Asymmetrical tolerances

RaphMORIN · ‎Mar 03, 2016

Hi,

How do you deal with asymmetrical tolerances ?

Sometimes customer give step files with nominal values and a drawing with some asymmetrical tolerances, do you model the part in medium values ?

Thanks.

This thread is inactive and closed by the PTC Community Management Team. If you would like to provide a reply and re-open this thread, please notify the moderator and reference the thread. You may also use "Start a topic" button to ask a new question. Please be sure to include what version of the PTC product you are using so another community member knowledgeable about your version may be able to assist.

KenFarley · ‎Mar 03, 2016

As with anything in life, it depends. What is the design intent of a specific tolerance? Is it plus-nothing/minus-something because it's a shaft diameter that needs to fit another component? If we have any information about what we're building, we'll try to accommodate that.

Otherwise, if there is no information given but the drawing, standard practice is to try to get to the middle of the tolerance range. That way if something goes wrong, you might have a bit of extra material there to correct it. It's the classic: "You can take more material off, but once you cut it, you can't put it back on."

RaphMORIN · ‎Mar 04, 2016

I understand what you and this is what we do

But I was wondering, when programming in Creo do you modify the reference part to fit to middle values or do you leave it as it is and play with parameters to get the dimensions you want ?

joshuataylor · ‎Mar 05, 2016

First complain a lot!!

I find modifying the reference model to be the most reliable. Playing with parameters and tool offsets, especially on parts that get side work, always seems to create more problems.

Josh

TomD.inPDX · ‎Mar 05, 2016

At some point, one of PTC's cash cows must have put a stake in the ground and forced limit dimensions to determine the nominal value.

PTC made this the default of all things! But it does give a hint to how some people think about dimension limits vs. nominal. I turn this option off in config.pro.

It is the practice of fit tolerance that guides my requirements. These are based on shaft or hole nominal value. In some cases, both values will be plus or minus. Therefore if I have a shaft-basis of 6mm diameter, I want that shaft to be modeled at 6.000000 even if the size range for a specific fit for that shaft is 5.996 to 6.000. Invariably, the mate too is nominally 6mm with assigned tolerances making it 6.010 to 6.025.

Invariably, people will switch from stacking + & - tolerance to limit tolerance. Doing so by default WILL change your nominal feature size. A very dangerous precedence because nothing would warn you if you changed it back again.

Personally, it is a call based on the design intent. A lot of features I define have an established minimum and maximum size. When I intend to have a single direction tolerance (plus something, minus nothing), I may model that at minimum (dim value). Edge chamfers and radii I do want to see what the max does in many cases, but I will model them nominal and assign a limit dimensions, say C .01-.015 will be modeled at .0125.

I am not giving you any guarantee with my export model other than being a reference. It is up to the shop to determine how it was modeled and how it relates to the drawing. Models are provided as a courtesy only (and I have that in writing). Many times these reference models are a crutch when your drawings are unclear or incomplete. The problem is, you never get any feedback on your drawing's short-comings either.

Bottom line, tolerances on fabricated parts makes a part quite different than anything you can anticipate while modeling. All I can say is to model whatever makes the most sense to help design high quality solutions.