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  • Gear rack linear accuracy?

    I haven't been able to find any info for the linear accuracy of gear rack.It's easy enough on threaded products like acme or ball thread,but I find dead ends when looking at rack.

    Any suggestions?
    I just need one more tool,just one!

  • #2
    Depends on if you get stamped, milled, or ground sections. Ground stuff is very accurate, milled is good enough for plasma cutters and router tables. Though most machines that use rack have closed loop control to keep lost motion under control.

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    • #3
      If the manufacturer doesn't quote some sort of standard, like an AGMA Q number, you don't really know. Standard "commercial rack" can be anything. An entry-level precision rack, like AGMA Q5 is (I think) about .003"/foot. Q10 is around 10 times better. You'd need the relevant AGMA standard, which I don't have handy. The standards aren't free and rather pricey. Not posted on the net. Maybe a gear handbook cites them. There's cumulative error, tooth-tooth error, center distance error, and a load of other stuff. Some people can make them accurate to tenths over 10 feet.

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      • #4
        You may be able to dig out some of the AGMA spec from www.wmberg.com

        I think this is right:

        AGMA 5 = pitch variation +/- 0.0025
        AGMA 10 = +/- 0.0004
        AGMA 14 = +/- 0.0001
        ----------
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        • #5
          Meshing qualities.

          Thanks SGW for the info. Is error on a "per inch" or "per foot" basis?

          Good question wierdscience. I checked Machinery's Handbook (27) and there is a lot of info on everything but rack.

          The fastening and support to/for the rack is paramount as any "out of line", "deflection" or poor "centre distances" will have similar effects as "backlash" and cyclic errors in and between conventional gears.

          Of course not all racks are "straight" or "spur" but many of the better or heavier loaded ones are "helical/spiral" with the "off-set angle" being equal to the helix angle of the mating gear/s.

          Cutting "straight/spur" racks using a DRO is relatively easy and quite accurate. Cutting "helical/spiral" racks is quite another matter.

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          • #6
            Thanks for the answers,rack for some reason is a black hole of information.I called the mfg of the stuff I am looking at.Their engineering dept didn't know,said they had never been asked

            I have some pieces of the same rack from two different mfgs.Both are milled,with some blue sprayed on they fit nicely together with consistent marking on each tooth over the 2' length.It should be good enough for the plasma cutter.

            Looking around anything with better than .009" per foot be it screw or belt gets expensive quick.I'm thinking the milled rack will suffice and at $56 for 6 feet the price is right.
            I just need one more tool,just one!

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            • #7
              Originally posted by wierdscience
              Looking around anything with better than .009" per foot be it screw or belt gets expensive quick.I'm thinking the milled rack will suffice and at $56 for 6 feet the price is right.
              Yep, same deal with Acme screw stock and ballscrews: the price goes up exponentially with accuracy.

              If this is for some kind of CNC, you could use a spring-loaded anti-backlash pinion gear, and then the remaining error would be mostly pitch error. If you're patient you can map the errors and compensate for them in Mach.
              "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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              • #8
                Originally posted by lazlo
                Yep, same deal with Acme screw stock and ballscrews: the price goes up exponentially with accuracy.

                If this is for some kind of CNC, you could use a spring-loaded anti-backlash pinion gear, and then the remaining error would be mostly pitch error. If you're patient you can map the errors and compensate for them in Mach.
                Error mapping will be in the future me thinks regardless of what I do.It's gonna be a 4x4' plasma cutter table.

                I have plenty experience building gantries,but not much in the electronics side of things.I've only done single axis stuff so far,mostly indexers.I have a lot of reading to do
                Last edited by wierdscience; 08-05-2008, 11:50 PM.
                I just need one more tool,just one!

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                • #9
                  If its for a plasma cutter than generic milled is all you need. Plasma cutting, at least entry level, is not very accurate.

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                  • #10
                    Originally posted by macona
                    If its for a plasma cutter than generic milled is all you need. Plasma cutting, at least entry level, is not very accurate.
                    That's about what I figured,I'll be making pad-eyes,base flanges and that sort of thing,not clock gears.
                    I just need one more tool,just one!

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                    • #11
                      Chains and sprockets.

                      WierdS.

                      Don't neglect considering chains and sprockets. They can sit in a channel/groove and can have adjusting screws at either or both ends. Some new chains and sprockets can be quite accurate.

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                      • #12
                        I've done chain and sprocket before and haven't totally rulled it out yet,#25 or 35 would be plenty.It would lower intertia since it would allow me to relocate the motor on the y axis to the machines base and it would also mean less wire tray to buy without that motor traveling.
                        I just need one more tool,just one!

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                        • #13
                          Wierd, I went looking for the same sort of info a while back on linear rack and on gears in general and did not find much. Ok, so you've got a pitch accuracy of 1 part in 10^14, big deal because what happens as you turn the mating gear and the teeth traverse thru their geometry while never losing contact.

                          It seems to be that there are numerous small errors to be had in the gears unless perhaps those errors almost disappear with very high precision (unobtainium and unaffordium) parts. Den

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                          • #14
                            Originally posted by nheng
                            It seems to be that there are numerous small errors to be had in the gears unless perhaps those errors almost disappear with very high precision (unobtainium and unaffordium) parts.
                            Normal commerical spur gears have AGMA ratings too, but I've never looked into it. If you look at the finish on a Boston or Martin gear, it sure isn't shaved or ground. So I'm guessing the gears you find in the Boston, Martin, SDPI etc have pretty loose tolerances.
                            "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                            • #15
                              Rack should be just fine

                              Gear rack is probably an order of magnitude better than that of the plasma arc. If the machine is light you will probably do just fine with soft gears and rack. Used to build 14" and larger gantry machines and we use soft rack with a hardened pinion. rack was 3/4 wide and showed no wear, but the pinion (1.5" dia.) would wear out in a year on a machine used 2 shifts a day. The hardened pinions never showed any signs of wear. We were running a 4000 lb. machine at a max of 600 in/min. ( at that speed in approx 1 foot). For a home machine - don't use the smallest pinion you can find - a slightly larger pinion will run smoother. Assuming you are using stepper motors - use a good gearbox - that will be where most of your backlash will take place, not in the pinion & rack. Spring load your pinion into the rack for 0 (zero) clearance - it doesn't need to be extreme - just enough spring preload so that the pinion doesn't bounce when you change direction on the axis.

                              Just as important is how you mount your torch. If the slide there is out of alignment it will add more location error into the process than will the accuracy of the rack. Are you going to run some kind of voltage follower for a torch height control? (100 - 125 V)

                              because plasma cutting is a non-contact process you can build fairly light, but watch for wobble in your cross beam, or run 2 motors on X, one on each end of the cross beam.

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