously AFU cross, 'company lathe' known POS, is to hang a bit of human muscle as a back-pressure on the TP to keep the nut tight-up agin' the 'push' side of the threads, not wandering off at random to see how much slack there is to the back side of the adjacent thread slope.

Company lathe, Union Shop, one JFDWT. YOUR lathe, you are allowed to actually FIX.

AFTER .. you figure out what you did to hide the root cause of the problem from yourself, of course!

:)
 
Thanks Guys, I put the dial indicator on the machine this afternoon. Looks like the cross slide screw and nut -more likely the nut, has some weird play in it. The tooling itself definitely pulls into the work .002-.004" when taking a .010" cut, although the cross slide itself is rigid and doesn't move. So iam expecting to find the crossslide screw and nut to be moving, within the cross slide itself. Still have to assess whether the compound slide has movement. Ilocked it down to isolate it when testing the cross slide movement. Now it's cold and late, so no more until the 'morrow.

Thanks for the tips! Looks like the guy I bought this from wasn't entirely forthcoming about why he wanted to part with it. Well, still hoping to remedy the underlying cause with new parts... other than this problem the machine is in pretty nice shape, overall.

Glenn
 
Thanks Guys, I put the dial indicator on the machine this afternoon. Looks like the cross slide screw and nut -more likely the nut, has some weird play in it. The tooling itself definitely pulls into the work .002-.004" when taking a .010" cut, although the cross slide itself is rigid and doesn't move. So iam expecting to find the crossslide screw and nut to be moving, within the cross slide itself. Still have to assess whether the compound slide has movement. Ilocked it down to isolate it when testing the cross slide movement. Now it's cold and late, so no more until the 'morrow.

Thanks for the tips! Looks like the guy I bought this from wasn't entirely forthcoming about why he wanted to part with it. Well, still hoping to remedy the underlying cause with new parts... other than this problem the machine is in pretty nice shape, overall.

Glenn

I *think* the last time I laid eyeball on South Bent's "How to ruin a Lathe" was around 1959, so can't say if later editions differ.

There was a page.. left lying if memory serves, on which, roughly centred vertically a diagram showing a round, indicating work, and a radial line to the left, or operator side, with an exaggerated upward incline indicating tool-tip height should be progressively above center as diameter, hence LEVERAGE applied to force-down the tool-tip increased. The text or legend specified a very small number in degrees.

Storal of the morey is that IF your tool height is set correctly AND does not unduly deflect downward under load, the tool will be at all times under modest push BACK, and will NOT EVER be 'drawn into' the work, even if the cross slide nut were to be one-sided, "push only" and return were to be left to a brace of rotten Chinese grocery red bungands, lantern TP top nut to crank handle bracket.

Look there and solve your immediate problem, then.

I'm not sure I really want to know if memory held for that many years, but Day Job practice on War One Niles lathes certainly proved the point SB had made, and I only ever forgot it the one time.
 
choose your poison.

the cutting force pushes away, or sucks the tool into the work.

there is a range of 30 degrees each way where friction will keep the cross slide from moving but once you add an interupted cut no clue what is going to happen.

so you tighten down the gibs until it doesn't move on you, and that wears out the cross slide.
 
Absolutely agree the tool bit will be pushed away from the work. And my tooling is set up dead center on the axis, plus a smidgen high, as you describe.

I think I have eliminated tool,height and tool geometry as a cause... However, the tooling always reduces diameter of the work .004 up to .010" more than depth of cut of my test piece - a 1.250" length of shafting. Setting up a dial indicator on the bed, with the point of the indicator on the TP, just under the work, I can watch the dial rack up .002" all the way out to .004+" movement over and beyond the depth of cut, sometimes more, as the bit travels down the shaft. Clearly somehow, the compound and crossslide upon which it mounts, is being set into the work. beyond the depth of cut I set up.

several people on another forum have suggested possible problems with the crossfeed screw and nut. So will be looking at that more closely tomorrow.

Glenn
 
Absolutely agree the tool bit will be pushed away from the work. And my tooling is set up dead center on the axis, plus a smidgen high, as you describe.

However, the tooling always reduces diameter of the work .004 up to .010" more than depth of cut I set up, when the reducing radius of my test piece - a 1.250" length of shafting. Setting up a dial indicator on the bed, and the point of the indicator on the TP, just under the work, I can watch the dial rack up .002" all the way up to .004+", sometimes more, as the bit travels down the shaft. Clearly somehow, the compound and crossslide upon which it mounts, is being drawn into the work. beyond the depth of cut I set up.

Check something for us, will you?

Presume you are using power surfacing feed, not manual, for long-axis traverse?

See if something in your surfacing feed system is dragging pressure against the cross-feed.

Easy test is to stop it all at the gearbox, hand-wheel your longitudinal traverse, see if the 'creeping' vanishes.

On-edit... AND.. set your compound right-on 90 degrees AKA parallel to the long-axis, as well. That way, IF it is prone to volunteer movement, it will not be reflected on the radius/diameter, only at a shoulder.. and you are now 'sneaking up' on that, manually, anyway.
 
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Could it be a poor mating condition between the bottom of the toolpost and the top of the compound allowing the post to rock slightly?

Or perhaps it's allowing the toolpost to twist into the stock under cutting pressure?
 
Well, the test you did with the indicator on the tool post is good, confirms the tool is actually moving closer to center as it cuts. Time to limit things a bit more. Do the same test but with the indicator contact on the compound, then on the cross slide itself to isolate exactly where the movement is happening. Until you figure out exactly what is moving relative to what it's all just guessing.

Some other questions: Could the taper attachment be dragging, pulling the cross slide in when it does?
You said you see oil coming from where the compound meets the cross slide, is the compound tightened down to the cross slide or could it be rotating slightly as you cut pushing the tool in?
 








 
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