Announcement

Collapse
No announcement yet.

new version of Caliper Accuracy article available

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #46
    I am still unconvinced as to the existance of a temperature compensation scale on a Starrett Master Vernier caliper. I have seen dozens, and possessed more than a few at one time or another, and none had any such device. It definitely does not appear in any of the catalogs. I do not consider someone's memory of something that may or may not have been correctly described on eBay several years ago as acceptable proof.

    The Starrett Master Vernier caliper 123 Series was introduced in the early 60's. Prior to that they sold only the 122 Series, which had the short, 25 division vernier.

    While the standard of length might be related to some imaginary distance traveled by a beam of light in some unmeasureable amount of time in an unachievable atmosphere, that has nothing to do with taking a measurement.

    All precision measurements are based on standard temperature conditions. A measurement of a part at 120*F will result in a different dimension than the same part measured at 0*F. If the measuring device did not change with temperature, it would indicate two different dimensions. If a part were mis-machined, all that would be needed to bring it into tolerance would be to heat or cool it. If the measuring device were hot and the object cold, or vice-versa, the error would be compounded. This is why all dimensions ultimately must relate to some standard temperature, with the measuring instrument and the part normalized at that temperature.

    There are several different protocols used by the various manufacturers in their digital instruments. There is no reason to believe that they would all behave the same.

    I have no idea who Nazakawa is, perhaps he was selling that caliper on eBay. He is referring to measurement, we are referring here to calibration. The standard method of calibration will rely on the use of the next smallest increment. In the case of metric and decimal measurement, that is a factor of ten.
    Jim H.

    Comment


    • #47
      Originally posted by Evan
      A couple of years ago there was a 72" Starrett Master vernier for sale on E-bay. It had a temperature compensation scale as shown in the photographs. As for the Starrett engineer, he probably wasn't born yet when it was made..
      What is the point of a temperature compensated caliper. All materials expand and contract togather, but at different rates.

      The best guess of size you can make is if part/instrument are the same temp unless calculations have made as respect to a certain situation.

      Even Invar has some thermal movement, It is utalized to point telecopes in deep space with solar powered heating coils.

      If you measure a part that is 100 degrees with a compensated caliper, you will get an exagerated error. The part is in an expanded condition, and the caliper is not, producing more of an error than exhisted with a warm caliper.

      My foreman many years claimed we should measure parts with a caliper
      at 68 Degrees, and the part at 140. This was big stuff measured with a 72 inch Master Starret Caliper.

      The parts hit inspection .03 undersize. The head inspector set him in his place.

      Later we stored the bore gages in pockets in the coolant tanks of cylindrical grinders to mantain reletive temperatures of parts, and gauges.
      This was to hold tenths tolerance.

      Today, some machining centers have water cooled ball screws supposed to mantain size. This will work when the parts do not absorb much heat from the machining, but not on heavy machining operations.

      I, in 38 years have not seen a compensated caliper but there may be one somewhere.

      I could imagine a caliper that can compensate for different materials and temperatures these days could be built, but not before that Starret Engineer was born.

      Kap

      Comment


      • #48
        Thank you JC your explanation is much more eloquent and precise than Mine might ever be.
        Evan , in my conversation with the Starrett engineer he made it clear that there is not and never has been such a thing. If there was , would Starrett have parts sitting around for 24 hours before measurement? How , I wonder would a caliper know what the temperature of the workpiece was? Even if it compensated for it's own temperature, (which means , I guess the difference from standard temperature,) how would it compensate for the workpiece temperature? And why would anyone develope such a thing when all you have to do is have everything at the same temperature? Again, the man said that there is not and never has been such a device. He did say that at one time there was a primitive electronic height gage that had a circuit to allow for heat from the electronics. It had no correlation to part or ambient temperature and was not successful.

        Comment


        • #49
          Thank you JC your explanation is much more eloquent and precise than mine might ever be.
          Evan , in my conversation with the Starrett engineer he made it clear that there is not and never has been such a thing. If there was , would Starrett have parts sitting around for 24 hours before measurement? How , I wonder would a caliper know what the temperature of the workpiece was? Even if it compensated for it's own temperature, (which means , I guess the difference from standard temperature,) how would it compensate for the workpiece temperature? And why would anyone develope such a thing when all you have to do is have everything at the same temperature? Again, the man said that there is not and never has been such a device. He did say that at one time there was a primitive electronic height gage that had a circuit to allow for heat from the electronics. It had no correlation to part or ambient temperature and was not successful.

          Comment


          • #50
            Originally posted by ckelloug
            Well guys,

            Putting aside my early aggrevation about style of discourse issues, I've actually put my gage blocks to use. The results are as follows.

            For the purposes of metrology, the gage blocks are B89-0 Starrett Webber steel blocks etched ZHH030 Calibrated 1/24/07 with calibration certificate 07-47943A-F from Starrett Webber Gage traceable to NIST via NVLAP partnership under lab code 200038-0. Room temperature was 72 degrees. The gage blocks had acclimated at that temperature for several weeks. The caliper had acclimated for several hours with the box open. Cotton gloves were worn during the handling of the blocks and the caliper to minimize contamination of the blocks and thermal transfer from the hand.

            The first column is the Gage block in use, the second is 5 measured values from the caliper for that block. The caliper was brought to the jaws closed position and zeroed for each measurement. I was blown away on the accuracy of the HF caliper. I tried graphing the raw measurements but the line is so flat that it shows nothing. I graphed the error but this board doesn't let you upload pdf files. I can e-mail it if anyone is interested.

            Finally, I got a fourth order regression fit of the data representing the caliper measurement vs. the absolute length of the item in the jaws which came out to

            M = -0.00017683 + 0.99985 * L + 0.0001182 * L^2 - 0.00011015 * L^3 + 2.3791e-05 * L^4

            Matching terms with the model I published earlier, epsilon is -.00017683, dl/dL is .99985 and d^2l/dL^2 =.0001182. This implies that the scale has about -.0002 offset built in and that it is about 1.5 tenths per inch shorter than ideal . The higher order terms are important as well and they indicate that the error in the scale is a bit nonlinear but that some of the error from low order terms cancels out error from higher order terms in some places. A touch of the error could be measurement technique on my part but it seems at least in part actual error.

            At any rate, here are the results:
            Block Caliper
            .0500 .050
            --------------------------
            ---------------------------
            4.0000 4.0005
            4.0000 4.0005


            In conclusion, the caliper was never more than .002 off and a bit of that could be technique. My only observation is that the .0005 readout on the caliper is actually worse than useless as it is almost purely noise and the since the caliper seems to read just a touch low, that digit gives a false sense of accuracy. It may be that once I have instrument oil and can wring blocks to measure at 5 and 6 inches that the error will go up more.

            Also hope I didn't get too annoyed on this thread. The comment I made about being right being the main factor in being right was only an indication that I support hard data over soft data even if it proves me wrong. I bailed from the thread initially because I didn't want to hang around to feed the trolls. Now that I know td isn't a troll I'll just say I personally favor polite discourse over the vaguely ad hominem stuff. Most people on this board are so helpful that it's a shame to be mean to any of them. Sorry td if I got a bit upset.

            Regards to all,

            Cameron
            Cameron,

            This data is pure gold. If you are willing, I would like to include this data in the article. You do use a few concepts that are foreign to me so I would want to work with you to explain them in more detail. Please contact me directly if this sounds good to you.
            Rick Sparber

            [email protected]
            web site: rick.sparber.org

            Comment


            • #51
              Originally posted by tdmidget
              Rick , my problem is that you froze the part, measured it , and called the difference "error". It is not. When you froze the part it became smaller and your new number reflected that. If you ground .0005 off and measured it would that be an error? Of course not. In either case the ppart as measured WAS .0005 smaller.
              Evan, I just got off the Phone with an engineer from Starrett who I'll be nice enough to leave his name out of the post. I asked him to settle this arguement. Has Starrett ever made a caliper with temperature compensation, is it possible? His reply- "What has that guy been smoking?

              You are right. This is not error and I have fixed the text. It will be available in version 4.

              Thanks,
              Rick Sparber

              [email protected]
              web site: rick.sparber.org

              Comment


              • #52
                I just bought a HF plastic caliper--can I do mission critical work with it?

                I'm an old fart and I wanted it for measuring stuff at the yard.
                Last edited by ligito; 06-06-2007, 07:01 PM.

                Comment


                • #53
                  ligito,

                  We only test prime grade steel measuring instruments on this thread. You kids with your plastic calipers: get off my lawn

                  In reality, I have no data on the plastic HF calipers though it might be interesting. I got involved here because I happened to have a set of gage blocks that are in calibration and itching for use.

                  The steel HF calipers appear to work quite well out of the box and I'd suspect the plastic ones do too to at least some extent. What I'd question is whether either continues to be accurate after hard use or use after a period of time. I'd also question whether they show warning signs and then start producing bad measurements or whether they just die a dusty death: out out brief caliper. . . The question is whether using them for mission critical applications can be considered a tale told by an idiot and whether it is full of sound and fury signifying nothing.

                  --Cameron

                  Comment


                  • #54
                    I do not consider someone's memory of something that may or may not have been correctly described on eBay several years ago as acceptable proof.
                    That's your perogative. I do clearly recall seeing it though. It was brought to my attention by a posting here.

                    While the standard of length might be related to some imaginary distance traveled by a beam of light in some unmeasureable amount of time in an unachievable atmosphere, that has nothing to do with taking a measurement.
                    That's true only if you wish to use your own arbitrary units of length. I will continue to use the official units.

                    All precision measurements are based on standard temperature conditions. A measurement of a part at 120*F will result in a different dimension than the same part measured at 0*F. If the measuring device did not change with temperature, it would indicate two different dimensions.
                    Of course it will. That's what it should do. The part has changed size unless it is made of super invar or zerodur. If we allow the measuring instrument to change size as well so that it still indicates the same value then it is lying to us. Why do you think that all CMM devices are temperature compensated?

                    Even Invar has some thermal movement, It is utalized to point telecopes in deep space with solar powered heating coils.
                    Invar and super invar are different alloys. Super invar has virtually no change with temp. If I mentioned invar I meant super invar. I also note that Mitutoyo makes calipers with a carbon fibre/epoxy body. This has the property of nearly zero CLE with temperature.

                    How , I wonder would a caliper know what the temperature of the workpiece was? Even if it compensated for it's own temperature, (which means , I guess the difference from standard temperature,) how would it compensate for the workpiece temperature?
                    The caliper doesn't know the temperature of anything (unless electronic), the operator does. It isn't the workpiece we care about when compensating for temperature. The objective isn't to compensate for the artifact (workpiece) temperature but to compensate for the reading error caused by the change in the caliper with temperature.

                    If that's what you asked the Starrett engineer then you asked the wrong question and the answer is meaningless.
                    Free software for calculating bolt circles and similar: Click Here

                    Comment


                    • #55
                      This thread has a life of it's own.

                      I've found a manufacturer of special temperature compensated gages which measure both the temperature of the part and the gage and factor them both into the readings.

                      http://www.albiondevices.com/Large%2...ter%20Gage.htm

                      I doubt such gages are common in the context of the argument on this thread but here's an existence proof.

                      --Cameron

                      Comment


                      • #56
                        Yes, I am aware of that company. The bar gauge is essentially a type of caliper and is temp compensated.
                        Free software for calculating bolt circles and similar: Click Here

                        Comment


                        • #57
                          Absent proof of some sort, your memory of an item listed several years ago on eBay is not valid. You certainly would not accept that from another person as proof. eBay is a worse source of information than Wikipedia. Starrett engineering, their catalogs and catalogs of other manufacturers all refute the existance of such an item. The experience of at least three people with a great deal of experience in the area reinforce that.

                          The unit of measurement and whatever means are used to determine it is meaningless in this context. It is only after some solid standard is created to some fixed dimension that it can be used for calibration purposes. That standard is kept at standard temperature and used as a reference. It is only that dimension at standard temperature.

                          Temperature compensating the measuring device, such as a CMM only compensates the device. The device normally has no means of determining the temperature of the part, and cannot relate it's dimension to standard temperature. Almost any CMM I have ever seen has been in an air conditioned, humidity controlled room. Since CMMs are not being discussed, they are irrelevant.

                          The Albion gage also is not a slide caliper as being discussed here. It is usually referred to as a special gage, bar gage or comparator. These and similar are available from most manufacturers, including Starrett. They are typically a fixed gage used for inspection. They are usually set with Jo blocks, and used with an indicator of some sort to measure over/under.

                          The ones pictured have an LVDT and computer which measures the temperatures of the gage and the part and compensates for any temperature difference. It simply calculates the inch/degree change for a given alloy and adds or subtracts that to or from the dimension to give it in absolute terms at standard temperature. The compensation would have to recognise the alloy of course. The gage itself is not temperature compensated. This is quite clearly explained in the descriptive information.
                          Jim H.

                          Comment


                          • #58
                            You said it!!

                            Originally posted by ckelloug
                            This thread has a life of it's own.

                            --Cameron
                            Thanks ckelloug.

                            And for that you have won the first (and only) 3 prizes for "The Understatement of the Year" award.

                            But seriously, the thread has, at last not only developed a life of its own but it has also developed a momentum of its own (a la "big - bl**dy big -"ship").

                            The content and people that are contributing and they way they are contributing is marvelous.

                            Rick Barber's pen or typing musty be flying!! ) you wanted good feed-back Rick - you've got it.

                            Thank you one and all for a tremendous co-operative and very professional "effort".

                            Comment


                            • #59
                              Evan and Ckelloug please note;
                              1. It's quite a stretch to call this thing a slide caliper.
                              2. The so called temperature compensation is a joke. It has a sensor that appears to be 1/2 inch or less in diameter for a part that is about 2 feet in diameter. Thus it will be "compensating" for a temperature reading of that small area under the sensor. The other side might easily be quite different in temperature. For example the man in the photos is handling it with his bare hands. What about his body heat being transferred into that area that he handles? What assurance do you have that it is same temperature throughout? Without that the "temperature compensation" is a merely a source of error. Now before you say that they would have to let it sit until the temperature is constant throughout, why not let it and the instrument sit in the same temperature. Then you eliminate the error and the need for temperature compensation. This looks like a hocus pocus sales tool to me. I don't think Starrett, Mitutoyo, or Browne & Sharpe are worried about these guys.
                              I have also seen a caliper of carbon fiber. It was a B&S and the reason for it has nothing to do with temperature. It was 72 inch capacity and was light enough to used by one person. It still needs to be at standard temperature as does the workpiece. It is even more critical with this material as the coefficient of expansion is not the same as the workpiece.

                              Comment


                              • #60
                                Absent proof of some sort, your memory of an item listed several years ago on eBay is not valid
                                It is to me. It wasn't the description I relied on in the sale, it was the photo showing the compensation scale. The operator estimated the instrument temperature and moved the vernier according to the compensation scale to correct the reading. Simple.

                                Temperature compensating the measuring device, such as a CMM only compensates the device. The device normally has no means of determining the temperature of the part, and cannot relate it's dimension to standard temperature.
                                So what? That isn't what I am talking about. I am talking about keeping the measuring instrument accurate at a range of temperatures.

                                Evan and Ckelloug please note;
                                1. It's quite a stretch to call this thing a slide caliper.
                                That's why I didn't bring it up. It does however serve a similar function.

                                The so called temperature compensation is a joke. It has a sensor that appears to be 1/2 inch or less in diameter for a part that is about 2 feet in diameter. Thus it will be "compensating" for a temperature reading of that small area under the sensor.
                                I believe they are relying on the fact that most metals are decent conductors of heat. I expect the joke is priced rather high, too.

                                It is even more critical with this material as the coefficient of expansion is not the same as the workpiece.
                                You don't seem to understand. Ideally we want the measuring instrument to NOT CHANGE with temperature so that it ALWAYS gives the CORRECT reading when presented with a given dimension, REGARDLESS OF THE TEMPERATURE. The correct reading is whatever size the artifact currently is. If the measuring instrument changes size with temperature then the reading is incorrect unless compensation is applied. If you don't compensate for instrument temperature then you have no idea what your instrument is telling you.

                                As for the carbon fibre, I designed my telescope to use carbon fibre/epoxy composite struts because the CLE of the struts is very slightly negative with temperature. This, combined with the positive CLE of the aluminum parts ensures that the focal length doesn't change with temperature enough to change the focus. This is important when electronically capturing a series of 160 images over a period of an hour or more.


                                [added]

                                If you measure a part that is 100 degrees with a compensated caliper, you will get an exagerated error. The part is in an expanded condition, and the caliper is not, producing more of an error than exhisted with a warm caliper.
                                It doesn't produce more of an error. The compensated caliper tells you the correct current size of the object at whatever temperature the object is at. It's up to you, the metrologist, to determine what is happening because of temperature. It's the correct size for that object at that temperature. The uncompensated instrument tells you something but what is it? Without compensation or knowing what the compensation should be you have no idea.
                                Last edited by Evan; 06-07-2007, 02:23 AM.
                                Free software for calculating bolt circles and similar: Click Here

                                Comment

                                Working...
                                X