Re: Z-Axis Knee Woes

Hi Mariss,

I suggested using a mix of servos and steppers in the Mach1 group a
couple of weeks ago for exactly this problem and one person brought
it to CCED shortly after. At the time the proposal was not well
received, but looking at the charactaristics of both type of motors
it seemed the logical step. Is there any practical reason not to do
so? the servos in question need a power supply of about 42 Volts and
if chosen carefully the stepper could live with this voltage. Any
further guidance would be appreciated.

Thanks

Bernard

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis
<mariss92705@y...>" <mariss92705@y...> wrote:

> Jake,
>
> This may be a situation where a step motor is a better match.
>
> Servos develop their maximum rated power at about 80 to 85% of no-
> load speed. The correct reduction will be the ratio of that RPM to
> the maximum RPM you expect to have on the jackscrew. That may be a
> rather large number.
>
> More than likely the 600 in-oz is the motor's stall torque. That
> means the motor's continuous rated torque will be between 60 to 120
> in-oz. The motor will overheat if you apply more than the

continuous

> rated torque to the motor for more than a few dozen seconds.
>
> The problem is not the motor's power but rather the gearing. If the
> motor is rated at 200W, then it would be capable of lifting a 500

lb

> weight at over 212 IPM vertically.
>
> The situation is similar to being in 5th gear instead of 1st when a
> red light turns green. The engine will lug and possibly stall; in

any

> case the getaway will not be nearly as spectacular as it would be

in

> 1st gear.
>
> On the other hand, steppers have the torque you need at low speeds
> with little or no reduction.
>
> Mariss
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "jbordens <jake@a...>"
> <jake@a...> wrote:
> > Hi folks. I'm retrofitting a G3102 Grizzly Knee Mill, a smaller
> type
> > mill somewhere bigger than a drill mill, but smaller than a full
> > bridgeport.
> >
> > I just finished my Z-axis, after success with X and Y. I decided

to

> > motorize the knee for various reasons. I'm using Camtronics 600-

in-

> oz
> > motors and a 2:1 ratio on the pulleys.
> >
> > The knee is driven by a shaft that is at 90-degrees to a

jackscrew.

> > The shaft is connected to the jackscrew via a set of helical spur
> gears.
> >
> > Here are my problems:
> >
> > (1) the motors aren't powerful enough, apparently, to raise the
> knee.
> > They stall and eventually cause my geckos to fault. I'm pretty
> sure
> > I have the "limit" set to Max on the geckos.
> >
> > (2) when lowering the knee, the motor turns and every now again
> > there's a 'thump' from the mill, as if the knee is not lowering
> > smoothly, but in little increments.
> >
> >
> > I guess I have two options:
> >
> > (a) go to a higher ratio, but 2:1 was the most practical. 3:1 is
> > possible but expensive, and anything more is really not possible

due

> > to pulley size constraints.
> >
> > (b) counterweight the knee. I think this is complicated and
> probably
> > not a good idea.
> >
> > In either case I'm not sure it would solve problem #2. I'm
> currently
> > at a loss on how to proceed. Any suggestions are greatly
> apprecaited.
> >
> > Thanks,
> > Jake