How-To: Use an indicator to find an edge?
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  1. #1
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    Hi: I know that many of you could describe the procedure for using an indicator to align the center of the spindle with the edge of the workpiece. Location tolerance of +/-0.0005.
    Please describe type of indicator and any other attchments needed. I REALLY appreciate your input!

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    If you had a precision hole of known radius somewhere, then center on this hole. Either accept the reading you get doing the centering operation, this will be the most accurate, or try to move the indicator and or dial to read zero.

    The radius, known to you, is the offset of the contact point on the indicator to the spindle center.

    Next go to your surface and adjust spindle position while sweeping the indicator until you get to the reading you had in the master hole. Now your spindle is the above known radius of the master hole away from the surface.

    .

    [ 10-26-2007, 08:03 AM: Message edited by: gar ]

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    Is this your homework?

  4. #4
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    There are a couple ways.

    Get a gage block and a 123 out. Wring the gage block to the 123 with a slight overhang over the side, and set it somewhere on your table. Bolt it down lightly, and square it up like you would a vice. This is more important than a vice, so get it right. Put the indicator on an adjustable arm, and put that in your spindle. Get the ball of the indicator centered somewhere close to running concentric with the spindle.

    Now, the tricky part.

    The idea is to get the indicator to read 0 on both surfaces by just rotating the spindle. So what you do is rotate the spindle to sweep the indicator against the 123, and register the highest spot as 0 on your indicator. Spin the indicator 180 degrees around, and crank your table so that the indicator now touches the overhanging gage block surface. Nudge the indicator back until the difference between the two surfaces is within the range of the indicator. Now, you should start moving the Y axis (if the blocks are lined up on X) half the reading between the two sweeps. Within a few iterations of this, you will have the indicator zeroed out at the point directly in line with the spindle's axis. Conveniently, the edge will also be under the spindle. For any other edges or features you need to verify, it just becomes an issue of moving the table, and sweeping the indicator against the surface until it reads 0.

    That process is sped up tremendously by using an edgefinder.

    There's another way you can do it, and naturally I saved it for after the complicated (but slightly more useful) way.

    Set the indicator in your spindle. Touch it against the surface you want to check, sweeping the spindle back and forth to register the high spot, set indicator zero and note your table's position off the DRO or handwheel. Move the spindle over the part, put a 123 block or gage block on the surface you want to check, and now move the table until that block's position reads 0 in the high point as you sweep the spindle. Note your DRO/handwheel reading.

    The edge of the part is halfway between those two readings.

    Both these methods are detailed in jig boring/grinding texts, as that's who mainly uses them. They take too long and most processes don't need that degree of location.

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    Use of a Moore Pick-up Chair should get you within a tenth or 2, provided you have a spindle and indicator that are within those specs.

  6. #6
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    I keep the edgefinder it the same drawer as the indicator.

  7. #7
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    A 1/2 inch tooling ball works good for that operation.Indicate the ball at the largest point & set indicator @ zero.Sweep EOP to read zero on indicator and preset .250 on readouts.

  8. #8
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    Many might disagree, but I'm not a huge fan (and never have been) of using physical objects + indicator to find edges. If you're going to do that, you might as well use an edgefinder.

    While some might argue that a flat surface is a physical object (sure, it is), there's less error using a geometric feature rather than relying on a dimension.

    Ignoring, of course, the error you have in perpendicularity and straightness in the axes. But that will be a limiting factor no matter what method you want to use.

    With an edgefinder + lapped surface, you can locate to the limit of your machine/environment plus whatever instability you have in your indicator. If you've got an electronic indicator of decent resolution (LVDT or an analog probe), your machine's inaccuracy will completely overshadow your location method. With a tooling ball, pin, edgefinder, or whatever you're limited by all those other factors plus your ability to accurately measure the device you're going to offset by.

    Now, my point is moot if you're using a bridgeport, or most other machines. But for technical accuracy, or if you ever find yourself in front of a jig grinder or slow tool/fast tool servo boring machine, it will make all the difference in the world.

  9. #9
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    This reminds me of a guy who worked at the same shop I did years ago.

    He scrapped a somewhat large block of tool steel because he thought he'd found the edge of the block when he approached the indicator with the block (Lucas horizontal) and simply stopped when the indicator read zero. That was it. He bored and drilled holes only to learn later what he had done wrong.

    I always used an edge finder to get close, then swing an Interapid tenths indicator on the edge alternating a 1-2-3 block to finish. Quick and simple.

  10. #10
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    Hi: Thank you all for the input. Not my homework, I know there are many talented machinists on this board and I'm trying to gain knowledge!

  11. #11
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    Back in the 60's, when I rolled my own cigs as an apprentice, this was how I was shown....

    Take a cigarette paper (Zig Zag etc.) measure it with your 0-1 mic. Apply spit and attach to the contact edge. Spin cutter and approach until paper splits. This will be approx 2/3 of the paper thickness (.001 - .004 depending on paper type) That + 1/2 the swept cutter dia will get most milling jobs done.

    (Swept dia = Measured Cutter + 1/2 of TIR of cutter)
    (TIR = Total Indicator Reading or excentricity)

    These days I still roll my own and sometimes use this technique, probes do a good job but have to be checked out constantly for more accurate work.

    Steve

  12. #12
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    Make sure you have a flat square reference edge, without burrs, on the workpiece. Get a piece of precision ground round stock that is longer than the workpiece is thick, face the end flat, leaving the rod about 3/8 or 1/2 inch longer than the workpiece thickness. mic the diameter. Say its 0.4004 or sumpin. Clamp the round stock against the flat edge, axis pointing "up". You could use a piece of scrap with a V cut in it to help stabilize the clamping. Center your spindle with the indicator. You should now be 0.2002 away from the edge.

    You want a lever-type indicator like a Interapid or a Bestest, or even a Starrett "last word".

    Jim


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