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Using " Size dimension " option in PCDMIS 2017 R2 ( Vision system )

Ajesh George
Using " Size dimension " option in PCDMIS 2017 R2.

I had a doubt regarding the Size dimension option in PCDMIS.

While doing ISO dimensions - I found this option which is useful in giving desired output.

But I got strange feeling about how PCDMIS calculating dimensions.

Since its a very small area , i had to use Vision system to create individual points and constructed as MAX.INSC circle.

Then i use " Size dimension " option to evaluate LP and GX together.

Dimension : Dia 8.54 + 0 (LP) , -0.03 ( GX )

Example : 1

Standard : ISO 14405-1

Here , 8.54 is nominal
LP is given to upper tol.
GX is given to lower tol.



So i got value as : UPPER SIZE - 8.549
LOWER SIZE - 8.528

Example : 2

Standard : ISO 14405-1

8.54 is nominal
GX is given to upper tol.
LP is given to lower tol.

So i got value as : UPPER SIZE - 8.528
LOWER SIZE - 8.531



My question here is :

what is UPPER SIZE ?
How that is measured in software ?
Why I'm getting different LP , if I flip the modifiers ? ( 8.549 and 8.531 )
How LP value helps the functionality ?


Waiting for someone to clarify this......
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  • What does it show on your drawing? Upper size is the upper tolerance boundary for your diameter. Changing the order of the modifiers or changing which tolerance they apply to (upper or lower) changes the meaning which is why it is important to enter them as shown on your drawing. When the modifiers are entered as per your example 2, it is the same as invoking the envelope requirement for an internal feature.

    Extract from iSO-14405-1 2016
    The envelope requirement, E, is a simplified indication to describe two specification operators where the local size exists on the linear feature of size. It is equivalent to express two separate requirements, one for the upper limit of size and another for the lower limit of size by using the GX modifier for an internal feature (e.g. hole), or GN for an external feature (e.g. shaft) for the maximum material side of the tolerance (upper or lower tolerance), and the modifier LP for the other side of the tolerance (see Figure 19).

  • Regarding the LP

    Think of it as measuring the part with a 2-point-micrometer.
    You measure the part in different locations and get different values.
    If you put the LP on the lower tolerance, it compares your lowest measured value against the lower tolerance.
    If you put it on the upper tolerance, it compares the highest measured value against the upper tolerance.

    The LP is not 100% the same as measured with a micrometer (according to the standard) but it helps visualize the concept.
  • in reply to Aaron Baldauf
    But , is there any way to find out in the software that which 2 points are considered in giving LP values ?
  • in reply to Neil Challinor
    Thanks Neil. I'm not fully aware of ISO standards.
    Apart from this , is there any way to know which all points were considered in creating GG , GX , GN circles ?


  • I don't think so, at least i haven't found out.
    What I usually do is look at the graphical analysis (with high multiplier) of the roundness but using legacy and a least square calculation.
    Like this I can try to "guess" what points are used for the LP.

    Edit:
    If you don't have many points you could also evaluate the 2 point distances of every point pair.
    One of the values should be close to the size calculation. (but not exactly)
  • in reply to Aaron Baldauf
    Yes we can try that 2 point distance. I tried it and the value is close only. Not exact as you mentioned. That's why i got confused how PCDMIS calculating LP dimension.


  • One more thing to keep in mind. PC-DMIS doesn't use the same calculation method for GX and GN (size command) vs max inscribed and min circumscribed (constructed feature).
    See my post (#2) in this thread. So you can get slightly different result between these two evaluation methods.
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