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Unwanted grooves when turning

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  • Unwanted grooves when turning

    I get concentric, evenly spaced grooves a few thou deep when turning. I tried an experiment to find out what is causing this. The tools HSS have been ground with a small radius on the end so I reground a tool with no radius. It is dead sharp (and leaves a crappy finish) and when I turn with it the grooves go away but if I stone on the smallest radius it comes back.The grooves appear to be about 3/32 in width. It would seem that something is pulling the tool in and out of the work some of the time and as far as I know nothing has changed on the lathe.

    Any ideas.

    P.S. this only happens when turning the OD under power the ID, turned under power and facing are OK.
    The shortest distance between two points is a circle of infinite diameter.

    Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

    Southwestern Ontario. Canada

  • #2
    Build-up edge on the tool?

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    • #3
      Are you power feeding with a leadscrew or a feed rod?

      -D
      DZER

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      • #4
        I get that on some materials. It might be that due to the rake and relief angles it is in fact pulling and then being pushed out. Or it may be the metal galling on the tip of the tool then breaking away.

        Look closely in the finish. Do there seem to be little rough buildups? If there is then I'd say it is galling and you might want to play with some cutting oils. You might also want to try slightly altering the clearance or back and side rake angles for a cutter that you use with that steel only.

        If the grooves are smooth, other than a rough to the skin feel, then it may be that your cross slide and/or compound gibs are a bit loose and the slide or compound can float in and out too easily. The fix there is to snug up the gibs a little.

        Often it's only the one material though.

        If you're using a little side rake or if the metal needs some side rake consider too what happens when you round the point. Now it is cutting around the whole radius and the back and side rake of the upper face are changing as it goes around the circumference of the rounded portion. So you may want to try different rake angles for this particular type of metal alloy.

        Try some hand feeding too. I find that it's often worse if I force a hogging cut. But sometimes it's worse if I try to slow down too much. Get a feel for it with some hand fed cuts first and then match that with a feed setting.
        Last edited by BCRider; 02-17-2020, 11:32 AM.
        Chilliwack BC, Canada

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        • #5
          If I power feed using the leadscrew on the Smart & Brown it is natural to get a helical cut equal to whatever thread pitch is set. The finest thread the lathe is capable of is 76tpi which will show up more or less depending what insert tip radius is used. The lathe is capable of much finer feed rates using the feedscrew which also runs the facing direction. Using oil makes a difference, and should always be used with HSS anyway. As MattiJ says, it could be build up and breaking away of metal on the tip of the tool, it would definitely change with oil, also with different types of oil, say soluble, or tapping.
          Last edited by old mart; 02-17-2020, 01:02 PM.

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          • #6
            If it truly is evenly spaced, it must be something mechanical, and not random, like a built up edge. Wobbly lead screw or drive rod, that sort of thing.

            Since it only happens on the OD, it may be combining with a wear pattern on the carriage. The carriage may be worn in a way that allows it to twist one way but not the other. Or, the ID may simply not be easy to see the marks on, they could still be there to some degree.

            You might start by checking the carriage for looseness in the up and down direction. If found, adjust the gibs as snug as can be done without jamming at some point along the bed.
            1601

            Keep eye on ball.
            Hashim Khan

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            • #7
              Any pics? That would help.

              JL...............

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              • #8
                You say: "this only happens when turning the OD under power the ID, turned under power and facing are OK". Are you using the same cutting bit on all 3 ops? If not, it sounds like the OD tool profile needs help. If using the same bit for all ops, the areas affecting feed would seem to be the cause. I had a similar issue with my lathe, and tried several ideas that didn't help. Finally, I found that the main feed screw is held into it's coupling by a roll pin made of rolled up thin metal. I think the PO may have had a crash since the pin was distorted enough to cause slack between the shaft and the coupling. The slack caused erratic motion of the carriage, which in turn caused groves or an otherwise lousy finish. Replacing the chincy pin with a larger spring pin solved the problem.
                “I know lots of people who are educated far beyond their intelligence”

                Lewis Grizzard

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                • #9
                  What is the feed? With a really fine feed (< 0.003 say), & a less-than-rigid lathe, the tool can be held back until enough force builds up and then breaks through. It would tend to be periodic: cut a bit, then pause (groove), repeat. A larger feed will give continuous cutting. Also consistent with sharp vs. radius-ed end.

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                  • #10
                    Originally posted by Bob Engelhardt View Post
                    What is the feed? With a really fine feed (< 0.003 say), & a less-than-rigid lathe, the tool can be held back until enough force builds up and then breaks through. It would tend to be periodic: cut a bit, then pause (groove), repeat. A larger feed will give continuous cutting. Also consistent with sharp vs. radius-ed end.
                    Yes, that too. That tends to be even worse if the cutter is not sharp, and if the material being cut can work-harden. Then the cutter rubs for a while, work-hardening the material, then it breaks through and cuts, releasing the pressure so it is not enough to cut, and the cycle repeats.

                    That might look pretty regular in spacing etc, but probably would not "really" be that regular if you were to measure it.
                    1601

                    Keep eye on ball.
                    Hashim Khan

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                    • #11
                      Is your lathe single phase?

                      -D
                      DZER

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                      • #12
                        Material,diameter and spindle RPM are not included in your post, this is very important.

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                        • #13
                          The short answer is- all of the above. Adjusting the height of the cutting edge will make a difference, and I would also check for play and flex in the path from the cutting tool to the lathe bed. This means the tool fixture, the stick-out of the tool, the fixture to compound mounting, the compound to cross slide mount, the slide to carriage mount, and the carriage to bed fitment. With my lathe, the most flex was in the compound to cross slide mounting, plus the carriage would rock slightly on the bed ways.

                          As far as play, the cross slide lead screw area and the compound lead screw area are wet noodles. I made up a custom mount to go between the tool holder and the cross slide, eliminating the compound altogether. That stiffened things up quite a bit- but I still have to cater to the cross slide lead screw play. I have the gibs snugged up without being tight, and that area hasn't required much attention. If they are loose, that's one thing, but properly adjusted there hasn't been any problem requiring any constant fiddling. I use the compound only when I need to.

                          During my whole process of tightening things up I found that the saddle would rock off the rear way. That motion moves the cutting edge in and out of the work, regardless of the lead screw play. I did a spring-loaded mod to keep the saddle in full contact with the rear way, and that made a significant difference to my work. There is still a tab in there to keep the saddle from lifting off if the tension of the spring is overcome.

                          Keeping the ways lubed helps too.

                          What else- are your spindle bearings adjusted correctly? Any play there can have the workpiece jumping back and forth under cutting pressures.

                          If all this checks out, maybe you're just not holding your mouth right

                          More things- is the motor vibrating the bed? This happened with me, and it telegraphs into the workpiece. I just lived with that until the motor died, then I fixed the motor and decided to replace the bearings at the same time. The new bearings lasted about two months before they got worse than the originals. At that point I went to a treadmill motor and mounted it off the lathe entirely. That made quite a difference and gave me more options than I had before.

                          The only area now where I have to cater to any looseness is the cross slide lead screw. Where the slide wants to creep forward, just because it can, I usually apply some 'pullback' by hand for finishing cuts. This has become automatic for me, I don't even think about it anymore. If I'm boring, this becomes 'pushforward'. The looseness here can be as little as 3 thou, depending on how well and how often I adjust the play, and as large as 20 thou- which is a lot of slop.

                          There's one more thing I'd like to mention that could affect the results you're seeing. If the saddle can rock side to side at all, then it will. This would usually be due to worn slideways, and would result in the cutting tool moving left and right, independently of the feed screw or handwheel. I don't know how common this would be, but if the machine is worn it's quite likely this will happen, at least over some portion of the saddle traverse.
                          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                          • #14
                            I'd suspect a broken or wonky tooth in the power feed mechanism. It might then move the tool normally until it hit the anomaly, which would cause a momentary pause in the feed, and thus a circular (as opposed to helical) groove.
                            http://pauleschoen.com/pix/PM08_P76_P54.png
                            Paul , P S Technology, Inc. and MrTibbs
                            USA Maryland 21030

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                            • #15
                              Loose nut, photos would be very helpful. Also, some clarity on OD turning results. You say the grooves are 3/32" wide (that's a very wide groove) with a slight radius on the tool and no grooves with a sharp pointed tool which produces a bad finish. Facing and ID boring operations work fine and there have been no recent changes to the lathe. Personally I can't think of a single lathe problem that would only effect OD turning with a radius tool. Is there something missing here that we need to know to help you? A video of the operation would be great if you can post one.

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