Plus/Minus dims - Possible bug?

DeanLong · ‎Aug 08, 2011

Disclaimer 1: I do not create that many drawings any longer. Most of of the work I do is Class A and organic surfacing projects.

However,I havehad an atypical amount of 2D work in the past month or so. While creating dims in WF5, I noticed that clicking the plus/minus button brings up the expected X.XX +.00 (superscript) and -.00 (subscript). All is right with the world.

However, the fun begins when I change the values for the plus and minus, especially the minus. If I do not explicitly type in the "-" (minus symbol),Pro adds a"+" inthe lower limit. So my dimension would look like: X.XX +.01 (superscript) +.01 (subscript).

To my knowledge there is never a case in ANSIwhere the plus/minus would/could both be a plus tolerance. This never is the case correct? Is this a programming oversight or is Pro allowing an allowable ANSI callout?

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msproe · ‎Aug 09, 2011

Believe it or not, I have had to do this in the past. I cannot remember the specifics right now but the callout required two "+" tolerances. Pro is actually designed to do that and it is, or was, documented in the manuals.

Kim

In Reply to Dean Long:

Disclaimer 1: I do not create that many drawings any longer. Most of of the work I do is Class A and organic surfacing projects.

However,I havehad an atypical amount of 2D work in the past month or so. While creating dims in WF5, I noticed that clicking the plus/minus button brings up the expected X.XX +.00 (superscript) and -.00 (subscript). All is right with the world.

However, the fun begins when I change the values for the plus and minus, especially the minus. If I do not explicitly type in the "-" (minus symbol),Pro adds a"+" inthe lower limit. So my dimension would look like: X.XX +.01 (superscript) +.01 (subscript).

To my knowledge there is never a case in ANSIwhere the plus/minus would/could both be a plus tolerance. This never is the case correct? Is this a programming oversight or is Pro allowing an allowable ANSI callout?

ptc-2607046 · ‎Aug 09, 2011

It is required by ISO standards for certain types of fits.

JonathanHodgson · ‎Aug 09, 2011

ISO fits do indeed sometimes use +/+ tolerances, but in WF4 the
behaviour is that the lower tolerance is assumed to be - unless you
enter the value as a negative - so, slightly confusingly, +0,10 / +0,05
is entered as upper: [0.1] lower: [-0.05].

It sounds as though this behaviour may have reversed in WF5, at least
for the settings Dean is using...

Jonathan

DeanLong · ‎Aug 10, 2011

We are standardized on ANSI 14.5 for the documentation. If ISO is the only way it can be specified and we have ANSI specified, Pro is allowing something it should not. Right?

I did not recall any situations where, even under ISO requirements, I have seen any situation where two +'s were used in a dimensional callout. You are correct when stating is is"confusing". I can appreciate the "need" for the callout, as in a heat induced press fit. But I highly doubt any inspector will measure a slip fit "at temperature".Doesn't itstill seems less confusing to use +/- with one being zero and specify the tolerances based on process (I.E. heat) than use a +, + tolerance?

Does anyone have an example of it's use?

thanks

JonathanHodgson · ‎Aug 10, 2011

Dean,

Do a web search for the "ISO System of Limits and Fits", or "ISO fit
tolerances", or similar - such as

BenLoosli · ‎Aug 10, 2011

Doesn't matter if it is ISO or ASME, this example shows +,+ tolerancing on the hole.
For example, if a shaft with a nominal diameter of 10 mm<">http://en.wikipedia.org/wiki/Millimeter> is to have a sliding fit within a hole, the shaft might be specified with a tolerance range from 9.964 to 10 mm (i.e. a zero fundamental deviation, but a lower deviation of 0.036 mm) and the hole might be specified with a tolerance range from 10.04 mm to 10.076 mm (0.04 mm fundamental deviation and 0.076 mm upper deviation). This would provide a clearance fit of somewhere between 0.04 mm (largest shaft paired with the smallest hole, called the "maximum material condition") and 0.112 mm (smallest shaft paired with the largest hole). In this case the size of the tolerance range for both the shaft and hole is chosen to be the same (0.036 mm), meaning that both components have the same International Tolerance grade but this need not be the case in general.

Thank you,

Ben H. Loosli
USEC, INC.

DeanLong · ‎Aug 10, 2011

Thanks for the explanation Ben. I was not thinking in terms of your explanation nor the fact that some companies produce components that clients may process further and use +, + as a way of indicating there is always positive stock for them to lap, mill or otherwise fine tune.

I have learned a new thing today.

ptc-593619 · ‎Aug 10, 2011

Here's as good a reason as I've seen for unilateral tolerancing. (From a
comment in Practical machinist archives)

When I am first designing parts and assemblies, I make everything
nominal size. For example, I draw a 1.000 shaft in a 1.000 hole. It's
easy to draw and design this way, alignments are easy, and who really
wants to think much about tolerances in the early stage of 3D design
anyway? Later, when I start making drawings from the 3D, I worry about
fits and tolerances. The drawings naturally start with the 1.000 shaft
and 1.000 hole. To get parts that will fit with clearance, at least one
part will have to have one-sided tolerances. I could take the effort to
go back to the 3D model and change the shaft to 0.998, say, so I could
have two-sided tolerances on my drawings, but that often will mess up
all kinds of mates, constraints, and alignments in the 3D solid model.

Richard A. Black

Lead Design Engineer

Eaton Corporation

440 Murray Hill Road

Southern Pines

NC 28387 USA

RaphaelCNascime · ‎Aug 10, 2011

Hi everybody.

Just wanted to chime in on this thread...

I think this change was made a little bit before I took over the
drawings area, but I believe the reason was to support the use cases
already mentioned by others for when +/+ or -/- tolerances might be
needed. Although they are somewhat rare, we do need to provide some
support.

So, from a functionality point of view we need to provide the ability
for +/+ or -/- tolerances, but from a usability point of view forcing
the user to type in the minus sign to get the 90% case isn't, in fact,
all that usable. One of the things we're considering for the future is a
smarter way of doing things that satisfies the 90% case with minimal
effort, but still provides the option to achieve the 10% case. Hopefully
we can sort this out in an upcoming release.

Thanks,

Raphael

ptc-2607046 · ‎Aug 10, 2011

Can you add it as a setup file option for the default operation?

dgallup · ‎Aug 11, 2011

I think the issue is not the need for +/+ or -/- tolerances. Pro/E has always had the capability to do this and there are valid reasons why. What has changed is the default behavior when you enter a value for the minus tol. In the past, Pro/E assumed that this would be a negative value and if you input a positive value it displayed as a negative. To get it to display as positive you had to input a negative value. This was perhaps a bit illogical to the programner but entirely logical to the user who will want the lower value to be negatinge 99.9% of the time. Somehow, PTC has flipped this behavior, requiring the user to enter a negative value to get a negative display. This is going to cause a great deal of confusion and drawing errors. PCT should go back to the old behavior ASAP. I for one would never put into production use software that is behaving as the original poster says it it is doing now. (FWIW, I always stay several releases behind because of PTC's abismal record of fixing one bug and creating 2 new ones.)

So you had a solution that satisfied the 99.9% case and you blew it.

In Reply to Raphael Nascimento:

Hi everybody.

Just wanted to chime in on this thread...

I think this change was made a little bit before I took over the
drawings area, but I believe the reason was to support the use cases
already mentioned by others for when +/+ or -/- tolerances might be
needed. Although they are somewhat rare, we do need to provide some
support.

So, from a functionality point of view we need to provide the ability
for +/+ or -/- tolerances, but from a usability point of view forcing
the user to type in the minus sign to get the 90% case isn't, in fact,
all that usable. One of the things we're considering for the future is a
smarter way of doing things that satisfies the 90% case with minimal
effort, but still provides the option to achieve the 10% case. Hopefully
we can sort this out in an upcoming release.

Thanks,

Raphael

http://www.ptc.com

Raphael Nascimento
Product Manager

T 781.370.5916
E -

PTC.com http://www.ptc.com