Re: [CAD_CAM_EDM_DRO] Ball screw load ratings

Jeff Swayze wrote:

> I'm in the process of choosing a ball screw (or lead screw) for a CNC
> retrofit of my mill/drill. Can anyone suggest a value for the maximum
> axial load such a ball screw would see? I would think only 100 lbs.
> or so, but I really have no idea.

On my small Bridgeport, the servo drives for the X and Y axes can develop
1000 Lbs of linear force at stall. You can figure out the forces involved
in cutting operations by converting motor HP to torque applied at a radius
at some speed. For low speed operations with larger diameter cutters, the
linear force applied to the workpiece becomes substantial.

> If the value is low enough I can use a modified acme screw with an
> anti-backlash nut (up to 125 or 250 lbs. depending on the style) and
> will save much $$. Otherwise I'll have to go with a ball screw and
> preloaded nut.

I wouldn't try this. You will have very rapid wear of the nut and screw
if you try to preload an Acme screw. An adjustable but locked Acme nut
will work for a while, but eventually the central part of the screw will wear
so that it is looser in that region.

I would think that a screw designed to hold at least 500 linear pounds of
force without bowing would be the minimum on your machine.
You need to keep in mind this includes table friction, inertia, cutting loads
all summed together.

Jon