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Checking Squares with a DTI (Forrest Addy)

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  • Checking Squares with a DTI (Forrest Addy)

    From Forrest:

    The gadget you see in the photos is the base I made to check squares following the calibration method I described above.. The device's base is mounted on adjustable legs to clear the square's stock. The mast is offset so the DTI gets a straight shot at the square's beam. The little lip at the bottom contacts the square's beam near the stock. Naturally you can take DTI readings at any height and compare them to the parallelism of the beam thus by interpreting the readings determine the straightness of the beam to the accuracy limit posed by the limitations of the DTI. If the DTI was changed out for an LVDT and gage amp you could work to millionths.




    If one needed to know the flatness and straightness of the stock in contact with the flatness reference on which this procedure is staged, one only has to apply a haze of skin oil on the surface of the stock, rub it on the plate, and examine the haze for removal much as though taking a blue reading. In grazing light the haze removal will be apparent. If the removal is uniform the stock surface can be taken as straight within a few millionths. A spin test will eliminate the possibility of convexity confusing the rub reading.

    If care with temperature, nulling of the indicator reading, and repeat zero is employed and sufficiently accurate indicating equipment is used, a hard square may be calibrated in a large number of increments along the beam in the home shop as accurately and reliably as a fully equipped calibration lab. Thus for the home and small commercial shops this apparatus and technique is definitive for determining squareness and straightness of the beam of a hard square.




    Forrest
    Last edited by lazlo; 12-28-2008, 05:40 PM.
    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

  • #2
    Thanks for posting this Robert! However, you might want to edit and place the images on separate lines vertically.
    Russ
    Master Floor Sweeper

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    • #3
      This is... much easier to understand.


      Thank you.

      Comment


      • #4
        wow. now that's a square!
        .

        Comment


        • #5
          The first order errors in this would be then

          1) the finish on the "bumper strip" in case the absolutely exact same spot is not hit each time.

          2) the precision with which the blade of the square is known to be parallel as far as its front and back surfaces.
          1601

          Keep eye on ball.
          Hashim Khan

          Comment


          • #6
            A few other method`s to check for square . first you check a master to set indicator then check part.






            With the surface gage you set a ball in the v notch at the bottom of the gage.



            The top gage has a ball made in the bottom of the very front The second one is ground with a radius at the bottom front . With all you set indicator against a master square are angle plate then check the part.
            Every Mans Work Is A Portrait of Him Self
            http://sites.google.com/site/machinistsite/TWO-BUDDIES
            http://s178.photobucket.com/user/lan...?sort=3&page=1

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            • #7
              hmmm i check my squares with other squares .......if three line up without letting any light through when interchanged with each other there is a very good chance they are spot on ....well as near enough spot on for the use i put them to.

              my thoughts

              Squares are only used for checking the work before you use another method..........so i cant see any point of using any other method than that Ive just described ......or i could be wrong please say.

              of course a large square like forests could be used as a master for all smaller ..then it would need checking .........perhaps with a couple of friends large squares..

              all the best.mark

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              • #8
                epsilon. That old square is about a zillion years old, home made by some olt toolmaker named Tucker when he emerged from his apprenticeship and I got it at a garage sale after he passed on. It was pretty rusty but I got it cleaned up. I'd never checked it before I took those photos. It's accurate as hell, within a tenth if I get to pick the right spot on the granite flat. Come to think of it if a man wanted to noodle out a good system...
                Last edited by Forrest Addy; 12-29-2008, 01:00 AM.

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                • #9
                  Let's see the figures

                  All very well and good.

                  I have no real problems with the principles - I have a few - but I'd like to see the amounts quantified in terms of parts of an inch/mm per inch/mm or in parts per million (PPM) by quoting or citing the appropriate "Standards" etc.

                  This is an extract of/from a post I made on a separate current thread.

                  Answers and amounts please.

                  Originally posted by oldtiffie

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

                  I think that as many here seem to either be pulling numbers and/or "standards" "out of the air" that it would be instructive and add to the credibility if the standards and quantities and acceptable limits of or within those "standards" were posted.

                  This is particularly so given that these "Standards" specify different "classes" of items such as surface plates and squares - to mention just a few.

                  The US and ISO standards are pretty well the same as one is usually a "hard" and/or suitably "rounded" copy of the other - the UK (British - aka BS) Standards are similar.

                  So how about all you knowledgeable gents who are "calling up" or "quoting" or "citing" those "Standards" posting them for the edification of any who either haven't seen them or don't have access to them?

                  Comment


                  • #10
                    I'm all for a thorough survey of the particulars of the square under calibration ie visiual inspecting check lineariity, flatness, and parallelism of the square's elemets etc. I'm also for providing stickers and certificates, where the error is listed as a slope, and the direction of error is referred to the interior or exterior angle of the square reference, etc as I mentions on the other thread.

                    Sometime i wonder if people read all the way through these posts. I know I often don't.

                    Comment


                    • #11
                      Any more for any more?

                      Well,
                      once again, no one either knew or chose (not?) to rise to the challenge.

                      I suppose I will have to make my all-be-it feeble "best shot" and "give it a go" myself then.

                      So here is the ISO standard for "squares" - ISO DIN875

                      http://www.stormspa.it/cms/index.php...mid=47&lang=en

                      http://www.google.com.au/search?hl=en&q=din+875&meta=


                      Surface plates:
                      http://en.wikipedia.org/wiki/Surface_plate

                      http://www.starrett.com/pages/860_gr...ace_plates.cfm

                      http://www.tru-stone.com/pages/faq.asp

                      http://www.tru-stone.com/pdf/Fed%20S...GGG-P-463c.pdf

                      http://dspace.dsto.defence.gov.au/ds...ndle/1947/9371

                      http://dspace.dsto.defence.gov.au/ds...86-TM%20PR.pdf

                      http://www.google.com.au/search?hl=e...G=Search&meta=

                      I suppose that while I am at it and apparently "on a roll" I thought I'd "have a go" at "slip guages" (aka "Joe blocks"):

                      http://en.wikipedia.org/wiki/Gauge_block

                      http://www.google.com.au/search?hl=e...+gauge&spell=1

                      Now, that doesn't seem to be too hard to get - or does it?

                      If some old, decrepit, senile old "dick" like "tiffie" can do it, why didn't or couldn't??? anyone/someone else do it too/instead?

                      Now if my fading memory is correct:

                      a.
                      Grade 2 is "shop" grade;
                      Grade 1 is "Inspection" grade;
                      Grade 0 is precision grade.

                      b.
                      checking/calibration should be carried out by/against a standard at least two grades higher/"finer" under specified controlled conditions.

                      I have a set of "Grade 2" ("shop") slip guages as well as two "Grade 1" and two "Grade 2" try-squares - all to the required DIN standards (we are "Metric" in OZ - in case anyone either "didn't know" or "(just??) forgot".

                      OK.

                      Now that this decrepit , senile old "dick" (like??) "tiffie" - who is in his dotage - can do it, why didn't or couldn't??? anyone/someone else do it too/instead?

                      Now.

                      How about a reality check and re-stating all the test requirements and results in terms of the standards above.

                      But, a caveat.

                      They should not be in esoteric terms, but in terms that EVERYBODY can understand and relate to - if you can stand dealing with or at the level of the lowest common denominator.

                      Careful there - that's me at the bottom of the heap.

                      No?

                      Go on - I'm sure you can.

                      The Brits, being the good-hearted chaps that they are will lend you their "stiff upper lip" and all that. Jolly fine chaps.

                      Norm (aka "Aviemoron") - are you listening??

                      Any other comments?

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                      • #12
                        Originally posted by oldtiffie
                        b.
                        checking/calibration should be carried out by/against a standard at least two grades higher/"finer" under specified controlled conditions.

                        I have a set of "Grade 2" ("shop") slip guages as well as two "Grade 1" and two "Grade 2" try-squares - all to the required DIN standards (we are "Metric" in OZ - in case anyone either "didn't know" or "(just??) forgot".



                        How about a reality check and re-stating all the test requirements and results in terms of the standards above.
                        I'm not quite sure where the demand for academic citations came from..... this IS a HOME SHOP forum, and advanced metrology is highly amusing, and even nominally "on topic", but quite out of the realm of reality.....

                        Since I get regularly bashed for getting anywhere past the 0.1mm/0.005 general area of precision, its a bit rich for us to be getting into millionths now.

                        However, let me ask you, Tiffie...................

                        If the standard of precision must be 2 orders greater than the object measured.......

                        Everything is jake until we need to GENERATE or CHECK those advanced standards.

                        NOW we have a problem. This problem can only be solved with measurements using "natural constants" (light), and by manufacturing methods which "automatically" generate the desired result.

                        Lapping three stones together can generate a plane as perfect as your grit can produce. It is "inherently so" through geometric truth.

                        The measurements only verify that the generating method did indeed work, and that no errors crept in. They don't "make" the part.

                        However, the generating methods can, independently of the measurements, "make" the part *perfectly*. You can't prove it without measurements, but it could in reality be the most perfect plane surface ever generated.

                        Note that the measurements are verifying that it worked, not necessarily showing where to do more work. Of course the measurements also can show a correction table, either for actual correction of the piece, or for use in correcting measurements taken with it.

                        And, of course, you can use a different method and at least theoretically make a plane surface using only measurements to correct it and show where to remove high spots. Scraping is one such method.

                        So.... eventually the self-referential academic methods employing means 2 orders better than the item measured must fall back on outside input to provide a reality check.

                        And, eventually one MUST take something on trust..... By definition, you can NEVER "check the checking means", because your methods are many times more imperfect than the reference.

                        Your argument is merely with the point at which things are taken on trust.

                        Which brings it back to the HSM, who has likely got no gage blocks, let alone a wavelength-based measuring reference.

                        The best measurement possible to the HSM is therefore 0.001 (0.25 mm). What price your millionths?
                        1601

                        Keep eye on ball.
                        Hashim Khan

                        Comment


                        • #13
                          I'm with JT on this, all of those standards, while having a place in industry, mostly for protection from liability, are close to meaningless to a hobby home shop guy.

                          "I have no real problems with the principles - I have a few - but I'd like to see the amounts quantified in terms of parts of an inch/mm per inch/mm or in parts per million (PPM) by quoting or citing the appropriate "Standards" etc."

                          Here we have people spending there own good time to demonstrate techniques usable by others that frequent this board and all that can be pointed out is that these techniques are not supported by some "standard".

                          My question to you is WHY? Please explain how some project you are currently working on will be compromised without "...amounts quantified in terms of parts of an inch/mm per inch/mm or in parts per million (PPM) by quoting or citing the appropriate "Standards" etc."

                          Do you bring this up only to cast dispersion on the offered (freely no less) techniques while not offering any usable counter technique even in verbiage, much less pictures of the actual implementation?

                          oldtiffie, if I didn't know better I'd label you a poser or noob so you might consider how others might react that haven't read your posts for so long.
                          James Kilroy

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                          • #14
                            Originally posted by oldtiffie
                            All very well and good.

                            I'd like to see the amounts quantified in terms of parts of an inch/mm per inch/mm or in parts per million (PPM) by quoting or citing the appropriate "Standards" etc.

                            Answers and amounts please.
                            I don't understand this comment. Forrest is showing a jig with which you can test your machinist square by the method that Lane shows for a flat object.

                            You can measure the square with a plastic ruler, a DTI, or with gauge head, depending on how much accuracy you want/need.
                            "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

                            Comment


                            • #15
                              This is a little off topic for the origanal post, but I myself find this very interesting. In that most of the work I do at this time can be machined to a "by eye" method and I don't need these tolerances most of the time. But I have contemplated the idea of creating more accurate work in the near future. I have also gone down the path of trying to create a square that is accurate. I have taken / hosted one of Forrests scraping classes and enjoyed it immensely. I do believe that upon occasion any machinist would question how square, square is, just because of the inquiring mind that most of us have. Most of these tecniques are based upon the experiences of our fellow members and I for one am always looking for ways to make machining easier and more fun. Knowing that a setup is accurate at the start is part of this experience.
                              Please carry on, and remeber learning is good even if it means change.

                              Jay
                              "Just build it and be done"

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