Fitting tolerance

 From:  Michael Gibson
3219.2 In reply to 3219.1 
Hi Danny,

> I'm starting to hear the fitting tolerance of 0.001 being mentioned
> more lately, can this tolerance be adjusted to handle smaller objects

As time goes on I have been converting functions to have a "relative tolerance", where it is calculated as a fraction of the object's bounding box, rather than fixed at one specific value.

That helps to automatically handle smaller objects (and larger ones too) - in the last beta release notes there were a couple of notes for some stuff that was improved to handle small objects better, those were ones that were converted to use a relative tolerance instead of the absolute tolerance.

Some programs let you manually adjust the tolerance value, but in practice I've seen that have several negative consequences, particularly if the tolerance value is stored in the file and when you open a file you received from someone else and start to create some new stuff in it you get weird results if the tolerance is a lot different than what you normally work with. It's also quite easy for someone to alter the value without really understanding all the ramifications of doing so and then getting poor results.

> and what effect will that have in MoI, will it take longer to calculate
> an operation, like if you made the fitting tolerance 0.0005 or even
> 0.0002 what will that do ?

Yeah as the tolerance is decreased it will cause some kinds of calculations to continue to higher levels of refinement, taking additional time and also creating heavier point counts in things like intersection curves. The additional point count then causes other side effects like increased drawing time and increased file size...

So it is not something that is good to have at too far out of proportion to the objects involved.

That's why I'm generally moving to have it automatically scaled to be relative to the objects involved. This has to be done on a kind of case by case basis to decide exactly how it works with a particular command. Like for sweep for example it would probably be best to have it based off of the size of the smallest profile curve and not around the rail, since you can have a small profile curve but a longer rail.

But every single command has not yet been converted to a relative tolerance.

> In NX6 there is a fitting tolerance that is adjustable by
> the user, currently our company standard is
> 0.0002"(0.005mm) is this the same thing?

I would have to examine the NX6 technical documentation to be certain, but most likely yes that is the same thing.

- Michael