1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.
  2. PLEASE: Read the FORUM RULES BEFORE registering!

    Dismiss Notice

Poly carbonate digital calipers

Discussion in 'QUESTIONS & ANSWERS (Get Help Fast Here!)' started by Tozguy, Apr 18, 2017.

  1. Tozguy

    Tozguy Active User H-M Supporter-Premium

    Likes Received:
    603
    Trophy Points:
    113
    City:
    Drummondville
    State:
    Province du Quebec

    -Return to Top-

    Last edited: Apr 18, 2017
  2. dontrinko

    dontrinko United States Active Member Active Member

    Likes Received:
    25
    Trophy Points:
    28

    -Return to Top-

    I have a $10 harbour freight, It works ok but only has 1 decimal place for mm and 2 for inches. I keep it in my desk drawer for quick measurements. Don
     
    bobl and Tozguy like this.
  3. f350ca

    f350ca Canada Active User Active Member

    Likes Received:
    1,315
    Trophy Points:
    113
    City:
    Calabogie
    State:
    Ontario

    -Return to Top-

    I had a dial one in fractions for the cabinet shop. It was ok but the jaws wore even on wood. They list the accuracy as .008.

    Greg
     
    Tozguy and Bob Korves like this.
  4. Tozguy

    Tozguy Active User H-M Supporter-Premium

    Likes Received:
    603
    Trophy Points:
    113
    City:
    Drummondville
    State:
    Province du Quebec

    -Return to Top-

    Oops missed that, thanks Greg.
     
  5. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

    Likes Received:
    2,668
    Trophy Points:
    113
    Location:
    USA
    City:
    Sacramento
    State:
    California

    -Return to Top-

    I have two Harbor Freight #47257 calipers that I bought 3-4 years ago. I use one every day for rough measuring in the shop, and it gets used on rusty stock, whatever needs a quick measure. The first one is still on its first battery, comes with a spare, and is still accurate to .001-.002", no issues. The second one is still new in the box. I bought them for $9.99 each, minus 25%, and including a freebie with each. I do not use them for work that needs to be accurate, more as an easier reading version of this:
    [​IMG]
     
    uncle harry, FOMOGO and royesses like this.
  6. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

    Likes Received:
    2,668
    Trophy Points:
    113
    Location:
    USA
    City:
    Sacramento
    State:
    California

    -Return to Top-

  7. Tozguy

    Tozguy Active User H-M Supporter-Premium

    Likes Received:
    603
    Trophy Points:
    113
    City:
    Drummondville
    State:
    Province du Quebec

    -Return to Top-

    Thanks Bob, good article.
    Being a target shooter I am fully aware of the difference between accuracy and precision.
    But it was interesting to read about cheater circuits in digital programming...

    When I compare a good dial calliper to a cheap digital one, the digital shows consistently .003'' to .004'' more than the dial caliper at 3 inches.
    I have more trust in the Mitutoyo dial caliper and enjoy it more anyway.
     
  8. woochucker

    woochucker United States Active Member Active Member

    Likes Received:
    172
    Trophy Points:
    43
    City:
    Pittstown
    State:
    New Jersey

    -Return to Top-

    Well how can it have a resolution of .0005 and an accuracy of only .008. Let's face it, there's something wrong with that. Those are at opposite ends of the accuracy /repeatability spectrum. The HF with res of .01 is probably about as accurate.
     
  9. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

    Likes Received:
    2,668
    Trophy Points:
    113
    Location:
    USA
    City:
    Sacramento
    State:
    California

    -Return to Top-

    Jeff, read the article in the link in my post above. Resolution, which sounds like a really cool and accurate thing, is only what the scales can show. If a ruler has 1/64th inch markings on one side and 1/8" markings on the other side, then the resolution of one side is 1/64" and the resolution of the other side is 1/8". If a caliper has digits on it that go down to .0001", then that is the resolution, even is the caliper is made of soft rubber. Repeatability is a completely different thing as well. If you keep closing that caliper to zero, but get different readings each time, then you have poor repeatability. All that stuff matters in getting good measurements, and more, such as ease of reading, and how comfortable the tool is to hold and use.
     
  10. woochucker

    woochucker United States Active Member Active Member

    Likes Received:
    172
    Trophy Points:
    43
    City:
    Pittstown
    State:
    New Jersey

    -Return to Top-

    Look,you can have .0005 resolution. it means nothing if the accuracy is .008.
    end of story. The unit is just .008... posting .0005 is a trap. what's the point of having that resolution if you only have .008 accuracy.
    I don't need to read it... having .0005 on accuracy of .001 is fine, you are now +- .0005, having a resolution of .0005 on a .008 means you are now +- .004 since you can't rely on the accuracy of .001 to begin with...
    Yes/No?
     
  11. Bob Korves

    Bob Korves H-M Supporter - Premium Content H-M Supporter-Premium

    Likes Received:
    2,668
    Trophy Points:
    113
    Location:
    USA
    City:
    Sacramento
    State:
    California

    -Return to Top-

    You are correct. It is misleading advertising. Nothing new about that, whatever you are looking for. The only way to make it through the mine field is to understand well enough to know how to see through the spin...
     
    Tozguy likes this.
  12. chips&more

    chips&more United States Active User Active Member

    Likes Received:
    965
    Trophy Points:
    113
    City:
    Danville
    State:
    California

    -Return to Top-

    Garbage in garbage out
     
  13. Tozguy

    Tozguy Active User H-M Supporter-Premium

    Likes Received:
    603
    Trophy Points:
    113
    City:
    Drummondville
    State:
    Province du Quebec

    -Return to Top-

    Actually the accuracy in the KBC ad is +/- .008'' for a total dispersion of .016'' but that might include hysteresis. :)

    It is what it is.
     
  14. Tozguy

    Tozguy Active User H-M Supporter-Premium

    Likes Received:
    603
    Trophy Points:
    113
    City:
    Drummondville
    State:
    Province du Quebec

    -Return to Top-

    The resolution versus accuracy thing is like having a hawk's vision to look at the car in your driveway.
     
    woochucker likes this.
  15. Tozguy

    Tozguy Active User H-M Supporter-Premium

    Likes Received:
    603
    Trophy Points:
    113
    City:
    Drummondville
    State:
    Province du Quebec

    -Return to Top-

    I think you are right. The problem with the KBC ad is that the word resolution actually refers to what Mitutoyo calls graduation;
    http://ecatalog.mitutoyo.com/Dial-Calipers-Series-505-C1387.aspx
     
  16. whitmore

    whitmore United States Active Member Active Member

    Likes Received:
    23
    Trophy Points:
    8
    City:
    Shoreline
    State:
    Washington

    -Return to Top-

    Oh, yes, it DOES mean something; a simple pair-of-points divider has
    very fine resolution, and no accuracy at all (there's no distance markings on
    a divider). It's useful because it holds a setting. The inaccurate
    calipers also hold a setting, and might be able to repeat it from
    the numeric readout.

    The metal points on a divider are a better choice for scratch-marking on
    sheet metal, though, than the plastic.
     
  17. Tozguy

    Tozguy Active User H-M Supporter-Premium

    Likes Received:
    603
    Trophy Points:
    113
    City:
    Drummondville
    State:
    Province du Quebec

    -Return to Top-

    Wouldn't a pair-of-points divider be very accurate once it was set, locked and handled consistently?
    Wouldn't its resolution depend on the width of the points (i.e. how pointed they are).
     
    whitmore likes this.
  18. whitmore

    whitmore United States Active Member Active Member

    Likes Received:
    23
    Trophy Points:
    8
    City:
    Shoreline
    State:
    Washington

    -Return to Top-

    I'm uncertain how 'resolution' applies; the blunt points get a touch-up every once in
    a while, but it's the scratchmark they make that holds the real precision of
    interest. Rounded tips still make a narrow line in the Dykem.
    Resolution should be high, if you think all settings in the opening range
    are distinct and spaced according to the mark width.

    The reason to use dividers, is to achieve symmetry. Same distance, on
    all the scribed arcs... That characteristic, symmetry, is the virtue of that
    instrument.

    I think the 'resolution' and 'precision' concepts relate to the measurement marks
    (resolution comes from the multiplicity of marks, measured in bits
    as the logarithm base two...) and to the dimension that separates the marks
    ('precision' is the temperature difference that separates adjacent marks on,
    for instance, a thermometer).
    After a thermometer calibration, there is accuracy (in degrees C or F)
    up to the level of the precision, in that the absolute numeric temperature
    is well-known after consulting the calibration (table or curve).

    There is also accuracy (lesser accuracy) in an uncalibrated thermometer,
    and (if you read the data sheet) sometimes you know that the ice point
    or triple point are maximally accurate because the factory did calibrate there.
     
  19. woochucker

    woochucker United States Active Member Active Member

    Likes Received:
    172
    Trophy Points:
    43
    City:
    Pittstown
    State:
    New Jersey

    -Return to Top-

    Bob Korves likes this.
  20. woochucker

    woochucker United States Active Member Active Member

    Likes Received:
    172
    Trophy Points:
    43
    City:
    Pittstown
    State:
    New Jersey

    -Return to Top-

    Not sure I understand. A divider is not measuring to a number, it either transfers , or it divides (arc), so it is not measuring like a caliper , mic, etc. So it's an apples to orange comparison. Let's stick to measuring units.
    Take a scale for instance. A scale marked in 8ths cannot measure to the 1 and 6/32nds yes I know it's 1 3/16 I'm trying to make a point. So this is the opposite of having a high resolution but low accuracy. If we determine that this scale is really accurate to the 8th. This scale is highly accurate to the 8ths but has a low resolution.

    The ad in question has something measuring to 5 tenths, but the claim is we are only accurate to the side of the barn. So who cares about the 5 tenths, when this thing is measuring the barn rather than the your piece of steel.. :) I am sure you care about being close to the .001 and this measuring tool is only giving you really .01 so why would I care that it has fine graduations / resolution.. I missed the spec.. The customer wants 3.151 and I delivered 3.161... I lose. But I had a device that had .0005 resolution.. uh.. ok..

    I am not making fun, but it's a serious issue. Having a high resolution, is garbage if you have no accuracy. Having accuracy is important. Because when you make parts for a friend , customer, yourself, you are basing it on a drawing, telephone call, whatever. You need to be accurate. Your .001 has to be matching their .001.. and thats all. If you have .0005 capability, but you are accurate .01 you cannot deliver.

    The Harbor Fright unit has a resolution of .01, to me that unit is better than this .0005 unit. Why , because you really know what you are getting with it. That .0005 says I am more than I really am. It's a poser.
     
  21. Tozguy

    Tozguy Active User H-M Supporter-Premium

    Likes Received:
    603
    Trophy Points:
    113
    City:
    Drummondville
    State:
    Province du Quebec

    -Return to Top-

    Take two identical dial calipers that are mechanically capable of an accuracy of +/- .001'' and resolution of .001'' with graduations of .001'' on the dial, if one reads 1.236'' and the other reads 1.235'' on the same piece what do you make of it?
     
  22. woochucker

    woochucker United States Active Member Active Member

    Likes Received:
    172
    Trophy Points:
    43
    City:
    Pittstown
    State:
    New Jersey

    -Return to Top-

    Depending on the actual size, using a certified caliper, if the size is one of those readings, then the accuracy for both is in spec. If the actual size is 1.234, then only one is in spec.
     
  23. Tozguy

    Tozguy Active User H-M Supporter-Premium

    Likes Received:
    603
    Trophy Points:
    113
    City:
    Drummondville
    State:
    Province du Quebec

    -Return to Top-

    And this is what I see through the spin:

    The number of decimal places in a readout is meaningless if the instrument itself cannot in fact resolve measurements to that number of decimals or to that fineness of graduation of the readout.
    Just like how a speedometer that reads to 250 mph does not garantee that the car can go that fast.

    The design of a sliding caliper does not lend itself to making measurements more precise and accurate than +/- .001'' (.002'' total). Plastic or composite calipers probably have special applications (i.e. measuring magnets or delicate materials) but might not be as durable, as accurate nor as precise as stainless ones.

    We should be aware how easy it is to be lulled into relying on a digital calliper, that reads to four or five decimals, for accurate and precise measurements. In fact the actual size of the object being measured might be quite different than what the readout is showing.

    For work to the last .001'' or less, a micrometer should be used.

    Some good reading here: http://www.hobby-machinist.com/threads/metrology-101.22521/
     
  24. woochucker

    woochucker United States Active Member Active Member

    Likes Received:
    172
    Trophy Points:
    43
    City:
    Pittstown
    State:
    New Jersey

    -Return to Top-

    I think there is a mistake in the link. The 1. should be +/- .5 I think, the decimal is not holding any precision other than units, since there is nothing after the decimal.
     
  25. Ebel440

    Ebel440 United States Active User Active Member

    Likes Received:
    65
    Trophy Points:
    28
    City:
    Long Island
    State:
    New York

    -Return to Top-

    I see no reason to buy them really. You can get metal ones for 10$. With the plastic you would have more flexing and faster wear. Unless you need them for a specific use where plastic may be of some benefit
     
  26. graham-xrf

    graham-xrf United Kingdom Steel Registered Member

    Likes Received:
    12
    Trophy Points:
    3
    City:
    Alton
    State:
    Outside US / Canada

    -Return to Top-

    Precision? accuracy? And how to tell if there are games going on in the software!
    It gets harder unless we doggedly insist on being hard-heads about it. I guess the temptation among those sales oriented folk is overwhelming, and gaming the software to give you a nice warm "Gosh - that must be accurate" feeling is only one small step away from going the whole 9 yards like Volkswagen did with the software "recognizing" that an emissions test was under way.

    For me, it is often about angular resolution using encoder kit in satellite tracking Earth station antennas
    Stuff like this --> http://www.renishaw.com/en/optical-encoders--6433
    One of the products offers 26 bits, and for an extra £15, one can have 32 bits resolution, of a full circle.
    No surprise then that a computer sampling at 40 times/sec (in between lots of other tasks) sees all the digits to the right of the true accuracy dancing about randomly.
    Worse, the numbers are being crunched by a hard-working DSP (Digital Signal Processor), hard-programmed hardware to do trigonometric math on these useless extra numbers.
    1/(2^32) = 1/4294967296 = 2.32830643654e-10 . Try 0.3 milli-arc-seconds! Is that about the height of a pencil in NY, if one is in London? I don't know. It is beyond the ability of the calculator trig function.
    OK - so it strains credulity. Maybe there is some kind of very special application that requires it.. (might be?)
    So I try the 26-bit product. Keep in mind these things are very expensive!

    1/(2^26) = 1/67108864. Even then, the display will not allow those random digits to stand still.
    We have to discard the last bit because of the +/-1 bit uncertainty about the state of the least significant bit.

    Eventually, finding that the communication to encoder protocol to the servos can only do 22 bits.. Hmm the digits still dance about sampling noise!
    1/(2^22) = 1/ 4194304 (of a circle), or 85.83 micro-degrees, or 0.309 arc-seconds. Better than the theodolite!

    OK - we can have the system working fine, but that does not stop the sales guys bragging that it has (looking at the encoder) 32 bits accuracy.
    In fact, the seismic vibrations of me stomping about on the concrete floor were making the last two digits jump!

    Getting the terms used correctly may be near impossible amid the sales culture (yeah - mostly telling sort of lies!)
     

Share This Page