Re: [CAD_CAM_EDM_DRO] Just How Accurate Is CNC?

> 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-

Ease for the most part.

To collect feedback from the servos means attaching something like a small
wind-mill disc to the shaft. As it spins through a counting system, it
produces feedback. The disc might spin over and over again for even a small
length of travel.

However, if you use a scale running along the length of the axis, you need a
much larger amount of accurate 'stuff', since this feedback method isn't
able to move like the wind-mill attached to the servo's shaft. Which means,
more money.

Another way of putting it would be that the servo shaft collected feedback
can be 'reused' as the servo pushes the table along the axis. Once the disc
does one rotation, it start a new one. So even if it only pushes the axis
25mm per rotation, it doesn't really matter as there's no limit on how many
times it can spin really. If you're collecting feedback from the axis it's
self, the majority of the feedback possible on that axis won't be in
continuous use.

As has already been said, big industrial machines that need to be very
accurate do use this method. They simply have a long slab of encoder style
material running down the length of the axis. As the carraige moves down it,
the feedback is produced from each unique position it runs over.
Essentially, you have a piece of material with lines drawn on it every
0.001mm. Each line is read as a position. If you have a few metres of this
material, it'll start costing moocho greens.

You need only apply the universal law in this case. If you're getting more,
you'll be paying more as well!

Best wishes,
John