On Tue, 26 Feb 2008 13:14:46 GMT, Dan@ (Dan ) wrote:

On Tue, 26 Feb 2008 03:31:55 -0800 (PST), wrote:

I need to get some parts made, and am going to approach a few of the
local and online cnc machine shops. I need 300 tubes made, and up to
now I have been doing these on my manual lathe in lower, "as I need
them" quantities.

On my lathe, I was able to hold a .0005 tolerance, which I think would
be called +/- .00025.

Is asking for +/- .00015 too much to ask for a cnc job? Would this be
deemed extreme, or add to normal costs?

Please, only helpful and productive replys.

thanks!

In my outdated opinion, the tolerance should meet the demand of the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.
====================
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).