Re: [CAD_CAM_EDM_DRO] Re: Newbie trying to help others make sense of it all

Doug Fortune wrote:

> Karl Peterson wrote Sat, 11 Aug 2001:
>
> Hi:
> This boils down to using linear encoders or rotary encoders.
> True linear encoders attached to directly sense the position of the
> table are the 'Rolls Royce' of position feedback methods. But
> they are pretty expensive (USDigital is a 'bargain') and have
> to be protected from swarf and shocks.
>
> Rotary encoders (much more affordable) can be used two ways:
> - directly attached to the screw or the motor. The downside is
> that you are NOT directly measuring the table movement, but
> are trying to interpret the table movement from the rotary
> motion. This means these inaccuracies are introduced:
> - if the threads are not true, then you will have displacement
> that varies along the length of the thread, from a low value
> of maybe 0.001" per inch, up to lots (for hardware store
> ready rod). Theoretically you can compensate this in software
> but no-one has done it for hobbiest software yet (that I know).
> - backlash of nuts (and maybe motor couplings) could range up to
> 10 thou or more.
>
> - attached to wires (attached to the table), or attached to pinions
> which run along racks. The errors include cable stretch, and for
> racks, the degree of trueness of the rack.
>
> IMHO, if you want feedback under 0.000 50" then linear encoders are
> the only way to go. The USDigital stuff can give you 1/1440" ==
> 0.000 6944" which is 1440 steps/inch (obtained by 360 steps/inch in
> quadrature).

I'm using shaft encoders on my mill with resolution of .000050" (no typo,
that is 1/10 th your figure above, or 50 micro-inches in the X and Y.
It does cause some error due to backlash and spring in the leadscrew
mechanism, but I get positioning of about .0015" throughout the machine's
motion envelope, including the backlash.

Jon