Checking a gage

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  • Checking a gage

    I am laying out a gage and would like a few ideas on doing this

    This is what I have done so far.

    I have cad for the part that goes on the gage. I created 3 spheres and set them to the coordinates by the spheres. I did this in manual mode. Then I did an Iterative alignment for the tooling balls. Then went and checked the 2-way & 4-ways on the gage and they were off to their nominal to the part cad. I then did an Iterative alignment with the 2-way & 4-way pins and a vector point to set the gage in the correct car coordinates. Then recalled the tooling balls and they were out. I changed my numbers at the beginning of the program to reflect what I had got from setting it to the car coordinates. I then redid the program with the tooling balls up dated and my 2-way & 4-ways were almost perfect this time. The 2-way for the gage is out (.16 long) in X so I think I have to lower one of the tooling balls to bring it in. All the other dims 4-way and vector point are almost perfect. The X direction for the part is a slot.

    Some of the questions I have are:

    When you get a gage in and are not sure of the tooling ball locations or the stamped numbers on the gage how do you begin with verifying the gage?

    If you layout a gage and your nominal for your 2-way & 4-way do not match your part print how do you go about setting it so they do?

    I use 3.5mr2

    Thanks for the help.

  • #2
    i think the fixture shop has to measure the locations of the tooling balls by aligning to ABC, then stamping the numbers on there. so i think your on the right track.

    this may also help...

    Ford Checking Fixtures General Construction Tolerances.

    These tolerances are to be used in the construction and certification of stamping and Assembly checking fixtures unless otherwise specified by Ford Motor Company Dimensional Control Department.

    1) CMM starts to machined edge of base = +/- 0.125mm.
    2) Surface locators to CMM starts = +/- 0.05mm controlling direction.
    3) Surface locators to CMM starts = +/- 0.2mm non-controlling direction.
    4) Positional tolerance of Pin locator to CMM starts = +/- 0.05mm.
    5) Nominal Pin diameter is .07mm undersized from product design.
    6) Pin diameter tolerance is +/- 0.0127mm.
    7) Check rails profile to CMM starts = +/- 0.15mm.
    8) -4- way (H) Locator Pin to -2- way (h) Locator Pin relationship = +/- 0.05mm controlling direction
    9) -4- way (H) Locator Pin to -2- way (h) Locator Pin relationship = +/- 0.1mm non-controlling direction
    10) Template (Flipper details) surface profile = +/- 0.15mm
    11) Probe bushing for Data Myte measuring points from CMM starts +/- 0.20mm non-controlling direction.
    12) Probe bushing for Data Myte measuring points from CMM starts +/- 0.10mm controlling direction
    13) Base must have a flat and parallel to +/- 0.10mm
    14) Base must be level to 0.l25 total overall tolerance.
    15) Generic holding fixture - Columns machined flatness and parallel hole to hole = +/- 0.05mm.
    16) Generic holding fixture - Columns square to base within 0.075mm
    17) Master Body Gage (Blue Buck) overall tolerance from front to back = +/- 0.13mm all directions.
    18) Master Body Gage locating pin relationships, Pin to Pin = +/- 0.13mm all directions.
    19) Check rail Flush check trim line 90 Degrees to surface of metal when angle is over 7 Degrees.
    20) Check rail Flush check trim line 90 Degrees to surface of base when angle is under 7 Degrees.
    21) Scribe line details to CMM starts = +/- 0.125mm

    EDIT: i have the offcial doc if you want me to email it to you.
    Last edited by Guest; 03-01-2006, 12:55 PM.


    • #3
      I don't use the tooling balls. Just the datums. I set up from them (dcc), and manual from a square corner.
      RFS Means Really Fussy Stuff

      When all you have is a hammer - everything looks like a nail....


      • #4
        Well, last first: What is master? The cad or the print MUST be master. Which ever is master is what you need to use for nominals.

        First time the gage comes in. Well, Iterative is NOT the way to go, as you are finding out right now. What number gets adjusted and which way to get the part locators in tolerance? So, start out by measuring the tooling balls. Construct a plane through the centers and a line through the two that are in line (in theory). Too many build sources are using Iterative alignments on the part locators, then measuring the tooling balls because they can't hold the required tolerance or don't want to bother trying, so this is their 'easy' way out (and it dumps more work and fuss and bother on the QC people). So, look at the numbers for the tooling balls. There should be a number that is common on all 3, or real close to it. If it is NOT common, then average it out and use that. Then, the two balls you used for the line should have a common number (or close to it). Now, you have a level feature (the plane) and a rotate feature (the line) and an origin feature (one of the balls). Do the Level, Rotate, Origin using those 3 features in a regular alignment, NOT iterative. Then do the OFFSET DISTANCES using the values on the ball you used for origin. Now, go check the part locators. EVERYTHING has a tolerance, USE IT! Most common for sheet metal fixtures is 10% of the part tolerance to a maximum of +/-0.002" (this is for the locators). If the nets all check 'low', you can simply adjust the offset value in the alignment by that amount, DO NOT UPDATE when it asks you if you want to after you change the number in the alignment (do this from the edit window, do NOT F9 the alignment). If both pins are off in one direction, do the same there. If you can get all the locators to balance within the tolerance, there is NO need for an iterative alignment and the regular alignment is MUCH easier to tweak. Also, after you change the offset in the alignment to balance the locators, AND BEFORE you use the cad data again for any features, you MUST do a CAD=PART AGAIN by placing the cursor just below the alignment, then click on CAD=PART (it will be under OPERATION ~ GRAPHICS DISPLAY WINDOW).

        I do this everytime a fixture comes in here. I do a regular alignment using whatever the fixture has, tooling balls, datum edges, whatever. I then check every part locator on the fixture, then tweak the alignment values until they all come within tolerance. After the manual measurements, I do make a DCC alignment and that is the only one I tweak around, the manual one does not matter that much.
        Originally posted by AndersI
        I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.


        • #5
          matt excelent post! i tried to add to your reputation points, not sure if it worked.

          do you guys check your parts on the fixture or freestate or both?


          • #6
            Well, that depends on the customer. About 95-98% of the time they are checked constrained on the holding/checking fixture. Once it a great while the customer wants to 'know' how something checks in a free state. And then sometimes, they just won't make a fixture for a part and I hold it with magnets. Right now, I have a part that I am trying to get someone elses program running on my machine to check. No fixture, no road map, and programmed almost 100% wrong. Vector points to check edges, about 48 alignments and alignment recalls that I can see in the program, some of them to get no more than a 3-D distance of a point to a plane (they had to align to the plane, then dimension a point! ???) and it is one that there is no fixture for and will not be a fixture for. Big part, about 2 foot square, about 6 inches high, and checks on all sides and top and bottom. And, one end if flat on the bottom, so it can sit on blocks, the other end is curved, and the part is very flimsy, so how do you hold it with any repeatability? I HATE working without a fixture! So, I mostly do fixture-held, but do some free-state.
            Originally posted by AndersI
            I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.


            • #7
              i was checking some gas tanks recently (~3 sq. ft.) and at first they wanted them checked in the free state. the tanks where so flimsy, i was resting on the four nets and you could lift the middle of the tank and see about 5 to 6 mm of sag. naturally the parts check like crap. i put a couple jack screws in the middle where the sag was, raised them close to nom and the parts checked perfect, offcourse. they ended up having to get fixtures made and put rests in the middle area because four nets on a part that size was clearly not enough. sometimes there is noway to check a part with out a fixture and get good results.

              as for the program you have to decifer, it sound like it would be easier to just rewrite it. lol vector point for edges , must have been newbie.


              • #8
                I always align right off the datums of the fixture. I figuire that is what is actually holding the part, so normally I can't go wrong. The only time I come off the balls are when I have no other options. The customers that have been in the CMM room while I have came off the datums have all agreed with me that it reduces room for error. I agree with Matthew though. 1st thing you need to do is find out what is "Gospel" as we call it here in Tennessee. The cad or the print.


                • #9
                  I will check parts in free stat or with a gage as a holding fixture. It all depends on what the print has for measurements. In car or from a datum on the part.

                  Thanks for the help. I am checking an F250 hood hinge final assembly gage to make sure everything is in the correct location. I like using the tooling balls because I can put the gage on the table and measure them and be set to go. I am using the cad as the master. I was always told in automotive that cad is the master. The gage has these numbers stamped on it.

                  Sphere 1 1650.05, -639.97, 22425.01
                  Sphere 2 1650.00, -1000.00, 2425.00
                  Sphere 3 2165.06, -1000.00, 2425.01

                  After I measure the 2-way & 4-way and do a vector point they read this

                  Sphere 1 1649.860, -640.099, 2424.854
                  Sphere 2 1649.989, -1000.057, 2424.798
                  Sphere 3 2165.038, -999.783, 2426.004

                  I changed them to this and my 2-way & 4-ways and vector point were all less the then .003mm. Except for the X direction of the 2-way pin. Matt I am going to give your way a try to see how it works. I am trying to find something that will work all of the time and try to do it the same way gage makers would do it.


                  • #10
                    One thing that I am sure you are aware of, but maybe only in the back of your mind, is the spread. I currently have 5 fixtures for little parts (less than 3" in X and Y) that sit on a big fixture (18X18 base) with tooling balls located on an average spread of 12". Now, you check those 3 little nets that are almost on top of one another and call them 'level' then check the tooling balls, you see error in the 0.5+ mm range. However, if you measure the tooling balls and call them level, then measure the nets, you see less than 0.01mm. Now, which way makes more sense? To me, it is the balls as level. The more distance between the master alignment features, the less error you are going to put into your measurement. I mean, what happens to your level plane when that floating piece of dust falls on the net just as you are measuring it, then you measure 2 more nets 1" away and call that level? A whole lot more than if that piece of dust fell on the tooling balls that have a 12" spread. This might be over the top to some, but that is the way I look at it.
                    Originally posted by AndersI
                    I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.


                    • #11
                      Winston: I would love to get ahold of the Ford Checking Fixtures General Construction Tolerances.

                      Thanks in advance.
                      [email protected]


                      • #12
                        OK matt

                        I set the gage up the way that you said to. Works great but now I measured my locators and they are off as follows.

                        Datum B is out in X=- 0.111, Y= 0.136, Z= 0.155

                        Datum C is out in X= 0.105, Y= 0.064, Z=- 0.093

                        Do I have to tweak the tooling balls now because the X & Z locations are out in different directions?

                        Thanks for the help.


                        • #13
                          What locators are those? Are they the locating pins or are they nets? If they are pins, and the Z axis is along the shaft of the pin, I would not ever report the Z value. As for the X, that is a simple shift, the Y is borderline, but both could be gotten within the 0.05mm (one would be +0.036 and the other -0.036). If those are Z axis vector points then you will probably have to tweak the alignment or go to an iterative alignment (YUCK) OR, you could put 0.010" of shim under the low one to bring it up to the same height as the other, then do a common shift. Without knowing which 'direction' those locators locate in, it is hard to say.

                          IF the Z axis is the locating axis for those pins, you can also do a rotate in the alignment to bring them into line. I would rather add a rotation to the alignment than use an iterative alignment. Simple trig will give you the angle you need and the command goes after the LEVEL and ROTATE and ORIGIN lines in the alignment but before the AXIS OFFSETS. Here is an example of the added rotation in the alignment, I just add it in the edit window. Yes, this way can take a little longer than doing an iterative alignment, BUT, it is SO MUCH easier to adjust the touch-offs than an iterative alignment. You will probably have to make a second set of adjustments to the ORIGIN OFFSETS after you add the rotation. This is because Pcdmis rotates around the CURRENT origin, which is why you have to add it AFTER you set the origins and BEFORE you offset them.

                          ALIGNMENT/ROTATE_OFFSET,15,ABOUT,ZPLUS <<<<<<added rotation
                          Originally posted by AndersI
                          I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.


                          • #14
                            Ok as a gage maker CMM guy let me shed some light on this subject about tooling balls and datums on a gage.

                            First The tooling balls are the ONLY thing that stay the same on a gage from initial check through final when a gage is being constructed. When the gage first comes in ALL of the nets will be full, ALL of the datum locators will be undersized qualification holes, ALL of the flush and feeler checks will be CNC cut but who knows where they REALLY are. So here is what happens:

                            The tooling balls are at some designers "nominal Car" position on the gage. The CMM guy picks up the tooling balls and origins on them to get into CAR. THEN he measures the flush and feeler surfaces and "bumps" the tooling ball numbers to get as much of the gage in tolerence as is humanly possible without going back to CNC and recutting the flush feeler rails. Then he will pick up the nets and locators and mark moves to get them on location as compared to the "bumped" tooling balls. The gage goes back out to the gage maker and he makes the adjustments to the gage to bring all of it in to tolerence. It then comes back into CMM and is checked AGAIN against the tooling balls. If its good, THEN and ONLY THEN can the CMM begin using the datums to check the gage. If your tooling ball numbers are messed up one of two things has happened. Either some bone head tried to START building the gage from the approximate datums, messed it all up and measured the tooling balls at the end and stamped BS numbers on the gage OR the gage has purposely been rotated out of CAR position.

                            I recommend using the tooling balls to get "in the ball park" on a gage. As soon as you are clsoe, go measure your datums and then adjust your alignment to bring in your datums after that, load your part and start checking.

                            If you're doing a second party cert or a recertification then aligning to your datums is MANDITORY. Using tooling balls for anything except initial build is just looking for trouble. On a gage that is 3 feet long, a change in temp of 3 degrees F can cause .002 -.003" change in over all gage size. How is that going to effect a 4 way that you're trying to hold at +/- .002"? My room STAYS at 70 degrees F. What is yours at? If it's at 73 then your checks are not going to be the same as mine. When was your machine last calibrated? There are a lot of variables to look at. Be smart folks thats what we get paid for.



                            • #15
                              I cut my teeth so to speak as a gage maker, then moved intot eh CMM lab, so I can appreciate what you say Bill. At my current employer, it is written policy that gage's be check to and aligned to from the datum simulators. no tooling balls, unless otherwise agreed upon.
                              The TB and datum argument has been going on for years. Know what ASME Y14.5M-1994 says?

                              4.2.1 Application
                              As measurements cannot be made from a true geometric counterpart that is theoretical, a datum is assumed to exist in and be simulated by the associated processing equipment. For example, machine tables and surface plates though not true planes, are of such quality that the planes derived from them are used to simulate the datums from which measurements are taken and dimensions are verified. See Fig. 4-10. Also, for example, ring and plug gages, and mandrels, though not true cylinders, are of such quality that their axes are used to simulate datums from which measurements are taken and dimensions verified...
                              Partial excerpt from ASME Y14.5M-1994 (Datum Referencing ch.4)

                              RFS Means Really Fussy Stuff

                              When all you have is a hammer - everything looks like a nail....


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