How close is close enough? / Digital calipers.

Just how tight tolerances do you really need? In a home shop environment making one-offs, with time spent not being a concern, you can fiddle as much as you want. When you are making parts for profit - not so. And in a jobber type shop where you are making repairs or replacement parts, you have to decide just how close you need to be.


Is the part for a tractor? Harvester? High end motorbike? Porsche 911? Granddads old something?


I have the feeling that people sometimes get carried away since its so easy to throw numbers around. The guy that asks for spot on probably doesn't know just how much, or little, a thousand of an inch is.

The hairs on my head typically measure just over two thousands.


Now that the price of digital calipers is so low, the same guy probably has them and thinks he is measuring to 0,0005". Oh yeah and the inside measurement must be what the calipers say, right?
Tell me, who pressed the "zero"-button the last time? Is zero really zero?


You can probably see how easy it is to misunderstand or miscommunicate. Your calipers and mine probably wont agree, and your way of measuring will probably differ from mine. Thats why we calibrate measuring equipment:

Checking micrometer calibration - Nikon D200, AF-s VR 18-200, on-camera flash, +something 

A quick check of the digital calipers is to measure some calibrating rods for outside micrometers. You might be surprised, not necessarily in a bad way. My Limit digitals are within +- 0,0005" with some care. Would I use them where +- 0,0005" matters? No way!


There are times when you really may want to be within a few tenths. Bearing seats can be such a time. The tolerance on high precision bearings can be just a few tenths of a thousand inch. But be aware that when we get down to these levels surface finish and temperature really matters. Shrink fits are an other example where thousands matters - miss with a few and your fit is gone. Thats why we have Loctite products heh heh.


There is no sense in trying to hit a diameter to a few tenths if you have a surface that's rough. You also have to know what you are measuring. Are you measuring just one peak, the average or the lows on the surface finish? Perhaps you have to grind the surface if the tolerance is going to be meaningful.
Roundness also comes into play, it it silly to try to hit a tenth if you aren´t truly round.


So how close is close enough? Most of the time the customer can provide you tolerances. If they seem fair, go for them. If he doesn't seem to have a clue, ask if "close as a hair is enough". Or look up a tolerance chart, and use the appropriate tolerance for the parts intended use.

Thats a hair, 0.05mm. Lines are 0.50mm apart - iPhone 5s, pocket loupe (10x) 

Chanses are you'll save some time and frustration, and spend a more reasonable amount of your customers hard earned cash, by working close only where it's needed. And you'll probably see more of him when he understands you're not trying to rip him off but instead provide quality work.