Limits of GD & T

[Mchel80]

Hi,

I have been trying to find a standard or a reference document that provides the possible limits of GD & T based on machining and material. For example if i have to manufacture a 1 square meter plate made of 316SS I would stipulate a flatness tolerence of 0.05mm on the surface. But for a aluminium material the same process would require a higher tolerence. I am involved with manufacture long bores with smaller ID and i need to stipulate practical GD & T. Is there a document that provides these limits. Please let me know if you want more detail

 

[MadMango]

There are no "built-in" limits to GD&T application. Proper and practical application comes from understanding your requirements and manufacturing processes.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?

 

[drawoh]

Mchel80,

This is not a theoretical question about the GD&T standards. This is a practical question about what your manufacturing process can do. In addition, you need to be able to inspect your parts to confirm that they meet your specifications.

Ask your fabricator what they are able to achieve. The materials you specify do affect accuracy.

There are tables in the Machinery's Handbook showing the accuracy of various processes. Look in the section on Limits and Fits. This is a crude, first order investigation. Your fabricator should have the final word on this.

How are you going to inspect your parts?

JHG

 

[MintJulep]

You need to understand the difference between design tolerance and manufacturing capabilities and variation.

For example if i have to manufacture a 1 square meter plate made of 316SS I would stipulate a flatness tolerence of 0.05mm on the surface. But for a aluminium material the same process would require a higher tolerence

Why? If you need 0.05mm flatness then that's what you need. The material or the process used do not change that need.

Tolerances are a design function. They are acceptable dimensional limits that are required for a part to function properly.

Manufacturing capabilities and part-to-part variation are what you can expect to get if you manufacture a bunch of parts with a given process.

 

[PeterStock]

You should the requirements for fit, then select your materials accordingly. If you have a part that needs to have a flatness of 0.05mm then you need to choose a material that can be machined to that. If you can live with a looser tolerance, you have more options to choose from. If your requirements can not be met, look into other ways to accomplish what you need.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[KENAT]

As others said, start with your design requirements, then verify they are within realistic 'process capabilities' for the manufacturing process.

There is no universal guide to tolerances based on process, let alone introducing the variable of material.

That said, for some specific industries/sectors/processes their may be common guidelines but even these aren't hard and fast and can vary by vendor.

I've found the information in Machineries to be of limited use, unless I'm looking in the wrong place or something.

Ask your vendors/machine shop as others say.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Mchel80]

Thanks for the input guys.

Looks like there is no guideline for GD and T. I will have to work closely with manufacturer to find practical solution.

Drawoh - I will inspect the parts using CMM. V-Blocks and clocks set accordingly in a granite table will enable measurement.

 

[KENAT]

+- Tolerances can in some cases be converted to broadly equivalent GD&T, for example with position.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​