What's causing odd pattern on finish cut on lathe?

[TDegenhart]

I vote for problems with collet and/or spindle bearings. Whenever I have had that type of problem it was a) stock that a couple of thousands or more off the actual diameter of the collet as well as sticking out several inches from the collet. Could also be that the chuck is masking loose spindle bearings by being heavy and acting as a flywheel. Also a big problem with trying to take a couple of thousands off will cause problems with carbides because the edges of carbide are not razor sharp like high speed steel and also because of our old enemy the built up edge. With heavy cuts there is enough pressure on the tool face to prevent BUE at slow cutting speeds so the tendency is beautiful surface finish on the heavy cuts and a terrible finish on the last thin cut. I have noticed this particularly with coated carbides.

Tom

 

[Limy Sami]

FWIW - when doing a 2 collar test, I use a SHARP HSS tool and no more than 0.003'' DOC, ....., anything more will give me false results.

The last tool I would use is a carbide insert of any description, no matter how sharp or fine / small DOC the makers state.

 

[michiganbuck]

QT [I use a SHARP ] We were taught to see the edge will shave your fingernail for steel and aluminum.

 

[Monarchist]

Also a big problem with trying to take a couple of thousands off will cause problems with carbides
.
.
...so the tendency is beautiful surface finish on the heavy cuts and a terrible finish on the last thin cut. I have noticed this particularly with coated carbides.

Tom

Point. And.. IF ever I figured out how to complete a task without having to TAKE a "final cut"?

Mebbe then I'll try carbides again.

 

[Leg17]

Did you catch the part about the 'outstanding' finish using the chuck?

Actually, I did miss that part.
Thanks

 

[sandiapaul]

OK time for a report. Despite setting up and running various engineering and physics experiments for years I broke one of the cardinal rules, change just one thing at a time!

I tightened up the gibs on crosslide and compound, not too stiff to turn easily but nearly so. I also re-adjusted the rollers on the carriage(this machine uses bearings at each corner, not a gib) I don't think this did anything really because I was overly fussy about this in the first place. For those who don't know this lathe is a Smart & Brown 1024.

I used a good sharp HSS tool for all cuts, it had a radius of about .030".

Using the 3 jaw on 2" OD 6061 with .004 per rev finish was really, really good, no banding. I looked at with a stereo microscope at 20x, it looks great.

I switched to a piece of 1.5 OD 303, also got great results, no complaints at all.
Similar results on a piece of O1 1" OD.

Now to the collet(1" nearly new Hardinge), banding again, but much less, you can see it but barely feel it.

Hmmmm, so I started messing with speeds and feeds. I was changing my feed with one of the gear settings on the dial in the same position(if you know the 1024 you will know what I mean) I did this because it was easy to change from .001, .002, .004 per rev. I changed to a different position on the dial, and used .003 and .005. Banding is now not detectable. OK that seems to have got it, there must be a problem with one of the teeth in that one feed setting. I inspected them all on tear down but didn't see anything.

I did try the indicator on the 5C spindle at 10 degrees and its spot on(5C is built into the spindle)

The only thing I have not tried and was suggested is the prybar under the chuck. I have never done that before, how much pressure should I use? It seems like you could get it to move if you wanted to with enough pressure. I'd like to try this test, so any advice on how to carry it out would be great.

There was one comment made:
"Wear in the saddle and bed means the feed pinion no longer engages the rack at the optimum position. That means irregular saddle motion whrn using power feed. I bet the banding has the same pitch as the feed rack"

I'm not sure I understand this, but when I first read it I thought "aha, a simple solution!" Especially as I replaced the pinion. But the pinion does not drive the rack, granted it goes along for the ride and I could see that it may wind up telegraphing onto the workpiece, but it seems doubtful. Maybe I am not interpreting this comment correctly??

Thanks again to all the commenters, it was a great help and it seems that it is mostly solved.

Happy New Year!

On edit: one more test I did, that was recommended, was to set up an indicator and watch the dial as the carriage moved. I took a slo mo video of this and can see no hesitation, regardless of feed position.

 

[DMF_TomB]

OK time for a report. Despite setting up and running various engineering and physics experiments for years I broke one of the cardinal rules, change just one thing at a time!

I tightened up the gibs on crosslide and compound, not too stiff to turn easily but nearly so. I also re-adjusted the rollers on the carriage(this machine uses bearings at each corner, not a gib) I don't think this did anything really because I was overly fussy about this in the first place. For those who don't know this lathe is a Smart & Brown 1024.

I used a good sharp HSS tool for all cuts, it had a radius of about .030".

Using the 3 jaw on 2" OD 6061 with .004 per rev finish was really, really good, no banding. I looked at with a stereo microscope at 20x, it looks great.

I switched to a piece of 1.5 OD 303, also got great results, no complaints at all.
Similar results on a piece of O1 1" OD.

Now to the collet(1" nearly new Hardinge), banding again, but much less, you can see it but barely feel it.

Hmmmm, so I started messing with speeds and feeds. I was changing my feed with one of the gear settings on the dial in the same position(if you know the 1024 you will know what I mean) I did this because it was easy to change from .001, .002, .004 per rev. I changed to a different position on the dial, and used .003 and .005. Banding is now not detectable. OK that seems to have got it, there must be a problem with one of the teeth in that one feed setting. I inspected them all on tear down but didn't see anything.

I did try the indicator on the 5C spindle at 10 degrees and its spot on(5C is built into the spindle)

The only thing I have not tried and was suggested is the prybar under the chuck. I have never done that before, how much pressure should I use? It seems like you could get it to move if you wanted to with enough pressure. I'd like to try this test, so any advice on how to carry it out would be great.

There was one comment made:
"Wear in the saddle and bed means the feed pinion no longer engages the rack at the optimum position. That means irregular saddle motion whrn using power feed. I bet the banding has the same pitch as the feed rack"

I'm not sure I understand this, but when I first read it I thought "aha, a simple solution!" Especially as I replaced the pinion. But the pinion does not drive the rack, granted it goes along for the ride and I could see that it may wind up telegraphing onto the workpiece, but it seems doubtful. Maybe I am not interpreting this comment correctly??

Thanks again to all the commenters, it was a great help and it seems that it is mostly solved.

Happy New Year!

.
many lathes with sleeve bearing the bearing cap has .001" shims. with indicator on part if you pick up lathe spindle and you see it go up over .001" then the loose bearing needs a shim removed so bearing clearance is less. the tighter the bearing overheating can be a problem at high rpm, got to watch for it
.
could be as simple as when cutting the lathe spindle and part is going up and down in the lathe spindle bearings from excessive bearing clearance. that can cause a random type vibration when conditions are just right
.
lathe gears often get damaged when spindle locked or put in back gear and you use a big wrench or hammer to get stuck chuck off. or trying to tap with a large dia tap using a big wrench and using the spindle lock or back gears to try to hold it steady.
.
i know from personal experience using a 18" or 24" wrench to turn a 1.5" dia tap the lathe spindle lock pin and the back gears were never designed for the load especially if the lathe is not a big high hp machine weighing tons

 

[iwananew10K]

i still think Gordon nailed it.

If it were a bearing issue I would think it would show when you used your chuck too, and even be worse since no chuck is perfectly balanced.

why not make a solid test bar out of 1.5" steel with the profile and thread of 5C on one end?
very easy and will remove the variables.

 

[Monarchist]

There was one comment made:
"Wear in the saddle and bed means the feed pinion no longer engages the rack at the optimum position. That means irregular saddle motion whrn using power feed. I bet the banding has the same pitch as the feed rack"

I'm not sure I understand this,

Involute gear against involute rack and even with both players brand-new and at "interference fit" - which you do not have - one has the residue of a cyclic advance.

That's the basic nature of toothed wheels, be they gears, roller-chains, or Gilmer & successor "synchronous" belts. Most other forms are far worse than involutes, FWIW, which is why they dominate.

More "bad news" - those are not the only two involute gears in-play when power-surfacing feed is at work. Far from it. Few will be Helical or Herringbone - most are the "worst type" vanilla straight-cut spur. And then there are keyways or splines, worm gear.. etc.

Take the entire lot of them out of the test by setting up for uber-fine thread pitch and using the half-nuts and the leadscrew for still not-quite-perfect, but far, far more regular advance. "Not quite perfect" because the leadscrew is ultimately geared to the spindle with involute gears ALSO.

Finish now depends on tool shape, akin to a planer or shaper, but there will be no mysteries at all as to pitch of any remaining artifacts. They'll match the thread chosen.

That simple.

"Sundstrand" found a way around it nearly a hundred years ago. Hydraulic carriage advance. And then... most hydraulic pumps of the era were ... meshed involute geared ones. Enter accumulators, snubbers... etc.

TANSTAAFL




And BTW, that's a "rightous" lathe, the Smart & Brown. A "keeper". So it is well worth the bother of learning its strengths (many) and weaknesses (few). All of them.

 

[DMF_TomB]

i still think Gordon nailed it.

If it were a bearing issue I would think it would show when you used your chuck too, and even be worse since no chuck is perfectly balanced.

why not make a solid test bar out of 1.5" steel with the profile and thread of 5C on one end?
very easy and will remove the variables.

.
not necessarily as i have often seen bearing vibration issues from load being just right to pickup shaft and let it down periodically and a heavy chuck can stop that as it heavier than collet. indicator will show bearing clearance by picking up spindle and measuring
.
i have seen ball bearing bearing housings with same problem. od of bearing clearance excessive a balanced load or near balanced the ball bearing can be picking up or bouncing around in the bearing housing and vibrations causing problems like stuff (screws) loosening up
.
i have job with 50 rollers 6 foot long conveying paper where we had 25 bearing housings and all with over .001" clearance i ended up applying polyurethane to fill the excessive bearing housing clearance. it looked like dirt or dried grease or it peeled off like electrical tape if you tried to remove. when the .002 to .003 bearings housing clearance was tightened down to <.001 vibration stopped. the balanced load pulling paper through machine was pickling up the rollers in the bearing housing allowing them to vibrate bad. all rollers where load pulled down even with excessive bearing housing clearance did not have vibration problem. it was only the rollers where load picked up just enough to balance the weight and the bearing was "floating" in the bearing housing

 

[Richard King]

You said on first post, "Rebuilt" What does that mean? You had a pro rebuild it? What was done?

What Tom, leg and Ill said about rocking of the saddle as it feeds seem like the issue to me. If it faces good, makes me think it's in drive train for saddle feed like rack or rack pinion has a burr. How does it cutting a long shaft using the tailstock?

I have heard of something like this once and it ended up being one of the windings in the motor was weak and it hesitated the rotor enough to cause a line on a part. Fortunately it was another rebuilder who was trouble shooting it, he pulled the spindle, made a new gear and went nuts trying to figure it out until the plant electrician tested the drive motor. Good luck

Edit:... You posted bad gear...maybe the key on that gear is worn and it wobbles in that feed if it works ok in other feed selections.

 

[DMF_TomB]

You said on first post, "Rebuilt" What does that mean? You had a pro rebuild it? What was done?

What Tom and Ill said about rocking of the saddle as it feeds seem like the issue to me. If it faces good, makes me think it's in drive train for saddle feed like rack or rack pinion has a burr. How does it cutting a long shaft using the tailstock?

I have heard of something like this once and it ended up being one of the windings in the motor was weak and it hesitated the rotor enough to cause a line on a part. Fortunately it was another rebuilder who was trouble shooting it, he pulled the spindle, made a new gear and went nuts trying to figure it out until the plant electrician tested the drive motor. Good luck

.
even a new motor timing or synchronize belt can cause vibration as rubber teeth go in pulley teeth.
.
i had problem with new belts and pulleys and i talked to gear engineer and he tells me often pulley teeth are too tight and he recommended i get pulley teeth recut so looser fit. it actually worked smoother exactly as he said it would.
.
engineer said pulley teeth can still be in tolerance on new pulleys he had been trying to get standards loosened or changed but there is no industry wide agreement on it. tight pulley teeth in theory less belt backlash if you can tolerate vibration until belts wear in. the time for belts to wear in can be days, weeks, months. on a paper coating machine the vibration was causing lines in the paper coating. at 1150 feet per minute we could not afford to wait for belt wear in

 

[Monarchist]

engineer said pulley teeth can still be in tolerance on new pulleys he had been trying to get standards loosened or changed but there is no industry wide agreement on it.

Nor should he expect one. Competitive field. Competing solutions.

Compare the legacy pre WWII trapezoidal shape of a Gilmer-pattern toothed belt with the more modern modified double parabola of a Pirelli patent.

What balance would you like of strong, smooth, accurate, durable, silent.. or CHEAPER?

Use a different belt and wheels.

There are more than just those two in the market.

ALL of them "cog". You want to shed that, you have to go to friction drive, PolyVee, MicroVee, flat belt, or Vee belt. And give-up "synchronous", of course.

 

[DMF_TomB]

Nor should he expect one. Competitive field. Competing solutions.

Compare the legacy pre WWII trapezoidal shape of a Gilmer-pattern toothed belt with the more modern modified double parabola of a Pirelli patent.

What balance would you like of strong, smooth, accurate, durable, silent.. or CHEAPER?

Use a different belt and wheels.

There are more than just those two in the market.

ALL of them "cog". You want to shed that, you have to go to friction drive, PolyVee, MicroVee, flat belt, or Vee belt. And give-up "synchronous", of course.

.
.
belts were old like over 10 years and i replaced the bad ones. just saying some new synchronice belts and pulleys can create problems til they wear in
.
getting brand new pulleys teeth recut looser was a new one for me. vendor offered to have future new pulleys special made to looser standard. i said dont bother machine not going to be in production 10 more years, lucky it runs 3 more years
.
often maintenance guys have a ideal how much longer a machine will be in production. you put in oil rated 5 years thats the last time you will put oil in on that machine. same with replacing any other parts
.
when boss says dont buy any more parts you know machine not going to be running much longer, you do odd stuff like adding paper or aluminum foil tape to loose stuff to keep it going a little longer. or pop bearing seal off and clean and regrease and pop seal back on to reuse as $10,000/yr in new bearings, boss says no money for that

 

[sandiapaul]

You said on first post, "Rebuilt" What does that mean? You had a pro rebuild it? What was done?

Just want to respond to you Richard since I read most all your posts and admire your knowledge. My machine was not "rebuilt" in the real sense, no scraping. I redid the entire apron, all new shafts, bearings and gears where necessary. Except for the headstock, everything nut and bolt was taken apart, cleaned, painted, new bearings installed, etc. Am I a pro? Not like yourself. But this is about my 10th lathe I have done similar with, so I am not a total greenhorn. I tried to teach myself scraping about 20 years ago and quickly realized there is a reason do it for a whole profession. I truly admire the guys who go the whole "nine yards" on a machine. I just have the time or physical strength(arthritis).

 

[DMF_TomB]

You said on first post, "Rebuilt" What does that mean? You had a pro rebuild it? What was done?

Just want to respond to you Richard since I read most all your posts and admire your knowledge. My machine was not "rebuilt" in the real sense, no scraping. I redid the entire apron, all new shafts, bearings and gears where necessary. Except for the headstock, everything nut and bolt was taken apart, cleaned, painted, new bearings installed, etc. Am I a pro? Not like yourself. But this is about my 10th lathe I have done similar with, so I am not a total greenhorn. I tried to teach myself scraping about 20 years ago and quickly realized there is a reason do it for a whole profession. I truly admire the guys who go the whole "nine yards" on a machine. I just have the time or physical strength(arthritis).

.
new gears ?? often gears have runout and if clearance not high enough they bind once a revolution. might want to manually turn see if you can feel binding hand turning stuff
.
old days you used paper between gear teeth to set gear spacing or used a lapping compound on gear teeth to accelerate them wearing in. after lapping you cleaned lapping compound off normally. you can also blue teeth turn and see if contact pattern looks ok

 

[omcgee]

OK time for a report. Despite setting up and running various engineering and physics experiments for years I broke one of the cardinal rules, change just one thing at a time!

I tightened up the gibs on crosslide and compound, not too stiff to turn easily but nearly so. I also re-adjusted the rollers on the carriage(this machine uses bearings at each corner, not a gib) I don't think this did anything really because I was overly fussy about this in the first place. For those who don't know this lathe is a Smart & Brown 1024.

I used a good sharp HSS tool for all cuts, it had a radius of about .030".

Using the 3 jaw on 2" OD 6061 with .004 per rev finish was really, really good, no banding. I looked at with a stereo microscope at 20x, it looks great.

I switched to a piece of 1.5 OD 303, also got great results, no complaints at all.
Similar results on a piece of O1 1" OD.

Now to the collet(1" nearly new Hardinge), banding again, but much less, you can see it but barely feel it.

Hmmmm, so I started messing with speeds and feeds. I was changing my feed with one of the gear settings on the dial in the same position(if you know the 1024 you will know what I mean) I did this because it was easy to change from .001, .002, .004 per rev. I changed to a different position on the dial, and used .003 and .005. Banding is now not detectable. OK that seems to have got it, there must be a problem with one of the teeth in that one feed setting. I inspected them all on tear down but didn't see anything.

I did try the indicator on the 5C spindle at 10 degrees and its spot on(5C is built into the spindle)

The only thing I have not tried and was suggested is the prybar under the chuck. I have never done that before, how much pressure should I use? It seems like you could get it to move if you wanted to with enough pressure. I'd like to try this test, so any advice on how to carry it out would be great.

There was one comment made:
"Wear in the saddle and bed means the feed pinion no longer engages the rack at the optimum position. That means irregular saddle motion whrn using power feed. I bet the banding has the same pitch as the feed rack"

I'm not sure I understand this, but when I first read it I thought "aha, a simple solution!" Especially as I replaced the pinion. But the pinion does not drive the rack, granted it goes along for the ride and I could see that it may wind up telegraphing onto the workpiece, but it seems doubtful. Maybe I am not interpreting this comment correctly??

Thanks again to all the commenters, it was a great help and it seems that it is mostly solved.

Happy New Year!

On edit: one more test I did, that was recommended, was to set up an indicator and watch the dial as the carriage moved. I took a slo mo video of this and can see no hesitation, regardless of feed position.

I haven't seen yet whether you have sleeve or roller,ball,needle bearings.
It does matter when it comes to checking the play in spindle.
To check the play in spindle bearings put enough blocks under chuck till you can pry up on chuck using about a 2-3' pry bar or piece of pipe.Put an indicator on chuck to measure play.For sleeve bearings you should you should have .001-.002 free play depending on condition of the bearings to hold oil.also you need about the same for end play.Too tight and the spindle will get hot and seize. For roller or Ball bearings you should have .000 Radial or end play.In my 55 years as a machinist I have seen about 3 machines that would have a pattern in the finish and each case it was due to the spindle bearings which were all rollers which were pitted. A little practice and you can tell whats free play and whats Deflection in machine .Make sure the Headstock bolts are tight. Re:HSS vs Carbide most modern machine shops graduated to carbide decades ago.My shop the only use i have for HSS is cut offs ,drills,Knurls, all form tools are carbide.My insert of choice for most light to med cuts is a G10 or cast iron grade uncoated.

 

[randyc]

I know as much about this problem as I do about how to keep the road from rising up and smacking my face at the moment ? So take this as a New Year's Eve suggestion for eliminating a variable. It took me a l-o-n-g time to type this so I hope it might be somewhat helpful!

Checking the tightness of the headstock to the ways is always a pretty good idea and perhaps measuring the running temperature of the front spindle bearing might provide insight - possibly but not necessarily to the cyclic problem but maybe a condition that amplifies it, since there have been suggestions related to the bearings.

Simple and cheap enough to do, un-nameable has cheap infrared thermometers. Spin the machine up to max RPM for 30 minutes or so, monitoring the front bearing cap temp. After cooling, adjust pre-load until a comfortable temp rise is obtained; loosen if too high and tighten if on the low side. Bearing manufacturer is best source of allowable temperature rise but note that there are at least four different spindle bearing configurations, not to mention the various bearing types and classifications !

If you adjust preload so that the temp rise is about 20-30 degrees C (and this is strictly a personal opinion) after running at max RPM for extended period and the spindle still exhibits significant movement (the deviations described previously sound good) with a sturdy load applied as described, there is probably work required within the headstock.

As noted, this may not have anything to do with the problem described but may reduce amplification and will be needed to restore good health anyway. And who knows, maybe tightening up the preload might reduce the problem to an acceptable magnitude. Sorry about the lengthu post, I'm practicing when and where to use semicolons.

Babble, babble ....

P.S. Success rate of this method is one in a row for me

 

[Bill D]

perhaps a priest and holy water are required... Not a good idea to use water around a machine tool causes rust . So ask him to use consecrated oil instead. I wonder if consecrated oil would stop rust from condensation on my lathe?
Bill D

 

[SeaMoss]

"Sundstrand" found a way around it nearly a hundred years ago.

SUNDSTRAND ! That's it ! I've been beating my head in trying to remember who built production lathes besides Gisholt (big) and Seneca Falls. It's as if they disappeared off the planet, nothing on the net and nothing in wikipoopia. Thanks !