Re: Digest Number 145

From: "CG" <cnk@...>
Subject: RE: Re: Scratch-built CNC mill

(conversation between Andrew Werby and Carlos Guillermo...Formatting this
reply to show who said what and when confuses me. Any tips? Carlos)

[You'll notice I always square-bracket my text- I find it helps me keep
things straight. Aside from that, those little ">" marks help; when there
are two of them: " >>" means the message is two generations old.]

> -----Original Message-----
> From: Andrew Werby [mailto:drewid@...]
> Sent: Wednesday, August 25, 1999 5:53 AM
> To: CAD_CAM_EDM_DRO@onelist.com
> Subject: [CAD_CAM_EDM_DRO] Re: Scratch-built CNC mill
>
>
> From: Andrew Werby <drewid@...>
>
> Carlos wrote:
> [I'd never heard of this material, (castable low-friction polymers -

Moglice,

> Belzona, Super Alloy) but it certainly sounds like just the
> thing for the job. Thanks, everybody who brought it to my attention. Has
> anybody tried using it for linear bearings, or is it uneconomical for

this?

> Where would I get some?]

I've been designing around Super Alloy Black 1500LFH from ITW Philadelphia
Resins (http://www.Phillyresins.com) They've got both liquid and putty
versions, as well as many other neat epoxy systems. Yes, I realize I'm
giving away all the secrets of my machine, but if someone beats me to it
with better results (and better pricing) I'll gladly buy one!)

[This sounds like interesting stuff, thanks. They do seem to say it is the
way to go for these "critically aligned sliding surfaces". How much does it
cost?]



Teflon alone is very soft and flows under load. There are many filled
versions, but the problem with glue-on sheets

[What do I use to glue this stuff? I thought Teflon basically resisted all
glue. Which of the filled versions is machinable and glueable too?]

is that you still need to
accurately machine BOTH surfaces, and they would have to match each other
precisely and be parallel to the other side via scraping, honing, grinding,
etc. I'm hoping to get away with making sure one piece is very straight and
smooth, and the other piece is very rough and very loose-fitting, and then
injecting the polymer to fill the void. Many times you can get away with
jacking screws built into the slide to align the parts, and a couple of
other alignment aids.

[Are you sure one can't find a sheet material that is adequately flat and
smooth on one side, that can be used smooth side to smooth side? This
doesn't sound that difficult, but then I haven't actually researched it
either. The other thing I was thinking of using is skate-board wheels
(don't laugh) mounted to enclose 4 sides of a big (4"x4" or more) square
steel tube. It seemed like this might be easier to do and more forgiving
(as well as cheaper than linear bearings) for moving the big heavy gantry
of my router.]

> > [This sounds like a good design for a gantry-type router, along
> the lines
> > of the Techno-Isel, but with more Z travel and less Y. I think
> there is a
> > market for something like this, especially if it had more Y. The routers
> > out there seem mostly oriented towards sign-carvers, but a more
> > general-purpose machine would be better for the rest of us. I'm in the
> > process of putting together something like this myself,
> although I wasn't
> > thinking of it as a steel-carving machine. I wonder if you will
> > really have
> > as much rigidity as you need without having a big "C" casting in there,
> > like the ones real milling machines all seem to use.]

> Well, with the modular nature of the extrusions, just such a machine might
> be possible. I considered a similar layout with the table moving
> instead of
> the gantry. I think you could get good rigidity that way

> [I've been going back and forth about this one myself. I think there might
> be a lot more slop in a gantry system when one starts adding the
> off-center
> force of an extended Z axis- perhaps that's why I haven't seen
> any machines
> like this.]

I've actually been finding alot of high-performance, high speed machining
centers using this layout. I think they call it a "portal" layout, where
the gantry is fixed and the table moves.

[The trouble with this is that it takes up twice as much space, and the
gantry has to be correspondingly larger too. If you're talking about a 4' x
8' bed, that about eats up my whole shop. I'm leaning towards a moving
double gantry with a square-section beam in the middle of it, which would
move back and forth on 4 linear bearings between the parallel bridge
members as well as moving up and down on another 4. It seems like it would
be more stable than the usual design, which hangs the moving Z beam off one
side of the moving bridge, using only 2 bearings for each direction. Does
this make sense to you?]

I've been fearing the oiling chores. I've got some ideas on lube ports, and
maybe I can integrate a simple plumbing system into my design.

[Don't the ball-nuts come with oil ports? ]

I've sketched out a simple spring-loaded oil reservoir that might do the trick.

[You can get simple hand-operated oilers from the Enco catalog for about
$100. But for a CNC system, automatic ones are generally used, which cost
more. They give the system a shot of oil every 15 minutes or so.]

Does your oiling system feed the ways and the screw nuts with the same
pressure?

[There are some needle-valves in the system so one can adjust the relative
amounts of oil going to the ways, nuts, and spindle.]

And are you using ballnuts?

[I think so, but I haven't taken them apart and looked at them...]


Andrew Werby


Andrew Werby - United Artworks
Sculpture, Jewelry, and Other Art Stuff
http://unitedartworks.com