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!

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In my outdated opinion, the tolerance should meet the demand of the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

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
increasing cycle time, or increase the scrap rate. Buy the accuracy you
need.

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In my outdated opinion, the tolerance should meet the demand of the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

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?

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

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!

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
4 places to the right of the decimal point.


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

On Tue, 26 Feb 2008 12:25:53 -0800, Gunner
wrote:


The most dangerous thing in the world, is a CAD package with more than
4 places to the right of the decimal point.


Hey, I've got all the decimals you'll ever need. g 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.

--
Ned Simmons

Gunner 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!

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
4 places to the right of the decimal point.


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

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

wrote in news:d72a6b58-ce65-4604-bd1b-
:

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!

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...

--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
better idiots.

Remove sp to reply via email

On Feb 26, 6:31*am, 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 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!

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).

On Tue, 26 Feb 2008 16:06:44 -0500, Ned Simmons
wrote:

On Tue, 26 Feb 2008 12:25:53 -0800, Gunner
wrote:


The most dangerous thing in the world, is a CAD package with more than
4 places to the right of the decimal point.


Hey, I've got all the decimals you'll ever need. g 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.

G

Gunner

On Tue, 26 Feb 2008 21:51:44 -0800, with neither quill nor qualm,
Gunner quickly quoth:

On Tue, 26 Feb 2008 16:06:44 -0500, Ned Simmons
wrote:

On Tue, 26 Feb 2008 12:25:53 -0800, Gunner
wrote:


The most dangerous thing in the world, is a CAD package with more than
4 places to the right of the decimal point.


Hey, I've got all the decimals you'll ever need. g 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.

G

Damn, that's almost as much as a new pair of basic Air Jordans!

--
Such is the irresistible nature of truth that all it asks, and all it wants,
is the liberty of appearing. -- Thomas Paine