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DROs cause good lead screws to go bad

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  • DROs cause good lead screws to go bad

    Nothing you can say will change my mind.

    Thanks to the Christmas break, I finally got around to a permanent install of my lathe DRO that has been waiting 2 years and in the meantime, has been half screwed, half clamped and otherwise half-a$$ed mounted. The longitudinal readout has been used quite a bit but the cross slide has been waiting for a more permanent mount.

    Until yesterday, the micrometer dial on the cross slide would say zero and I KNEW it was zero. No matter how many times I cranked the slide in or out, when the dial said zero I knew it was an EXACT multiple of 0.200.

    I can still start with the dial at zero but now when I crank it in or out until the DRO reads a multiple of 0.2000", the dial no longer says zero

    This proves that DROs cause otherwise good lead screws to develop errors ... in this case overnight

    On a more serious note, the good news is that with the proper install, my tests of OD turning are yielding results within one or two tenths of an arbitrary target OD on several steel and aluminum samples from 0.1250" to 1.5000", a few extra tenths on plastics probably due to the material recovery and improper tool angles. To get this accuracy, the gibs had to be adjusted from what I thought was a good setting. They have now been set for 0.0005" dry clearance when side pressure is applied. Without this, an error is introduced between the tool location and the scale reader head location.

    Den
    Last edited by nheng; 12-28-2009, 09:12 PM.

  • #2
    Den, they ruined my lead screw too and that was on a new lathe!

    I made a Heath Robinson DRO installation using tensioned flexible cables to move the scales and had low expectation of its accuracy but I was suprised at how the errors showed so I put a DTI on the slide and found the cable operated DROs agreed with the DTI.

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    • #3
      Ditto, went thru the same thing yesterday with a Starrett 1" range indicator. The lathe is a 1976 Harrison M300 with light wear but if you start scrutinizing the tenths digit, the wear shows up quite nicely.

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      • #4
        All measurement are in error, the trick is to know by how much, and then that is also wrong by some smaller amount, etc., etc.. If you do not detect any error in your measurement, your measuring device is just not sensitive enough to show it. Sometimes it is better to just not know! ;>)

        I once had a friend who had no electrical problems with his car until he put an ammeter and voltmeter on it. After that, it was "never working right".
        Don Young

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        • #5
          I guess I need to go to the shop and see how much the DRO has worn the screw on my mill.
          It's only ink and paper

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          • #6
            Pity the man with two watches. He never knows what time it really is.
            Lynn (Huntsville, AL)

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            • #7
              In keeping with lynnl's post, precisely how would you know if the error is in the lead screw or in the DRO. I know DROs are modern and flashy and electronic, but that does not necessairly mean they are more accurate than the lead screw.
              Paul A.
              SE Texas

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

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              • #8
                Paul, in my rig I have two DRO scales that are operated by taut cables and I tested them by mounting a DTI on the cross slide and comparing movement relative to a piece in the chuck. The DTI compared quite closely with the DRO readings.

                I have opened a topic on what I did.
                Last edited by The Artful Bodger; 12-29-2009, 03:46 AM.

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                • #9
                  And when you've finished pratting about with the readouts, how many actually trust implicitly what the DROs are telling you?? So you NEVER check with an off machine measuring system???


                  Regards Ian.
                  You might not like what I say,but that doesn't mean I'm wrong.

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                  • #10
                    This one is going to get a quick check with a tenths or micron dti and gage blocks sometime soon.

                    But for now, any diameter I targeted within a pretty wide range was within a few tenths. The test diameters were all turned with one or more roughing passes and a 0.002" final pass. I'm happy

                    The actual parts always get check with a mic both on and off the machine. With luck, the part is either on target or oversized

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                    • #11
                      my DRO has worn my X traverse screw on my mill. In 10" of travel with the hand crank the DRO showed 10.0016 of travel.

                      I think I will check the entire length of travel next.
                      It's only ink and paper

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                      • #12
                        Originally posted by nheng
                        This one is going to get a quick check with a tenths or micron dti and gage blocks sometime soon.

                        But for now, any diameter I targeted within a pretty wide range was within a few tenths. The test diameters were all turned with one or more roughing passes and a 0.002" final pass. I'm happy

                        The actual parts always get check with a mic both on and off the machine. With luck, the part is either on target or oversized
                        You never heard of plastic deformation, sheering force, rebound, or deflection? Then there's edge wear, and thermal expansion to go with those, and the machine being cold or warm... It's endless the "errors" induced upon your beloved electric display device.

                        Your readout only tells you where your slide is in relation to the head stock. You still need to be a machinist to know how the metal is going to respond to your attempt to cut it and how the machine will react to your implemented plan.

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                        • #13
                          Originally posted by Jim Shaper
                          You never heard of plastic deformation, sheering force, rebound, or deflection? Then there's edge wear, and thermal expansion to go with those, and the machine being cold or warm...
                          You are absolutely correct, Jim. The DRO is only a readout and you still need to know what you are doing. All of these factors and more are affected by your techniques, the material characteristics, how hard you are pushing the machine, quantity being produced, etc.

                          In many ways a DRO could be compared to CNC. With competent setup, the CNC has correction factors applied for all of the items you've mentioned. Applying good metrology to the parts coming off the CNC, at least on a sampled basis, insure that they meet specs.

                          Using a DRO without machinist skills would be akin to operating a CNC without it having been first set up by a CNC machinist.

                          Den

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                          • #14
                            I got curious yesterday and ran 20" of X travel on the dial and the DRO displayed 20.0006". The shop was about 55-60 deg F. All it tells me is my X feed screw is pretty accurate assuming the DRO is accurate.
                            It's only ink and paper

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                            • #15
                              Originally posted by nheng
                              You are absolutely correct, Jim. The DRO is only a readout and you still need to know what you are doing. All of these factors and more are affected by your techniques, the material characteristics, how hard you are pushing the machine, quantity being produced, etc.

                              In many ways a DRO could be compared to CNC. With competent setup, the CNC has correction factors applied for all of the items you've mentioned. Applying good metrology to the parts coming off the CNC, at least on a sampled basis, insure that they meet specs.

                              Using a DRO without machinist skills would be akin to operating a CNC without it having been first set up by a CNC machinist.

                              Den
                              I was just trying to point out that what the dial on the hand wheel says, and what the number on the DRO says are only a fragment of getting the part to come out that size.

                              When I set up a tool on my DRO, I make a cut at the depth and feed rate I intend to do my finish pass at. That tells me what the tool/machine/work is producing under those circumstances. It will change things if you hogged a bunch of material off first - it can even be a lot like .003" or more depending on the material. Even so, I still usually end up compensating with regards to the mic readings off the actual part.

                              I don't have generic presets programmed into the DRO for all my tools. None of those have offset adjustments (like cnc) for wear and such, so I just make it a point to do a test cut every time - just like I was setting up a new job on a cnc.

                              I'd trust my scales more than my machine dials. But there again, temperature differences between the two will likely cause errors. Neither is important, it's how well you can use the tool to meet your tolerances.

                              If you want to hold .0002, you're using the wrong machine anyway.

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