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

Jon Elson wrote:

>Stephen Wille Padnos wrote:
>
>
>>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.
>>
>>
>No, you could mount some bars near the top of the knee that project back
>a few inches
>past the ways to the side of the column. That area is not usable for
>workspace
>anyway. The pulleys could be mounted to the big bolts that hold the turret
>from turning. If you put garage door springs above the weights then
>even the
>increased mass would not be much opf a concern.
>
>

Ah - I see. Of course, that would get in the way of my control cabinet,
which I intend to mount on the side of the column :) (I could do it on
one side, though)

>>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.
>>
>>
>Right. But there's no NEED to stay at 10 TPI with a CNC conversion.
>Use what
>you can get. But, don;t fogret you'll need a power-off brake to prevent the
>knee from slamming to the bottom when the drive is turned off.
>
>

True. I was just thinking about the weight and upward travel. Using a
5:1 or 10:1 drive ratio and a 5TPI screw could work, but that's way more
of a PITA than it's worth.

I have no intention of doing anything other than possibly adding a power
drive to the knee on my machine. The only advantage I've seen in this
discussion is a longer Z travel under CNC, which isn't important to me
(yet). I may modify one of my manual pulse generator cards to allow me
to move the knee a specific distance, but I would still re-zero after
the move, since I wouldn't trust it to be accurate enough.

>Jon
>
>

- Steve