How to determine tolerance and what tolerances are reasonable?

[nwong103]

I'm an entry level engineer and hopefully a future designer. When I tolerance things, I just tolerance things by the default (everything is +-.05 inches) and for more accurate items, I just tolerance it +-.005 inches. I do some basic calculations to check worse case scenarios and make sure it will still work. This is definitely the WRONG way since doing anything in engineering without understanding why it is, is wrong in my book. There is a better way to do this and I'd like to know.

I'm often wondering what defines the limits of tolerances and how are those limits determined?

For example, if I set a tolerance of +-.0001 inch for a circular cut, will that be achievable? I'm guessing it all depends on the type of CNC machine and how accurate it can make these cuts.

I have very limited machining experience but when I do machine something at the shop using a lathe, mill, or drill press, I can't even imagine getting +-.05 inch accuracy from eye ball measuring. I'm sure machinist almost never eye ball things as eye balling measurements is hardly accurate enough. So back to my question, how do you know what is a proper tolerance that is actually achievable?

If possible can you please show me some resources so I can educate myself? Does the 14.5Y GD&T book go over these details? I always thought that guide just defines how to properly dimension your work so it's not a huge mess, follows standards, and makes it easy to communicate drawing to machining. I'm guessing it doesn't teach how an engineer properly tolerances things and explain why an engineer choose that specific numerical value over another value.

Do you think taking a Machine Shop class will teach me this stuff? Or will I just waste my time learning how to use the mill, lathe, drill press, etc that doesn't have the best accuracy? I used all the machines in the shop at campus and something tells me only automated machines like CNCs can produce accurate results.

 

[Ranger Mike]

Welcome to the reality production world. Y 14 GD&T is a good start. Take that machine shop class. I recommend you spend some time in a machine shop owned by an old guy who can read a vernier micrometer. The biggest sin in tolerancing is to tighten up the tolerance to an unrealistic and many times unobtainable level. Putting a 0.0001" size tolerance on an I.D. is expensive and may even be fatal to the parts life. I run into this all the time when selling high accuracy measuring equipment to measure these requirements. You have a good mind and are asking the right questions and I can see only a good and promising future for you!

 

[Mech_Engineer]

ASME Y14.5 will tell you how to dimension and tolerance parts, but it can't tell you what tolerance to use- that's left up to the engineer.

As a general rule our default tolerances are +/-.005" and/or GD&T True Position of .014". These numbers are relatively easy to achieve on most standard machining equipment with proper setups. The true position tolerance of .014" is based on manufacturability and interchangability studies, basically achieving a 95% interchangability between parts. Since we usually only do "production runs" of 1-10 components, I haven't run into any problems. For "precision features" like locating holes, press-fit pins, datum surfaces etc. I may use +/-.001" or even +.001" -.000 depending on the application (a hole for a precision bearing may be even tighter, like +/-.0001"); locating pins also usually get a GD&T true position of .009" or better depending on requirements of orientation and the distace between two pins.

+/-.050 is a pretty loose tolerance in my world, that's bidirectionally .100" which would be ok for maybe outside dimensions on a large plate, but is too loose for hole positioning or precision features. But it all depends on what you're building. If you're building a ship, +/-1.0" over it's length is a very tight tolerance; if you're building an interferometer, every micron counts.