Lathe: Every pass removes more material?

I finally got most ly my Myflrd ML7 back together (Feed gears waiting for some housing bolts)=20

I decided to do a rough alighment test with some 1" aluminum bar stock. I set about 10" in the chuck, with the other end supported by a live center in the tailstock. =20

The problem: After a finishing pass, if I make successive passes down the bar without touching the crossfeed or compound knobs, it takes off more chips on every pass. I've done as many as ten additional passes, and it shaves off more every time. =20

I thought it might be due to thermal expansion, so, after one pass, I cooled the bar with some alcohol on a rag, and then cranked the carriage across one more time, and it took off more material. (!) =20

The cross and compound feed knobs hold position, they're not creeping inward. If there were slop in the carriage, I'd expect it to perhaps take off additional material when traversing in one direction, but not the other. If I let the carriage sit in one place, it stops producing shavings after about 30 seconds, so the creep only seems to occur with carriage movement. =20

Does anyone have any idea what's happening? =20

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Reply to
Doug Warner
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Yupp..its called variously Springpass, deflection, thermal expansion and whip.

Try the same bit with a shorter chunk of 1" steel. You will find that the second or third pass no longer brings up a chip, generally. On lighter springier lathes, this is fairly common and MUST be accounted for when making finish cuts as its far to easy to think you are almost there, and finding out that you are past the diameter. Ask me how I know.....

As explained to me, the lathe has a certain amount of spring in it, bed, head, ways, compound etc etc. So that tends to deflect both the lathe away from the part and the part away from the tool. Each successing cut changes the balance of the spinning bar and it will tend to whip a bit as the balance changes, and as internal stresses are relieved. For me, aluminum is the worst offender, brass hardly so, and steel, almost none. YMMV, and it is worse or better as you change the spindle speed. This is VERY common with carbide tools that are not razor, scary sharp.

When I was first starting this fascinating and often times frustrating hobby, I turned an aluminum bar lengthwise and then upped the spindle speed to make the final finish. I was boggled to find out that the part was now oval, by a few thousands.. more tear dropped shaped in cross section. I tried steel, brass and didnt have the same problem. An old time tool and die guy in one of my customers shops explained it to me when I showed it to him. The machine was a very light SB.

Just my $.02USD

Gunner

"War is an ugly thing, but not the ugliest of things. The decayed and degraded state of moral and patriotic feeling which thinks that nothing is worth war is much worse. The person who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself."

- John Stewart Mill

Reply to
Gunner

My first two lathes were just like the original poster's lathe. Guess I'm not as lucky as you

Then I got a 10EE and installed DROs on it. It really does it just like you say. Cuts what I tell it to, no more no less. Has me plumb spoiled.

Karl

Reply to
Karl Townsend

I finally got most ly my Myflrd ML7 back together (Feed gears waiting for some housing bolts)

I decided to do a rough alighment test with some 1" aluminum bar stock. I set about 10" in the chuck, with the other end supported by a live center in the tailstock.

The problem: After a finishing pass, if I make successive passes down the bar without touching the crossfeed or compound knobs, it takes off more chips on every pass. I've done as many as ten additional passes, and it shaves off more every time.

I thought it might be due to thermal expansion, so, after one pass, I cooled the bar with some alcohol on a rag, and then cranked the carriage across one more time, and it took off more material. (!)

The cross and compound feed knobs hold position, they're not creeping inward. If there were slop in the carriage, I'd expect it to perhaps take off additional material when traversing in one direction, but not the other. If I let the carriage sit in one place, it stops producing shavings after about 30 seconds, so the creep only seems to occur with carriage movement.

Does anyone have any idea what's happening?

To reply, please remove one letter from each side of "@" Spammers are vermin. Please kill them.

Reply to
Steve Lusardi

Thanks. This was with aluminum, and a cheap carbide bit. =20 I took a 3/8" bolt, a fresh bit, and proceeded to make the bolt head round.. Much better. Barely a trace of cutting on the second pass, none on the third.. =20 Now, I remember where I've seen this effect before: When milling in the drill press. When I stopped feeding, it would keep feeding. With the far more rigid milling attachment on the lathe

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could take a MUCH heavier cut with no run-on due to spring. =20

BTW: This is a good way to check for looseness. With the bit against the work, I applied some torque to the carriage in a direction that would increase the depth. It caused a slight increase in cutting. Same thing to the cross slide. little difference. Compound: much more cutting. Only one of 4 gib screws was locked, one missing. Tightened them, much less effect. =20

Hmm, now I have to buy more stuff:=20

  1. HSS bits
  2. A better toolholder than the Myford's clamp and shims.
  3. And, I finally have an excuse to buy a better grinder..

Have any of these ancient machinists wrttten good books, containing all the little "gotchas" like this? (Do CNC operators have to know this stuff, or has all the knowledge of machinists already been extracted and programmed into the machines?)

To reply, please remove one letter from each side of "@" Spammers are vermin. Please kill them.

Reply to
Doug Warner

Exactly like the one I have for sale.

Most CNC operators are just that..button pushers. Operators know enough to load, run, make offset changes, replace a tool, and usually do "at machine Q&A measurements".

Programmers should know this stuff, though the machines tend to be incredibly rigid and seldom have problems of this type. And a lot of programmers know dick about machining.

CNC Machinists on the other hand..are the guys in small shops whom do everything, program, setup, run, measure etc. They are the ones whom have to know everything. Like any computer..a CNC machine is nothing but a highspeed moron, that does exactly what you tell it to do. Sometimes with embarasseing consequences...and very fast. And the damage can be really expensive

Im just a newbie too, and learn something everytime I turn on the lights in my home shop. Sometimes expensivly...sigh.

I figure Ill still be learning til the day they kick dirt over my box.

Gunner

"War is an ugly thing, but not the ugliest of things. The decayed and degraded state of moral and patriotic feeling which thinks that nothing is worth war is much worse. The person who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself."

- John Stewart Mill

Reply to
Gunner

I had this sort of problem with a small hercus lathe. It wasn't "spring" as such but loose. Try clamping the saddle slightly and see if it still does the same thing also check the your slides. Another thing you can try is to clamp a long peice of steel (say 2') in the tool holder and get a buddy to move it up and down. You will soon see where the movement is.

best of luck

Marc

Doug Warner wrote:

Reply to
Marc Hanssens

Jim,

This is a subject that is going to be very close to my heart shortly (I hope). I have recently ( about a month ago) bought a 13 x 40 lathe and a dovetail mill-drill. I haven't cut a single chip on either yet! The supplier is taking the lathe away and replacing it with one from a different manufacturer (all Chinese of course). The M-D is having the second gearhead rebuilt (casting sand in the bearings, etc). But I made a few quick measurements last week with a DTI mounted on various points of the M-D and got a real scare. The one that really got me was how easily I could deflect the vertical column, and by how much, just by leaning on the gearhead. When I finally get my lathe and M-D to keep, I'll be making a bunch of measurements like this on both machines. But then I'll need to know just how acceptable or otherwise those readings are. It would be nice if a few people who own 'good' machines could document the sort of numbers they see, and maybe a few others who own Chinese gear like mine could post their own measurements.

Ideally, everyone would be working to a common list of procedures. Of course, everyone has their own idea of just how tight they consider acceptable when adjusting gibs etc, but some measurements with one or more gibs completely locked could also be informative.

Are there others out there who would be interested in such info? Willing to make a few measurements, and post them? In any case, I will do so, and invite comments.

Roger

jim rozen wrote:

Reply to
Roger Head

I don't know about 'good' machines. With my Atlas 12X36 I won't lean on the tail stock cause I've had a dail indicator set up and its scary how much the bed deflects. Its ok for my first lathe and I'm amazed that it cuts straight enough , but when I replace it I'll make sure the next one has a stronger bed. On my Logan 7" shaper I set the gibs too tight and broke the advancing pin. On my P&W planer I lock the gibs on the arch and set the others so that it takes some effort to turn the handles. I'm sure with other types of power feeds that you'd have to be careful how tight you set the gibs so that the motors don't burn up. At least the Atlas has a clutch for the lead screw so that it doesn't break when it crashes.

Reply to
Sunworshiper

Chinese cast iron isn't noticeably more "rubbery" than US cast iron. What mostly determines how much a machine deflects is how much iron the machine has. A 4000 pound lathe is much more rigid than a 2000 pound lathe or a 1000 pound lathe. That's because thicker sections deflect less than thinner sections under the same load.

As Jim said, even a high quality light lathe will deflect noticeably under light hand pressure. A heavy machine, even a fairly crudely built one, would deflect much less. Same with mills. Mass matters, because it equates to thicker sections, and mass matters because it supplies inertial resistance to dynamic loads. So a heavy machine will have less springback and less of a tendency to chatter than a light machine (both with similar work envelopes).

Light machines are always more of a challenge to run. You can achieve good results, but you have to be aware that you don't have the rigidity of a big industrial machine, and modify your techniques accordingly.

Gary

Reply to
Gary Coffman

"Micro Lathes" like the Unimat 3 are particularly prone to this. The guy who had mine before me (I bought it from an estate liquidator, so here's to you, whoever you were and wherever you are :-)) had an neat solution to the problem. He took a piece of 6" wide x 2" deep structural steel channel, about 1/4" wall, had the surface ground flat, and bolted the lathe to that. Since the U3 has the middle of the bed supported (by the milling column mount) this makes for one *rigid* little puppy. 0.05 mm cuts in 303 stainless using power feed curl a nice long swarf without chatter. I know, no big deal to you *real* lathe owners, but it sure made me smile :-)

Cheers,

Earl

Reply to
Earl Boebert

Thanks for pointing that out , I forgot that I had it set up for Gunner cuts to see what it could do. When I went out to his place he had me backing away from his mill a couple of times , didn't have any safety glasses for one. I'll re-adjust it for lighter cuts.

Reply to
Sunworshiper

Thanks for all the useful tips. I'm hoping to get my ML7 back in service by November. I dismantled it and moved it from a factory where it was no longer needed in the summer. I'm lucky to live just a few miles from the Myford factory where it's going home for a bed regrind in October. Never occurred to me that it would be so springy.

What's the group's opinion on raising blocks? Will they make levelling much easier? Worth the 50 UKP/$75? What about the effect on spring? And finally, on machine height? I'm 6'1" and I've mounted the cabinet stand on a 2 1/2" thick cast concrete slab on the concrete floor in the cellar (bolts set in the concrete for the stand's feet). I can't stand on a box to use it, my head just fits between the joists holding up the kitchen floor now!

John

Reply to
John Fletcher

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