Centroid Community CNC Support Forum

Is the knee counterbalanced? That would greatly reduce the motor effort.

No Tom, the knee isn't counterbalanced. That's a good idea though. I must admit I hadn't given that any thought.
Is counterbalancing widely used?

Counterbalance is used universally in machine tools to counteract gravity. Most bed mills and machining centers use cable connected weights. Most knee mill refits use gas struts or pneumatic cylinders.

Thanks for that info Tom. I'll look into it. The knee is V heavy and I have installed a largish servo motor and driver to lift this. If the latest suggestion that I'm trying (a larger synchronous timing pulley to increase the mechanical advantage of the servo) doesn't work, gas struts or a pneumatic system, will probably be my next port of call.

while not a knee mill. mike posted great info on how he counterbalanced his mini bed mill.
video link of it working


photo album.
https://goo.gl/photos/EddCrajE9wguNNAy7

thread
https://centroidcncforum.com/viewtopic. ... 748#p31748



how he did it.https://centroidcncforum.com/viewtopic.php?p=8525#p8525

Thanks for that Keith. The logistics of counterbalancing my knee mill are a bit more involved than those in the video, but it looks look a good system.
I am 3/4 of the way through modifying a larger upper timing pulley for the existing setup, so I'll finish that 1st, then if moving the knee still causes servo errors I'll look into counterbalancing.

dmbgo wrote: ↑Mon Oct 09, 2023 6:14 pm Thanks for that Keith. The logistics of counterbalancing my knee mill are a bit more involved than those in the video, but it looks look a good system.
I am 3/4 of the way through modifying a larger upper timing pulley for the existing setup, so I'll finish that 1st, then if moving the knee still causes servo errors I'll look into counterbalancing.

yeah for sure, wasn't suggest that

Bridgeport had a pneumatic system on some of their old CNC knee mills.
but most others just belt way down and put a big motor on it.

gas struts go bad,
air cylinders are nice but require air
whatever you do, leave some weight, meaning only counter balance a portion of the weight,
and use a ball screw for sure to drive the knee (don't use acme thread)
it will work quite well if done rigtht.

My knee does have an acme screw Keith, I have ball screws in the X and Y axis. I realise that acme screws are less accurate and harder to turn than ball screws, but unless I go down the counterbalancing route, I think I would need a big, expensive ball screw.

I was hoping that, when it's time to time the axis, I would be able to compensate in the Acorn software for backlash etc.

Tune