Re: [CAD_CAM_EDM_DRO] Servo/Stepper on a Surface Grinder

Thanks for the reply. You bring up a lot, so let me answer in point form.

. Forgive nme for not being more clear, yes, this is a uController
project. The board is already wired wrapped (too simple to justify
the expense of a PCB) and the code 99% written.

WAY easier to do this on a uC than a PC .. and I'm a PC programmer !

. Steppers or servos .. the issue here is two conflicting criteria. The
first is that on occasion, a FAIR bit of force is required to turn the
crank to move the table. The wheel hits something high or something hard
and 10 times the force is required for a second or two. So a whack of
power is required every once in a while, meaning a fairly big motor.

Now consider the saddle. It merely sits on top of two V ways, with nothing
holding it down but the weight of the table. And with the table at either
extreme, there is a fair lever trying to "lift" the saddle off the far
V way.

Any motor I use is going to add to this lever effect, therefore, I want
the lightest motor that will do the job, to minimize, as much as possible
this effect.

Steppers make the best power/weight at these speeds, but cog .. only a bit
to be sure, but cog. When removing 1/10 of .001", I'm going to believe
that this cogging will be noticeable.

DC motors should be smoother, but don't make a whole lot of power at low
speeds, so I'm back to using a big one, with the problem mentioned above.

A servo, though, controlled by something like a Gecko (that I have a fair
bit of experience with) would allow a small motor, with the Gecko watching
the speed and bumping up the current for that short period of time, should
it sense a slowdown.

A simple PWM controlled _small_ DC motor would require additional electronics
(and a tach) to control the speed.

So the Gecko/servo is the easy way out.

. I like the 3 phase idea, given that I have a 1/3 HP VFD sitting around
(eBay madness). But unless I run across a 1/30 HP or 1/50 HP 3P motor,
anything else is going to be too darned big.

Double-stack NEMA23 is about the size and weight I'm comfortable with.

Mind you, I am starting to stare at the thing with an eye to mounting a
counterweight on the side opposite the motor. But there just isn't a whole
lot of machine to bolt anything too; even the NEMA23 mount is going to
be tricky ...

. Belt to reduce the speed would be nice, but there is a limit to the size of
pulley that will fit under the table. This is the one you'd want to be
large but will have to be small. I think a 16 tooth XL pulley is as large as
I can go here, which doesn't give a whole lot of potential for reduction.

. As attractive as CNCing the Z axis is (I call it the Z axis, I have no
idea what the grinder guys call it), I cannot imagine doing so anytime
soon. The amount of "down" is very hard to predict and highly dependent
upon a lot of not easily calculated factors (hardness of the material,
wheel composition, wheel loading, wheel grain sharpness, moon phase).

There is no formula for flutes/material/diameter/surface_speed_constant as
there is when milling or turning, or anything like it .. your eyes and ears
tell you when it's safe to go down.

So while it would be nice to program the "down" in the same manner as I
program the across, I just can't see it happening. Still, for the sake
of two more Step/Dir signals and two more BCD switches (down increment and
down distance) ...


Anyway, thanks again, and glad you don't see me as excessively mad !


Alan

On Wed, Apr 26, 2006 at 10:24:39PM +0100, Abby Katt wrote:
> Hi Alan,
>
> Wow, I really appreciate your kind of thinking. It certainly appeals to
> the inner-geek in me ("What can I automate about my house?!") from what
> you're describing though it sounds like you're willing to make your own
> controller for this purpose. To be fair, a microcontroller or PLC is by
> far the most optimum thing for this solution as using an entire PC for a
> back-and-forth job really is overkill. But by this thinking, I'm lead to
> wonder if you really need gecko's or other servo/step drivers to control
> the main back/forth motor?
>
> It seems a simple DC motor speed/dir contoller would do quite well (A
> homebrew H-bridge with a bit of PWM to set the speed) - or even a really
> cheap 3 phase motor and an inverter. Most inverters have a control line
> for direction, so your PLC could just flip that and you could use a
> potentiometer to set the speed. This way you'd have nice smooth motion
> (with lots of simple adjustment for speed). Maybe even use a belt/pulley
> system to reduce the speed? For the Z-axis (is it called a Z-axis on a
> surface grinder? Well, for the "mill Z-axis" anyway) I'd use a gecko
> stepper driver. No point in servo's here - steppers are nice and cheap
> and have the perfect "hold" characteristics you'd want for raising and
> lowering the grinder.
>
> Sound workable?
> I certainly rather like your idea of CNCing this type of machine. :)
>
> ~Abby
>
>
>
>
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--

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