Tolerance - decimal places not rounding up accurately

[ptc-3139256]

Have an issue with Wildfire not rounding up accurately

The properties of the dimension are as follows.

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Duel Dimensions primary = Metric & secondary = Inch

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Dimension Properties menu

tolerance mode = +- Symmetric

tolerance = .38

Format = Decimal
Number of decimal places = 2

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When I use these settings the tolerance on the inch dimension displays as
.014

It should display as .015 because it is actually .0149606


Robert Altman
General Dynamics Amphibious Systems
CAD System Management
703.490.7533
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[JonathanHodgson]

Hi Robert,

This was discussed fairly recently - can't remember whether it was here, or over on the PTC forums.

The answer is that the secondary units tolerance must not be greater than the primary units tolerance - so the rounding is altered to satisfy this.

±.015" would be ±0.381 mm, which is greater than ±0.38, so the software displays .014" instead.

HTH,
Jonathan

[dgschaefer]

I believe this is intended. Using 0.15 would actually give the inch
dimension a 'looser' tolerance than the primary metric dim. Rounding
down is the safer solution.

Doug Schaefer

[STEVEG]

I forgot where I got this from. It might have been from this forum.



Pro/E Dual Dims: Why Truncate, not Round?





Pro/E has some "intended functionality" that truncates secondary tolerances. This may seem suspicious at first when you look at a drawing and see different numbers from what you'd expect... let's investigate.





















From what we understand the reason for truncating was that a model could never be larger than the original intent by changing the number of decimal places. Because of this particular issue, dual dimensioning (English and Metric) was almost impossible to use because with very tight tolerances you could end up with zero tolerance on the secondary dimension.





















Example: a part is drawn with an imperial tolerance of +/-3 thousandths (0.003in). Here are the relative sizes of this tolerance when truncated & rounded:



















[cid:image003.jpg@01CACB2F.F3DF9CA0]



















































































































































Truncation keeps the tolerance closer to the nominal value, thus keeping within the tolerance specified by the primary tolerance.



















If you rounded all secondary tolerances, some sizes could be inside the secondary tolerance, but outside the primary tolerance.

[jflores-3]

Robert,

Pro/E is rounding the tolerances accurately. There are two acceptable
methods of converting units. One (method A) where the conversion of
secondary units is allowed to exceed the value of primary units, and
another which is being employed by Pro/E (method B) where the conversion
of secondary units results in a smaller value that the primary units.



Most people think that Pro/e is not converting accurately because
generally method A is applicable unless expressly indicated to the
contrary. Typically the increase is within 2% (inches to millimeters)
or 2.5% (millimeters to inches) of the value of the tolerance. This
increase is considered acceptable for interchange ability (depended on
the magnitude of the tolerance) and serves as a basis for inspection.



Method B is the preferred choice when gages that have been manufactured
to the primary units will be used for inspection. Method B also
simplifies manufactures conversion process of a basic size and two
deviations in order to avoid accumulation of rounding errors.



Thank you, Jim Flores

Mech. designer/CAD admin.

Clinical Care Systems

Philips Healthcare

2271 Cosmos Court

Carlsbad, CA. 92011

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