Geometric Tolerances - reference values for different mechanical process

[Mateus_R]

Dear colleagues,

I would like to understand if there are some reference values for the definition of geometric tolerances depending of the manufacturing process, just some guide values to not ask for suppliers something not feasible, for example if is reasonable to consider decimals or centesimals...

Case a: using manufacturing process like turning/milling/drilling for single components, which guide values for flatness, circularity, straightness are reasonable?

Case b: welded component that is going to be machined after welded.

Thank you for your attention.

Kind Regards.

 

[chez311]

Mateus,

Attached are a few figures which I have seen cited in different incarnations. They don't specifically apply to geometric tolerances but could be roughly extrapolated to such - ultimately it will depend on your suppliers and their processes, equipment, and inspection capabilities and if tight or critical tolerances are required in a certain area the feasibility of such tolerances should be discussed with them.

 

[pmarc]

In ISO world there is a group of standards, called complementary standards, that refer to specific manufacturing processes or to specific machine elements.

In this group the following standards can be found:

-- ISO 2768-1:1989 - General tolerances for linear and angular dimensions without individual tolerance indications;
-- ISO 2768-2:1989 - General geometrical tolerances for features without individual tolerance indications.
These two standards mainly apply to the parts that are produced by material removal, but may be also applicable to other types of manufacturing processes (e.g. parts formed from sheet metal).

-- ISO 13920:1996 - General tolerances for welded constructions - Dimensions for length and angles - Shape and position.

With that said, while it is definitely recommended to look at these general tolerances to get better understanding of what can be achieved with customary workshop accuracy, the main driver for selection of tolerance values should primarly be functional requirements of the component/assembly.

 

[Mateus_R]

Thank you for your replies, both were very helpful!

 

[mkcski]

Mateus R:

I understand your post and agree with pmarc about part function driving the dimensioning and tolerancing. But please be aware of the following:

Y14.5.-2018
4.1 FUNDAMENTAL RULES
(e) The drawing should define a part without specifying
manufacturing methods. Thus, only the diameter of a hole
is given without indicating whether it is to be drilled,
reamed, punched, or made by another operation.
However, in those instances where manufacturing,
processing, quality assurance, or environmental information
is essential to the definition of engineering requirements,
the information shall be specified on the drawing or
in a document referenced on the drawing.

Certified Sr. GD&T Professional