So many choices - Best way to make Sherline CNC

   From: mszollar@...

>I'm know that this has been asked over and over again but cannot find
>one with this twist.  There are basically (as I understand it) 2
>options to convert a Sherline Mill to CNC.  1 would be to use stepper
>motors, and another (?) to use servo motors.  My question is which is
>better and more accurate?  In my browsing I noticed that the servo
>kits (e.g., G320) and motors use feedback, this verifies position,
>move, etc.  I've seen/heard of stepper motors that have encoders on
>them but haven't seen a driver that supports feedback directly
>(unless I'm missing something).  So my question holds; what is the
>best for the $ if you want accuracy (is feedback really
>required/best)?  

These points are offered with the assumption that you mean accurate to the
ability of a common Sherline.

1 - If your steppers are hefty enough to get you past obstructions and
drag then you do not need any kind of feedback.

2 - The EMC does have a stepper driver that uses real feedback from
quadrature encoders. (smdromod.o) For this kind of operation the
encoders are connected to a Kulaga/Mauch type of isa card that sends
position signals directly to the PC running the motion.

3 - The feedback loop used by Gecko 320's and 340's does not affect the PC
or program driving the axis. This loop is between the amp and the motor
that turns the encoder -- motion signals from the PC are outbound only.
This feedback does not affect the motion software at all unless an estop
is wired to the trip on the drive. This is not intended to be a criticism
of Gecko. They are great for their purpose and do overcome some of the
problems of lost steps.

4 - The tightest control of accuracy requires that the PC supplying the
motion signals be included in the feedback loop. This is normally
accomplished in commercial machines, and often in hobby machines by the
use of servo amps that monitor motor power and velocity and a cnc that
monitors position and computes the motion signals required to put an axis
where it thinks it ought to be. The EMC can also do this.

Number 4 is probably overkill for most hobby work with Sherline like
machines.

HTH

RayH