Re: [CAD_CAM_EDM_DRO] Re: DRO encoders- Rotary or Linear?

gittt2000 wrote:

>Linear - without a shadow of doubt. Here's why :
>
>http://www.heidenhain.com/posmeas.html
>
>
>

Well, yes, in $100,000 CNC machining centers, no argument! Notice, though,
that the graphs are in uM units, the ballbar plots are 5 uM/division!
To be of much use, the linear scales must have resolution on that scale,
or these errors
would lie undetected underneath the basic resolution of the scale. What
is the standard
resolution of typical DRO scales? .0005" ~= 12.7 uM if I computed it right.
So, the backlash errors they showed would be barely one quantum on the
typical
DRO scale. Linear errors due to screw expansion depend on how hard you run
the machine. Note the thermograph of the hot screw was run continuously for
6 hours at 24 M/Min. That equals 945 IPM. Are you planning to keep
your machine
running at almost 1000 IPM for 6 hours? If I tried to move my Bridgeport at
1000 IPM, if would hurl itself over so hard it would break the floor slab!

So, while the things stated in the Heidenhain article are real concerns
for makers
of dedicated, HIGH-precision CNC machining centers, and more important as
size and speed increase, they may not be so significant for small machines.
If you price linear scales with sub-micron resolution, you'll find them
pretty
expensive.

Jon