# Surface Finish Issues with CNC Lathe



## JLeather (Nov 27, 2012)

I'm just getting familiar with my CNC-converted South Bend "Fourteen" 14x40 lathe. I'm having some strange issues with surface finish. It's not that the the finish isn't smooth, but there are regular waves in it. Basically the diameter of a finished part wanders in and out .001"-.002" every 1/4" of length or so. It's most exaggerated on a high-rake cutter (like for aluminum). I've included a picture of a pair of aluminum parts I turned (about 1" OD) with a nice CCGT high-rake aluminum insert. It left a smooth finish, but you can see the color striations which correspond to a diameter variance. The dark bands are lower diameter, about .001"-.002" smaller (diametral) than the light bands. Even another light pass yielded the same pattern.  I only recently acquired this machine and most of the converting and setup work was already done.  I've written the previous owner and he never encountered this before, so it's likely something that happened during the move or setup at my house.

Last night I had had enough and I took the lathe partially apart to investigate this.  It seems to be something to do with the leadscrew, leadscrew bearing, or leadscrew ballnut. The striations turn out to be the same pitch as the leadscrew. It turns out that if I loosen up all the bolts that hold the headstock-end leadscrew support bearing in place the striations go away. This leads to other problems, however, so I can't run it that way. I cut a few inches of test bar (aluminum) while tightening and loosening those bearing screws and you could actually see the problem start and stop. Tightening the upper screws on the bearing support make it worst. I believe that every revolution the leadscrew is applying slight upward pressure on the saddle. That's why it makes no difference what the feedrate is. It's a mechanical issue. I've temporarily reached a happy medium with the bearing support screws just snugged (and one of them removed entirely) and the lathe is cutting a mostly un-noticeable pattern that I can polish out.

It seems to me that there are a couple possible causes for this. I can't really tear the lathe down until I'm done with the current project (a Christmas present) but for now I'm bouncing ideas around. I thought that one of the three bearings in the leadscrew system had to float? You can't build it aligned well enough to run 3 fixed bearings, right? Does anyone know if it's the left support bearing that is typically supposed to float, or is it the ball nut that's supposed to float? It could be that whichever bearing is supposed to float has stopped floating; possibly gotten a big chip lodged in it or a little surface rust. I thought that the ballnut was supposed to be the one that floated axially to isolate the saddle from the leadscrew system? Unfortunately the more likely culprit is a bent leadscrew, probably during moving the lathe, that is exceeding the axial float limitations of the system.

I can't quite figure out how the leadscrew comes apart on this lathe. I looked at it last night and it has me a bit stumped. The servo motor comes off readily (I had it off last night) but I can't figure out how the left side bearing carrier comes apart. It looks like I have to drop the leadscrew, apron, and both bearing carriers off in a lump and then take it apart after that. A job that big will have to wait until after Christmas for sure. The leadscrew is driven by a nice poly coupler, BTW, and I didn't see anything wrong with it so that doesn't appear to be the problem.


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## OakRidgeGuy (Nov 27, 2012)

My first thought is that the lead screw is warped.


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## JLeather (Nov 27, 2012)

Yeah, that's pretty much what I'm expecting to find when I get a chance to pull it all apart. Was hoping maybe it could be something else. I would imagine that a leadscrew could be straightened enough to be used again? Some careful time spent with a press, some delrin blocks to protect the threads, and a long machinist's straight edge?

EDIT - Just thinkng out loud, but I think it looked like the holes that mounted the entire leadscrew drive system (servo, thrust bearing, etc) were slotted for adjustment. I wonder if it's possible that during transport that entire assembly was bumped out of vertical alignment?  Of course I would expect that this wouldn't produce a pattern but rather a constant upward pressure on the leadscrew/saddle but who knows?


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## jumps4 (Nov 27, 2012)

hello
on my 9x20 lathe conversion i had the exact same problem i had to remove the bearing on the spindle end of the lead screw it was lifting or moving the carriage as the screw turned. 
pics would help a lot.
steve


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## JLeather (Nov 27, 2012)

I'll get some pictures of the parts later.  Unfortunately the leadscrew is protected by a set of spring covers so I'll have to remove it before I can verify if it is or isn't bent.  I do have the advantage of knowing that the lathe *was* working fine at one point with this setup.  The spindle-end bearing is the thrust bearing (probably 2 bearings really, but I haven't gotten it apart).  As such I can't really run the lathe without it.  I've lost a few thousandths of accuracy in the Z-axis by running it with the bearing support loose.


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## Ray C (Nov 27, 2012)

Yes, had this same problem on a manual lathe when I first bought it.  Combination of two problems:  Leadscrew was bent and halfnuts not adjusted properly.  Also, helps to have the carriage jibs adjusted properly and if the ways are belly worn near the headstock, it's time to resurface the ways or get a new lathe.


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## JLeather (Nov 27, 2012)

Fortunately the bed is fine.  It's a flame-hardened bed and since this was an early CNC conversion it didn't get much use (probably spent most of it's first decade or two down with 1980's control problems ) ).  I'll double check the saddle gibs, although I don't know if I can reach them with the enclosure on and it's tought to remove the back half.  Sounds like everyone's anectodal evidence points towards the leadscrew.


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## Ray C (Nov 27, 2012)

Definitely a leadscrew -or related problem (bent LS, or the drive-pin in the collar gear is sticking in the lengthwise LS slot, halfnuts misadjusted, bigh hunk of swarf embedded in the halfnuts etc).  BTDT myself -and have seen it on other lathes too.  




JLeather said:


> Fortunately the bed is fine. It's a flame-hardened bed and since this was an early CNC conversion it didn't get much use (probably spent most of it's first decade or two down with 1980's control problems ) ). I'll double check the saddle gibs, although I don't know if I can reach them with the enclosure on and it's tought to remove the back half. Sounds like everyone's anectodal evidence points towards the leadscrew.


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## JLeather (Nov 28, 2012)

Took another look at the lathe last night. I removed the servo again and tried to adjust the headstock end of the leadscrew. While there were in fact adjustments built in (see pics) I discovered that after proper adjustment was obtained the mount was drilled and roll-pinned in place. It could not have shifted during transport and can not be adjusted now.

I also cut a 16" test bar. The pattern was present the entire length. In the middle of the test bar the pattern becomes very faint, but I think this is due to the bar flexing. If I had larger diameter material I believe that the pattern would be present and unchanged for the entire length. It seems too coincidental that it became faint at exactly the center of my test bar and returned as the carriage approached the spindle.

I forced back the leadscrew covers and moved the carriage to both ends. I don't have a long machinist's straightedge, just a long woodworking straightedge, but there was no detectible bend along the length of the leadscrew. I would think that if this were a bent leadscrew that it would have to be bent severely enough to be detectible with even a reasonable straightedge, unless it is bent right at the drive end where it enters the thrust bearing assemble (entirely possible, but I have to remove it to check that). The carriage gibs appear adjusted, but that is tricky to tell on a CNC. I need to remove the servo and turn it by hand, and I may need to remove the enclosure to reach the gibs well enough for a fine adjustment.


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## Ray C (Nov 28, 2012)

Can you clarify?  Is your LS being driven by a CNC motor or, driven synchronously through gears in the headstock?  Once you rule-out glitches in the screw, gib etc. it's time to look at what's driving the LS.  I've seen this same symptom in several lathes and if it's not the typical bent or mis-adjusted LS, you usually end-up taking the whole darn lathe apart.


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## JLeather (Nov 28, 2012)

It's a full CNC lathe.  It was sold by South Bend with no gearbox and no apron, and an aftermarket company installed servos for the X- and Z-axis.  The leadscrew is only driven by the servo motor, shown in the first picture from my last post.  The servo drives the leadscrew through a zero-backlash poly coupler, so no misalignment of the servo can be translated to the leadscrew.

I'm not certain the leadscrew isn't bent.  It may be that the apron/ballnut is holding it in alignment.  I'll probably have to take it totally off to verify 100% that it is not the issue.  Just trying things in the mean time.

Also, is it possible that one of the balls inside the ballnut has become damaged?  It would stand to reason that each time a ball recirculates it may be forcing the ballnut off center?

I'm also going to try turning a test pass by hand with the z-axis servo disconnected, and I need to try a pass with the x-axis servo disconnected as well although that it much more difficult (it's mounted behind the apron).


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## OakRidgeGuy (Nov 29, 2012)

I have read over on CNC Zone of having to shim the apron.


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## JLeather (Nov 30, 2012)

On a side note, when I pried back the leadscrew covers I doscovered that this machine has a 1 1/8" leadscrew.  The force required to bend it would be immense, and I would expect that whatever caused it would have to have damaged the apron and/or ballscrew covers (which are thin sheetmetal) as well.  I can find no damage at all to the lathe, apron (which also is only sheetmetal), or covers.  I'd bet that nothng short of lifting the lathe by the leadscrew, or tipping it over onto the apron, could bend it.

The other thing I'm struggling with is the fact that the previous owner never experienced this.  I've been corresponding with him and he is as stumped as I am.  It would seem that the most likely explanations, a poorly-machined or bent leadscrew for instance, or a gib adjustment problem or alignment issue with the LS mounts, should have been a consistent problem before and after the move.  As well done as the conversion is, and the fact that the previous owner had no similar issues with it, I have to believe that it's not a problem with the leadscrew alignment, apron alignment, etc.  If it's a mechanical problem then it has to be something I jarred loose during the move, or a part that has worn out/failed.

Of course I've been wrong before


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## Ray C (Nov 30, 2012)

Are you using the same bench as the prevous owner?  Is the lathe properly leveled and the ways parallel?  Are you using leveling pads?  Mabye there's a twist in the bed.

Does the problem happen at all ranges of the carriage travel?

And finally, is it possible the servo motor is having a problem?  I'm assuming it's highly geared and in those cases, minor fluctuations in motor RPM make big changes in the outcome at the final output.

I'm just tossing as many ideas as I can think of...

BTW:  I think leveling pads are pretty useful in a lot of machinery -especially if you're on a concrete floor.


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## JLeather (Nov 30, 2012)

It's a floor-standing lathe (I'm attaching a pic of the overall machine). There are levelling feet built into it, and I levelled it as per the South Bend setup instructions. This is especially necessary in my garage as the floor is sloped approximately 1/8" per foot towards the garage doors for drainage. I have a Craftsman digital level, not a machinist's quality instrument by any stretch but it does read to tenths of a degree. I levelled the headstock end across the ways, then the tailstock end across the ways, and then the entire lathe lengthwise. I don't know if the previous owner levelled it, and in fact based on the position of the feet when I acquired it I doubt it (one was much higher than the other 3), but it's level now.

The problem happens for atleast the entire 16" nearest the headstock. I would like to cut a test bar the entire length of the lathe, but I don't have a follower rest so workpiece flex would probably negate the value in such a test. I could get a long piece of stock and chuck it at various lengths with a tailstock center and make a cut for a few inches at various points of the bed. Based on the 16" bar it looks like it is unchanged for the whole length, but it would be nice to prove that to myself.

I would like to disconnect the x-axis servo and lock the gibs and make a pass, I just haven't had time. I would think that a servo issue would be more random than this, but it pays to check everything of course.


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## Ray C (Nov 30, 2012)

Man, that's a big lathe...  Anyhow...

I know this is a stretch and it's probably not the cause of the problem but, at some point, maybe some neoprene-type pads between the floor and leveling bolts will help.  Case in point...  On my 1200lb surface grinder, I would occassinaly get a strange pattern on fine finish passes.  It would come and go. It drove me nuts.  One day, I bought neoprene leveling pads and the problem went away and never came back.  -Go Figure...  It was a weird vibration.

Try those easy things first, even if they don't make sense.  You might just stumble upon the solution.  It's imporant thought to just try one thing at a time so you can learn what the problem was.

... How about the leadscrew bearings and tolerances?  leadscrew lube the right kind?  Is it very cold?

Again, just tossing more thoughts out there.  I had the exact symptom on my old manual lathe but, the solution wasn't as hidden but these are things I tried.


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## Tony Wells (Nov 30, 2012)

If you have a long travel indicator, set it up and watch it during a cut, or even just moving in air in -Z- to see if there is a "jump" in the travel. A 1" indicator might do it, but longer would be better. Also I'd want to look at the shaft as it was being cut by running an indicator on it behind the tool. When the "jump" occurs, I'd expect to see it on the indicator. I don't mean as the indicator passed over the spot, but as a result of the cross slide being moved in some direction. Might want to run the indicator on the ways trailing the carriage too, so see if it is jumping up or something. All could be clues to help zero in on it.


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## JLeather (Dec 1, 2012)

I put a mag base and a DTI on the carriage tonight, referencing the flat way on the bed, and ran it manually up and down the ways.  I noticed that when I ran the carriage towards the headstock I got a repetitive .0015" jump.  Here's where it gets really weird.  I also noticed that when I ran the carriage away from the headstock the DTI stayed steady.  To prove this wasn't a fluke I put an identical tool in the toolpost that was a LH insert holder and took a pass away from the headstock.  No surface variation!:thinking:


Now I'm thoroughly confused.  The front edge of the carriage jumps in time to the leadscrew pitch when travelling towards the headstock, but not when travelling away from the headstock.  How can that be?  Atleast it seems I can rule out bent leadscrew, since that would have to give me trouble in both directions, but now I'm kind of stumped.  My guess is an issue with the leadscrew bearings, although I'm not sure in what manner they could have failed to cause this issue in only one direction.


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## Ray C (Dec 1, 2012)

Sounds like a gear in the apron doing that jumping.  I wonder if you put the DI on the right side of the carriage and move toward the TS, if you see that jump again.  This would be a different problem than the other one because in auto mode, the half nuts are grabbing the LS.  In manual mode, a gear off the handcrank is driving off the underside of the way.

Hmmmm... Q:  Does your handcrank spin by itself when in autofeed mode?  Pull that gear out of there that engages the handcrank to the LS and see what happens with an auto-fed cut.

Have you taken a real close look at the LS and is it clean as a whistle?  How about that handcrank gear, is it packed with swarf?

Doubtful, but is that handcrank gear adjustable or out of adjustment by sliding off the shaft it rotates around?

Oh, if your handcrank rotates while in auto-feed mode, does it kinda plop over when it rolls past TDC?  Maybe try balancing the handcrank.

EDIT:  Had to use the word "handcrank" just one more time.


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## Ray C (Dec 2, 2012)

We're less than an hour apart.  -Don't make come up there and start tearing your lathe apart :nono:


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## Tony Wells (Dec 2, 2012)

That change of direction could point to a thrust bearing. Any end play in the leadscrew? Or it could be in the apron, as Ray mentioned.


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## JLeather (Dec 2, 2012)

Ray,

There's nothing inside the apron except the ballnut.  The apron on this lathe is only a sheetmetal cover for the ballnut.  There is no manual mode.  The only way I've been able to manually move the apron around is through a knob on the tailstock end of the leadscrew.  There's no handwheel, and the gear rack on the lathe is no longer attached to anything.  There's only the leadscrew.





Tony,

You're right that it's something related to leadscrew thrust.  My money is still on the headstock-end bearings, but I can't figure out how to take them apart and don't have the time to really dig deep until after Christmas.


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## Red_SC (Jul 16, 2014)

Sorry for bumping an old thread- did you ever get this fixed?


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## JLeather (Jul 31, 2014)

Actually I think I just recently solved the issue.  The lathe had no front gib. Back when it was a manual lathe there was 40 pounds of apron and assorted gears hanging from the front edge of the carriage, so a front gib wasn't really necessary. Now that it's a CNC there's no weight on the front of the carriage, and with little effort you can lift the front of the carriage off the ways. I found that the carriage easily racked when sliding along, causing the front edge to slightly ride up on the v-way which may well explain the issues I've been having with especially light cuts.

I decided to add a front gib to the lathe. I machined it out of 3/4" 7075 aluminum and attached a strip of 932 bearing bronze to the front edge for the bearing surface. The carriage does seem to have a good bit of drag now, but on the plus side I can no longer lift the front edge at all.

Completed Gib Assembly:




Installed Gib:









Initial results are very promising. I took a 2.5" test cut on a piece of 3" OD solid 6061. This picture was a pass taking a .002" cut and at an exaggeratedly-slow .001"/rev feed rate just to see what would happen:






Took a nice long pass on a test bar. 1 1/4" 6061, a 16" long pass without a follow rest. No sign of the striations I have been fighting. I think I'm good.


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## Bishop (Aug 1, 2014)

Nice, glad you got it sorted. Issues like that can be maddening. 

Shawn


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