Profile of a surface without a datum reference.

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

  • Profile of a surface without a datum reference.

    I apologize in advance if this has already been covered, I was unable to find an answer. I have very little experience with profile dimensions.

    I have a drawing that calls out the profile of a surface 0.8 . The surface just happens to be datum A but there are no datums called out in the FCF. My understanding is that this would mean it is a form only callout and that the tolerance should be in the positive only. The customer i am dealing with is checking the profile as ± 0.4.

    The last time that i had to deal with profiles i had found information that stated F&L profiles split the tolerance + and - and that a form only profile was just + unless stated as unilateral. I printed this out to keep on hand for future reference, but stupid me did not note where i found the information.

    Am i correct in my understanding that it would be a positive only tolerance?
    Last edited by bbennett; 08-07-2020, 09:57 AM.

  • #2
    Think of it like flatness, no particular place it has to be, doesn't relate to anything else but itself, so the hits taken is will create a zero, but calling it out to itself, the Max and Min will be equally split highest and lowest because nothing to control location, orientation, it controls itself.

    Capture.JPG
    Last edited by KIRBSTER269; 08-07-2020, 11:33 AM.
    (In Memory of my Loving wife, "Ronda" who I lost March 7, 2016. I love you baby.)
    They say "Nobody's Perfect." I must be Nobody.

    Comment


    • KIRBSTER269
      KIRBSTER269 commented
      Editing a comment
      Also to add DON'T MISTAKE T-Value for results, that will relate to your alignment, when you start creating a FCF, it eliminates your alignment, all six degrees do not exist, so it's free in space. When you start adding Datums it starts creating a new Trihedron, this is XactMeasure and technically in Legacy when you choose Form Only it eliminates your alignment, so it's in a free state. that's why I said it's like Flatness
      Last edited by KIRBSTER269; 08-07-2020, 11:00 AM.

  • #3
    Thank you, I misspoke earlier about the surface i am measuring being datum A. Datum A is made up of 3 targets at the extreme ends of the part. The customer supplied us with a fixture that they want it inspected in. I picked up A B and C and have aligned, set the origins to the fixture. The surface profile in question is the same surface that the A targets are on. The reason that i am asking is that the part is in spec per both ways of reporting if i use form only. They are using an identical fixture with stands and indicators and a ± 0.4 tolerance. The only way i am seeing that i can use a ± tolerance is to switch it to form and location. If i do this the parts are out of spec.

    Comment


    • KIRBSTER269
      KIRBSTER269 commented
      Editing a comment
      Read #2.1 above. If you call it out with no Datums, I don't care if you have 3 hits or 23,000 hits, no datums in your FCF, then Datum A,B, or C doesn't exist, nor does your alignment.
      Last edited by KIRBSTER269; 08-07-2020, 11:03 AM.

    • KIRBSTER269
      KIRBSTER269 commented
      Editing a comment
      Also show Max and Min in your report. Post #1 edited
      Last edited by KIRBSTER269; 08-07-2020, 11:34 AM.

  • #4
    Back to my original question, should i use a unilateral tolerance or should it be bilateral?

    Comment


    • KIRBSTER269
      KIRBSTER269 commented
      Editing a comment
      When it calls for form only, it's going to be bilateral automatically. which is what you want. It doesn't say anything for a unilateral. It's an envelope to itself, equally.

    • VinniUSMC
      VinniUSMC commented
      Editing a comment
      This isn't technically correct. Form only is "unilateral" in the sense that the size of the zone is value, like TP. When you are not dealing with a feature of size (things get wonky dealing with features of size), you should expect only positive values, like with flatness (which is just a form only profile of a specific feature type, plane).

    • KIRBSTER269
      KIRBSTER269 commented
      Editing a comment
      VinniUSMC my mistake on my lingo when I said bilateral, I was referring to the Max and Min, but yes total value in form only

  • #5
    when i try to put the tolerance in the minus box it wont let me unless i switch it to form and location. running 2015.1

    Tried to switch it to bilateral again and get a message
    Form only profile dimension must have zero minus tolerance
    Last edited by bbennett; 08-07-2020, 02:25 PM.

    Comment


    • KIRBSTER269
      KIRBSTER269 commented
      Editing a comment
      you don't put that in there, report MAX an MIN. It doesn't care what size it is. Envelope doesn't appear until you call it out for form only. because it doesn't know how big the feature yet. you put the total and it will distribute it evenly. based off of the results.

  • #6
    Lets say I told you I need a form only sphere, You say "what size?" I say "I don't care" most of the time customer will state what size. The size could be bad, but the profile to itself can still be good. Lets say I need the form to itself to be .010 profile, I touch off on sphere several points. It creates that sphere based off of the hits and creates an envelope around it, and none of the points taken can be more than -.005 or +.005 That envelope doesn't appear until I establish the sphere.

    Untitled.jpg
    Last edited by KIRBSTER269; 08-07-2020, 02:17 PM.
    (In Memory of my Loving wife, "Ronda" who I lost March 7, 2016. I love you baby.)
    They say "Nobody's Perfect." I must be Nobody.

    Comment


    • VinniUSMC
      VinniUSMC commented
      Editing a comment
      Form only of a sphere is different than form only of a plane, because it is a feature of size. Yes, the best fit sphere is going to be in the envelope somewhere, and may be used to calculate min/max, but with form only, the value of significance is still a positive value, max minus min.

      For form and location, the min and max are important (at least, until the new revision of Y14.5 apparently...), for form only, there is 1 value, the size of the envelope.

  • #7
    Ok, I thought that was like any other tolerance and i had to input it bilaterally.
    Thank You

    Makes a lot more sense now

    Comment


    • #8
      There are a few things to unpack here.

      First, yes, a surface profile with no datums (what we call "Form Only" in PC-DMIS) is a positive value. It's just the size of the region that envelopes the actual measured points. It is possible to have negative values, as Kirby illustrated in post 2, but that is something related to how PC-DMIS calculates the envelope by first fitting to the feature.

      You cannot check this by securing it on a fixture. Once the part is secured into a fixture, then the check becomes "Form and Location", to whatever datum reference frame it is locked onto. If it is a plane, it could be measured as flatness, and various features of size can be checked for form, but a free form surface couldn't be checked without a CMM or scanner of some sort. If you put a perfectly flat plane onto a fixture skewed at a 10 degree angle, you'll still have a perfectly flat plane, but the extremities of the plane will be way out of tolerance to form and location (key part being location).

      Form only is not unilateral but it is a single value, the size of the region. And the region is free to float in space, in 3D, to fit the smallest zone possible.

      Min/max for form only is not of any significance.
      "This is my word... and as such is beyond contestation."

      Comment


      • KIRBSTER269
        KIRBSTER269 commented
        Editing a comment
        Thanks for putting it in more detail. I am terrible at explaining this. The only thing I could relate it to was flatness, No not the same thing. I figured if since I su-ck at explaining this, if he saw Max and Min showing it was an equally distributed envelope, again kinda like flatness, it would help the bigger picture.

    Related Topics

    Collapse

    Working...
    X