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  • #16
    A dial indicator or a dial test indicator by itself, lying in it's case, is incapable of doing anything.

    Either can be used for comparative measurements when installed in the appropriate equipment. The comparators illustrated are set with gage blocks or other fixed standard, and the indicator is used to measure the deviation if any. The second eBay item is commonly referred to as a snap gage over here, but the function is the same. Either of the two gages would be useless without the dial gage.
    Jim H.

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    • #17
      A car has wheels. That does not make a wheel a car. A comparator may have a dial indicator. That does not make a dial indicator a comparator, especially as that is not the common role for the instrument.

      This is all I have to say on this matter as any attempt to argue otherwise is clearly an attemp to argue, not to inform.
      Free software for calculating bolt circles and similar: Click Here

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      • #18
        Originally posted by Evan
        This is all I have to say on this matter as any attempt to argue otherwise is clearly an attemp to argue, not to inform.
        An extraordinary attitude.
        "I'm right, don't bother disputing it"

        Tim

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        • #19
          Evan how would you go about testing the acuracy of the DRO in the OP's circumstances? His test seemed fair to me.
          I think constructive input is what'll do the guy most good not telling him how wrong his methods are.
          Peter - novice home machinist, modern motorcycle enthusiast.

          Denford Viceroy 280 Synchro (11 x 24)
          Herbert 0V adapted to R8 by 'Sir John'.
          Monarch 10EE 1942

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          • #20
            I did not say the op's methods were flawed, others did.

            To test fix a micrometer to the table on an insulated block. In a drill chuck fix a piece of music wire. Connect an LED to the micrometer and the drill chuck (with appropriate resistor and battery) and very slowly wind the axis toward the moving anvil of the micrometer until the LED illuminates by touching the music wire to the micrometer. Zero the DRO and back the micrometer off a known amount and continue winding the axis of the machine in the same direction until the LED illuminates again. This test will be as accurate as the micrometer. The DRO should agree.

            Then back off the touch with the micrometer until the LED extinguishes. That will characterize the back lash. The DRO reading should not change more than one least significant unit. If it does the mounting of the DRO is suspect.

            Incidentally, I use this method to set the Z axis on my mill. My mill table is insulated from the rest of the machine by virtue of having PTFE linear bearings and an acetal leadscrew nut. It is repeatable to within 0.0002 or so.
            Last edited by Evan; 04-13-2009, 06:17 AM.
            Free software for calculating bolt circles and similar: Click Here

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            • #21
              DRO's should be good to go

              Now to cut back to the chase and the topic of and question in the OP:

              Originally posted by loose nut
              I have the generic digital scales, with the simple display, mounted on my RF 45 type mill.

              If I mount a dial indicator on the mill and move the table in the x or y directions and check the movement with the indicator I get a bit of a discrepancy, maybe a couple of thou up to .005" and even .010" after a while.

              Today I put in a wiggler, the type with the ball on the end of a shaft, and ran it into a parallel mounted in the vise, zeroed it and then ran it back and forth several inches, jogging it in both directions so there would be lots of backlash added into the movement, and back into the parallel again. I repeated this 5 or 6 times in the x and y directions and every time I got a 0.000 reading against the parallel.

              These scales are suppose to be accurate to .001" which is good enough for my shop, mostly.

              Now dial indicators are comparators and not really meant for measuring because they can hang up a bit when the plunger is returning to rest so there could be some error there and a bit of tangential error in the way it is mounted.

              So which is right the scales or the indicator or is there something wrong with my methodology in checking this out. It would seem to me that if it returns to the same place with the same readings the scales should be right. :
              Hi loose nut.

              I would give odds that your indicator is at fault and that your DRO scales are fine.

              The DRO seems to have performed very well in "X" and "Y" while the indicator was at best suspect in both.

              Forget about the dial indicator and just use the dials on your "X" and "Y" hand-wheels. Turn the hand-wheel clock-wise, zero the dial and the DRO. Wind on 2", 4" 6" 9" 12" (all clockwise). Check the DRO at each point. Now do the same in an anti-clockwise direction.

              Compare the DRO readings with the hand-wheel dials. I will be very surprised if there is a substantial difference between the DRO and the hand-wheel dial readings.

              I think that you can be confident in the accuracy of your DRO scales to the degree of accuracy you require - probably a lot better.

              I would either overhaul that dial indicator until it works satisfactorily, or if it cannot be repaired - scrap it.

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              • #22
                A comparator must have some means of measuring deviation from a set measurement. That comparative measurement can be made with a variety of devices, dial indicators and dial test indicators being among them.

                A plunger type dial indicator measures linear dimensions and will not have cosine error. Cosine error can be present only in a lever type DTI and can occur when the probe is set at too great of an angle to the direction of movement.

                I have not criticized loose nut's set up, as I do not know what it is, and I am not sure of which type of indicator he is using. His results would seem to indicate some mechanical problem with the indicator or the set up, as neither the indicator or the scale should have the degree of error he is seeing.

                If it is a plunger type and he is not providing enough preload, the error described can result. I would recommend setting the indicator up so that there is one complete revolution of the dial before setting it to zero. If a lever type of DTI is being used, he could be seeing cosine error, but it is not usually that large of an amount.

                Again, to prove the accuracy of the scale, the best method is to use gage blocks. The wiggler, or an electronic edge finder should repeat zero within 0.001". If you do not have gage blocks, accurately measure several parallels with a micrometer. Another parallel can be clamped to the table, it will have to be carefully trammed to ensure it is at 90 degrees to the travel of the scale to be checked. Locate the edge of the fixed parallel and set that as zero. The measured parallels can now be set in place and their width measured with the scale by locating their edges one at a time with the edge finder. Several parallels of known width can be put in place, and the distance measured incrementally as each is put in place. This will give a reasonably good idea of the accuracy of the digital display provided the parallels are truly parallel.
                Jim H.

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                • #23
                  A plunger type dial indicator measures linear dimensions and will not have cosine error
                  Of course it will if it cannot be placed square to the item measured. My Starret DI has a rounded button to allow such placement and the cosine error must be taken into account if an absolute measurement is desired.
                  Free software for calculating bolt circles and similar: Click Here

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                  • #24
                    Cosine error is the term used in to describe the error introduced when using a lever type indicator with the probe at too great of an angle to the direction of movement. It is a common term in metrology.

                    A plunger indicator, if grossly misapplied will also display cosine error, but it is much more of a problem with the lever style of DTI.
                    Jim H.

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                    • #25
                      Cosine error is a general term used to describe the error produced when a linear measurement is made at an angle to the direction of motion and results in an error reading that is less than the degree of motion.

                      A plunger indicator, if grossly misapplied will also display cosine error, but it is much more of a problem with the lever style of DTI.
                      A plunger indicator will display cosine error in direct mathematical proportion to the degree that it is not square to the work measured. It will exist regardless of how small the angle departs from 90 degrees and has nothing to do with being "grossly misapplied" as a cause. Gross misapplication will more likely result in binding of the plunger than cosine error.
                      Free software for calculating bolt circles and similar: Click Here

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                      • #26
                        Whatever........

                        The difference is in the design of the two instruments. The plunger of a plunger type dial indicator moves in a straight line, there is no cosine error inherent in the gage itself. Cosine error can be induced in the measurement taken by misapplication.

                        The probe of a lever type DTI moves in an arc, and cosine error is always present. It can be compensated for either mechanically or by jiggering the dial graduations, but if the lever is moved outside of the range of compensation, cosine error will rear it's head.
                        Last edited by JCHannum; 04-13-2009, 10:22 AM.
                        Jim H.

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                        • #27
                          Originally posted by oldtiffie
                          Compare the DRO readings with the hand-wheel dials. I will be very surprised if there is a substantial difference between the DRO and the hand-wheel dial readings.

                          Don't be so sure.....
                          A couple years ago I was taking a test on a manual mill, was down to the point where I was about to start sneaking up on final dimension so my cuts had to be what I intended.

                          Measure, move the table, cut, measure again.....WTF??? hmmm??? Let's try that again......

                          Measure, move, cut, measure again.....WTF is going on here??
                          Something isn't right, I'm cutting about 25% more than I should be??

                          Scratch head, think, mutter, scratch some more etc.....

                          Hmmm, lets try this. Zero dial, zero DRO, turn dial .100", look at DRO..... .075"??

                          Huh??

                          Try again, re-zero, move dial .200", DRO reads .150"

                          WTF??

                          Call over the guy that's administering my test, demonstrate and ask him if he expects me to run the test with or without the DRO, and how the hell I'm to do this in the allotted time if I have to requalify the machine before continuing??

                          Test cancelled for today........

                          Turns out the shop was moving out some old equipment and the guys decided that the display on one of the machines that was being moved was better than on the machine that was staying, so they swapped the displays.
                          Problem was, the display didn't match the scales on the machine.

                          Handy part was, I got a 2nd chance at the test after now having seen it and knowing how to approach it better the 2nd time around.

                          I passed.

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                          • #28
                            Originally posted by tdkkart
                            Turns out the shop was moving out some old equipment and the guys decided that the display on one of the machines that was being moved was better than on the machine that was staying, so they swapped the displays.
                            Problem was, the display didn't match the scales on the machine.
                            Man, that sucks!

                            Glass DRO scales are also relative measuring devices: the reader head is just counting hash marks on the glass scale as they go by. But that means the reader head needs to know the resolution of the hash marks it's counting

                            Twice I've seen DRO's reporting inaccurate results. Both times it was because the mount brackets were loose and I was either getting a cosine error (because the scale moved to a diagonal w.r.t. the table) or the reader head was flopping around with the leadscrew motion.
                            Last edited by lazlo; 04-13-2009, 01:53 PM.
                            "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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                            • #29
                              Boy I think I'll quite asking questions if it's going to stir up this kind of hornets nest.

                              When I said the DI was a comparator I didn't mean in the literal sense that Evan means only that it isn't a tool that should be use for linear measurement, which most of us are guilty of if for no other reason then they are convenient.

                              Now before someone jumps down my through for that statement, it's not mine, I have read it in many books and magazine articles on the subject and told this by "real" machinists (I'm holding on to my amateur standing). As someone said earlier in this thread DI's compress OK but not so much when they are returning back towards the at rest position, hence error can creep in.

                              When you put a piece of bar in a 4 jaw and set up a DI to center it, you take readings at points 180 degrees apart and compare the readings to find the amount of adjustment towards center, so you are using it to compare not to measure. Even if the plunger is at an angle to the work, the readings on both sides will be accurate even with a sine error because the actual distance doesn't matter, only the difference in the reading that you get compared to the reading from the opposite side, any sine error will be the same for both.

                              If one wants to be a real stickler, any measurement tool is a comparator, Mic's and verniers compare the thickness of a work piece against a scale, if a DI is mounted on a pillar set into a surface plate for inspection work is it not comparing the thickness of a part against a known standard. Everything else is just semantics.

                              All I really wanted to know is, if I get the same reading at the same point every time I try can I consider the repeatability of the scales to be adequate. I believe the predominant answer is yes. Thanks, now play nice.
                              The shortest distance between two points is a circle of infinite diameter.

                              Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

                              Southwestern Ontario. Canada

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                              • #30
                                If you are referring to a plunger type dial indicator, you are mistaken in stating that they are not for linear measurement. They are indeed made expressly for that purpose. I have just finished installing bracketry for using dial indicators with 2" & 5" travel for table positioning on my milling machine. Dial indicators are regularly available in lengths from less than an inch up to 5" or so in length. They should and will return to zero repeatably if not damaged or allowed to become gummed up.

                                If you are referring to a lever type dial test indicator, the statement is only partially correct. A lever type dial test indicator will read dimensions accurately in a much smaller range as mentioned. They are limited by cosine error if used outside of that range. When used for tramming or dialing in in the lathe chuck, the error is of little consequence as the aim is for zero movement. However, these gages are used for accurate measurement in inspection and other applications and are capable of excellent accuracy if applied properly. They also will return to zero reliably if they are maintained properly.

                                You have referred to DI and DTI in your posts, and I have asked which instrument you are using. I have not seen a reply. The terms are not interchangeable and this makes it very difficult to make a meaningful reply to your question.
                                Jim H.

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