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

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
>
>

Hi there.

I'm not an expert, but I'm also starting a Bridgeport retrofit, and have
thought about powering the knee as well.

I'm not sure a counterweight system would work. You'd need a cable that
goes over the top of the machine or something - this would probably get
in the way of the machining envelope.

I haven't seen ballscrews for this purpose. Not that they don't exist,
I just haven't seen them. I'm sure you could get a custom screw made
(or turn the correct ends if you have a lathe). The thing about the
knee is that it uses a 10TPI screw because of the weight. If you think
about the load, you have a 150lb saddle, a 225 pound table, a 50 pound
vise, and possibly a 50-150 pound rotary table - that's about 500-600
pounds before you put a workpiece on it. Add the knee, which is
probably 400-500 pounds, and that's one heck of a load. As far as I've
seen (and from what I can figure out), you would need a pretty thick
ballscrew - like the X/Y screws, which are 1.25" diameter. I haven't
seen ballscrews that thick with more than 5TPI.

As for pros and cons, I can't think of any pros other than the fact that
you have a power feed on the knee :) As for cons, the quill is meant to
be used for depth control, and is much less massive.

Hope this helps.

- Steve