SELECTION OF TOLERANCE 1

[ASHWA]

Hai,
I have a doubt in basic selection of Tolerance for a part.
Tolerance is classified into FIT, FORM & FUNCTIONAL.

for, FIT we can select by means of using FIT CHART.
FUNCTIONAL TOLERANCE can be attained by STACK-UP ANALYSIS.

How, FORM TOLERANCE is selected? also by STACK-UP ANALYSIS?

Pl. help SME's.

With Regards,
ANBU.

 

[Burunduk]

If you decide the tolerance value for fit by looking at a FIT CHART, also select the tolerance value for form by looking at a FORM CHART.

 

[ASHWA]

Thanks Burunduk ,
Do you have a FORM CHART? any standard pl.?

 

[Burunduk]

No. There is no such thing as form charts.

When they say "fit & form and function" it means that these are the general considerations for tolerancing. You won't find your tolerance value in a "fit chart" other than in the specific case of selecting size tolerances for a pin and a hole that are only constrained by each other at assembly.
Fit could mean for example the fixed or floating fastener calculations. It could mean other things.
Form could be related to many things. Form tolerances are often applied to primary datum features, to ensure they are adequate as main interface features at assembly and for simulating datums for inspection in a repeatable manner. In the end, form is also linked to function, and/or it could be based on aesthetic considerations.

 

[drawoh]

ANBU SARAVANAN,

What do you need to accomplish? What is required for your part to work?

Specify tolerances that make your part work.

--
JHG

 

[Belanger]

There are forumlas to calculate many tolerances (such as VC for position as well as fixed and floating fastener formulas). Form tolerances are often a judgment call, but should still be based on function.

If you're referring to form tolerances on a feature of size (flatness and straightness), then a stack analysis can indeed reveal the correct tolerances -- you'd be looking for virtual condition or a worst-case boundary calculation.
Sometimes form tolerance on a surface can also be derived by a stack if that surface has a known parameter such as a leakage/flushness requirement.
That said, I suspect many designers just choose numbers based on previous designs or what they think manufacturing can hold.


John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[ASHWA]

Thankyou all,
Doubt some what cleared. If any further arises, will let you know.