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Tolerance, accuracy, how much is enough?


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Some background if you will......

I used to work for a machine shop assigned to Texas Instruments. My job was not a machinist at all, but as an assembly tech and/or electronics tech. We were in an equipment development group and were constantly building and re-building and improving semiconductor process machines. When we were doing the assembly work, we often used the machining drawings for the various parts to aid in figuring it all out. I remember reading, almost always as I recall, the tolerances of the parts were +/-.005

I tell that only to ask what is 'usual' or 'reasonable' in a hobby shop? I know if will depend on lots of things, but what is the goal for a hobby guy with a 12" Logan and a bench top mill/drill?

I remember being in the shop once, and the owner was asking a new machinist "how close can you work?" to which he replied "All the way to the chuck"


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For me it depends on what I am working on. Sometimes I shoot for my mark and sometimes I am happy to be within 2-3 thous.

Bob Korves

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I find that if I try for consistently good tolerances in my ordinary work, it improves my chances of success when I have to machine something to a tight tolerance. Paying attention to the details even when doing very rough work comes back to help us on all our work, without wasting time. It actually saves time and effort when the fussy work and the ordinary work are all done mostly the same way. None of which is to say I am a great machinist, far from it...


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Tolerances depend totally on the part being made and what it mates to. +/- .005 is easy to machine to but probably a waste of time if your going to weld it, but unacceptable if your planning to press fit a bearing. For general work where parts have to mate to others in an assembly I work to .001 but figure on .0002 for bearing fits. I don't have any 1/10ths mics.



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Most of my hobby work does not have a tolerance spec but I work to the tightest practical fit just for the challenge. It is truly satisfying to get a smooth and close fit even when it is not necessary. Learning how to accomplish that with a machine that is less than perfect is fascinating. Sometimes it requires buffing, lapping, etc. to remove an amount that is not measurable with common hobby level instruments. Although I still have a long way to go, I have had a taste of getting the most out of the machine and out of myself and it is addictive.

PS: Working all the way to the chuck is fairly easy to master. Going beyond that can be a chore.:)


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I agree with everyone above and they've left me nothing more to say so I'll say it anyway.
Parts that can be within a sixteenth can be whipped out by checking with a scale.
Parts that need +/- .005 I try to adjust my infeed to hit dead on and then check to see how close it comes out.
This teaches me how the machine reacts and how well I have done my job.
Parts that need to be +/- .0001 are easyer to hit if you practice on all those other parts.


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You need enough accuracy and precision to ensure the part/assembly will function properly. Beyond that, accuracy and precision are for benefit for you or other's esthetic sensibilities.

Like Tozguy, I get satisfaction of working to closer tolerances than are needed for a functional part. And like Bob and Jim, I believe that practicing on the less demanding parts is good training for the times when you will need to make precision parts.


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.005" is the standard tolerance for "non precision parts in most industries. I worked in both the Engineering Design and Machining departments of a large company that made most of it's own processing and packaging machinery. We built machines ranging in size from smaller than a bread box to several hundred feet long and several stories high. Almost every non critical print was dimensioned to +/-.005. Bearing mounts, shafting and other critical moving parts were an exception. In cases where there would be multiple machines built we designed and built the first as a prototype. Once it operated and could be maintained as expected prints were sent out to vendors for bids.

The standard warning from management was to make the specifications "practical". There's no need to specify a dimension to .0001" when .005" will work. The standard joke when specifying dimensions was "for each digit to the right of the decimal point you move the tolerance, you move the cost one digit to the left of the decimal point". In other words when specifying tolerances the tighter the tighter the tolerance the higher the cost. In my own shop I follow the same guidelines. In my case the actual dollar outlay doesn't multiply by 10, but the time to fabricate the part is certainly extended.


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I often find I've not left enough clearance in certain parts. My wanting to machine to tight tolerances sometimes bites me, as in using the part it is too tight and must be clearanced to function properly.
It's all a learning curve for what needs what clearance/interference.