Re: [CAD_CAM_EDM_DRO] Positioning

On 06/09/2014 06:51 PM, Steve Blackmore steve@...
[CAD_CAM_EDM_DRO] wrote:

>> Actually, Steve, zero backlash IS possible, but the caveats which go
>> >with it are non-trivial. It usually only works over a very limited
>> >range of loads and zero ERROR is not possible.
> Actually - it's not, unless you weld the components together. Some
> allowance must be made to allow for thermal expansion, rolling/sliding
> clearance and the lube film to prevent said components really welding
> themselves together or wearing out very quickly.

Let's look at your points in order:

Thermal expansion should not affect backlash, it should affect
positional accuracy. Since backlash is clearance or lost motion in a
mechanism caused by gaps between the parts, and such lost motion is
possible to eliminate, thermal expansion may or may not matter. The
joint may not be rigid, but that is another matter.

Rolling and sliding friction can cause backlash, it is true, but that
assumes material contact. Magnetic suspension in a vacuum does not have
rolling or sliding friction. It may not be practical for your
operation, but it exists.

Lube films do not need to be matter. Magnetic suspension in a vacuum
does not have any lube film, nor does it have a wear issue.
Electrostatic suspension in a vacuum is similar.

As I said, the caveats are pretty severe, and the positioning system may
be too compliant, but the backlash can be zero.

For a common, backlash free system used in real life, look at a speaker
voice coil mechanism. It has no backlash, but it is far from rigid.

Dave 8{)
--

"A word to the wise ain't necessary - it's the stupid ones that need the
advice."

Bill Cosby