Re: [CAD_CAM_EDM_DRO] Re: Looking for short ballscrew

Hugh,

Well, I spoke up too soon on this. After starting to do some more
serious looking at how I would build it, I realized the trucks for the
linear slides I was going to use are attached with 4mm screws. Using all
three trucks, that'd be 36 puny screws holding several hundred pounds of
iron to the ram. Also, after looking at the Milltronics unit, there's a
fair separation of the trucks, vertically. To use my rails, I'd have to
pack them together very close. Probably no big deal for a compressive
load, but for a large cantilevered load, I didn't think it was enough.
So, add up the large steel plates for each side of the slide, ball screw
and nut, properly sized linear rail bearings, pulleys and belts, and
eventually a larger stepper motor for the Z, and it's not any cheaper
than a ready to install Z drive. Factor in labor, it's no contest.
I'm going to go with a Z drive from Elrod Machine, this unit allows the
drive to be quickly disconnected so I would gain manual use of the
spindle, which would work great if I had to tap a lot of holes. CNC
posititioning, manual "feel".

As for the size of the ball screw, I went by what Milltronics did.
While it's in tension, it bears quite a load all the time, and I would
imagine there are significant peak loads when trying to drill holes for
example, accelerating several hundred pounds.
There's the issue of a brake for the screw, so the head doesn't slam to
the bottom of it's travel when the stepper is powered down.

To address the issue of round linear bearings: I know these are used
frequently in home build projects. However, these bearings are primarily
designed to take compressive loads. Used in a vertical fashion and
holding something like 250 lbs, the upper bearings would be more in
tension and only using the two tracks adjecant to the rail support. Home
built machines with routers are much lighter, and the load is closer to
the bearings, not hung 6-8" out in front of the bearings. For a small
machine, round shaft bearings are probably enough, but for a full size
mill, they simply aren't rigid enough to
handle say, a 1" end mill.

I still like the idea, especially the extra travel that can be had.
There is one design consideration if you want to try this. The linear
rails must attach to the head, with the trucks attached to the ram.
You will quickly see, if you look at a knee mill, that you can only go
down maybe 2", the rest of the travel has to be had by traveling up.
With 6" travel, your head would sit about 4" higher, and if you are
somewhat short, it might make reaching the drawbar difficult. Just
something to think about. The Milltronics I looked at, the spindle nose
was chin and shoulder height for me (5'10").

Well, should have thought things out better before posting, but since
you expressed interest in what I was doing, I thought I'd explain why I
didn't go further. And I'll add that I'm a one-man commercial shop.
My entry into CNC was a MaxNC, and I made parts on that which are now on
Boeing aircraft, as well as parts used in clinical surgical trials on
live humans. Struggling to build up a business from almost nothing, I've
made what I needed because I had the time, and no money for commercial
stuff. That is finally starting to turn around, and there lies the major
decision for me, time. I can make far more money making parts for
customers than I can save by trying to make a Z drive.
And I'm really sorta disappointed, I'd really like to build one...

Jon