Having trouble measuring a part

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  • Having trouble measuring a part

    Some of my results are inconsistent.
    On one pass a threaded hole will report .025" out-tol on true position.
    Measure it again, and it'll show out by .009".

    I'm aligning to the part itself, using the datum features specified on the drawing. I have CAD and am using version 3.7MR3.

    The main problem are the two threaded holes...especially the one farthest from the A datum. We (the engineers and I) decided to add in another datum, (D) not pictured, on the surface that this threaded hole is drilled into.
    That improved the results some, but not a lot. The material is also giving us trouble. It is G10 (Epoxyglass). We decided to try to measure the threaded holes with the inserts installed and a 6-32 threaded plug run into the hole. I measure the plug as a cyl, and then I project a circle onto its surface (3.636" from B) and dimension that for TP. Still getting shaky results. Repeatability seems to be an issue. I am currently re-calibrating my tips. ( I use two during the program.)

    Attached is the print:

    Is there a better way to align this? The A and B datum surfaces are SMALL. A is .545" X .350" and the new D datum, into which the second threaded hole is drilled, is .472" X .350". AND both of these datum surfaces have holes drilled in them, which reduces the available real estate from which I can take hits.
    This has been a pain in the ****.
    Any new ideas are welcome...

    The weirdest thing, is if I lay the part on a surface plate and measure the .175" basic dimension to the hole centers, it shows almost perfect (around .174 to .176). The report says that it meaured .1844" of something crazy like that. I've got a bunch of these to measure, which is why I'd like to use the CMM. The height gage and surface plate says some of my CMM numbers are not very believable. I'm confused.
    Last edited by d.evans; 01-29-2009, 05:13 PM.
    ** "Well, ain't this place a geographical oddity. Two weeks from everywhere!"~ Ulysses Everett McGill **

  • #2
    have you considered using a cylinder probe, and not useing the plug gage? It should help with aligning to -B- and -C-
    sigpicSummer Time. Gotta Love it!

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    • #3
      I'm not familiar with the material but if there's possibility of distortion I would probably try to simulate mating part by restraint and not measure in freestate.

      Looks like a simple part - datum features seems insufficient in size to spread points out - try to take more probe hits or simulate datums somehow.

      I would also not project a circle but pierce the plane creating a point or construct a circle using intersect (second opt may not be available depends on software?) much more reliable than projecting.

      Check setup - LOL
      PC-DMIS CAD++ 3.7 from 4.2 MR1

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      • #4
        This sounds like a pretty light material. How are you holding it? It is very possible it is moving when you take hits! I would use double sided tape or clay to hold it in place. I would use a small ball ruby (1 or even .5mm), and enter the pitch of the holes. I do not like plugs, they almost always kick to one side or wiggle. HTH
        sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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        • #5
          When they machine this part, are they only locating on just the -A-B-C- datums or are they also locating from lets say a datum -B1- locater on the face at the 3.136 dimension?

          I would ask eng., if this is an in house part, to call datum -C- datum -A-, ask that datum -B- become two locating points -B1- where it is now and -B2- on the 3.136 and change datum -A- to -C-. this way datum -A- can be created using 3 points on a larger surface from top to bottom and datum -B- will be a better rotation for you to hold.

          I am not positive on this, but by using your print the way it is now, why can't you level datum -C- using 3 points or more over the whole surface. Datum -B- is probably the one that is causing allot of the trouble because there is such a small area to level, but if manufacturing is using another area to hold rotation, you should also be able to use it. Since there are not any set target points for your datums this should be acceptable.

          If you were to place this part in a square corner for the datums, besides datum -B-, you would lye on -C-, butt up against -A-, with -B- being the little bugger that will be the hardest to rotate to accuratly every time.

          Good Luck.
          sigpic

          B&S ADVANTAGE 12-22-10, EXCEL 9-15-9, ETC.
          PCDMIS 4.1, 3.5mr2,

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          • #6
            I agree with Vondy that the datum structure is not the best setup. Rather than changing the datums they could just change the datum order in the FCF to CBA instead of ABC. Good luck getting them to do this as they probably have good reason for setting it up like this.

            I agree with Wes that you need to be carefull to hold the part rigidly without distorting it. I use hot glue quite a bit for plastic parts - it will usually peel off without too much trouble. Also, Wes is correct about using the pitch option in an autocircle or autocylinder to measure the threads. I do not necessarily agree with Wes on the ball size though. My first thought when you said you were getting non-repeatable results was that you were shanking on the holes. Use as small a tip as you can but be absolutely certain you aren't shanking.

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            • #7
              Originally posted by Goodluck
              I agree with Vondy that the datum structure is not the best setup. Rather than changing the datums they could just change the datum order in the FCF to CBA instead of ABC. Good luck getting them to do this as they probably have good reason for setting it up like this.

              I agree with Wes that you need to be carefull to hold the part rigidly without distorting it. I use hot glue quite a bit for plastic parts - it will usually peel off without too much trouble. Also, Wes is correct about using the pitch option in an autocircle or autocylinder to measure the threads. I do not necessarily agree with Wes on the ball size though. My first thought when you said you were getting non-repeatable results was that you were shanking on the holes. Use as small a tip as you can but be absolutely certain you aren't shanking.
              Goodluck, shanking is always a concern. Even more so with a cylinder probe, which is why I do not recomend or use those. But a 6-32 hole is pretty small, (.110" or 3mm roughly), and you would have no choice but to use a 2.5mm or smaller tip. I have measured some 6-32 holes with a 2mm tip, but if they are out of location at all you will foul out before you ever get into the hole. A smaller tip helps you be able to measure the hole even if it is .005-.010" off location. I do not use surface hits or find hole because I do not do sheetmetal and have never needed those functions, but I can see they might help with this part and permit the use of a larger ball than I suggested even if the hole is off location.
              sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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              • #8
                Originally posted by Wes Cisco
                Goodluck, shanking is always a concern. Even more so with a cylinder probe, which is why I do not recomend or use those. But a 6-32 hole is pretty small, (.110" or 3mm roughly), and you would have no choice but to use a 2.5mm or smaller tip. I have measured some 6-32 holes with a 2mm tip, but if they are out of location at all you will foul out before you ever get into the hole. A smaller tip helps you be able to measure the hole even if it is .005-.010" off location. I do not use surface hits or find hole because I do not do sheetmetal and have never needed those functions, but I can see they might help with this part and permit the use of a larger ball than I suggested even if the hole is off location.
                I would not use a cylinder probe either. I use a 2mm tip all the time. I would just be hesitant to go much smaller - especially on a thread where you could end up with your hits half way between two thread peaks. I also use find hole quite often. It works great!

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                • #9
                  Originally posted by Goodluck
                  Also, Wes is correct about using the pitch option in an autocircle or autocylinder to measure the threads. I do not necessarily agree with Wes on the ball size though. My first thought when you said you were getting non-repeatable results was that you were shanking on the holes. Use as small a tip as you can but be absolutely certain you aren't shanking.
                  I am using a 1mmBY20mm probe. Originally, I was using a larger probe (3MM) to for the alignment and then switched to the smaller probe for the holes. I have now gone back to the beginning of the program and deleted the 3mm probe and am using the 1mm probe for the whole program.
                  I have been using "PITCH" in auto circles for these holes.
                  QUESTION- Is the pitch the same for holes threaded for helical inserts as the pitch for straight tapped holes? I know the diameter is bigger for helical inserts, but is the pitch the same? That is what I was dealing with. The epoxy-glass material is extremely rigid, but when tapped for threads, it is nasty. Lots of garbage left in the holes.
                  That's why we decided to install the inserts and measure them that way, so there would be no garbage in the tapped holes.

                  I am going to go back and add in some additional hits on the datum surfaces and see if that improves things.
                  I'll let ya know how it comes out.
                  Thanks for all the suggestions.
                  ** "Well, ain't this place a geographical oddity. Two weeks from everywhere!"~ Ulysses Everett McGill **

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                  • #10
                    Originally posted by Goodluck
                    I would not use a cylinder probe either. I use a 2mm tip all the time. I would just be hesitant to go much smaller - especially on a thread where you could end up with your hits half way between two thread peaks. I also use find hole quite often. It works great!
                    Well with a 32 pitch you are looking at less than 1mm between crests. If you key your data in correctly all of your hits will land in the same location on the thread form, whether at the crest or in between. By that I mean if you use a cylinder the length of the cylinder needs to be a multiple of the pitch. I will experiment with find hole and see if it can help me in places.
                    sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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                    • #11
                      Originally posted by d.evans
                      I am using a 1mmBY20mm probe. Originally, I was using a larger probe (3MM) to for the alignment and then switched to the smaller probe for the holes. I have now gone back to the beginning of the program and deleted the 3mm probe and am using the 1mm probe for the whole program.
                      I have been using "PITCH" in auto circles for these holes.
                      QUESTION- Is the pitch the same for holes threaded for helical inserts as the pitch for straight tapped holes? I know the diameter is bigger for helical inserts, but is the pitch the same? That is what I was dealing with. The epoxy-glass material is extremely rigid, but when tapped for threads, it is nasty. Lots of garbage left in the holes.
                      That's why we decided to install the inserts and measure them that way, so there would be no garbage in the tapped holes.

                      I am going to go back and add in some additional hits on the datum surfaces and see if that improves things.
                      I'll let ya know how it comes out.
                      Thanks for all the suggestions.
                      The pitch should be the same for helical inserts.

                      As for the probe... A 1mm probe's diameter is .03937 inches. I don't know what the shank diameter of the probe is. The distance between peaks of a 6-32 thread is .03125 inch. If your probe is attempting to take hits in between peaks of the thread it is entirely possible that you are shanking. As the hole is so small I don't think you would be able to see this. I would try switching to a 2mm probe and see if that helps. Also, do a hole re-meas or find hole to help ensure hit vectors are good.

                      Another posibility - Check to ensure the probe is spiraling around the threads and moving in or out of the pitch correctly. For a right hand thread if the probe moves around the hole counterclockwise the probe should take hits progressively closer to the surface (move out of the hole). Set your pitch to .1 and measure. Watch the probe to make sure it moves the correct direction. Then you can change the pitch back to .03125.

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                      • #12
                        Originally posted by d.evans
                        I am using a 1mmBY20mm probe. Originally, I was using a larger probe (3MM) to for the alignment and then switched to the smaller probe for the holes. I have now gone back to the beginning of the program and deleted the 3mm probe and am using the 1mm probe for the whole program.
                        I have been using "PITCH" in auto circles for these holes.
                        QUESTION- Is the pitch the same for holes threaded for helical inserts as the pitch for straight tapped holes? I know the diameter is bigger for helical inserts, but is the pitch the same? That is what I was dealing with. The epoxy-glass material is extremely rigid, but when tapped for threads, it is nasty. Lots of garbage left in the holes.
                        That's why we decided to install the inserts and measure them that way, so there would be no garbage in the tapped holes.

                        I am going to go back and add in some additional hits on the datum surfaces and see if that improves things.
                        I'll let ya know how it comes out.
                        Thanks for all the suggestions.
                        When I said I suspected the part of moving, I meant more the whole thing sliding on the table than it flexing. I have had this problem with some aluminum and plastic parts that I would have thought were heavy enough to just sit on the table.

                        You do need clean holes. Can you blow them out well with air? Maybe even carefull chase the thread a couple of times with a sharp tap. Yes the pitch for the Helicoil should be the same. Maybe you could have the inserts installed and then measure the holes, this would hopefully eliminate the trash problem. HTH
                        sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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                        • #13
                          Originally posted by Wes Cisco
                          Well with a 32 pitch you are looking at less than 1mm between crests. If you key your data in correctly all of your hits will land in the same location on the thread form, whether at the crest or in between. By that I mean if you use a cylinder the length of the cylinder needs to be a multiple of the pitch. I will experiment with find hole and see if it can help me in places.
                          Yes, if you use a cylinder the length needs to be a multiple of the pitch to ensure you hit the same place on the threads.

                          I was trying to point out that if you hit precisely in between two crests that if your ball is too small you will shank. Think about a 1mm ball (.03937 in), now you wouldn't use that to measure a thread with a pitch of 8 (.125 in between peaks) would you? If you did you would need to make sure that you hit on a peak because otherwise the ball will "fall" between the threads and you will shank.

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                          • #14
                            Originally posted by Goodluck
                            Yes, if you use a cylinder the length needs to be a multiple of the pitch to ensure you hit the same place on the threads.

                            I was trying to point out that if you hit precisely in between two crests that if your ball is too small you will shank. Think about a 1mm ball (.03937 in), now you wouldn't use that to measure a thread with a pitch of 8 (.125 in between peaks) would you? If you did you would need to make sure that you hit on a peak because otherwise the ball will "fall" between the threads and you will shank.

                            I agree with your post entirely if we are discussing UN threads. Shanking must be considered and avoided. But I must point out because it may help someone out there, that I would measure an 8 pitch with 1.5X30mm tip, I do it all the time, but ONLY if it is a hobbed stub acme thread, because in that case the thread will occur at the same place on every part and the stub acme form means there a "largish" land at the crest for me to hit on. I will fine tune this on the first run so I am getting good hits. I can almost always see at least the top few hits, and you can tell by size if you falling off the crest.

                            I love that 1.5X30mm tip. It is my most used. The first .87" are the .059"dia ball and .04" dia stem, this gives almost .01" to a side clearance. But you have to be careful as it is easy to convert these from measuring to fishing equipment.
                            sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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                            • #15
                              Originally posted by Goodluck
                              The pitch should be the same for helical inserts.

                              As for the probe... A 1mm probe's diameter is .03937 inches. I don't know what the shank diameter of the probe is. The distance between peaks of a 6-32 thread is .03125 inch. If your probe is attempting to take hits in between peaks of the thread it is entirely possible that you are shanking.

                              OK, I did a little research and a little trigonometry. Looks like I was wrong. I won't get too math heavy here so I will just give you results.

                              For a 6-32 thread the minor diameter can be as small as .104 and the major diameter was listed as .138 (machinery's handbook). This gives us a thread depth of .017 in.

                              Now, The distance between the crests would be .0196 (crests are truncated). Using half of this and the radius of the ball we can calculate the distance from the center of the ball to the point at which the ball contacts the thread crests (.0220in). Doubling that number gives you the maximum diameter (.044in) of a shank that would allow you to measure with the shank just touching.

                              According to the renishaw catalog a 1mm tip has a shank diameter of .03in. So, if the hits occur exactly halfway between two crests the shank will have a clearance of half of .014in (.007in). Now, are the vectors of the hole and the shank parallel enough to avoid shanking?

                              Edits made in Red
                              Last edited by #2#; 10-06-2006, 12:38 PM.

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