Re: Knee for z-axis control?

Ron,

Thanks for the help, considering the retro with no ball-screw
(w/brake) or counterweights, I figured it would take a considerable
amount of servo/stepper power to raise the knee at an acceptable rate.

Do you have a good `economical' source for the 40in/lb servos? As I'm
sure you know, these larger servos can run a small fortune when
buying new!

Thanks, Eric




--- In CAD_CAM_EDM_DRO@yahoogroups.com, R Rogers <rogersmach@y...>
wrote:

> 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@y...> 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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