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Thread: Wire dimensions for measuring a gears.

  1. #1
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    Default Wire dimensions for measuring a gears.

    A recent post on gear measuring tools prompted this post.

    I picked up some time back a digital gear PD and OD measuring system that uses two opposing anvils of sorts on a THK rail interfaced with a digital readout.

    My question is, does every gear have to have a specific anvil diameter or does each size anvil have a range?

    The anvil diameter on it now are both 4mm

    Thanks
    Ken







    Thanks
    Ken


    .
    Last edited by Ken_Shea; 07-31-2010 at 03:14 AM.

  2. #2
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    Default Tooth PA

    Ken.

    The angle of the anvil is 2 x the gear pressure angle and theoretically will be tangent to the gear involute sides/flanks at the pitch point so there will be different anvil details/sizes/diameters for a range of gears.

    Each anvil will have a range of gear teeth (usually DP, number of teeth and/or circular pitch if "inch" or module if metric).

    They will need to be set to a datum or reference as they are in effect a comparator.

    They have a similar principle to a thread micrometer which has different anvils for different tooth forms and pitches.

    Setting it for an even number of teeth/gaps will be relatively easy but setting it for an odd number of teeth/gaps will requires a lateral ans perhaps angular off-set so that both anvil axis are radial with respect to the gear.

    Any luck with Google?

    Any more details that might assist?

  3. #3
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    Default

    Quote Originally Posted by oldtiffie
    Ken.

    The angle of the anvil is 2 x the gear pressure angle and theoretically will be tangent to the gear involute sides/flanks at the pitch point so there will be different anvil details/sizes/diameters for a range of gears.

    Each anvil will have a range of gear teeth (usually DP, number of teeth and/or circular pitch if "inch" or module if metric).

    They will need to be set to a datum or reference as they are in effect a comparator.

    They have a similar principle to a thread micrometer which has different anvils for different tooth forms and pitches.

    Setting it for an even number of teeth/gaps will be relatively easy but setting it for an odd number of teeth/gaps will requires a lateral ans perhaps angular off-set so that both anvil axis are radial with respect to the gear.

    Any luck with Google?

    Any more details that might assist?
    Thanks,
    Did not glean much help from Google, did find some text describing the benefits of this equipment from the manufacturer but not much that did not read as totally confusing with regards to the anvil size.

    Was hoping that this equipment would help me identify rather then just inspect, guess that's why they call it an inspection gauge

    To be sure I have it right, every gear will have a different anvil diameter then?

    Thanks OT.

  4. #4
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    Default reverse eng

    Ken

    I never used one of these gages, however it appears the end of the avil has pins. It would seem to me that it would require a set for each Diametral Pitch.
    hence van kuren gear wires established by 1.728/ DP = wire diameter.
    or 1.92/ DP = wire diameter for enlarged gears.

    DP = gear diametral pitch

    What does this gage calculate?
    your best reverse engineering tools would be
    optical comparator to physically to measure the pressure angle.

    Measure the base pitch of the spline & or gear(span check of 3 teeth then 2 teeth then subtract) = base pitch. do this at least 3 times.
    then calculate the dp using the base pitch formula. it will get you close.

    just so you know
    the most accurate method is to use an involute inspection machine like the fellows involute checker. if it obtains a fairly straight involute then you have the correct Diametral pitch & Pressure Angle.

    HTH

    Leesr

  5. #5
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    Default

    Leeser,

    It only measures the distance between the pins.
    They are pins, was calling them an anvil because I don't know what I'm doing

    Thanks for the info.

    Ken

  6. #6
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    Default

    Lessr-

    Your comment on using an involute tester is interesting, but I am struggling to see how it would work.

    I assume we are talking about the same type of machine - i.e the gear under test is mounted on a spindle which also carries a disk which is ground very accurately to the base circle diameter of the gear. A straight edge is held in contact with the base circle and the two rolled in contact. Meanwhile a probe in contact with the tooth face and attached to the straight edge measures deviations from the required path.

    Assuming that we are talking about the same type of machine, how would you go about establishing DP and pressure angle for an unknown gear? I have used this type of machine but only for checking a known gear and I can't visualise the setup for an unknown gear.

    Bill
    Bill

  7. #7
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    Default

    Much has been written about making gears while little has been set down on measuring them. Measuring a newly made gear to insure that it meets specs is relatively easy. You know all the target numbers and only need to verify them. If you used a 12 DP cutter, then it will at least be close to the tooth form for 12 DP. You can calculate the various values and just check them. Verify the tooth thickness with a gear caliper or mike. Then use pins sized to touch at the pitch circle and verify the pitch diameter. OD is easy: may require some calculation if there are an odd number of teeth and you want to check to tenths, but it can be done. All of the above can be done with precision. There appears to be an unworn section on the gear Ken pictured so this "new" assumption can probably be applied there.

    However, if you have a worn gear and no information on it, no specs, then you have a delema. If you are stuck with a gear that is worn all over like badly worn section of Ken's gear, then you must use approximate measurements and made assumptions. What if you measure the OD? It is obviously worn quite a bit. Without the unworn section, you would not know how much. So it will not be easy to get a DP from the OD and teeth + 2 formula. The number you get will be low. Suppose you get a calculated 15.5 DP. Is it a 16 DP? Could there be enough wear to make it an 18? Probably not. But perhaps an unstandard 17? Perhaps. But 16 DP = module 4.987 so could it be metric module 5? With that much wear, how could you tell? You could measure other features and see if they are even numbers in either inches or mm. But this is far from certain. Other measurements like PA will be equally unsure.

    Is my example just a single coincidence where the DP comes out close to a module value. Well, no. Any gear in the DP sequence 1, 2, 4, 8, 16, 32, 64, etc. will have the exact same close ratio to a standard module gear. And a 5 DP = module 15.959 so other sequences of DPs will also be close enough for confusion. 20 DP is especially close to module 4 and gears of the two, if the same PA, could probably inter-mesh.

    If you have a heavily worn gear, you are are going to have to make some assumptions, some guesses. Fact of life.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  8. #8
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    Default VK wires

    Quote Originally Posted by leesr
    Ken

    I never used one of these gages, however it appears the end of the anvil has pins. It would seem to me that it would require a set for each Diametral Pitch.
    hence van kuren gear wires established by 1.728/ DP = wire diameter.
    or 1.92/ DP = wire diameter for enlarged gears.

    DP = gear diametral pitch

    What does this gage calculate?
    your best reverse engineering tools would be
    optical comparator to physically to measure the pressure angle.

    Measure the base pitch of the spline & or gear(span check of 3 teeth then 2 teeth then subtract) = base pitch. do this at least 3 times.
    then calculate the dp using the base pitch formula. it will get you close.

    just so you know
    the most accurate method is to use an involute inspection machine like the fellows involute checker. if it obtains a fairly straight involute then you have the correct Diametral pitch & Pressure Angle.

    HTH

    Leesr

    Very interesting post Leesr.

    I thought the "circle" that you refer to on the ends of the anvils was a jpg or similar artifact.



    On having a closer look, I agree with you - it was careless of me - many thanks.

    I never used one of these gages, however it appears the end of the avil has pins. It would seem to me that it would require a set for each Diametral Pitch.
    hence van kuren gear wires established by 1.728/ DP = wire diameter.
    or 1.92/ DP = wire diameter for enlarged gears.

    DP = gear diametral pitch
    If they are similar to VK gear wires they would not apply to shop-made gears with the standard store-bought cutters as they are made and used for high-class work possibly more for use in the "Inspection/QA" section than at the machine (shop). This is due to the vagaries and inherent inaccuracies due to store-bought cutters:





    For those that are interested, in theory, the number of VK wires needed is huge but can be limited to the range of PA/DP/CP being manufactured or checked.

    See Machinery's Handbook27 page 2125 onward.

    The VK/wire method of measuring does not include clearance or back-lash which can be calculated as or for a separate "wires" measurement - same applies to measurement over several teeth with calipers etc. (MH27 page 2141).

    Ken,
    you'd need the right wire/anvil and an unworn part of the gear to get the best out of that machine.

    [Edit]

    As they are cylindrical anvils it should be quite possibe to calibrate/set the VK wires/rollers centre-distance with slip-guages and then zero the read-our scale to get relative or absolute variations from the pre-set distance when measuring the gears.

    [End edit]
    Last edited by oldtiffie; 07-31-2010 at 09:23 PM.

  9. #9
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    Default Fellows Gear Checker

    It's been a while since I ran a fellows involute checker, but I will wing it.

    The Base Radius of the gear is adjusted at the machine by a micrometer dial.
    the correct pointer is set to the left or right flank of the gear tooth.
    the pointer is set near the root diameter(TIF) total involute form dia.
    it is where the maximum contact of the mating gear TIP diameter.

    A dial is cranked that follows the gear tooth profie it is then transpsoed to a chart that is set at a magnification so so it is easier to read.
    "for example" there are lines in the same direction of the chart, each line can be set at diffrent magnification. for example one line can equal .0002" change.

    The Base Diameter is calculated from the # of Gear Teeth, Diametral Pitch & Pressure angle of the gear. that is the relationship. so if the involute is correct then the base diameter is correct.

    That is how the correct DP & PA is established.

    fars as the Inards of the maqchine I would have to study the original Manual.
    it's been a while since I have reviewed it.

    HTH

    Leesr

  10. #10
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    Default

    Leesr -

    Your description is very close to my recollection, with one exception; the machine that I used had to have a master Base Circle disk ground for every different base circle diameter. That would make it a bit clumsy/expensive for assessing an unknown gear.

    If I understand the method that you describe you would need to make an intial guess of the DP (perhaps not too difficult) and the pressure angle (a few likely choices) then run the trace against the flank looking mostly at the middle part of the trace to see how curvy and angular it was, then retry with a better guess until you found a combination that gave the most linear trace?
    Bill

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