Re: [CAD_CAM_EDM_DRO] Knee for z-axis control?

Hi Eric,

I'm using the knee for Z on my Bridgeport retrofit. You dont need a ballscrew for the knee. The acme screw is 10:1 reduction and due to the weight of the knee there is zero backlash. It would be dangerous to use a ballscrew because if you ever broke a belt the table would fall. Ballscrews when pushed against will "feed" themselves unlike an acme thread which has an inherent binding effect. I'm using a 40lb/in servo @ 100 volts driven by a gecko 320 @ 72 volts with a 2.5:1 belt reduction. As has been stated before you can run large servos at less voltage sacrificing speed but not torque providing you are supplying the necessary amps. I dont have any counterweights or gas springs for assist. That motor and Gecko whips the knee up and down like it was a feather. I've ran like profile programs that are approximately 2 hours long and the knee was whipping up and down all the while. I did that just to test the motor and drive. They stayed right with it and got slightly warm but were not
overheating. Originally the machine had an 1100 oz Slo-syn stepper and it would move it but very slowly.

The cons to using the knee? you cannot accelerate as quickly as you do with a quill. So it is a bit slower however, not a huge difference. And you need a larger motor for Z than X and Y.

Now for the pros: Driving the knee gives you approximately 14" of linear programmable Z travel coupled with the quill its about 19" (my machine has a 4" riser). That comes in really handy when doing deep bores or tall weldments. The quill is limited to 5". I dont know if you've ran a Bridgeport much but it doesnt take long to get tired of cranking the knee up and down. Invariably, I'd be working on something .25 thick and the next job was 12.0" tall. I remarked after my retrofit was complete and I was jogging the knee up and down with the keyboard "If it didnt do any more than that I'd be satisfied" What a relief to not have to crank that chunk of iron up and down anymore.

Ron

pondindustrial <pondindustrial@...> wrote:
Hi all,

I will be retrofitting my Bridgeport mill soon and know that some
have used the knee for z-axis control. Does anyone here have any
experience with this method? If so, what should I take into
consideration if I decide to go this route? The mill has a 9" X 48"
bed.

Is a counterweight to offset some of the lifting load preferable?
Did you use the original acme screw/nut, or retro with a ballscrew?
What would be a reasonably sufficient drive in terms of motor torque
and gear reduction?
What are the pros & cons of using the knee opposed to the quill?


Thanks, Eric




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