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  • #16
    Originally posted by oldtiffie
    I agree with what you say John, but there is another that is rarely discussed and almost certainly rarely if ever allowed or compensated for.

    That is the lateral movement or difference between a table position in the "clamped" and unclamped modes. For example, "dial in" an accurate straight edge (eg. vice fixed jaw or a 2-3-4 block) along the table/slider axis in the "free" (unclamped) state. Now clamp the slider/table clamps and see if table has moved laterally. It almost certainly will have. If it were the table "X" slide, the lateral ("Y") movement will not be picked up by the "Y" DRO but it would be picked up by a dial indicator on the side of the table with the base on the apron or "Y" slide fixed part (ie on the apron).

    The same can be replicated on the "Y" and "Z" slides - particularly on a knee-mill for "Z".

    It also applies to movement in the "X" and "Y" planes between the clamped and free conditions of a mill quill - ie between "centreing" a quill axis to say a rotary table MT bore - in the "free" state and then the "locked" state. But indicators with bases on the table and indicators in the "X"and "Y" planes on the quill will certainly pick up the movement - but the table DRO's will not.

    I seem to recall that some CNC/NC packages have a "clamp" feature but I don't know if common-use ones - such as Mach3 - have it.

    That a DRO may be able to indicate/measure accurately the relative movements between it fixed and moving parts, it does not necessary guarantee the position of a machine table or quill.

    A lathe cross and top/compound slide has the same ("dove-tail") problem where the carriage "Vee" ways do not as they are self-aligning.
    There should be a realization between theory and practice. Practically speaking, sometimes my table moves .0002 to .0005 if I twist the clamp quickly. If I twist it slowly, it won't move. On the Y axis, for one inch moved on the dials, the DRO is off by .0007. Now that could be an error in the mounting of the glass scale, or an error in the manufacture of the lead screw. I took a DTI along the length of the glass scale and made sure it was within .0005 from end to end along the table movement. Now who is to say the casing of the glass scale is square to the glass scale inside? It probably is not. The X axis has much less error.
    Now further on the practical end of things, it is PLENTY accurate to make steam locomotive parts and anything else I have dreamed up.

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    • #17
      Better machinists - or not?

      Bill,

      I agree with you as regards the practicalities of using dials and DRO's as the "problems" I mentioned either will neither show up nor affect most jobs on a fair quality machine within the limits that most will be working under - say +/- 0.001" (sometimes) to +/- 0.0005" (less often).

      It was just to point out that DRO's will not make a better machine of a better Machinist - but they will make life easier.

      If I was really fussy I'd be using slip guages and good indicators - but I'd be scratching to remember what I used that for now.

      I suspect that some may use DRO's to get around having to use basic shop math 101. Same applies to CAD.

      But having said that, I am in the process of fitting DRO's to my HF-45 vertical column mill and have to re-learn or "brush up" on my CAD.

      Comment


      • #18
        Originally posted by oldtiffie
        Bill,

        I agree with you as regards the practicalities of using dials and DRO's as the "problems" I mentioned either will neither show up nor affect most jobs on a fair quality machine within the limits that most will be working under - say +/- 0.001" (sometimes) to +/- 0.0005" (less often).

        It was just to point out that DRO's will not make a better machine of a better Machinist - but they will make life easier.

        If I was really fussy I'd be using slip guages and good indicators - but I'd be scratching to remember what I used that for now.

        I suspect that some may use DRO's to get around having to use basic shop math 101. Same applies to CAD.

        But having said that, I am in the process of fitting DRO's to my HF-45 vertical column mill and have to re-learn or "brush up" on my CAD.
        Ah yes, I've been redoing all my solidworks 2d drawing dimensions to take advantage of my new DRO it changes the way I do dimensioning.

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        • #19
          Originally posted by Allan Waterfall
          I'm in the process of fitting a new Sino DRO to my old Tom Senior mill,I did have a Shumatec fitted but got fed up with its unreliability.

          I fitted the long "X" axis first and then had a bracket set up in the mill vise to drill a 6mm hole and didn't bother to lock the table. Started drilling the hole and noticed the last number on the display was flickering as I drilled,locked the table and the display was solid.

          This amount of sensitivity to table movement suprised me to say the least.
          Think I'll be happy when the all scales are fitted and I've got an accurate read out at last.

          Allan
          Allan,

          What was that last digit?

          WHat was happening to the Shumtec that made it unreliable?

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          • #20
            Originally posted by Richard-TX
            Allan,

            What was that last digit?

            WHat was happening to the Shumtec that made it unreliable?
            It was the fourth digit and the scale was set for imperial so I'm guessing that it was the 1/10 of a thou flickering.

            I got really brassed off with the Shumatec.
            When I switched it on it usually took several unplugging and plugging back in of the power supply and swapping outputs to get all the readouts to light up and work.

            Varying between 30 minutes to several days one of the displays would freeze,a damn nuisance if you're in the middle of a job when it happens.
            If I zeroed the absolute and incremental display there was always a discrepancy switching between the two afterwards.

            TBH I've had it about four years IIRC,but it's always been a bit temprementel. I suppose like most people the price was what I wanted to pay at the time and I don't think the "Sino" scales I've got now were available.
            For a three axis setup it cost me £510.00 delivered complete with covers and loads of fixing brackets,screw etc.as against about £283.00 that I paid for the Shumatec set up.
            Everything seems very well made and the covers I got use the cover backplate to fix the scales onto.
            The Sino set up actually takes up less room that the Chinese scales that I had to use for the Shumatec.It also comes with proper test certificates.
            There is a cheaper set up available but the three axis mill set up is not available at the moment.
            .

            http://www.digital-caliper.co.uk/3_axis.php

            Allan

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            • #21
              Originally posted by Allan Waterfall
              It was the fourth digit and the scale was set for imperial so I'm guessing that it was the 1/10 of a thou flickering.


              I suppose like most people the price was what I wanted to pay at the time and I don't think the "Sino" scales I've got now were available.
              For a three axis setup it cost me £510.00 delivered complete with covers and loads of fixing brackets,screw etc.as against about £283.00 that I paid for the Shumatec set up.
              Everything seems very well made and the covers I got use the cover backplate to fix the scales onto.
              The Sino set up actually takes up less room that the Chinese scales that I had to use for the Shumatec.It also comes with proper test certificates.
              There is a cheaper set up available but the three axis mill set up is not available at the moment.
              .

              http://www.digital-caliper.co.uk/3_axis.php

              Allan

              The flickering last digit on the Sino when in Imperial mode is a bit of a pain, and the last digit is actually fairly meaningless because of the way the metric-imperial conversion is done. I still like the Sino units for the price, though had to rule out a 3-axis kit for my 'new' mill because it was too much money, at least for the moment. Maybe if someone buys my old mill I can run to it
              I've got some secondhand 2-axis gear which I could adapt, but I fancy having the third axis this time.
              The 'cheaper' 3-axis kit you refer to is out of stock 'until the exchange rate improves'.

              Regarding general use of DRO on a mill, having had one in the past it's a high priority for me to have one again soon. Counting dials is OK for simple one-axis moves, but beyond that there's always doubt creeping in. OK, some pros may be proud of their ability to get it right every time with dials & handles, but I'll be much happier when I can see the numbers on a readout again. Although I use my machines as part of my work, I'm essentially an amateur and not ashamed of using modern aids

              Tim

              Comment


              • #22
                DRO sensitivity

                Allan,
                With a Shumatech on my( err- cheap and nasty) Warco MillDrill, I was shocked to read your problems. I mean that I get flooded with Shumatech forum stuff which is way over my head. It's like the tides in Southampton- four a day. I have an x and y set up on my miserable effort and it's OK- for my modest needs.
                Sometime back, I didn't want to involve myself with the lathe which is workable but will work- but that's it. So I bought a pair of these £25 now £30 digital readouts from Uncle John who was fronting for ArcEuroTrade.
                With a pair of digi calipers with their jaws ground off, it is working and able to read in 3 decimals Imperial.

                All this begs the issue of whether we can or cannot or need or need not accuracy of 4 places of dismals. Does your Tom Senior- I know it it is a better piece of machinery than mine- actually work to 4 places of doodahs now?

                I've looked as best as I can at other more obvious mills which came out of Woolworths or wherever- and wondered- a bit.

                Summarising, I know what 4 places of decimals is- or I did. They are the things that one uses in Quality Control to set mass production limits in MASS production.

                Humm

                Regards

                Norman

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                • #23
                  A few years ago a local company doing nice work decided they needed accreditation to Bull Shït 9000 and had to remove the old 3 digit Heidenhain DRO's for the new 4 digit at great expense.

                  Actually the originals were 4 digits as they read 0, 0.0005, 0, 0.0005 etc., the new ones were genuine 0.0001" readings.

                  Next time I went in they all had a band aid stuck over the last digit as it was driving the machinists mad

                  I had a Shumatech on my lathe, not for accuracy as it is still very accurate on the dials but because it has a very fine feed screw it's easy to be a full turn out.
                  Dropping a shaft down from say 120mm to 25 you think you are close, do a quick measure only to find you are at 24.5

                  I had the same problems as Allan, dancing about all over the place, freezing etc and then not working.
                  I had 4 different scales on over a period of time, it went back for repair 3 times, in fact the last time it came back it's still in the bag.

                  I fitted just one axis glass scale and a 2 axis readout [ may put the long axis on later but it's over 40 " long ] from the same people Allan linked to, and it's been brilliant.

                  I also bought one of the out of stock 3 axis DRO's from the same guy about a year earlier to replace the ageing Heidenhain on the Bridgy as I rely on these working in metric on an imperial machine.

                  At one time you could get a reasonably cheap 'homemade' DRO but with the advent of the cheap glass scales the home shop type have actually become as expensive and if you factor in cost v accuracy then there is no contest.

                  .
                  .

                  Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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