Re: [CAD_CAM_EDM_DRO] How do I interpret accuracy

khalique_rehman 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".
>
>

Yes, this is almost meaningless. 0.0005" over the full travel of the
Taig is completely preposterous. Many $100,000+ machining centers
can't hold this level of accuracy over a foot of travel unless installed
by millwrights and calibrated by laser interferometer by the manufacturer.
The very high end machines have hollow ballscrews with temperature
stabilized water flowing through them, and have the spindle head filled
with a constant temperature bath to prevent thermal expansion.
I doubt the leadscrews in the Taig are machined to .0005" precision,
and once in the machine, the other sources of error will all combine
to make them worse. Probably the worst errors in the small machines
are orthogonality errors, ie. the X and Y axes are not square. High
end machine tools are hand scraped to bring the axes into proper alignment
relative to each other, and very precise measuring tools are used to get
the orthogonality down below .001" over the full travel on modest sized
machines. I'm quite sure the lower cost machines like the Sherline and Taig
are not hand fitted at all for orthogonality, and that error alone is
probably
several times the 0.0005" spec you quote above.

My guess is the spec you refer to might be the basic leadscrew pitch error,
in units of inches per INCH! IE, .0005" per inch of travel.

A precision ground leadscrew with .0005" per foot travel would cost more
than
the whole Taig machine!

Jon