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Thread: Setting Starrett depth mike

  1. #1
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    Default Setting Starrett depth mike

    I need to calibrate three new rods for my Starrett depth mic. they are 6-7, 7-9, and 8-9 lenghts. Does anyone know how to go about this without having a standard for those lenghts? I have a DRO on the mill I though maybe I could use but I have not come up with a set up for that. Any help would be apprecated.
    Last edited by rolland; 06-17-2009 at 09:22 PM.

  2. #2
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    You're going to have to have some sort of standard, if the calibration is to be at all accurate. Even if your DRO is absolutely accurate, it'd be a tricky project to measure the rods- an edge finder or other indicator may introduce it's own error.

    If it were me, I'd try to find a local machine shop that has a decent metrology lab, and see if you can slip the tech twenty bucks to set it.

    That or maybe a board member here might do it for you for tyhe cost of shipping. I'd volunteer, but my plates are already way too full.

    Doc.
    Doc's Machine. (Probably not what you expect.)

  3. #3

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    Do you have outside micrometers in that range that you trust? If so, you could make a comparative measurement to anything that comes to hand with good enough surfaces to be repeatable. 1-2-3 bocks, even if not gauge accuracy could be compared as setting gauges.

    The mill DRO wouldn't be gauge standard either but it would give you a head start. My best shot would be making up a block within the range of each of your new rods and if necessary screwing a block to one end for a step like you would measure with the depth mic. Set horizontally on the mill table, you can use an edge finder, zeroed at one end to find the length of the step as the DRO measures it. Then use the depth mic and set the rod so the mic gives the same reading.
    .
    "Opportunity is missed by most people because it is dressed in overalls and looks like work." Thomas Edison

  4. #4
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    You could buy a surface plate and a set of gage blocks for under $100 and it would get you close enough for most home shop purposes.

    If you have a high quality glass scale DRO (.0002) you could probably come within .001 or so using a probe in the spindle and slip feeler to measure from the base to the tip of the depth gage. It would be a big pain to use this method to try to adjust the rods; and with little certainty of accuracy.

    What equipment do you have? Standards for large mics? A height gage? High quality 1-2-3 blocks? What accuracy do you need?

  5. #5
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    Default Deep "in it"??

    Quote Originally Posted by rolland
    I need to calibrate three new rods for my Starrett depth mike. they are 6-7, 7-9, and 8-9 lenghts. Does anyone know how to go about this without having a standard for those lenghts? I have a DRO on the mill I though maybe I could use but I have not come up with a set up for that. Any help would be apprecated.
    Rolland.

    For larger (or any!!) calibration or setting or checking of depth micrometers, I find that having the micrometer hanging over the side of a "block" guage to be a PITA. Its much easier using a cylinder as you have support all round and the micrometer cannot "rock". Realistically, even using a standard micrometer "stick" is pretty well impossible. Finding and using larger height stacks (2 minimum) of guage blocks is a PITA as well.

    Using a cylinder solves all or most of the problems. I have made some cylindrical parts that I use as "Master Squares" and they work very well for setting or testing a depth mic.

    If you get a cylinder - say at least about 1" OD - and cut it to be within your depth micrometer measuring range (say, in your case, 6>7, 7>8 and 8>9" ranges). Set it up in your lathe and face off both ends - length is not important - at all - but finish and "squareness" are!!.

    I assume here that you don't have a set of slip guages - but no matter, you can do the job pretty well without them for most shop work.

    If you have the matching outside micrometers (pre-set) in those ranges, you can measure the tube lengths directly. Record the lengths - etching on the tube will be best - and store them away for later use.

    If you don't have the matching size outside micrometers then your suggestion of using your mill DRO's can be very effective with care. I will assume that your DRO has the normal mill DRO accuracy of 0.0002" - so we are well on the way.

    Put a good solid "stop" - a straight edge (a parallel strip or a 1-2-3 block will be ideal) - accurately squared across your mill table (use a good dial indicator) and over one of the mill table tee-slots.

    Put the tube to be measured/checked into/onto the tee-slot (use the tee-slot as a vee-block) with one end hard up against the stop. Put a very good (0.0005" or better) dial indicator or test dial indicator (aka TDI) onto the stop. Zero the DRO and the indicator. Now use your mill and DRO to go to the other end of the tube/"guage" and set your indicator onto the end of the tube. Move the table until the indicator is zero. Read off the DRO - that is your tube length. The accuracy of the dial indicator is only required for use as a comparator - not for actual measurement.

    I'd suggest several checks and use the most predominant or the average/mean of the DRO readings as your size for setting/guaging purposes.

  6. #6
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    It looks like I may have to see if I can find a shop that can set them or else I can use it for an excuse to get a height guage and a surface plate.
    The worst part is I just realized ordered the wrong ones as I need shorter, I needed the 3-4, 4-5, 5-6 inch. That is what happens when you don't write things down when ordering. So reorder and go from there perhaps I can find someone that needs the long set.

    Pete
    As far as accuracy I would like to get them within .005. My mic don't go past 4-5 inch altho I have some good 1-2-3 blocks.
    Last edited by rolland; 06-17-2009 at 09:23 PM.

  7. #7
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    Default Block it

    Why not use the 1-2-3 blocks as they should be within "tenths" (of a "thou")?. Just poke the depth mic spindle down a block hole onto the mill table - and away ya go!!

    It couldn't be easier or better - or quicker.

  8. #8
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    Default

    I have used step blocks and 1-2-3 blocks but mine only go to 6".

    Rolland, I think you really want to get closer than .005" for them or did you mean to type .0005"?

    If you use the 1-2-3 blocks measure them with a mic that is set as close as posible and record the readings and then set the depth mic to get the accumulated stack reading. Take all the time it needs and don't get in a hurry.
    It's only ink and paper

  9. #9
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    Default

    You probably need a "transfer standard"...... something you can measure with a different instrument AND also with the depth mic.

    You can make one.....

    Tiffies cylinder is one such, and it is a very good idea.

    if you trust your calipers, you can do an initial check by setting the calipers to a known setting, and comparing the depth mic reading to the caliper setting (lock the calipers). It's a nuisance due to the overhang, but not impossible by any means.

    Not everyone has 12" calipers, but if you do, that will cover all. If not, you can zero the smallest rod, at least.

    You can get at the larger lengths by using your measuring devices to check added blocks in addition to the 123 blocks.

    Or, using the DRO might be fine if you trust it......

    An extra set of 123 blocks is cheap these days, and you can measure them with other instruments for a check.

    Ultimately the answer is a cal lab...... that's what they do.

    I find depth mics a pain, since they don't have the "stop" that an OD mic has.

  10. #10
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    Default

    Quote Originally Posted by rolland
    Pete As far as accuracy I would like to get them within .005. My mikes don't go past 4-5 inch altho I have some good 1-2-3 blocks.
    Good 1-2-3 blocks should be within .0002 or so if clean and undamaged. You can get every inch from 1 to 6 with them and a fairly easy job of adjustment. Put them on a $20 granite surface plate and you're good to go.

    The .005 you've noted is easy. If you meant .0005 you'll probably be within +/- .0005 if you take care and the blocks are good. Normally, of course, you'd want your standard to be 7-10 or so times better than the needed accuracy. As a precaution, I'd check the blocks on the plate for flatness and with calibrated mics for dimension.

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