acceptable tolerances for cnc work

In my outdated opinion, the tolerance should meet the demand of the part and should have nothing to do with the machine it's made on. If .0005 tolerance is what is required for the part's performance, that is the tolerance. The machinist needs to be able to meet that tolerance. If you've been making them at .0005, and that has been acceptable, that should be good enough.

I agree with Dan, but think which direction the tolerance must go. Think of a hole. Must it fit on a ground shaft, and must be +.0005, or does it fit within a hole, and must be -.0005?

The closer the tolerances you specify will add to the cost, possibly > On Tue, 26 Feb 2008 03:31:55 -0800 (PST), snipped-for-privacy@hotmail.com wrote: >

This tolerance is by no way "extreme", but believe me, it WILL add to the cost. If you don't need that kind of tolerance, don't ASK for it. I rarely try to push general "stuff" much tighter than +/- .003", but for shaft fits and such I do go a bit tighter.

Jon

Why do you need 4 digits tolerance? Particularly if you have been doing them on a manual lathe?

Depending on the part, material, setup AND machine, you could reasonably expect .0002 0/+ repeatability with CNC

The most dangerous thing in the world, is a CAD package with more than

The big question is...how "accurate" can you afford? Gunner

Hey, I've got all the decimals you'll ever need. I'm up to revJ designing a pair of parts that are less than an inch square and have approx 60 features with a +5/-0 micron tolerance. They're running my customer around $4000/pair.

And how accurate do you really need? The two most expensive errors in tolerancing are asking for more than you need and paying through the nose for the machining, and asking for less than you need and donating some expensive bits of metal to the land fill.

Sometimes it's cheaper to pay the machine shop for tolerance than to adjust things after the fact. One of my clients does aerospace work, and the mechanical engineers there are demons on tolerancing -- to make sure that the final assembly's tolerance stack up will be acceptable, to make sure that no parts are being specified out to be more expensive than they need to be, and to make sure that the basic design keeps the really significant tolerances limited as much as possible.

They must do their jobs well -- they put together some impressive mechanical designs that, for the most part, locate the important parts with hardened pins into slots. A few of the most critical parts get glued in from fixtures, but there are almost no adjustment screws or shims in the system.

snipped-for-privacy@hotmail.com wrote in news:d72a6b58-ce65-4604-bd1b- snipped-for-privacy@d5g2000hsc.googlegroups.com:

To be honest, unless you have some very nice, expensive gauging, you probably aren't really holding 0.0005 on your manual lathe. Second, remember that decimal places directly == $$$$$$$$. If you need 0.0005 and are willing to pay for it, no problem. If you want your parts at a reasonable price and +/-0.003 will work...

In addition to what others have said, I like to pose the question another way.

Ask yourself, "What would happen if this part was .010" out? What about .050" out?" And then make sure to *answer* these questions! Yes, you'll need a calculator.

Only once you arrive at a limit where your product no longer functions correctly have you found your *actual* tolerance. Specifying ANYTHING tighter is costing YOU and your customers money - period.

If you look deeper into manufacturing techniques, there is more to the story. Based on your question, these topics probably aren't important to you at this point, however.

Regards,

Robin

Thanks everyone.

Since this part is part of an assembly, the OD has to be +/- .0005 as it is a press fit... with a little green loctite, the rest I decided was fine at +/- .0025.

I had a few quotes on the work before specifying the tolerance, and when I humbly added my tolerance, both places said it would not matter and the price would not change.

thanks for the insight fellas!

==================== Good observation, but remember accuracy costs money.

Use as much as you need, but more than you need adds nothing to the product and simply costs more. It also takes longer, and this may be important when you need 100 parts ASAP for an order.

If +/- 0.005 inch will work then that's what you need to specify.

Also the feature that you are trying to hold will have a big affect on the cost, that is it is harder to hold overall length than it is an ID. What are you wanting to hold 0.0005 on? Also be reminded that at 0.0005 inch you are into accuracy/measurement/repeatability where temperature starts to have a significant effect, so you will need not only a very accurate micrometer (and a good feel) and a method for checking/setting the micrometer such as a gage block, but also controlled temperatures or very accurate temperature measurement for correction. The parts will have to "soak" for several hours for the temperature to equalize/stabilize, and your hands may warm the parts when you measure if you touch these.

Unka' George [George McDuffee]

------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?

Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).

Gunner