What's causing odd pattern on finish cut on lathe?
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  1. #1
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    Default What's causing odd pattern on finish cut on lathe?

    So I got my rebuilt lathe set up and in place and mostly leveled. I was in process of making a 2 collar test bar when I noticed an odd finish pattern on the surface. It is like a series of parallel "rings". You can feel them with your fingernail, my guess is they might be a couple or less tenths deep. Frequency of the rings are about .030, regardless of speed and feed.

    Lathe is running on a VFD(Teco 7300), 3 HP 1800 rpm motor drives a 2 speed gearbox, output of gearbox drives the spindle. Flat belts throughout, all new, scarfed and glued. Everything runs nice and smooth.

    Here was my setup: 1" collet with 1215 steel. Part was sticking out of the collet about 4.5". Cuts were all light, .005-.020. HSS insert cutter, brand new. There was essentially no chatter. Speeds are a guess as I don't know exactly since VFD is not displaying RPM. .0016, .004 and .008 per rev all made the same pattern. Cranking the speed way up and down made no difference in the pattern.

    I changed to a piece of 6061 and it was improved, not gone but much less visible.

    Before I put the machine in place I ran it a fair amount playing with cutting a piece of 2" D 6061 held in a 8" 3 jaw and I saw none of these patterns.

    So I put the 3 jaw on and cut the same piece of 6061 and the finish is outstanding.No rings even under 10x magnification.

    I put the steel in the 3 jaw and it too has a great finish.

    So I read a post by Forrest taking about this phenomenon with single phase motors, but that is not what I have here. Could the VFD be causing this? Is the large mass of the 3 jaw "evening it out"?

    Any thoughts? Kind of a bummer as I spent the last 18 months working on this machine!

    Paul

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    Chuck okay - collet bad? .....have you checked A, the actual dia of the steel, spring collets don't like much more than a few thou off isize.

    B, the condition of the collet? .bellmouthed, worn etc etc etc.

    C, the condition of and the seating of the collet.


    Lotta variables with collets.

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    Sami,
    Collet should be good, its a Hardinge and has been used maybe 20 times from new. All else seems good collet wise, steel measures pretty much spot on 1", but to your point it is cold rolled and not ground stock. I've got some 303 ss that is ground I can try.

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    What Sami said. His list would be the first things I'd check given your symptoms and description. If the problem is present with the collet chuck and not with the 3 jaw chuck I'd say something is out of whack with the collet setup.

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    Quote Originally Posted by sandiapaul View Post
    So I got my rebuilt lathe set up and in place and mostly leveled. I was in process of making a 2 collar test bar when I noticed an odd finish pattern on the surface. It is like a series of parallel "rings". You can feel them with your fingernail, my guess is they might be a couple or less tenths deep. Frequency of the rings are about .030, regardless of speed and feed.

    Lathe is running on a VFD(Teco 7300), 3 HP 1800 rpm motor drives a 2 speed gearbox, output of gearbox drives the spindle. Flat belts throughout, all new, scarfed and glued. Everything runs nice and smooth.

    Here was my setup: 1" collet with 1215 steel. Part was sticking out of the collet about 4.5". Cuts were all light, .005-.020. HSS insert cutter, brand new. There was essentially no chatter. Speeds are a guess as I don't know exactly since VFD is not displaying RPM. .0016, .004 and .008 per rev all made the same pattern. Cranking the speed way up and down made no difference in the pattern.

    I changed to a piece of 6061 and it was improved, not gone but much less visible.

    Before I put the machine in place I ran it a fair amount playing with cutting a piece of 2" D 6061 held in a 8" 3 jaw and I saw none of these patterns.

    So I put the 3 jaw on and cut the same piece of 6061 and the finish is outstanding.No rings even under 10x magnification.

    I put the steel in the 3 jaw and it too has a great finish.

    So I read a post by Forrest taking about this phenomenon with single phase motors, but that is not what I have here. Could the VFD be causing this? Is the large mass of the 3 jaw "evening it out"?

    Any thoughts? Kind of a bummer as I spent the last 18 months working on this machine!

    Paul
    slides are probably on the loose side and pivoting back and forth when moving and many machines get slip stick. its where pressure builds up the slide jumps a bit and moves and maybe stops til pressure builds up and it slips. basically jerky motion

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    Pre Load on spindle bearings maybe?
    The weight of the 3 jaw could be enough to eliminate the problem. Just a thought.
    Im assuming your using a center since its 4.5 inches.

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    Check for a bent lead or feed screw. Put a few test indicators on the carriage, cross slide, etc. and check for relative motion when individually pushed/pulled. You can tighten gibs to check effects, just return to optimal position before further use.

    You can also check the gear train for a missing tooth, either in the headstock or carriage. A long travel dial indicator aligned with the carriage may show a regular "jump" if this is the case.

    If none of these are the reason, perhaps a priest and holy water are required...

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    What kind of collet? 5C? 4+ stickout with a 1" od is too far for the limited grip length of a 5C, and how much material is behind the collet (is there significant mass back in the spindle bore that could be whipping around?) Your chuck jaws have a far greater grip length than any collet which will minimize pivot error during a cut, and also help stabilize any mass back inside the spindle. Your machine is fine if it works with the chuck, your collet is probably fine too.

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    Either tighten the collet more or use the three-jaw chuck for the test (it'll work identically to a collet, since it's checking headstock alignment).

    If you have some spare time after verifying the headstock alignment, set the top slide over to match the 10° 5C collet taper, mount a tenths idicator in the toolpost and verify that the taper in the lathe mandrel/spindle is not worn.

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    ever hear of dampening grease ? its comes in different viscosity and one use is camera lenses for smooth rattle free action
    .
    many lathe gears rattle pretty good and a quick spray with grease and they quite down in seconds. just saying vibration rattling if it cannot be tightened, often the right lubricant can help smooth things

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    +a zillion to Gordon Heaton.
    5C at 4x plus with no tail support? No way.

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    thanks for all the ideas guys, I have a lot of stuff to look at. I'll report back.

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    We had a lathe that gave a pattern form a bad gear..Not a Hardinge problem.
    I would check to see no vibrations from motor, pulleys, table, machine set.. also would get a simple RPM tester..that machine can run smooth enough that you might turn at a high RPM and the balance of the material can make a surface marking..Stick out 4.5" running too fast could cause a whip from balance. A machine set on a wheeled table can vibrate... as cam a table with heavy load on three feet and light load on one, the one can bounce..

    RPM tester | eBay

    Yes and all the good advice above.

    Qt [output of gearbox drives the spindle] so there are gears involved..but Flat belts should reduce that as a problem unless the gears vibrate the table.

    1215 to 6061 cutting forces would change so checking tool post for being solid.insert edge and height to center..and not likely but a hard side of the 1215..

    still the 3jaw ran better than the collect so perhaps your draw bar or the collect the problem..

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    Quote Originally Posted by Milland View Post

    If none of these are the reason, perhaps a priest and holy water are required...
    Sometimes that's very true.

    It's a hard to track problem. Some lathes are prone to it, some are immune. 35 years ago I worked for a lathe rebuilding plant and this exact problem was a major headache - they had a specific procedure to track it down.

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    I would look more towards the gear train / lead screw areas rather than the work holding areas.

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    Quote Originally Posted by Leg17 View Post
    I would look more towards the gear train / lead screw areas rather than the work holding areas.
    Did you catch the part about the 'outstanding' finish using the chuck? At 3hp, It can't be a huge lathe (with a correspondingly massive chuck) so its doubtful the 8" chuck is damping some mechanical problem to a perfect finish.

    One thing I didn't see mentioned by the OP is whether the collet was mounted directly in the spindle nose or in a collet chuck or adapter. If there is any significant protrusion past the spindle nose added to that excessive stickout, I don't think it'll ever produce an acceptable finish.

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    Don’t rule out and wobbly drawbar assembly. Your not running a pneumatic collet closet are you. I have 2 that I need to figure out. I’ve had a sneaking suspicion about our 2 pneumatic 1s for a while now

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    Quote Originally Posted by Leg17 View Post
    I would look more towards the gear train / lead screw areas rather than the work holding areas.
    Yes. It was rebuilt, so it is an older lathe. 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.

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    Quote Originally Posted by Illinoyance View Post
    . . . I bet the banding has the same pitch as the feed rack.
    Again. That same drive train is used with the chuck, yet no banding. Please help me understand why you believe the feed drive assembly is a problem with a collet and not with a chuck.

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    Try a carbide insert with a little cutting oil.
    Make sure tool is on center.Is spindle bearings bronze or tapered roller?
    Check spindle clearance using pry bar under chuck with an indicator on test piece.


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