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the amazing apparently twisted lathe bed - help!

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  • the amazing apparently twisted lathe bed - help!

    almost every day for a month or so i`ve gone to my Sheldon, stuck a piece of 3/4" 12l14 steel or aluminum about 4" out and took a skim pass...
    and always get .002 taper getting smaller toward the headstock.

    so i monkey around with the levelers, use a two collar bar with identical size collars, until an indicator in the tool post reads the same on the collars.
    then take another test pass and voila!!! i get no taper!

    BUT.... then the very next day the exact same .002 taper will be back! always the same!!

    i feel like i`m stuck in a time loop and repeating the same day over and over.

    at this point i can only conclude the bed has taken a "set" and overnight keeps going back to where it was.

    any thought or help here is appreciated, i really like the machine but can`t keep doing this every day.
    is re-grinding the only answer?? twist the snot out of it in the opposite direction??
    get a big hammer?? set it on fire?? try to find someone that needs parts with .002 taper?

    start making lathe mandrels??

    i don`t know... anyone sucessfully dealt with this problem?

  • #2
    What size Sheldon?
    Is the lathe mounted to a bench, or is it on a Sheldon cabinet?
    What is the condition of the floor?


    Rex

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    • #3
      sorry... 10" Sheldon on the Sheldon cabinet, levelers on both the cabinet and the bed feet(3 points on each foot).
      floor is concrete in good condition.
      Last edited by 1200rpm; 10-20-2012, 08:34 AM.

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      • #4
        Sheet steel cabinet or cast iron cabinet? Under-drive at the headstock end? Might consider bolting the cabinet to the floor (roughly level) and shimming the bed feet. How sensitive a level are you using for setup?

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        • #5
          Sell me the lathe and let someone else deal with it?

          twist the snot out of it in the opposite direction
          , risking potential flaming...often advice is to not deliberately put a twist in but IMO, is not the primary concern of turning to get a true cylinder when you are done? And given that more than one of us have equipment that has been through who knows what simply due to age...let alone how it came out of the factory in the first place. If it needs it what other choices do you have?

          Again, just an opinion, I think you have to start from the floor and work up, sort of at each level (floor to feet, top of cabinet/stand to feet/base of lathe) and at each stage reduce the amount of degree of adjustment (e.g. if the floor is 2" out, get rid of say 1 3/4" of that at that connection, not further up the chain).
          I don't know Sheldon cabinets or later models than mine re: adjustment points of the lathe feet or bed. Is there a way of holding down as well as pushing up as it were? What I mean is mine is just bolted to a steel plate but I have a nut both above and below the attachment point both of which need to be changed when I need to adjust, both of which need to be re-tightened when adjustment is complete. Basically, for my situation, there can still be movement if a nut only holds "down" or holds "up" since there are more than enough forces to move along those studs.

          If factory cabinet, could one of the attachment points be weak? e.g. rusted through so not providing support on that corner

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          • #6
            Originally posted by 1200rpm View Post
            almost every day for a month or so i`ve gone to my Sheldon, stuck a piece of 3/4" 12l14 steel or aluminum about 4" out and took a skim pass...
            and always get .002 taper getting smaller toward the headstock.

            so i monkey around with the levelers, use a two collar bar with identical size collars, until an indicator in the tool post reads the same on the collars.
            then take another test pass and voila!!! i get no taper!

            BUT.... then the very next day the exact same .002 taper will be back! always the same!!
            Of course there is a tendency for that type taper to occur even if the machine is perfectly aligned. The pressure of the cut can let the free end bend away and result in a taper where the free end is bigger. This does not HAVE to be what is happening, but that tendency is always there, especially when the work is stuck out a tad more than 5 diameters (0.75" diameter, out 4").

            If the taper keeps coming back, then something is happening to shift the machine somehow.

            The adjustment may be not tight, so that it can settle back with vibration. Lock nuts on leveling feet, or the like, are good to have to prevent this.

            The adjustment may be pulling a base 'foot" off the floor, so that the entire base eventually settles back, re-applying a twist to the bed. You would need to check that the machine is still solidly set on all 4 feet. Small lathes don't usually have the weight necessary to settle down by themselves.

            The "feet" may have rubber under them, which means they never are solidly on the floor. Rubber is OK, but the thickness on most leveling feet is way too much. My Logan has 2" square feet with very thin rubber glued to the bottom. Might be 30 thou thick of fairly hard rubber. which doesn't compress much at all. Just enough "rubbery-ness" to keep it from sliding around.

            Floor might be shifting.... probably not if concrete, but temperature changes and moisture under a slab floor can affect it. My garage floor moves with the seasons.

            Some other cause????
            Last edited by J Tiers; 10-20-2012, 10:14 AM.
            1601

            Keep eye on ball.
            Hashim Khan

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            • #7
              Originally posted by J Tiers View Post
              Of course there is a tendency for that type taper to occur even if the machine is perfectly aligned. The pressure of the cut can let the free end bend away and result in a taper where the free end is bigger. This does not HAVE to be what is happening, but that tendency is always there, especially when the work is stuck out a tad more than 5 diameters (0.75" diameter, out 4").
              My thoughts also. While it is possible the bed has taken a set, I find it doubtful that the same set would return after biasing it with the leveling adjustments.
              Jim H.

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              • #8
                I had the same problem with a wet noodle Asian lathe until I used the tail stock on anything over 1.5" out of the chuck (stock diameter factored in).
                Len

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                • #9
                  not daily prehaps, but floors do move...concrete moves. I guess you have to start looking at what's changed, room temp, an electric heater behind it, adjacent to a window covering it in sunlight.

                  I've always consider lathe twist to mean slight changes in shape induced by its own weight vs twist to a wood worker meaning the the boards at rest shape. One could say the lathe could have a real twist to the bed, like a board might, but how's this cause it to spring back?......and if that were the case, why don't all the properly done beds just go "boing" back to the correct shape without any twist?

                  I think that's why all the Q's on base and floor....that too is where i'd be looking first.

                  I also don't get the slow reaction; a deflected piece of metal goes back to shape right away; not overnight. It suggests something the operates more slowly, ie change temp.

                  do you have a precision level, like a starrett 199? might help to start to eliminate things. without doing that first, its hard to know if the taper is tailstock or twist induced.....if it is tailstock and forcing the bed out of shape to compensate, maybe it is springing back some, especially if its just sitting there vs bolted.

                  Another idea occurs on how to isolate things; set things up on three points. try lathe on three points on the stand, then stand on three points. Might eliminate if twisting action is coming from chance in the floor or stand. This wouldn't be a safe set up for permanent use but could be done for experimentation with light cuts.
                  Last edited by Mcgyver; 10-20-2012, 10:32 AM.
                  .

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                  • #10
                    As others have suggested, the only way to find the issue is to start leveling and eliminate possible issues. I would start with the cabinet itself getting it as close as possible, then move to the bed. Once you have that, you can begin to consider wear and adjustment issues. Keep in mind that not only does the tailstock move forward and back, but also can move up and down. It can also be cocked/angled slightly in 4 directions, in which case you will find no taper with the tailstock extended at a single extended point, but taper at all others.

                    Considering your machine is likely 40+ years old, you need to be rather systematic about this, not only looking for wear, but also an inaccurate "fix" or "regrind" at some point in the past.
                    "I am, and ever will be, a white-socks, pocket-protector, nerdy engineer -- born under the second law of thermodynamics, steeped in the steam tables, in love with free-body diagrams, transformed by Laplace, and propelled by compressible flow."

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                    • #11
                      I have a 13" Sheldon and had the same problem. Turns out it wasn't the lathe at all. I did use a center in the tailstock, and after putzing around for an hour seemingly unable to get the tailstock tru I discovered that my carbide insert in the cutting tool had loosened! Couldn't see or feel it, but I turned the retainer screw half a turn and the problem was gone.
                      Southwest Utah

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                      • #12
                        Originally posted by QSIMDO View Post
                        I had the same problem with a wet noodle Asian lathe until I used the tail stock on anything over 1.5" out of the chuck (stock diameter factored in).
                        +1 on that.

                        4 times the diameter is about all you can turn with out a center in it without getting some amount of taper.
                        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

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                        • #13
                          One thing I have learned, the hard way of course, is that concrete floors are NOT flat. And the people who do concrete work do NOT even have any idea of what a thousandth of an inch is, much less work to that kind of tolerance. A concrete floor can vary by 32s or even 16s of an inch over a fairly short distance. The FIRST thing you must do for any lathe is lock it down to one and only one position on that concrete floor. If it moves, even a fraction of an inch, it WILL change it's level.

                          The next thing I would check is the size of the leveling feet. Concrete will abrade and if the feet are too small, they can wear down into the concrete from normal machine vibration. It is also a good idea to run a brick or small flat block of concrete over the area where the feet will sit to be sure there are no high points there (in other words, sand it flat). These high points can wear down from vibration and that leg will settle over time.

                          Be sure that your levelers are locked down. Sounds basic, but it is important. I like a leveler that has the lock down nut ABOVE the threaded hole that it sits in. If the nut is below, it will pull the leveler out/down when you lock it and that destroys the work you have just done to level things. If the locking nut is above, it pulls it up and since the machine weight is already pushing it in that direction, it will not move very much.

                          Oh, another point: if your concrete floor has any kind of tile or vinyl covering on it, the levelers' feet can settle into that tile or covering. It would be best to make cut outs for the feet to sit directly on the concrete. If that is not possible, I would suggest using steel plates that are at least 1/4" thick and at least 8" in diameter for the levelers to rest on. I used a strategy similar to this for a lathe in a trailer.

                          Only after you get a ROCK SOLID footing can you even start to level a lathe.
                          Paul A.
                          SE Texas

                          Make it fit.
                          You can't win and there IS a penalty for trying!

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                          • #14
                            Yeah, I'm thinking that if you are continually making the adjustments, the lathe bed is remaining fairly true but you are flexing the stand. The bed does not want to be 'twisted', and is overcoming that by forcing the pressures elsewhere- the stand, the floor, the feet- anywhere that a movement can happen, including in the floor itself. There is no guarantee that a concrete floor won't flex.
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                            • #15
                              Firstly, while I agree that foru or five times diameter sticking out is sort of acceptable, there's a vast difference in deflection between a 20 thou cut with carbide and a 2 thou shaving cut with a sharp HSS tool.

                              Secondly, if you're convincing yourself that your bed has a twist, give the bed the freedom to show you. I mean that you should support the bed at three points - one front and one back at the headstock end, and only one point at the tailstock end. Where shoul I put the support at the tailstock end, you ask. Well, perform three tests, with the support at three positions, at the front, in the middle and at the back.

                              If all three tests are identical, then your bed weighs so little that it doesn't mind if it's supported at the front or the back. I suspect any lathe would should some difference. The question is, which position begins to, or completely, removes your taper.
                              Richard - SW London, UK, EU.

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