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TCT, RDM, test bar, etc.......

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  • TCT, RDM, test bar, etc.......

    Since the other thread got deservedly closed, for un-needed nastiness on several person's parts, including mine, here is some explanation of the TCT as I understand it from my reading and use. I'll be nice if you will....... 'nuff said

    I will NOT call it "TCM", which would be the "two collars "method".....

    NEITHER the Rollie's Dad's ****** NOR the two collars ******* should really be called a "method", in my biased opinion....

    BOTH are "tests", giving "results".....

    What you DO with those test results is your own business. I do not believe it is possible to generalize on what to DO with the results, because machines vary, and what is true of one may not be true of another.

    One machine may be worn in such a way that shimming and twisting into a semblance of straightness is not possible, because the leveling surfaces are not aligned with the slideway surfaces.

    The slideways may be worn such that both front and rear slope down towards the headstock, and the front is worn more, adding a "twist", the total perhaps being as much as 15 thou or even more wear (I have seen that).

    Without a geometrical proof, my speculation is that there is no practical way to re-align by twisting and shimming to remove that problem, since there will always remain a "knuckle" in front of the headstock where the slideway starts to slope up towards the tailstock. A reasonable average may be as good as it gets....and that in turn may cause issues with the contact of carriage to slideway as it moves along the twisted bed.

    Let that be an argument for later.......

    OK.... so my point is that conflating "the test which reveals the fault" and "the solution to the fault" is where RDM and other "methods" or "systems" fall out of touch with reality.

    lets stick to the "test" part of each.....


    1) RDM test..... this is a test using an arbitrary bar mounted in an arbitrary manner to the spindle. Let's say it is chucked, but it could be in a collet, it could have a taper end, etc.

    The test is done by rotating the bar (and spindle), with bar in contact with an indicator secured to the bed (probably on the carriage), noting the high and low readings, and averaging to get a single number. This is done at say two places along the bed, and the result is to arrive at a reasonably accurate relation between the spindle axis and the bed. Usually horizontal measurements are of most interest, but the vertical is also measurable.

    Error sources include indicator repeatability, flex in the bar (they are typically but not required to be, thin), errors reading the indicator each time, roundness of the bar (affects true center) and error of bar straightness, which may force use of a less sensitive but longer "throw" indicator, plus errors in finding the true max and min readings. I am sure there are numerous others (mostly smaller) as well.

    Typically, the results of this are used to advise on shimming of the bed, or headstock adjustments to improve alignment. I don't propose to get involved with this aspect. I would advise leveling the lathe first, but many of the RDM descriptions tout the "method" and suggest that it will replace the level. I dispute that, and I have company......

    net result is that you get numbers relating to the alignment of the H/S spindle axis with the *effective* bed (including all wear)


    Test bar........... First you level the lathe. Then a precision bar with proper taper end is inserted in the spindle taper, and the indicator is traversed along it on the carriage to assess the error of carriage movement vs spindle center.

    Since things are rarely perfect, a variety of the same averaging routine may need to be done to arrive at a proper assessment. This becomes the same thing as RDM, only the bar costs more.

    Naturally, if you are the manufacturer, things are different......The socket is being tested as much as the alignment.... wobble out of spec means the spindle socket is bad, and needs re-working. But for the rest of us, it is RDM with expensive parts.


    TCM (Forrest's)...... This *new innovation* is essentially the same thing as having a test bar, except that you can MAKE the bar, the key being identical diameters of two (or three etc) collars which you turn to suit, after which you use the bar as-is. You would naturally level the machine first.

    As with the test bar and RDM, averaging may be required. Again, if so, same thing as RDM, but more trouble to make the bar.....


    TCT......... In this test, you again hold a bar (or tubular piece) in a chuck or any other convenient holder on the spindle. (See any machining text for a description of this test, BTW)

    The first time of use, you turn it to a "dogbone" shape with two (or 3) collars on it as with Forrest's test. However, instead of checking with indicators, you actually take a slow, fine, cut, with power feed, across both collars, using a tool (and bar material) that will give best finish possible. When you have completed the pass, and a full cut was taken on both collars (no "winking", or spots not cut), you measure the diameters with your best mic.

    If they are the same, you win... your machine turns spot on over that distance, and the H/S is aligned with the slideways sufficiently to do that.

    if the T/S end is larger, either the H/S is aimed towards the back, away from the tool path, OR the bar flexed, OR there is a gross vertical misalignment, OR there is a bed wear issue. You can check the vertical alinement, and you can use a large not-very-flexible bar or tube, which leaves the aiming, or a bedwear issue.

    if the H/S end is larger, the H/S is aimed towards the FRONT, or else one of the other issues is at fault, with the same remedies.

    Error sources include accuracy of turning (at least partly a feature of the machine), surface finish of the cut, and inconsistency of the mic or the user's technique.

    These can be controlled to some degree, plus they will also affect your accurate turning results, so they are pretty much a 'fact" of the machine..... What you get is what you will get in using the machine..... complete with all it's "assignable causes of variation" and limits of accuracy. It is an "honest test".

    However, there is only ONE measurement at each point, not two..... half the chances for errors, easier to be careful. And, the check is a comparison..... no absolute measurement, and the mic only needs to have the accuracy of resolution plus repeatability, doesn't even need to be calibrated.

    This is the classic "two collars test" as described in many machining texts.

    A short test bar (tube) chucking end at left. As you may notice, it's been used some, the collars are down a bit.........

    Last edited by J Tiers; 05-30-2012, 01:03 AM.

    Keep eye on ball.
    Hashim Khan

  • #2
    For reference, my position on RDM is copied below. From what you've written we may agree on more things than either of us realized. Separating the measurement method from the correction method is a major step because Wasser's inclusion of it confused the issue immensely.

    Clearly, there are differences remaining so I'll make a subsequent post with questions so you can clarify areas I don't understand.



    My view is all three headstock/bed alignment methods are in essence the same: if you have a precision test bar then you use it. If you don't have a precision test bar, you simulate it. With TCT you turn two accurate test areas on a bar and use them to simulate the precision test bar. With RDM you remove the error sources using arithmetic thus simulating a precision test bar. However it's done, the same information must be extracted from the test used. The goal remains: measure the deviation between spindle axis and bed ways.

    I consider John Wasser's paper on RDM to be a concept paper rather than a test plan ready for execution. The paper identifies error sources that make chucking a generic test bar differ from measuring a precision test bar fitted in the headstock taper. And he indicates how these errors could be compensated for mathematically. Finally, he describes how the results might be used to improve the situation.

    In reducing the concept to practice, one need not (in my view should not) simply follow what he wrote. I believe the reason the error sources are identified is so the user can figure a way to minimize them. For example, providing a method to allow using a rod whose diameter isn't constant and removing the difference mathematically would be plain silly - I think it is included so people understand that rod diameter is critical to the measurement. Using an accurately ground rod with a major bend in it would be silly too. But, if the rod used has a slight bend the calculation should prevent it from affecting the result, where his logic on this seems reasonable.

    The test bar will have some runout from chucking which is unavoidable. Runout at the far end can be minimized easily by snugging the chuck in increments, measuring the high point and pushing it down with your thumb, tightening more, etc. It's easy to get TIR within a thou at the far end. This makes it more closely simulate the perfect test bar we wish we had, making the mathematical corrections needed smaller. Then, set the indicator up for horizontal or vertical measurement and set indicator zero so the reading for runout at the chuck end goes equally above and below zero, making John's Near End Average = 0 so it can be ignored in the calculation. Now, the RDM calculation is reduced to adding the high and low readings at the far end together and dividing by 2. (This result is analogous to the TCT result except with TCT the result reads directly off the indicator since runout has been removed by machining.)

    A detail John's paper ignored is test bar sag. All test bars sag, even precision test bars. A larger diameter test bar sags less. In all of the methods sag should be accounted for. Measuring the actual sag isn't easy so generally it is calculated and then used to adjust the vertical result.

    John Wasser's paper suggests shimming the feet of the lathe to correct bed twist. He didn't say to check your common sense or experience at the door. It is reasonable to use whaterver means are available to ensure the bed isn't twisted before using any spindle/bed alignment method. Nor does his paper consider the effect of bed wear on the measurement so common sense and experience are needed there too.

    Does RDM provide perfect results? Certainly not - although it looks like you're taking only two measurements per axis, setting indicator zero at the chuck end is really 2 more readings that enter into the calculation invisibly. So now there are 4 readings, each with a little error and if the errors don't cancel then it would be easy to be off half a thou. The ground test bar could have a diameter difference between the measurement points of a tenth since I can't measure better than that. The calculated sag in the test bar gets added in too and it's a calculated value which could be affected by the type of steel in the test bar. And so it goes -there is some noise in the result. It's random so sometimes it cancels and sometimes it adds. The question is, is the error in RDM greater than in other methods. And does it matter in a home shop - that is, there are some real world limitations imposed by Mother Nature when you try to use the results to adjust a lathe.
    Location: Newtown, CT USA


    • #3
      Originally posted by GadgetBuilder
      From what you've written we may agree on more things than either of us realized.
      I've found that's often the case Jan
      "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."


      • #4
        Or as is often (sadly) the case "we agree on more than we'd like to admit"


        • #5
          In reading what you've written, you agree that the measurement method and the use of the results are separate issues and what we're considering is simply the measurement aspect.

          Further, you seem agree that all three home shop methods: TCT, TCM, and RDM can use whatever size test bar is convenient so sag is equivalent for all.

          I am impressed that you said the test bar method and TCM are really RDM in disguise - I speculated that this was the case but wasn't sure I could defend that so I didn't raise that issue.

          The major issue seems to be consideration of measurement noise and how it affects the calculated error value in the real world.

          If there are other areas where we disagree it would be helpful if you could identify them at this point.


          Edit: change "measured value" to "calculated error value"
          Last edited by GadgetBuilder; 05-30-2012, 05:16 PM.
          Location: Newtown, CT USA


          • #6
            cutting the barbell works for me. i have a short and sturdy lathe.

            however, i can see problems with it. firstly cutting pressure. i suspect it will always be larger than the force of an indicator. thus, the lenghth you can check is limited. my largest micrometer is 75 mm, so i am pretty much limited to 300 mm. yes, i could chuck up a 150 mm ally pipe and have it stick out 600 mm, but i would have to get the pipe and the micrometer first.

            secondly it depends on what you want to do. if you want to make sure, your chucked work comes out straight, its fine. when taking off material, you will have a much smaller stickout. but lets suppose you have a long bed and want to make an assesment on its wear. you want to check it in 10 locations over 1500 mm lets say. then i agree, that the "measurement approaches" might work better.

            has anybody tried to elliminate sag by supporting the bar by a spring or piece of rubber?

            test bars: my understanding is, that "test bars" are ground to a tolerance of 0.005 mm, whatever that exactly means. i also understand, a mt3/300 mm bar will set me back 300$. ( i would need a mt5, dont even want to think about cost.) also it will be only as good as the spindle taper. straight bars are out of the question, because of the runout of the chuck or collet. btw, i have four test bars, one even has a certificate and none of them are true, especially since i have a miutoyo 0.001.

            idea: hard turn the jaws (or get soft jaws) for the diameter of the test bar.

            gadget, the sag will not depent on the type of steel, i believe it will even not be substantially be influenced if you use steel or ally, as the weight is roughly proportional to youngs modulus.
            Last edited by dian; 05-31-2012, 04:45 AM.


            • #7
              Sag wouldn't generally be a factor unless you're testing over a great length. For instance, a 1" diameter aluminum bar 12" long would sag 4.5e-7 inches at the unsupported end. The same size steel bar would be 1.6e-7 inches. I would just be careful of how large you make the "dumbell". It shouldn't be much larger than the minor diameter. If you wanted to go extreme I suppose you could press fit aluminum dumbells onto a steel shaft.


              • #8
                Originally posted by dian
                gadget, the sag will not depent on the type of steel, i believe it will even not be substantially be influenced if you use steel or ally, as the weight is roughly proportional to youngs modulus.
                +1. In Harry Beckley's outstanding "Wreck" rebuild, he calculates the sag for his TCT test bar. It wasn't insignificant.

                Connelly, IIRC, recommends a hollow tube, which will have less sag. A tapered tube would be even better, if you can calculate the moment correctly.
                "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."


                • #9
                  Figuring the sag starts at post #126 in the "Another New Toy" topic on PM.
                  I had some help with the numbers.
                  Last edited by beckley23; 05-30-2012, 06:25 PM.


                  • #10
                    To clarify....

                    Test bar, TCM, RDM are, or may be, essentially similar. TCT (distinct from TCM) is different.

                    Actually, the test bar is also different. I say this because the test bar, as commonly used, has the spindle taper on it, and is always used placed into the spindle.

                    Therefore, it is really testing the accuracy of the spindle taper, in terms of centering, as well as concentricity.... i.e. absolute overall off-center, AND "off center aim".
                    The bar may be perfectly parallel to the rotational center, but offset from it, so the whole bar is moving identically off-center. The bar may also be set at an angle to the rotational center, such that the far end is "out" more than the near end.

                    The test bar can*also* be used to check aim, in which case your errors mean you must treat it as the bar in RDM is treated, averaging errors.

                    Now, RDM is no new innovation in measurement..... In truth, while I am separating the *test* from the *correction method*, this is NOT the standard case with RDM....they are presented as a package, and touted as a "method" for alignment needing no level. However, the *measurement* part of RDM uses standard methods which pre-date the emergence of RDM itself.

                    Therefore it is more correct to suggest that RDM uses the same techniques as the test bar would for very precise measuring, rather than suggesting that the bar test is based on RDM.

                    The key issue in TCT vs TCM (Forrest's method) is that you have no standard "test bar", you use the bar only to test the alignment by actually making a cut. In contrast, Forrest suggests making the bar, and then using it as a standard, which simply shifts it over to being a "test bar" type measurement.


                    before discussing ANY of the issues raised, note that one is not (at least I am not) trying to make a full-bed survey with the test. The most precise printed guarantee I have seen given for a lathe is the 0.0001" in six inches given in the specifications for the Rivett 608. Note that is a "turning" guarantee, i.e. making a cut, not using an indicator on a test bar.

                    One presumes that this is primarily for the first six inches away from the headstock, although I suppose nothing precludes it from being for ANY six inch length along the bed.... So presumably, this was tested with a cut as per TCT, with the test bar being about 6 inches long outside the spindle. Assuming the bed is straight, that simply comes down in the first place to the sum of the "aiming error" PLUS the bed straightness error being no more than 0.0001" over 6 inches.

                    Don't try to make the test more than it is, or can be.

                    So deflection and sag were raised as problems.......


                    Deflection of the test bar is handled by making it bigger in diameter ....and using a very fine low-pressure cut, so that it does not deflect enough to be a problem. Considering the construction of the machine, this should be possible to reduce to an amount less than exists elsewhere in the structure, by upsizing bar diameter.

                    Deflections in parts OTHER than the bar itself are *fair game*, a part of the machine, and form the inevitable "noise floor" that stops you from getting more precise in your measurements, OR your work, with that machine.

                    Trying to exceed the noise floor is not productive...... which is why RDM manages to work out decently... most machines it is used on have problems and issues that raise the noise floor enough to swamp much of the measurement error sources in RDM.

                    ***Sag in the bar......

                    This is again handled by a larger bar, so that it sags very little. Also the larger bar has less diameter difference resulting from a constant sag, because the periphery is closer to being a vertical plane at the cutter, the larger the bar diameter is. The diameter difference per thou of up or down error is vanishingly low.

                    While sag is not irrelevant, it is, with a large bar and a shorter distance, a secondary effect. If you want to go out a half meter with an unsupported bar, well, you will need to deal with it, but that is extreme. I don't suppose any cutting-type test is useful with that amount of stickout on any lathe most of us will have or want.

                    You will need to use a test bar of some sort to do that type extreme length test..... and you should expect to deal with a larger absolute error than over a shorter distance. An error of 0.001" at 20" is, if it is smooth, the same as an error of 0.00025" over 5", and is nothing to sneeze at.
                    Last edited by J Tiers; 05-30-2012, 11:44 PM.

                    Keep eye on ball.
                    Hashim Khan


                    • #11
                      forest, can you explain your calculations? i wonder, because last time i had a look at the formula, i saw density in the first power above the line and the modulus in the first power below the line. since aluminum has a modulus and density that are roughly a third of the value for steel, i figured deflection would be similar for both.


                      • #12
                        Originally posted by beckley23
                        Figuring the sag starts at post #126 in the "Another New Toy" topic on PM.
                        I had some help with the numbers.
                        I get 4.9e-7 inches in the case you referred to. Quite a bit different than .0007". I'm using ml^2/3 for the moment and ml^3/(8EI) for the deflection. Specified rod was 1018 1.34"x15".


                        • #13
                          Originally posted by J Tiers
                          ***Sag in the bar......

                          This is again handled by a larger bar, so that it sags very little.
                          A larger bar will sag more. The two things you can do to reduce sag is to reduce the length/diameter ratio, but then you're losing resolution on the two collars test, or use a hollow test bar, with a tapered hollow having the least sag of all.

                          Connelly has a whole chapter on making and measuring test bars in Chapter 15, starting on page 109. It's a really good read, in a very dry book
                          "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."


                          • #14
                            Originally posted by Forestgnome
                            I get 4.9e-7 inches in the case you referred to. Quite a bit different than .0007". I'm using ml^2/3 for the moment and ml^3/(8EI) for the deflection. Specified rod was 1018 1.34"x15".
                            I'm getting 5 tenths -- what DaveE got. Are you using PiD^4/64 for the circular moment?
                            I'm using the beam calculation from Machinery's Handbook...
                            "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."


                            • #15
                              Originally posted by lazlo
                              I'm getting 5 tenths -- what DaveE got. Are you using PiD^4/64 for the circular moment?
                              I'm using the beam calculation from Machinery's Handbook...
                              I could certainly be using the formulas wrong, but I'm using ml^2/3 for the moment. That's the moment for a slender rod. I'm using the deflection formula for a uniformly distributed load on a cantilevered beam.