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Lathe problem - seems to cut too much?

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  • beanbag
    replied
    Maybe the rake on your insert is TOO high, and pulls the tool and the work towards each other. If you want to use an indicator to check deflection, you need the indicator mounted directly on the tool holder, and the other end touching the actual spinning stock. Note the measurements both with the lathe on and not doing anything, and while cutting metal. Once you have this figured out, you can mount the indicator in other places and measure this deflection with respect to other deflections.

    It is also possible that you are getting "built up edge" on the tip of your carbide tool, which causes it to cut deeper. Typically, a coated, slightly positive rake insert is used for steels.

    Leave a comment:


  • Jaakko Fagerlund
    replied
    Originally posted by loply
    A few days ago I was turning a rod of stainless down to 10mm and as I approached the size I mic'ed the rod and found I had 0.32mm to take off, so I popped a dial indicator on the toolpost and wound it in 0.15mm and took a pass. Prior to this I had taken a few spring passes and the finish was even.

    I mic'ed it afterwards and it was 0.03mm undersized
    There is your major problem - taking spring passes and/or not taking proper depth of cuts to get to the diameter needed.

    Generally you turn to about 1 mm oversize, mic it, take half, mic it, take the rest and it comes spot-on. In different materials and with different tools you may be required to take smaller finishing chips than this example.

    Leave a comment:


  • saltmine
    replied
    When I first got my lathe, I assumed(you know what happens when you assume....you make an Ass out of You and Me)

    It wasn't long before I ran into the same problem, and sometimes a severe chatter when cutting 4140 steel. I switched over to carbide with no luck.
    Then, as luck would have it, I was re-adjusting the compound, one day and I noticed the carriage move. A closer look revealed that the carriage was lifting up about .020" off of the ways, and I could easily rock it back and forth. So much for factory set-ups. I cleaned everything off, and took it apart. The gibs were sloppy as all get-out, there was swarf and dirt everywhere, even the cross-feed nut was loose, and the apron allowed the drive gear .025" of slack (instead of .010") I spent all day, and part of the next adjusting everything. This included removing the bottom of the apron, clamping it in my mill and machining off .015". I also fabricated an aluminum cover for the back side of the apron to keep swarf out of the gears (yes, there must have been a whole bunch of it in there, the gears and handwheel would hardly move.) After lightly oiling and adjusting everything, the machine has been cutting quite well for over three years.
    I use both HSS and carbides depending on what I'm cutting. I've even machined stainless steel and hard chromed parts.
    Were I to do it all over again, I'd take the thing apart and clean and adjust everything before I even tried to use it.

    Leave a comment:


  • franco
    replied
    It might also be worth checking whether you actually have a metric pitch screw and not an imperial one which has been fitted with a metric dial. Stranger things have happened. Some common metric pitches are very close to imperial pitches, e.g. 2.5 mm pitch is very close to 10 TPI, but if a metric dial has been fitted to a 10 TPI screw, there will always be a small but consistent error in the indicated cross slide travel.

    franco

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  • darryl
    replied
    It's definitely worth setting up an indicator to compare with the readings on the dial. Once you know that your dial does advance the slide by the amount indicated, you can discount that as a source of the problem. If you find that it doesn't, then IMHO it would be best to do something about this before you get into too much making of parts.

    I've checked machines on the showroom floor and found some dials that were way off. On one machine, the crosslide leadscrew had a periodic error of 12 thou- how could you possibly be happy using such a machine?

    The leadscrew error can be from the threads themselves being drunken, or from the collar that positions the leadscrew in the crosslide. If the collar is off, the leadscrew will move back and forth as its rotated. This isn't too hard to fix, and the machine would be way more friendly to use. If it's the threads that are bad, get a new leadscrew under warranty if possible.

    You might want to do this same test on the compound. Align the compound parallel to the spindle axis, set up an indicator to bear against the chuck, then wind the leadscrew (and hope the dial markings coincide with the indicator readings).

    My advice is to get this out of the way first, then try to find where the problem lies. Another test that's worthwhile to do is to turn a test piece, using a sharp tool, then see how many spring passes it takes before the tool stops removing material. This might show that there's a source of play still there somewhere.

    On my lathe, I found that the rear of the carriage easily raised from the guideway. The effect of this is to back the tool away from the workpiece. If the carriage was touched down on the rear way, the tool would cut more. There is a tab at the back that prevents the carriage from lifting off too far, but unless the play there is zero there is going to be an effect on tool position. If the tab was shimmed to eliminate play, then the carriage would be hard to move in some spots. My solution was to make up a spring-loaded wheel that would run on the underside of the rear way and keep the carriage touching the rear way at all times.

    Every time I eliminate a slop somewhere, the lathe gets nicer to use-

    Leave a comment:


  • Hopefuldave
    replied
    Originally posted by saltmine
    It may just be the metal cooling off that accounts for the dimensional change. Aggressive cutting, especially with carbide cutters, can cause this.
    I've found this when cutting lots of 8" cast-iron backplates down to 6" (don't ask, wasn't me who ordered the wrong size!) - positive-rake carbide tool at high speed (300SFM!), repeated 1.5mm DOC, fine feed (3 thou"/rev)... the workpiece gets too hot to handle, and I've found leaving them about 8-9 thou" oversize puts 'em right on 6.000" once they're down to room temperature...

    The hot metal hailstorm's worth standing back from, too!

    Dave H. (t'other one)

    Leave a comment:


  • philbur
    replied
    That doesn't account for the tool nodding forward.

    Phil

    Originally posted by loply
    Re the tool nodding forwards or the work climbing, that's what I thought and is why I tried on a 32mm bar, I figured that would be too stiff to climb the tool.

    Leave a comment:


  • jugs
    replied
    Originally posted by Gravy
    In addition to the other ideas, measure the actual travel of the cross slide in comparison to the numbers on the handwheel scale. Some people have found significant mis-calibration that way. You may be getting .032" movement when the scale says .030"
    Thats why I use DRO, measure the tool movement, not the dial movement.

    Your cross slide screw maybe worn/badly made

    Leave a comment:


  • Gravy
    replied
    In addition to the other ideas, measure the actual travel of the cross slide in comparison to the numbers on the handwheel scale. Some people have found significant mis-calibration that way. You may be getting .032" movement when the scale says .030"

    Leave a comment:


  • loply
    replied
    Hi folks,

    Thanks for the very useful contributions.

    Just to answer some of the queries-

    I am using a carbide insert which is meant for aluminium, it has a very high rake angle and seems to require less cutting force than my HSS tools, I say that because it cuts more silently and leaves a better finish, I do have and use some HSS tools but the finish from these high rake carbides is a lot better, I will repeat my experiment with HSS.

    Tool is definitely on centre.

    Re the tool nodding forwards or the work climbing, that's what I thought and is why I tried on a 32mm bar, I figured that would be too stiff to climb the tool.

    No taper is present and I did try to ensure the part wasn't hot enough to shrink it's way undersize.

    I will take some of this advice onboard and have another play with it tomorrow.

    Cheers,
    Rich

    Leave a comment:


  • Alistair Hosie
    replied
    Please also remember the advance rule when requiring to remove 2mm you advance the tool 1 mm as it removes from back and front at the same time .Most metal lathes are set to take this into account .Ok I just wondered ? Alistair

    Leave a comment:


  • 90LX_Notch
    replied
    Check the alignment of the indicator. It needs to be perpindicular in both planes. When you went to take .30, you were .03 under. When you went to take .20, you were .02 under.

    Leave a comment:


  • J. R. Williams
    replied
    Cut

    Go for a HS tool and have it sharp. Carbide tools have a feed/speed cutting window for each tool. Work outside the window and you will have problems. Your problem may be the tool and not the machine.
    JRW

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  • GadgetBuilder
    replied
    I use a 7x12 for much of my work and mostly use HSS because it takes less power, something the 7x12 lacks.

    When size needs to be exact I get close to the target diameter, within 0.3mm say, and then switch to the vertical shear bit:
    http://www.gadgetbuilder.com/VerticalShearBit.html

    The vertical shear bit takes predictable small cuts (while leaving an excellent finish) so it allows sneaking up on the finished size in very small increments. I use the dial for this rather than an indicator because tiny infeeds are easier for me to judge this way (infeed varies slightly vs material), YMMV.

    Edit: Fix link
    Last edited by GadgetBuilder; 10-02-2011, 08:43 PM.

    Leave a comment:


  • saltmine
    replied
    It may just be the metal cooling off that accounts for the dimensional change. Aggressive cutting, especially with carbide cutters, can cause this.

    The other thing is possible taper cutting. The spot where the measurement takes place might be closer to the axis of the taper, and may actually be the dimension you set it to.

    I discovered a long time ago that I was always cutting things undersized, until I started "sneaking up on the final number, and then working the final dimension down with a file and emery cloth. Of course, if you're one of those CNC whiz-bang guys, hand filing and polishing with emery is out of the question (I have yet to see a G-code for hand filing or polishing)

    Being an Asian clone, I would suspect either a mis-alignment or a lack of rigidity. Even though tool bit placement can play heavily into a tool "digging in"

    Leave a comment:

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