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Like many here I'm slowly doing a CNC conversion. I've read through a ton of threads about quill vs knee for a Z axis. I had a brainstorm, or not LOL, and thought about using a powered feed like this: Power Feeds It's a prepackaged and actually designed to move a knee.
I'm assuming it's an induction motor and doesn't appear to have any feedback, so what sort of problems would I run into trying to control something like this? Has it been done before? Thanks for any input in advance...
Your link doesn't work.
There is no feedback.
The motor is probably a "universal" type, I am not aware of any drive which can operate a universal motor in a ervo application. You could however replace the motor with a brushed dc type which could be easily servo controlled.
Matt
I'm not sure why the link didn't work for you. It's a link to a Servo Products power feed, but that was just an example of what I had in mind. The type 200 feed from them is listed as a "half-wave series motor" another is listed as "full-wave SCR control compound DC motor".
Admittedly I don't full understand those terms, but I think they refer to DC motors being driven by an AC circuit. In which case they could be driven by a brushed DC type controller, or am I totally lost?
As for feedback. I would more about where or if there was room for an encoder once the unit was disassembled, but that's a moot if the drive can't be controlled without breaking the bank.
The link is working now, maybe it was my work computer.
It sounds like either configuration uses a dc motor, th cheap one with just a diode in series with the motor and the more expensive one with the full wave control. If it is a normal dc motor, then you can use a normal servo drive on it after you fit an encoder. I don't have experience with these units so I don't know about backlash in the drive mechanism but I know that you shouldn't have any backlash in the actual knee due to the weight of it always keeping the nut push down on the threads.
Matt
That's pretty much what I was thinking. I think that's the direction I'm going when I gwt to that part.
I see some issues; back lash, the difficulty in turning the motor into a servo and speed. The power feeds have failry low HP motors. The cost of converting the power feed would out way just making a knee feed.
On my J-head Bridgeport conversion I have both a quill feed and a knee feed. Both use steppers and timing belts. For the knee I added a timing pulley to the lead screw just below the bearing support up inside the knee. The belt runs through a small window cut in the side of the knee casting to the stepper mounted with a bracket on the outside of the knee. Initially I was was using the old acme screw but just finished an upgrade to a ball screw. Most of my cutting is done with the quill feed but almost all tool changes requre the knee to be adjusted. In the quill feed the stepper drives a ball screw using a 2:1 timing belt reduction. The ball nut is attached directly to the quill so there is no backlash like would come from driving the rack gear. I get 100 IPM on both quill and knee.
Craig
Originally Posted by vidio1
@79tigerpilot I've seen you Z axis thread. Impressive stuff, I especially like the quill drive. I hope to do something similar. I have an Alliant mill that was converted on X and Y with Millport controls and a power feed on the knee. For the time being I'd like to convert the power feed to control and use it as a Z axis. As time and more importantly money permits I'll try to come up with a quill drive design......
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