huangyhg

positional tolerances
where do i find the "standard" positional tolerances used in the aerospace/aicraft industry? that is, if i want to locate a tooling detail, is there an industry-accepted minimum tolerance applicable for a specified position (e.g the hole must be 5 inches from a datum with a tolerance of +/-0.0x inch)? is there a table of common tolerances used in the aircraft industry?
there are many standards out there but it almost depends on which company you work with. it depends on the engineering drawing of the part itself. if the feature of the part is tightly controlled on the drawing or is a locating feature i've seen as small as +/- .006 positional and as great as +/- .050. there are mil standards out there for datum and control features but it kind of depends of what kind of part it is (ie sheet metal, casting, forging, etc). plus it also depends on the jig type itself (tooling feature with drill bushing, scribed tooling feature, edge locator, removable detail, etc.) can you be more specific with what feature you are trying to locate on the part?
i do not think that there are standard positional tolerances. what i know is that many standard like din have tables of tolerances that apply in the drawing when the designer doesn't define a custom tolerance.
for example in my drawing i use concentricity of dia 0.1 mm unless otherwise specified. tolerances should be functional and only should ensure that the parts and assembly will function. if the standard/general tolerance is what needed then we do not write it within the dimension otherwise always put the maximum tolerance that will still do the job.
a good advice, always try yourself to figure out what is the needed tolerance before you rely on cooked rules. this is the only way to get better understanding.
old joke - "the nice thing about standards is that there's so many to choose from."
the military is getting out of the business of maintaining standards. they're relying on commerical standards published by industry and even handing over mil specs for industry to adopt and maintain for the world to use and buy. like whiterabbit said, it depends on what company you work for. i know when i worked for lockmart most if not all joining methodology was spelled out in lac3600. but another spec that's packed with information on aerospace structures is mil hdbk 5.
best of luck

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