Re: [CAD_CAM_EDM_DRO] Sawyer motor

Jon Elson wrote:

>
> Anne Ogborn wrote:
>
> > Anybody on this list been sick enough to contemplate
> > making a machine based on a sawyer motor?
> >
> > A Sawyer motor is a sort of 2D linear stepper motor.
> > I used to run a giant pen plotter that used one.
>
> These are quite fine, but actually hard to make. There apparently
> is a machine tool made with them. We have a roller coaster at Six
> Flags amusement part that uses them. They had a $5 million cost
> overrun because they ran too hot, and they had to redesign the
> whole system to use twice as many feet of armature to keep it
> running for a full day of rides.

Yep, though that machine uses them simply because no friction braking system could
absorb all that energy.

>
> One problem is that the huge magnets will attract all sorts of metal
> chips. They tend to run hot, and draw huge currents at very low
> voltages. The attractive forces between the stator and armature
> are not balanced (as in round motors) and so the entire design
> has to be made with this in mind. The attractive forces can run
> to several thousand pounds even in small motors.
>

Good point. I'm familiar with one that ran a plotter, hanging down from
an overhead platen and running about on air. Incredibly fast, I think you could set
the speed to up to 400 IPS and the acceleration up to 120 g's.
I was scared of the machine. If it'd gotten away it wouldn't have stopped at the edge.
Hadn't thought about the swarf problem either.

> If you were designing a machine from the ground up, you could
> probably design linear motors into it well. I'm not so sure how well
> you could retrofit linear motors into an existing machine tool.

They'd have to step awful fine. But then that plotter managed to step,
and the grooves in the motor were a couple mm.