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  • loose nut
    replied
    The DI is the plunger type, and as for using them as a measurement device, we will have to agree to disagree, 'nuff said.

    The DI is removed from the machine and I'm going to check the accuracy of the scales against gage blocks.

    Tiffie you seem to have it right.

    And that's all she wrote.

    Have a nice day.

    Leave a comment:


  • oldtiffie
    replied
    As he sees fit

    Jim,

    I will answer in the negative here in that I do not consider your comments as being uninformed - but they should tell people the extent of the applicability of, in this case, "Cosine error" and by extension the relevance and use of either comparators and direct measurement.

    I hear a lot of stuff quoted here that someone "heard" or "their mentor said" or some long-gone author of some renown said too - much of which may be either or both wrong or inapplicable.

    Indicators versus comparators was a red-herring as well.

    My main point was to show the OP that if his readings on his lead-screw dials was pretty close to that of his DRO scales, then it was near enough to both identify that the indicator was inadequate or faulty and to validate his DRO scales such that he could proceed onward to such other additional measures and measurements as he required to suit his purpose.

    Further, the accuracy required of the mechanical set-up of DRO's relative to the machine are mainly a mechanical requirement. The "errors" of a set-up at or less than o.25 degrees (which if "FAR" better than a good DRO set-up to the manufacturer's specifications will be pretty well irrelevant as the cosine "error" will be pretty well insignificant and certainly within the +/- 0.0005" (or better) for most commonly used DRO's on mills and lathes. Surface grinder DRO's (down-feed) are quite another matter as they are an order of accuracy above those normally used.

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  • JCHannum
    replied
    Tiff, I don't know whose comments you feel are uninformed, and would appreciate you clarifying that statement.

    loose nut has asked a couple of questions, and there have been a couple of sugestions as to how to proceed. Lane suggested using 123 blocks, mic standards or something other of known dimension to measure at intervals, I suggested the same using measured parallels.

    Simply checking that the scale returns to zero or the same number at different points will not tell the accuracy of the scale. Actual measurements of known dimension will have to be taken at several points along the travel of the scale to verify it's accuracy.

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  • oldtiffie
    replied
    "Cosine rule error" garbage

    Originally posted by loose nut
    Boy I think I'll quite asking questions if it's going to stir up this kind of hornets nest.

    ...........................................
    ..............................................

    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.
    Loose nut.

    I think that you've got it right. Your set-up of your DRO scales seems to have no looseness in it. Your repeatability is OK.

    Now just to set the "cosine error" bit to rest - as I think there is more quoting of it than knowledge of its importance or extent.

    Cosine - aka Cos in a right-angle triangle is the ratio of the the length of the side adjacent to the angle to the length of the hypotenuse.

    Or Cos angle = adjacent/hypotenuse

    Or Cos = A/P

    http://en.wikipedia.org/wiki/Cosine_...etric_function)

    Here is a good explanation:
    http://www.cncexpo.com/IndCosine.aspx

    In theory, the arm of a dial indicator (with a swing arm)should be at right angles to the surface/part being measured/checked/compared where-as the plunger of an axial indicator should be normal to the part being checked.

    As Cos 0.00 deg = 1.0000 it follows that the adjacent side and the hypotenuse are the same.

    Now let's have a look at the situation at varying angles:

    Cos 0.25 deg = 0.99999
    Error = 0.00001"/inch

    Cos 0.50 deg = 0.999962
    Error = 0.000032"/inch

    Cos 0.75 deg = 0.999914
    Error = 0.000086"/inch

    Cos 1.00 deg = 0.99985
    Error = 0.00015"/inch

    Cos 2.00 deg = 0.9994
    Error = 0.0006"/inch

    Cos 3.00 deg = 0.9986
    Error = 0.0014"/inch

    Cos 4.00 deg = 0.9976
    Error = 0.0024"/inch

    Cos 5.00 deg = 0.9962
    Error = 0.0038"/inch

    Cos 10.00 deg = 0.9848
    Error = 0.00152"/inch

    The lever arm on my "adjustable finger" indicator is about 0.700" long so any so-called "Cosine error" is 0.700 x the values above.

    So if my "arm" was off-set by 3 degrees from "square" or "normal", the cosine error at the end would be:
    Cos 3.00 deg = 0.9986
    Error = 0.0014"/inch x 0.700 = 0.0010" - jep, just a "thou" at 3 degrees.

    For my axial indicator the error will be as above ie
    Cos 3.00 deg = 0.9986
    Error = 0.0014"/inch for each inch of plunger travel.

    So a lot of this "cosine error" is not the problem it is made out to be in some or many circumstances.

    It is easy to set a 90 degree/normal angle to within 2 degrees by eye and most times, that will be good enough.

    So, loose nut, that is why I think you are doing OK, despite some of the adverse, and I suspect largely uninformed, comment.

    I hope this has set your mind at rest.

    Leave a comment:


  • lane
    replied
    And as far as this so called cosine area is concerned. Just make sure the stem of your indicator is parallel to the movement in both side to side and vertical and horizontal . If you have some mic standards set a 123 block on the table and clamp down indicated it square lay a long mic standard in a v block and push it up to the 123 block . Pick up end of standard with edge finder ZERO your measuring system remove mic standard and put in one shorter move till it zero out on edge finder check reading on system repeat with shorter standards . You can use any thing of known length . When done move back to 0 and see if numbers return to 0.

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  • lane
    replied
    I read most of this but not all . Something to check is . I have seen the nut the screw rides in being loose in its housing and slipping back and forth are the screws are loose that clamp it down. This will give faults readings with a read out are what ever measuring system you are using. So check this before you go any farther.

    Leave a comment:


  • JCHannum
    replied
    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.

    Leave a comment:


  • loose nut
    replied
    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.

    Leave a comment:


  • lazlo
    replied
    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.

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  • tdkkart
    replied
    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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  • JCHannum
    replied
    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.

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  • Evan
    replied
    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.

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  • JCHannum
    replied
    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.

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  • Evan
    replied
    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.

    Leave a comment:


  • JCHannum
    replied
    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.

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