Re: [CAD_CAM_EDM_DRO] Just How Accurate Is CNC?

At 8:30 PM -0600 11/8/04, Jon Elson wrote:

>softtechinit wrote:
>
>>
>>What sort of repeatability and positioning accuracy should one
>>expect from a CNC mill? Would you call 0.001" typical or
>>unacceptably sloppy?
>>
>>
>There are a whole raft of things that cause loss of accuracy. You can't
>lump them all together. There are things like axis straightness and
>orthogonality, which cause increasing errors as you move farther.
>There are things like screw pitch error and variation, which can
>be compensated for. There are things like backlash that are quite
>repeatable, but can cause poor finish, out-of-round circles, etc.
>.001" on a 1/4" part is pretty poor. .001" on a 12" part is getting
>into the stratosphere, when you include stress release in the
>part, temperature variation, etc. To hold .001" over 12", you need to
>keep the part in a temperature controlled room. So, it depends
>on what you are doing, and the size of the parts.
>
>I have a Bridgeport with high accuracy screws and .001" backlash,
>and it is more than accurate enough for me to make some fairly
>exotic parts at times.

I am still new to this, but one thing that has been bothering me. If
you have a full feed-back system, why is the feedback taken from the
lead screws? Seems like you would eliminate the blacklash completely
if you measured the table location directly instead of through the
slop in the screws. The controller would just need to move the table
till it was in the correct spot. seems to me that when you want to
measure something, you measure as closely to the thing as you can,
not through a lot of linkages. -Chuck-