Re: Just How Accurate Is CNC?

I wrote this for my site a while back, you might like it.

Thanks
Aaron Moss
www.IndustrialHobbies.com


In search of Tolerance


Tolerance
Tol´er`ance

Noun
5.
tolerance - a permissible difference; allowing some freedom to move
within limits

leeway, margin, allowance



Tolerance is without a doubt one of the most misunderstood terms when
people talk about CNC. To be exact tolerance has noting to do with
CNC, the proper term is resolution (ability to return to a known
location) when discussing CNC. But, tolerance is the work that folks
like to use.

Maintaining tolerance is the ability to produce something within
specifications, no more no less. In the hobby (and small business)
arena this is a loaded question, because for the most part you
the "designer", the "machinist" and the "quality inspector"; the
tolerance can be whatever you want +/- 1' (one foot) +/- 0.000050 (50
millionth) or anywhere in between. It's your call.

Reasonable Expectations

0.050 With a tolerance of 50 Thou' you can produce most things that
are used around the home and shop, some model projects but nothing
too serious. Some of your friends will be amazed with your results,
some will not. Basically, you threw the machine on the bench, threw
down a few beers (not a good idea) and "fired it up".

0.020 With a tolerance of 20 Thou' you'll be able to do some of the
more difficult hobby stuff but still nothing too serious. You did
everything as above but without the beers.

0.010 With a tolerance of 10 Thou' you'll be able to do most hobby
stuff, except engines and extremely complex mechanical projects. You
trammed (squared) the machine and played with the gibs (adjustment
bolts). You think about machining more than once a week, and look
for ways to improve you tolerance.

0.005 With a tolerance of 5 Thou' you'll be able to do some simple
engines and fairly complex mechanical projects. From time to time
you re-tram your machine. You realize that more expensive cutters
will more than likely help and you think about popping for a "good"
set of gauges. Your CNC code now includes a finish pass. Your
friends are impressed.

0.001 With a tolerance of 1 Thou' you'll be able to do most model
engines, except turbines. You can check the square-ness of you
machine in less than 5 minutes and adjust as necessary in another 5.
Your code now has "lead in's and lead out's" and you know whether you
need to use a climb or conventional cut. You've lapped, scraped, or
re-ground the ways of your machine and know when it doesn't "feel
right"

0.0005 With a tolerance of half Thou' you'll be able to complete any
model in any book you find new or old. Your CNC code is poetry in
motion and you are considering leaving your real job in order to be a
machinist. When you read Home Shop Machinist, and say, "I can do it
better"

0.0001 With a tolerance of Tenth' you have left your day job and do
this full time. You know what temperature you machine needs to be at
to "hold tol" and you adjust your machine accordingly. You are
considered a skilled craftsman. Your friends have no idea what
you're talking about anymore.

So on and so forth …

Now of course there are many in between stages, for example when
you're making something and say, "I can be loose here and I need to
be tight here", knowing that just comes with experience for which
there is no substitute. There is no law against doing a test run on
material to see if you get the results you want and then do the
finial pass. It's just one of those things.

As for the question at hand, "What's the tolerance?" the answer is
simply another question. "What kind of tolerance do you want?"





--- In CAD_CAM_EDM_DRO@yahoogroups.com, "softtechinit"
<JFlanigan@T...> wrote:

>
>
> What sort of repeatability and positioning accuracy should one
> expect from a CNC mill? Would you call 0.001" typical or
> unacceptably sloppy?
>
> Regards to all.
> John