Re: [CAD_CAM_EDM_DRO] Re: Z axis drive

spc_aux <aux34@...> wrote: --- In CAD_CAM_EDM_DRO@yahoogroups.com, R Rogers
wrote:

>
> Both of my Bridgeports drive the knee for Z axis and they are in

excess of 1000 pounds (knee, table, saddle, ballscrews, 2 servos, 2
mounts and vise). Getting 60 ipm Z rapids on both and use no assist
whatsoever. 40 lb-in DC brush servo 2.5:1 reduction on a G320 Gecko.
I ran one job recently on one of these that consisted of 1280 .875"
reamed holes 3 inches deep in 1018 steel. Alot of pecking. The servo
would have to rest every 4 hours to cool a bit. The Gecko would've
run continually it seems. Great job Mariss..

>
> During the retrofit and setting up the electronics, I had

determined that drilling for prolonged periods would be out of the
question. I was wrong.

>
> Ron

Since there is plenty of room under the knee on a Bridgeport and the
travel isn't too far, you might benefit by sticking a good sized
(bore) pneumatic cylinder under there connected to a small tank and
regulator. Take some of the load off the leadscrew and you should get
better performance and less heating for very little cost.

Pete C.
/.////////////
Pete,

Both of these get used alot. Under the knee is a very dirty and abrasive environment. Doesnt seem a pneumatic cylinder would last long. As far as the travel that is another issue, these have almost 14" of programmable Z. Not much room for a cylinder of that stroke. These are the knee type with just a hole for the stanchion. Not an open casting. Another consideration when counterbalancing is the alignment of the force relative to the Z axis. The leadscrew or jackscrew is in perfect alignment, dead center of the ways. Mounting an apparatus off to one side of center may impede accuracy and may definitely increase friction, i.e. sideload on ways, binding motion. May be easier on the axis motor but may be damaging the machine over long periods.

I've looked at several of these methods of counterbalancing the knee and none of them seem very practical. Cables, gas struts, springs, cylinders, counterweights, etc. Also, have to remember, storing that much energy as in springs, is very dangerous. Actually the weight of the Z removes all backlash on the leadscrew. The closer it comes to zero balance, the closer it is to an axis with a ton of backlash.

Alot of things to consider.

I've opted to just use large servos and forget counterbalancing. 60 ipm on Z is ample. No cylinders or airlines or tanks to maintain. No springs to fly across the shop. I've set three different knee-mills up this way and no counter-balancing was used.

Ron







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