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  • Lathe test bars

    I need to "level" (yes I know it doesn't actually need to be level) my lathe again. The preferred method is to put a bar in the chuck and turn the outboard end and then a bit near the chuck and adjust the feet until it turns parallel. The size of bar usually quoted is 1" in diameter but it is also suppose to be extended about 1' from the chuck without any tailstock support. With that amount of overhang it seems like there will be a lot of chatter and the bar will spring which makes turning a bitch.

    Is this right or wrong and if so what is a better way and don't say get a "master precision level" and do it that way, my pockets don't run that deep.
    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
    Supposed to be done with tailstock support. You gotta dial in your tailstock first (with the tailstock center against the headstock center, or as short as you can make it). This gives you the case where the tailstock is coaxial with the headstock at that point only. Then with the bar between centers (and the tailstock some significant percentage of the length of the bed away from the headstock), you turn your gauge rings. This gives you the general case to test - the difference in axial location between 'nil' gauge length between headstock and tailstock and 'large' gauge length is the twist of the bed.

    The difference in diameter of the gauge rings should be caused by the offset of the tailstock from the headstock, caused by the twist of the bed; the 'handedness' of the difference tells you which direction the bed is twisted.

    You then adjust the 'level' of the bed, recut your gauge rings, and measure. Repeat ad nauseum until the gauge rings turn the same diameter.

    When the rings turn the same diameter, the 'twist' has been taken out of the bed, and the tailstock center should be coaxial with the headstock.

    Recheck the initial case (tailstock center against headstock center); if it still holds, you're done. The tailstock can then be moved to any point along the bed and will remain in the same axial relationship to the headstock. The lathe is 'levelled'.

    Edit: The first test is done between centers because it mostly eliminates errors in headstock and tailstock axis with the bed. You can then do that test again with a shorter bar, unsupported, to test how axial the spindle axis is with the new levelled bed. And around and around in circles you go until you've reached a level of axial parallelism you're happy with. I normally go for a thou per foot or better.
    Last edited by Sun God; 03-05-2017, 10:08 AM.

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    • #3
      Keith Rucker just did his lathe; https://www.youtube.com/watch?v=5bWpgElMm8c
      Len

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      • #4
        The test needs to be done without the tail stock support; the tail stock comes in AFTER the head is aligned to the bed. Use an aluminum tube as big as you can measure and grab well, (weight sags), super light cuts with sharp tooling. Remember in the real world your chuck jaws can cause misalignment (ok, so once the part turned it goes away, but...); using the spindle MT as a cross check or even your initial check is a good idea. You can heat shrink an AL tube onto an MT stub.

        Tons of long arguments over on PM about the best way to do this. If all you are concerned about is 1 foot from the chuck, you can fake it, but for that go for a few 10ths, not thou. If you want 60 inch... yikes.. not really practical with bar and mics. IMO... you need to level the lathe or you have no real reference. Level the lathe, then test cuts to confirm whether other items need adjustment. Adjusting the base to compensate for a misaligned tail or head stock is backwards. I have real levels, and it's amazing how much a 5000lb lathe bed follows the floor, and how the 8 jack screws can change things with the smallest adjustment.

        I have an $80 level, and a $zillion level, the latter I got from an auction for $300. I use the $80 level most of the time to stay sane. The other stays pristine in its beautiful wood box, just as it did with the prior owner
        Last edited by lakeside53; 03-05-2017, 11:39 AM.

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        • #5
          Originally posted by lakeside53 View Post
          ... IMO... you need to level the lathe or you have no real reference. ....
          +10

          If the machine is not at least roughly aligned, you have no idea where you are starting. If you CAN align the head, and on most common US machines this can only be done by scraping, then you must level first to be sure the bed is straight, in order to have some consistent reference for alignment.

          There are endless arguments about "RDM" as a substitute for a level.... but he level is needed to give an outside reference. It can be a simple medium level, like a Starrett 98, no need for a "master precision" as a prelude to removing twist with "two collars" test. In fact a sensitive level will drive you crazy if there is much error.
          1601

          Keep eye on ball.
          Hashim Khan

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          • #6
            I do not even have a real level.

            My " precision " level is a ww 2 inclinometer, presumably from a large gun. I was taught that once the bed is levelled as best as possible, to take a piece about 1" diameter, stick it out about 4 " and without tailstock support, light cuts and sharp, very slightly radiused hss tools see what the results are. If satisfactory then to move on to the tailstock alignment with longer pieces. I got within two tenths in 4" on my Colchester Bantam, and now my Southbend 9" gets within half a thous in 4" I seldom have to turn all along pieces longer than 3" or so nowadays, and I am my own inspector just building steam engines . Regards David Powell.

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            • #7
              The master precision level I bought 30 years ago on ebay (?) is graduated .0002" per 10", I think I paid $80 or something like that for it. It's been a good level, but looking at cheap master levels in ebay recently I didn't see any of them that were that sensitive, more like .0005" per 12". If I were setting up a lathe I'd find a way to get access to one. Beg borrow or rent.

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              • #8
                I leveled my lathe using this machinist level from Shars: http://www.shars.com/8-x-1-9-16-x-8-...-spirit-levels. After leveling, I machined a piece of steel and measured 8 microns per 300mm (roughly half a thou per 18"). I am now a fan of that machinist level, particularly considering the 80 bucks price tag.

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                • #9
                  I don't know where you heard that the bar needs to be only 1". It is better if it's up around 2" diameter for all the reasons you are worried about. The ONLY downside to this is the need for a 1-2" micrometer instead of a 0-1".

                  It's well worth hogging off .04 to 06 through the middle and leave just a collar at the end so you can traverse quickly to the collar by the chuck without touching the part between.

                  For just getting into the ballpark even a builder's level will be close enough that the lathe doesn't rock corner to corner.
                  Chilliwack BC, Canada

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                  • #10
                    Most of the old books and newer videos use a 1" bar. I can use 2" no problem.
                    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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                    • #11
                      Originally posted by partsproduction View Post
                      The master precision level I bought 30 years ago on ebay (?)...
                      Nope, eBay didn't exist 30 years ago. The World Wide Web wasn't even around. The technology is so pervasive that it feels like it's been around forever. I was discussing this with my wife today. I've got a computer science degree, but I never had an email address at university.

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                      • #12
                        Originally posted by loose nut View Post
                        Most of the old books and newer videos use a 1" bar. I can use 2" no problem.

                        At 12 inches there will be sag.. all depends on what "accuracy" you are aiming for. Try a 2 inch aluminum tube - less sag, and lighter tooling pressures "pushing" at the end point. As a test case, mount up something with 12 inches of overhand and measure deflection when pushing/pulling by hand. Tooling pressures are way higher.
                        Last edited by lakeside53; 03-06-2017, 12:59 PM.

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                        • #13
                          Sag will have nearly NO effect on a larger diameter workpiece, due to the geometry. So go large.

                          It may also have less sag to start with, of course.
                          1601

                          Keep eye on ball.
                          Hashim Khan

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                          • #14
                            Originally posted by J Tiers View Post
                            Sag will have nearly NO effect on a larger diameter workpiece, due to the geometry. So go large.

                            It may also have less sag to start with, of course.
                            That's true. But a 1" bar sticking out a long way will certainly flex enough to make a big difference. That's why I used a large diameter round bar when I did my own test cuts. As pointed out above even with a large bar we are able to pull on a 12" long piece enough to easily show on a dial gauge. The trick is to reduce the flex that occurs to something below an amount that can be a source of significant error. And the easy way to do that is to make the diameter larger.

                            If using pipe for this you'll want to use a piece with a thicker than normal wall. Otherwise the jaws will distort the pipe. And even with a rounding cut the change in wall thickness can cause a release of the pressure and reverse clamping distortion. So if you have your druthers it should be solid round bar. And the larger you can go the better. Whateevr you have micrometers that can fit.
                            Chilliwack BC, Canada

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                            • #15
                              My mics go to 9" so 2 or 3" bar isn't a problem.

                              I was always told that unsupported projection of a bar was 4 x the diameter. so 4" for 1", 8" for 2" etc. I suppose 8" can be stretched to 12" for this.
                              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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