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GTOL Dual Dim: Conversion Accuracy

GTOL Dual Dim: Conversion Accuracy

EDIT: This is expected behavior but I have yet to find the reference for this in ASME and ISO.  At the very least, this should be a config option: round_dual_dim_tol standard/*minimum allowing the minimum to remain default.

Please see comment below for PTC's documented behavior justification.

This really shouldn't be an Idea, but rather a "Quality Oversight Bulletin!".

This error should never have existed outside the halls of PTC, much less considered for release into production.

Why is it that dual dimensions round properly, yet GTOL dual dimensions truncate the secondary dimension?

I will let the image speak for itself... ref - .0030 [0,07620]

GTOL_ROUNDING_ERROR.PNGmodel annotation shown.

detail config options:

dual_digits_diff -1

dual_dimensioning primary[secondary]

dual_secondary_units mm

This makes for yet another obstacle in 3D annotation not to mention using relational GTOL throughout development.


Reply from CS


Tolerance values of Secondary Units are tighter than Primary Units in Creo Parametric


  • With Dual Dimensioning, Tolerance values of Secondary Units are rounded incorrectly.
  • The Secondary Dimension tolerance values are Truncated instead of rounding
  • Why are the secondary units rounded differently than the primary?

Applies To

  • All releases of Pro/ENGINEER and Creo Elements/Pro
  • Creo Parametric 1.0, 2.0


  • Works to Spec


  • Secondary values are rounded to fit within the range indicated by primary units
  • See also Help files at #Help# Help Center > <Product> Functional Areas > Detailed Drawings > Detailed Drawings > Annotating the Drawing > Dimensioning the Model > Formatting Dimension Display > To Format Existing Dual Dimensions in a Drawing.
  • Product Help Extract:


"When you are using dual dimensioning with dimensional tolerances, the system rounds the secondary values so that they always fit within the range indicated by primary units (that is, so that they are tighter than primary values), and retain the desired design intent".

I have requested an ASME and ISO reference for this response.

EDIT: Reference to standard is not available from PTC.  Can anyone please provide appropriate behavior specifications from the standards perspective?


Further information:

Statement from All-Seals Inc. regarding their use of the SAE J390 dual dimension standards, and an example of their rounding.  According to the statement, there are "methods" which allow for rounding up.  I do not have access to SAE J390.


And a sample section.  Please note the -tolerance- is .003" (left) and SI is 0,08 (right)


Know that these are inch-based orings.


My understanding is that secondary unit tolerances (SUT) are truncated to ensure the toleranance band is smaller than the primary unit tolerance (PUT) band. Under unfortunate circumstances the SUT's can essentially get cut in half if you don't have the tolerance display digits set to account for truncating.  So if your PUT's are tight and you don't realize truncation is occuring, the SUT's become unmanufacturable or very expensive (and don't match the PUT's).

You can deal with this if you know truncation occurs, although it creates a somewhat confusing drawing due to the large number of SUT digits required.  But we are used to, and generally assume, that rounding occurs.  I suggest a note or warning in the Dimension Properties box which simply lets us know what will happen to secondary tolerances.  for example: "Warning: Secondary Unit Tolerances are Truncated"

An even better solution would be to round the secondary unit tolerances and automatically set the secondary unit tolerance digits to ensure they do not increase more than 5% over the PDT's

Note: this does not only apply to Gtols, I am relatively certain it also applies to all non-Gtol secondary unit dimension tolerances.

Status changed to: Archived