Re: Z-Axis Knee Woes

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