Re: [CAD_CAM_EDM_DRO] Z axis drive

Ron Ginger wrote:

>Im doing a conversion on a large, square column, mill, like a RF-45, but
>from Wholesale Tool
>
>The Quill drive has a rewind spring to keep the quill up, and is driven
>down by either a drill press style wheel or a worm gear and small handwheel.
>
>I am considering driving the handwheel with a belt and stepper, and
>hoping the rewind spring can be set strong enough to keep backlash down
>to a workable level.
>
>Has anyone tried this? It seems to me the forces are always cutting down
>on Z, never when retracting, so the spring ought to do it.
>
>

If this is like a traditional Bridgeport or drill press arrangement, you
can forget any hope of accuracy with the rack drive. The axial forces when
milling are so much greater than the weight of the quill, I doubt any
reasonable
force will do what you want. Have you ever had a 1/2" end mill "sucked"
out of the collet? Do you have any idea how much axial force that takes?
It has to be several hundred pounds, even taking into account the impact
nature of the cutting tool.

Most of these rack drives have very sloppy gears and there may be .010"
or more of cyclical error on the rack and pinion, plus the pinion is
often not
constrained except by a hole in the head casting (ie. no ball bearings).

So, as far as I'm concerned, the rack is totally useless for quill
drive, unless
a Z axis position uncertainty of at least .010" is OK.

Jon