Tolerances of different process 1

[Parsnip]

Hope this is the right forum.
I need to specify flatness, straightness etc but I never know what is a realistic value to place on the tolerance. Most things are machined and ground if need be. I understand that for straigntness/flatness the overall length must come into it but I have no idea what to put on.
Are there any web resources etc that may help?
Thanks in advance
Parsnip

 

[KENAT]

thread1103-198341

A lot of people will say it varies so much from shop to shop/tool to tool etc that's it's almost meaningless to have typical values.

I'm not sure I'd go that far but for sure, one of the best ways is to talk to your machine shope/vendor to see what they can do.

KENAT, probably the least qualified checker you'll ever meet...

 

[PSE]

What tolerance will maintain the safe & functional integrity of the component/assembly? Tolerance accordingly. Manufacturing methods will change according to what is needed to achieve a tolerance (and costs will change as well). You will get feedback from them when tolerances approach the limits of their capabilities. It is then a choice on whether or not to persue additional equipment that extends their capabilities.

I appreciate engineers who take current manufacturing capabilities and methods into account when they are designing/tolerancing components. Obviously, the looser the tolerance the easier it will be to meet and I prefer as big of a "window" as I can get. However, never sacrifice safety and try not to adversely affect functionality just to meet what manufacturing may want to see on a tolerance as the cost of potential downline or field failures could far outweigh the additional manufacturing expense.

Regards,

 

[KENAT]

Well said PSE, tolerancing needs to be driven by function with manufacturability a very important secondary concern.

KENAT, probably the least qualified checker you'll ever meet...

 

[MintJulep]

Processes have VARIATION.

TOLERANCE is an engineering design requirement.

 

[powerhound]

If you are using ASME Y14.5M-1994 then straightness, flatness, circularity and cylindricity are all indirectly controlled by general rule #1. In certain cases you don't have to specifically call out any of the form callouts as they are already indirectly controlled. You would specify form controls if you need them controlled to a lesser value than general rule #1 already does. You have to identify which standard to interpret the print to in order to invoke those rules though.
Maybe do a google on General Rule #1 and see what pops up.

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[Parsnip]

Thanks all for your help. I do understand the fact that the end use will define the actual tolerances required but for example when mounting a pair of linear rails for a test rig I do not want to specify a ridiculous tolerance that will make the part too expensive. If a 5 micron flatness is easily achievable where as 2.5 doubles the cost I can then do an assessment to see if that tighter tolerance really is a requirement.
Parsnip

 

[KENAT]

Parsnip, take a look at iso 2768 (there are excerpts on the web).

I would not invoke it directly as I disagree with the certain ways it's invoked etc but...

The typical values by size of feature & precision of machine shope may be usefull.

Did you look at the other thread I referenced, there is stuff that may be of use.

KENAT, probably the least qualified checker you'll ever meet...