Servo/Stepper on a Surface Grinder

I have a small little Sanford Surface Grinder, 4" x 7", fully manual, and an
absolute jewel of a machine (after my friend Marcus spent a whole day of his
valuable time regrinding worn things back to wonderful with his nice big SG).

The problem is, being fully manual and with a rather small motor, there is a
LOT of hand cranking involved, and that gets old in a hurry.

It's clearly time to automate it.

Add a motor to the table, add another to the Y axis, add some (moveable)
travel limit switches, build and program a controller, done. Really not much
more than a long weekend project.

The controller will have a number of BCD switches

table speed (TS)
Y axis step distance (YSD)
table traverse count before step (TTC)

The usage is this. Set the switches and press start. The table will move
forward then back, at the speed specified by the TS switch, as many times as
specified by the TTC switch.

It's been my experience that when taking off 3-5 tenths, about 4 passes are
required before there are no sparks .. so set it to 6.

Once 6 passes have been made, stop the X motion, then move in/out the YSD
distance (.025", .050", etc). Begin traversing again. Repeat until a Y
traverse limit switch is hit, then reverse Y direction and continue.

Simple and should work fine.

The real question is this .. I want a SMOOTH motion, and at the very low RPMs
the traverse motor will be running at, everything in the house (NEMA 17 - 34
steppers, Sherline-sized to Bridgy-sized servos) seem to "cog" a wee bit more
than I'd like.

(The electronics are designed to issue Step/Dir pulses for use with either a
Gecko G320 or a stepper controller)

I like the idea of a servo to supply extra current if the grinding gets a bit
tough. I like the idea of a stepper because they have lots of power at these
sorts of speeds. I'm not as crazy about a simple DC motor, as controlling
and maintaining the speed is not so easy as just wiring up a Gecko.

Also, too big a motor is problematic .. this IS a small machine, and so the
size and weight of the traverse motor is a consideration.

(There's a tach being added to the spindle motor; should it slow because the
grinding got really tough, power will be removed).

But all the motors in the place of suitable size jerk jerk jerk at the 5 RPM
or so rotational rates I'm talking about.


I _could_ use a gear head motor (the lash in the gears not really being an
issue in this application), running the motor fast (and therefore smooth) and
using the gears to slow it down, but now manual use is severely compromised.

A clutch might be possible, but I'd rather not, if I can avoid it.

So I'm kinda stuck, in my own mind anyway.


Any thoughts appreciated.



Alan






--
Alan Rothenbush
Academic Computing Services
Simon Fraser University
Burnaby, B.C., Canada


The Spartans do not ask the number of the enemy, only where they are.

Agix of Sparta