RE: [CAD_CAM_EDM_DRO] Re: Just How Accurate Is CNC?

Aaron has put together an excellent overview for the novice, but I feel
compelled to add that the perspective he puts on precision applies mostly to
metal removal with cutting tools. If you're doing metal removal by
grinding, 0.0005" is sloppy, 0.0002" is typical minimum, and better than
0.0001" is a common request.

People should expect that sort of precision in spindle centers, ball bearing
size, tool holders, and cutting tools. My own tool holding system
(http://www.tormach.com/tts_products.htm ) has no tolerance specifications
wider than 0.0002" and the actual delivered products have all come better
than 0.0001". Grinding is different than cutting. If you try to use your
lathe or mill to reproduce a part out of your lawnmower, power tool, or
model gas engine which had a ground finish, you will probably not be able to
meet the performance needed. Grinding is not just used for final finish on
hardened parts, it's also used where cutting processes cannot achieve the
required precision.

Greg

> -----Original Message-----
> From: industrialhobbies [mailto:admin@...]
> Sent: Tuesday, November 09, 2004 8:50 AM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] 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
>
>
>
>
>
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