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Thread: Magnetic vs glass DRO scales

  1. #1
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    Default Magnetic vs glass DRO scales

    Hi guys:

    I'm tired of trying to keep track of handle rotations and am going to get a 2 axis DRO for my mill.
    I've read a bit about magnetic scales and am leaning in that direction.

    What I've gathered:
    Magnetic are:
    Immune to coolant, swarf etc.
    Very durable.
    Easily cut to length.
    Thinner, so easier to mount.

    Glass are:
    More accurate if clean but .0002 seems enough for me.

    What would happen to a magnetic scale if a strong magnet got near it?

    Any suggestions?
    Mike

    My Dad always said, "If you want people to do things for you on the farm, you have to buy a machine they can sit on that does most of the work."

  2. #2
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    Default

    "If clean"...well, you really have to try hard to get a properly mounted glass scale dirty enough so it won't function.

    No comments on magnetic scales as I haven't ever even seen a picture of them.

  3. #3
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    Default

    Quote Originally Posted by MotorradMike
    Hi guys:

    What would happen to a magnetic scale if a strong magnet got near it?

    Any suggestions?
    Just tried it on a piece of magnetic scale, with a couple of neodymium magnets.

    Magnetism is weaker, but maybe a neodynium magnet goes beyond the definition of "strong magnet".

  4. #4
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    When you read the specs, be careful not to confuse the accuracy implied by the number of decimal places displayed by the readout with the actual accuracy of the DRO. When I researched DROs a few years ago I found some that displayed to +/-0.0002" but could repeat only to +/-0.001". Naturally, if such is the case that fact will be highly obfuscated and deeply buried in the DRO's literature.
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  5. #5
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    Quote Originally Posted by SGW
    When you read the specs, be careful not to confuse the accuracy implied by the number of decimal places displayed by the readout with the actual accuracy of the DRO. When I researched DROs a few years ago I found some that displayed to +/-0.0002" but could repeat only to +/-0.001". Naturally, if such is the case that fact will be highly obfuscated and deeply buried in the DRO's literature.
    Yup, resolution is what you see and accuracy/repeatability is what you get

  6. #6
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    There is also the Newall inductive system that uses hermetically sealed stainless or carbon fiber tubes as scales. Scales and reader heads are completely immune to just about any external influence( IP67). Accuracy is .0004" or .0002" depending on scale selection. The scales are extremely easy to install on machines which is why I picked it for my Jet 13x36 lathe.

    RWO

  7. #7
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    Default

    I went thru this not long ago searching for a DRO for my lathe.
    When you read the specs, be careful not to confuse the accuracy implied by the number of decimal places displayed by the readout with the actual accuracy of the DRO.
    SGW is correct on this part. What you should look at is the resolution of the scales. A 1 micron scale will have greater accuracy than a 5 micron scale. Both glass and magnetic scales come in different resolutions, with an increase in price for higher resolutions. I opted for the DROPROS Electronica model for my lathe. am happy with it so far. Like anything, set up and calibration (linear/non-linear) are where attention to detail is required to get the most accuracy from your unit.
    DQ

  8. #8
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    I'm contemplating designing a DRO headunit for a magnetic scale, as well as a cheap read head. I would offer it for sale if I do it, not to get rich but to fund the habit... I mean....AH .... hobby... I can't stomache the high prices folks are charging for the readouts and the caliper readouts seem like a crappy way to go. The problem is the good magnetic tape has quite a large minimum quantity. If anyone might be interested, let me know as it is hard to get an idea of if there is a market for this kind of thing.

    One vendor I'm looking at offers 20um or 40um accuracy grades. Keep in mind that the rating is over a 1m span, so it is hard to imagine an application in a home shop needing better than 40um in that span. Keep in mind that you will have very fine resolution. Repeatability is often much better than accuracy so that in extreme cases the true accuracy can be taught or mapped like a ballscrew on a high end machine tool. I don't recommend this as you would have to home your readout before using it, just noting that this can be done.

    KEJR

  9. #9
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    Much ado about nothing.
    Unless you are precision grinding , the ranges you will get are far better than most machinists can hold on a machine tool.
    If you are a perfectionist, then you must also consider the expansion rate differences between Glass and Magnetic scales, and the variable temperatures the unit may experience.

    You will be far better off getting a unit that you can find the manufactuurer of after the sale ! IMHO
    Rich

  10. #10
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    Quote Originally Posted by KEJR
    I'm contemplating designing a DRO headunit for a magnetic scale, as well as a cheap read head. I would offer it for sale if I do it, not to get rich but to fund the habit... I mean....AH .... hobby... I can't stomache the high prices folks are charging for the readouts and the caliper readouts seem like a crappy way to go. The problem is the good magnetic tape has quite a large minimum quantity. If anyone might be interested, let me know as it is hard to get an idea of if there is a market for this kind of thing.

    One vendor I'm looking at offers 20um or 40um accuracy grades. Keep in mind that the rating is over a 1m span, so it is hard to imagine an application in a home shop needing better than 40um in that span. Keep in mind that you will have very fine resolution. Repeatability is often much better than accuracy so that in extreme cases the true accuracy can be taught or mapped like a ballscrew on a high end machine tool. I don't recommend this as you would have to home your readout before using it, just noting that this can be done.

    KEJR
    You might look at http://www.shumatech.com/web/products/opendro

    Better than starting from scratch yourself. Put whatever quadrature scales you want on it.

    I use 20um on my lathe. That is a pretty standard accuracy for a machine tool. Gives you .0002" increments. On a lathe that is .0004 of the diameter when turning. Really, thats the minimum I would get.

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