Re: Just How Accurate Is CNC?

As others have stated, accuracy is not clearly defined. Most CNC
machines can be programmed to within .0001". Precision ground ball
screws can function to within .0002" (in my experiences). At that
level of precision, you must have climate control and preferrably
the machine on an isolation pad.

Most of the specs you will see from machine tool builders are tests
made without cutting. Adding cutting tools to the equation is
altogether a different matter and much more dynamic.

For greater accuracy, many machine tool builders used to (Probably
still do) offer an option for "Glass scales". While we all tend to
think of servo driven machines as closed loop, this is actually not
the case. Remember, the feedback encoder is located on the servo
which means the loop is only closed upto the servo - not beyond it.
This is where "Glass scales" come into play. These are linear
scales (although not necesarily glass anymore) that provide feedback
to the control along it's entire travel. These scales provide
feedback as to actual table positions not just the rotary position
of the servo motor.

The mfg claims were that with scales, you could double your
accuracy -- usually from .0002" to .0001"

Again, this is all without the effect of actual cutting taking
place. Machining to tolerances better than .0005" is very difficult
to achieve.

On my shop built router, I ran some positional tests yesterday. I
wanted to verify repeatability (point A to Point B). For the three
axes, they were all within .0005" repeatability. When measuring
from home position to a fixed point, my repeatability dropped
to .001" - my limit switches are your basic micro switches. While I
am running ball screws, they are of very low grade (Rolled, not
ground).

Hope this helps you a little

Chris






--- 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