Re: Accuracy

Jason,

Intersting word, accuracy... Most machines have far LESS of it than
many would have you believe. Thankfully, most applications(parts)
REQUIRE less of it than many would have you believe<G>

First thought is that you might be confusing RESOLUTION with
accuracy. They are NOT the same thing. Resolution is the
smallest "command-able" movement of a given axis. Accuracy is whether
ANY movement (large or small) is of the correct length, as determined
by international standards and conventions.

The third leg of the "table" is REPEATABILTY. This one may be easiest
to understand, and is usually easiest to determine as well. It is
simply whether a repeated move "ends up" in the EXACT SAME position
each time (whether this position is accurate or not).

Yes, the leadscrews and drive motors are COMPONENTS of a machines
overall accuracy. But there is FAR more involved than these.

Here are a few more items contributing to the overall accuracy of a
given machine:
Motor electronics(drives, including power supplies); controller
software, firmware, and hardware; Wiring, and grounding practices;
temperature (and controls OF temperature, like leadscrew chilling);
material chosen for the structure/parts of the machine; geometrical
accuracy of the relationship(s) of parts to each other(ie,a leadscrew
mounted farther away at one end introduces an error); leadscrew
MOUNTS, meaning the bearings used and their arangement; Spindle
runout; tooling; coolant/lubricants for cutting; control panel, or
screen layout (since an operator making an error due to a poorly
designed interface will DEFINATELY affect accuracy1<G>. This aspect
is called the MMI, for Man-Machine-Interface;

Remember that programming/machining "style" can have a major impact
on overall accuracy attained. Which is why a GOOD operator can make
accurate parts on a "crummy" machine(relatively speaking); and a POOR
operator will have trouble making accurate parts on an EXCELLENT
machine. (the "assumptions" made by the better CAM packages are
helping this as they contain essentially the results of thousands of
hours of machinist expertise.)

It is the SUM TOTAL of ALL of these (and more) which will determine a
machines' overall accuracy.

Hope this helps,

Ballendo

P.S. re-read that 1st paragraph above :-)

P.P.S. I'm sure Mariss is better to answer your Gecko question. I
will say that generally glass scales are considered more accurate.
However, in light of the above written points about accuracy, simply
putting glass scales in place does not ensure an accurate machine.
Certainly a well-designed rotary encoder-based machine would be
preferable to a poorly designed glass-scale-based system...

--- In CAD_CAM_EDM_DRO@y..., "Jason Cox" <sinergy@o...> wrote:
> Hello,
>
> Just wondering what exactly determines the final accuracy of a cnc,
is it a combination of the leadscrews and driving motors?
>
> Also can a gecko servo drive be used with a glass scale instead of
a rotary encoder and would this give greater accuracy?
>
> Thanks!
>
> Jason Cox
>
>
> [Non-text portions of this message have been removed]