Cant figure why lathe tool is grabbing while facing

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts
  • Croy9000
    Junior Member
    • Dec 2016
    • 18

    Cant figure why lathe tool is grabbing while facing

    I have a LMS 7x16 lathe and having an issue I cant figure out, total newb here. Often when I face 6061 alum, when the tool gets toward the center it wants to "grab" and pull down on the tool gouging the work.

    For instance today I am am facing 3" round cutting about .010". I start off the outer section at about 650 RPM, which cuts nicely, and as I move toward the center I increase the RPM. At about 1" from center I have it up to about 1800 RPM, but thats where it starts this grabbing thing.

    I suspect maybe I need to cutting faster that 1800 RPM? I can never find a calculator for facing, only turning. Is turning the same as far as speeds go? Is .010 too aggressive for a lightweight mill like mine?
  • mattthegamer463
    Senior Member
    • Jul 2016
    • 754

    #2
    Is your tool on centre or is it above the part centre line? If above, any deflection downward from cutting forces will cause the tool to dig in more, making it worse. Does the issue get better with a slower feed rate?

    EDIT sorry just realized you said facing the end.

    Maybe try some lubricant? Slowing down the feed helps me get a better finish near the centre.
    Last edited by mattthegamer463; 03-10-2017, 10:37 AM.

    Comment

    • pinstripe
      Senior Member
      • Nov 2015
      • 1633

      #3
      You will never have enough RPM to get the recommended surface speed as you approach the center. Is the surface just a bit rough, or are the gouges making rings? A photo would help.

      Comment

      • lakeside53
        Senior Member
        • Nov 2007
        • 10511

        #4
        Buildup on the tool? (Use a light oil like wd40 or whateve ron aluminum anyhow)
        Tool above center?
        Too much tool overhand?

        As explained above, SFM deceases to zero at the center so just be light-handed with the feed as you approach.

        Comment

        • BCRider
          Senior Member
          • Dec 2015
          • 11536

          #5
          How big is the diameter of the tit left when it starts to grab? I know that if my cutter is a few thou lower than dead on center that when I get down to around .050 to .030 the tit will try to grab at the cutter pull it forward and crawl up and over the cutter if there's any flex at all. I used to guess at my center height setting before and adjust as I did the first face cut. But a few months back I took the time to make a center height gauge and find that ends up being a lot faster to set the center height.

          Flex or play in all the parts can lead to the cutter being flexed down by that last little tit of metal too. Take steps to minimize your cutter overhang and be sure your compound and cross slide gybes are set correctly. On the compound I prefer a light amount of drag. On the cross slide a little less drag but there should still be a positive feel that each screw is only a tweak away from producing a fair degree of drag so you know that the whole length of the gybe strips are giving support.

          Next up is what sort of cutting tool are you using? If you're using carbide inserts a lot of them are purposely dull and require a high degree of pressure. On a smaller and more flexible machine this could be a problem. Be sure you're using a higher rake angle insert with a fairly small nose radius and a keen edge that is intended for finishing and other lighter cuts. A "light" cut for most inserts being equal to a hogging cut on a mini lathe after all.

          As mentioned you simply cannot turn the spindle fast enough near the center. It's SOP to slow down the feed rate as you near center regardless of the machine. Just watch the chips coming off and slow it down progressively through the last 1/4" of the cut and really slow down as you get down to the last .06 worth of the cut. Even so the material will be quite thick and crumbling off at the end but just try to keep it looking as normal a chip as you can.
          Chilliwack BC, Canada

          Comment

          • J Tiers
            Senior Member
            • Jan 2004
            • 44281

            #6
            What lakeside53 said. Same list I thought of.

            I would add a suggestion to check the slop in the gibs for crosslide and compound, since that can permit a slight tilt that can move the tool inward and allow it to "grab".

            Also, pull the compound back so that the toolpost is more nearly over the swivel. That will reduce flex.

            And, if you continue feeding at the same rate, you may get into trouble taking too big a "bite" for the SFM (speed of work past tool feet per second) as the cut diameter reduces. That increases force on tool, and so can cause flex.
            CNC machines only go through the motions.

            Ideas expressed may be mine, or from anyone else in the universe.
            Not responsible for clerical errors. Or those made by lay people either.
            Number formats and units may be chosen at random depending on what day it is.
            I reserve the right to use a number system with any integer base without prior notice.
            Generalizations are understood to be "often" true, but not true in every case.

            Comment

            • Yondering
              Senior Member
              • Feb 2014
              • 1077

              #7
              Originally posted by Croy9000 View Post
              At about 1" from center I have it up to about 1800 RPM, but thats where it starts this grabbing thing.
              Guys, read the post more carefully. This comment above has nothing to do with the little tit in the center, or approaching zero surface speed.

              Croy, something's definitely not right there, but it's hard to make any accurate guesses from what you've said. It sounds like you're doing the right things, and 1800 is plenty fast at 1" diameter to make nice cuts.

              Without knowing any more, it sounds like either chip welding (try some WD40 for cutting oil) or your little lathe is flexing for some reason and digging in. Post some pics of the tool and workpiece, maybe show the effect of this "digging in". Is it actually chatter? If so, slow the speed down; it's just aluminum, you can maintain constant speed for the whole cut if you want, just feed slower.
              Last edited by Yondering; 03-10-2017, 12:35 PM.

              Comment

              • BCRider
                Senior Member
                • Dec 2015
                • 11536

                #8
                Good point. Don't know why I glossed over that.

                So I'd want to read more about the setup and tool bit or insert being used. And maybe see the sort of chips coming off.
                Chilliwack BC, Canada

                Comment

                • mars-red
                  Senior Member
                  • Jan 2012
                  • 1059

                  #9
                  Originally posted by Croy9000 View Post
                  I have a LMS 7x16 lathe and having an issue I cant figure out, total newb here. Often when I face 6061 alum, when the tool gets toward the center it wants to "grab" and pull down on the tool gouging the work.

                  For instance today I am am facing 3" round cutting about .010". I start off the outer section at about 650 RPM, which cuts nicely, and as I move toward the center I increase the RPM. At about 1" from center I have it up to about 1800 RPM, but thats where it starts this grabbing thing.

                  I suspect maybe I need to cutting faster that 1800 RPM? I can never find a calculator for facing, only turning. Is turning the same as far as speeds go? Is .010 too aggressive for a lightweight mill like mine?
                  Croy, could you put up some pics, or even better a video, so we can better see the issue and your setup?
                  Max
                  http://joyofprecision.com/

                  Comment

                  • Mcgyver
                    Senior Member
                    • Mar 2005
                    • 13403

                    #10
                    Originally posted by Croy9000 View Post
                    Is .010 too aggressive for a lightweight mill like mine?
                    are you turning or milling???

                    as for speed forget about what machine it is, the formula for for speed is always the same - rpm - cs x dia / 4. Nothing fancy required, its very simple and can be done in your head as you are setting up the work. CS is the optimal cutting speed - tool going across the work. Same for drilling, milling etc. you look up CS and quickly memorize the few you need - ie steel 100 sfm, brass 200, AL 400, chrome moly 70, etc (all for hss)

                    what exactly is grabbing - tool be violently pulled into the work and stalling the machine, or a bit of scoring?

                    I suspect maybe I need to cutting faster that 1800 RPM? I can never find a calculator for facing, only turning. Is turning the same as far as speeds go?
                    going fast in AL isn't a requirement for good results in AL....just makes the job go more quickly. so you won't hurt things slowing it down. Speeding up will only improve cycle time.

                    There many things to try. When a lathe, esp light, is wound out there can be vibration that could make the things prone to move, build up tool as was mentioned and so on. Is the compound locked, cross feed gib tight, carriage locked? whats the tool geometry? whats the DOC, whats the feed?

                    Give a few more details, try the good suggestions so far and try just face the whole thing at 600
                    located in Toronto Ontario

                    Comment

                    • J Tiers
                      Senior Member
                      • Jan 2004
                      • 44281

                      #11
                      A REAL "dig-in" is just about 100% of the time due to either:

                      1) Flex or looseness that allows the tool to move in under the pressure of cutting

                      2) Feeding too fast so the tool takes too big a bite and is flexed inwards toward the work, even of it is not loose enough to have a dig-in with normal cutting pressure.


                      DO YOU HAVE THE COMPOUND LOCKED? if it is not locked, or is not stiff enough in movement (as many are) so as not to self-feed, then it may actually feed in until it digs in. For some perverse reason, self-feeding seems very often to be inward, and not outward as one might expect.
                      CNC machines only go through the motions.

                      Ideas expressed may be mine, or from anyone else in the universe.
                      Not responsible for clerical errors. Or those made by lay people either.
                      Number formats and units may be chosen at random depending on what day it is.
                      I reserve the right to use a number system with any integer base without prior notice.
                      Generalizations are understood to be "often" true, but not true in every case.

                      Comment

                      • aboard_epsilon
                        Senior Member
                        • Oct 2004
                        • 6107

                        #12
                        I would go through the whole lathe checking for slop ..gibs ..fit of tool post

                        and there is a plate that fits under the rear ways...that may have loosened off.

                        also with aluminium at that speed you may get the metal actually melting onto the tool tip and adhering to it ..use neat cutting oil suitable for aluminium, spread it out evenly over the surface ..I use Rocal..........but others use Tap Magic...others use wd40

                        Not sure if your lathe may suffer from the same weak compound slide and tool post design as the rest of them, there is a whole culture modifying them with different designs ....that weakness shows up as the tool digging in and rocking.

                        Comment

                        • BCRider
                          Senior Member
                          • Dec 2015
                          • 11536

                          #13
                          For some perverse reason, self-feeding seems very often to be inward, and not outward as one might expect.
                          Ain't that the truth....

                          To be honest though I suspect it's related to the usual and highly desirable top rake used on most cutters. It's not easy to see on some of the insert tooling because it's hidden in the chip breaker groove configuration but it's usually still there.


                          There are certainly no lack of videos on You Tube related to improving the common mini lathes. And certainly with the small surface area in the dovetails of the cross and compound slides there's not a lot of area to provide desirable drag against being pulled forward. Which makes setting the gybe screws spot on to achieve a slight drag and zero out any and all play all the more important.
                          Chilliwack BC, Canada

                          Comment

                          • Paul Alciatore
                            Senior Member
                            • May 2002
                            • 17491

                            #14
                            I would check:

                            1. The height of the tool tip. As someone stated above, if the tool's tip is above the center line of the lathe it can dig in. But, if it is below that center line, it can also dig in when it tries to go under the tit of metal that it will leave in the center of the work. It should be as close to being on center as you can make it.

                            2. Check the gib adjustment screws on the cross slide. If the cross slide gibs are loose, then the cross slide can tip sideways and it will be more prone to digging in. These gib adjustment screws should be as tight as you can make them while still allowing the cross slide to move in and out. The same thing applies to the compound slide gib adjustment screws. Both of them must be properly adjusted.

                            3. Is your tool dead sharp. And does it have the proper clearance on both the front and the side that is doing the cutting.

                            As for the cutting speed, the spindle speed, for facing I usually just set it for the outer diameter, the largest diameter of the cut and leave it there. The surface finish may change as you approach the center, but the speed does not usually lead to digging in. A good cutting oil/coolant will help here. For aluminum I usually use WD-40. It helps to prevent buildup of aluminum on the tool's cutting edge and that buildup can increase the cutting force needed which would also increase the tendency to dig in.
                            Last edited by Paul Alciatore; 03-10-2017, 03:52 PM.
                            Paul A.
                            s
                            Golden Triangle, SE Texas

                            And if you look REAL close at an analog signal,
                            You will find that it has discrete steps.

                            Comment

                            • Croy9000
                              Junior Member
                              • Dec 2016
                              • 18

                              #15
                              Thanks for the great tips. Based on this I decided to halt my project and do a teardown and adjustment of the lathe. I still need to test, but I think I found the issue. Im pretty sure its what was mentioned by many here, and Frank Hoose here https://youtu.be/spgGXLR3gs4?t=441.

                              I just got everything back together and plan to test again sometime today.

                              Comment

                              Working...
                              X