Re: How do I interpret accuracy

Hello,

I'll give this a go...

You wrote "the accuracy of a machine really is", and I'll add...

Elusive. <G>

The accuracy figures quoted in this hobby/desktop CNC market are not
even close to correct. What you often see called "accuracy" is REALLY
just the machine's RESOLUTION. And resolution IS NOT accuracy!

Your next example is similarly misleading. This ".003/ft." is a
statement of leadscrew pitch error, and is taken directly from the
leadscrew manufacturerer's documentation.

Both cases are examples of incorrectly stating a machine's capability
by using a number that "sounds good", but really tells you very
little.

As for your SPECIFIC question: "exceeds 0.0005inch" means that
the "accuracy"(SIC) is going to be AT LEAST 0.0005, and implies that
it will be better than this. So now you know what they were trying to
tell you. But to tell you the TRUTH, that statement is pure BS!

If we take the "exceeds 0.0005inch" at face value, it means that when
the Taig leadscrew, axis alignment, bearings/bushings, structure,
spindle, and handwheel graduation errors are ALL figured together,
the machine will be no farther from the desired location than 0.0005.
Which is ludicrous. (you CAN get to A GIVEN desired location within
0.0005 using a Taig.) But to imply that you can get to any arbitrary
location to this degree of accuracy by simply turning the leadscrews
is BS...

But let's not blame Taig, because ALL the mfrs. do this sort of
misleading marketing. They are just competing in the way the market
requires. They DO have a very nice little series of machines.

So to answer your initial question: The accuracy of a machine really
is the sum total of ALL it's inaccuracies. These will include the
list in the paragraph two above this one. Add to that list
temperature, deflection (both cutter AND machine) and we're getting
close to having all the items needed to discern true accuracy...

Fortunately, YOU ALMOST NEVER NEED ACCURACY OF 0.0005!...

You WILL need REPEATABILITY of .0005; but repeatability is MUCH
easier to get.

You may need RESOLUTION of 0.0005, and with our current CNC setups
this too is pretty easy to achieve.

So realise that when you read the specs of machine tools, you are
often being told what they think you want to hear. And also realise
that your lack of understanding is being used against you, to make
you believe that first, you NEED something you don't. And second,
that you will get it from the item being spec'd...

But that's marketing, not REAL machine specifications...

Hope this helps,

Ballendo

P.S. If I were reading the two examples given, I'd expect the Taig
leadscrew error to be less than .0005, and the CNC jr. leadscrew
error to be less than .006/foot. Nothing more, and nothing less. But
I suspect the Taig claim is not actually true. Simply raising the
temperature of the leadscrew by friction during use can cause
expansion of the screw itself which will exceed the stated accuracy.
The good news is, you don't need what they've said to BE true to make
EXCELLENT parts using their product. They build nice machines, IMO.



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "khalique_rehman"
<khalique_rehman@y...> wrote:

> Hi everyone.
> Please help me understand what the accuracy of a machine really is.
> The Taig specifies its accuracy as "exceeds 0.0005inch" while
> CNCMaster's CNC Jr has accuracy of "+/-0.003inch in 12inch".
> While I can understand the meaning of "+/- 0.003inch in 12 inch"
> (seems more like maximumm error), I have difficulty in
> understanding "exceeds 0.0005 inch".
>
> TIA for responding to my dumb question.
> Khalique Rehman