Re: [CAD_CAM_EDM_DRO] Z-Axis Knee Woes

Hi Jake,

I had many of these problems on my BP Clone once... I initially fitted
two 175lb gas springs from McMaster Carr but recently upgraded to two
250lb units - for a smaller machine, probably 2 x 125 or 150 should be
ok.. Even with the 175lb gas springs, I didn't need a brake, but it was
getting marginal with a 60lb Kurt Vice on the table. Now the motor/belt
friction can easily hold the static load.

I also fitted a ballscrew to the knee lift in place of the acme. This
made a huge difference to reducing the friction when raising the knee.
I also shimmed the large gear (on top of your acme screw, inside the
knee) very slightly to reduce backlash. Too much shimming and you'll
hear a grinding vibration as the screw shaft flexes - this is most
undesireable. Once I had things running, I had to fine tune the gib
adjustments to reduce a high speed vibration. My servo motor is 880
oz-in torque and is reduced 2.4:1 via belts and 2:1 via the helical spur
gear. I can get 48 IPM rapids on the knee no prob but at a lower
acceleration setting to X & Y. I'll post a pic of the gas springs
tomorrow as I can't find the last pic I took.

Cheers, Peter

jbordens 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
>
>
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