Re: Scratch-built CNC mill

Carlos wrote:

>...for what I want to do, I really need a reworked quill feed and fast

feeds. I saw the CNC Junior a few months ago, and I'm curious as to why
they only get 50 ipm with such beefy motors. I would think the drivers may
be under-rated for the motors, the pitch/gearing to the ballscrews is not
optimized, and/or the mill/drill just has alot of friction in the dovetails.
I've seen alot of systems that just don't match the motors/drivers/gearing
very well, and end up losing alot of potential performance.

[I wonder if this is the case- how would one figure this out? My guess
would be that there is a lot of inherent friction in the dovetail gibs of
the imported mill-drill they use, since it wasn't designed to be a CNC
machine. But optimization of the motor/gearing would be a step forward, I
agree.]

I would
probably want to do the conversion myself if a mill/drill did the job for
me.

>

[You probably would want to do so much work to it - replacing or rescraping
the dovetails, redoing the quill, etc, (it seems like you're rather
particular about this stuff) that you might decide it's not worth it and do
what I did- get an older but better-built CNC milling machine and
concentrate on replacing the obsolete control system with a modern PC-based
one. This in itself turns out to be a fair amount of work...]

>
> [Have you found someone who does this, or are you planning to do it
> yourself? Most of these polymers (teflon, etc) come as blocks or sheets,
> but I haven't seen any castable versions.]

Yes, I'm experimenting with the stuff, and hope to do it myself. There has
been some feedback on the material/process recently on CAD_CAM_EDM_DRO.
There was mention of Moglice and Belzona, which are pretty much the same
thing.

[I'd never heard of this material, 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?]

> [Why go through all this work and then use aluminum for the working parts?
> You didn't believe in it above, in the case of Sherline and MaxNC- why put
> it in your scratch-built machine? Or am I misunderstanding you?]

The main problem I have with Sherline and MaxNC is (I believe) that the
sliding parts are anodized aluminum against anodized aluminum.

[This is true.]

If this is
true, wear will be much greater than with low-friction polymer cast onto one
of the surfaces, especially for the speeds and loads I hope to see on my
machine.

[Oh, I see what you mean. But for flat-to flat, wouldn't using sheet teflon
make more sense than casting in place? I can understand using it for
matching odd contours, making nuts for odd threads, etc, but if you were
just trying to make a sliding gibbs, would it really be economical?]

Also, the damping characteristics of the polymer is one of it's
biggest selling features for use in new machine construction and machine
tool rebuilding. I realize the stiffness of aluminum is much less than iron
and steel, but I'm hoping that if I reduce or eliminate the "weak links" in
the slide assembly design, such as gibb adj. design and fit-up of surfaces,
and I mount the slide assemblies to a welded or cast steel frame, I can have
a reasonable well damped, stiff machine.

[You're using aluminum because you want to save weight on the moving parts,
or is it because of the design conveniences afforded by the use of aluminum
extrusions? ]

> * rails and sliders are designed as potential aluminum extrusions and
> have
> integral limit/home switch mounting channels
> * each rail/slider system has a mated cross-sectional
> envelope of 8" X 3"
> * sliders/saddles are 8" X 8"
> * .631" X .200" pitch ballscrews and 2 ballnuts (preloaded
> against each
> other) on each axis
> * motor mounting plate bolted to bearing end blocks; couples motor to
> screw
> via toothed belt. Pulleys are changeable from 2:1 ratio to 1:2 ratio
> * bearing endblocks, ballscrew nut mounting block, and screw support
> block
> are based on common design (extrusion-ready)
> * each linear stage is identical with the exception of rail length and
> corresponding ballscrew length
> * X-axis: 16" travel x 24" table
> * y-axis & z-axis: 8" travel x 16" rail
> Motors/drives system:
> * 315 oz-in steppers, 200 full steps/rev, geared down 2:1
> * .200"/rev screw
> * microstepping driver: 40V, 5A/phase, set to half-stepping
> * 110 ipm at 354 lb thrust (without losses)
> CNC control system (currently available):
> * FlashCut CNC (max 7300 steps/sec)
> * serial cable to signal generator (MPU) box
> * Windows-based
> * lookahead capability
> * reads G-code and DXF
> CNC control system (if/when it is available):
> * Windows-based CNC control interface to IndexerLPT device driver
> * parallel port step/direction output
> * 90,000 steps/sec (allows more steps/rev for microstepping)
> * unlimited lookahead
> What do you think?
>
> [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.]

>
>
> [This sounds cheap. Where are you getting these inexpensive
> ball-screws and
> linear bearings? Last I checked, they cost more than that for a machine
> this big. The source I asked wanted about $1000 per axis for the
> screws and
> nuts to retrofit a Bridgeport. If you really can build all that for $1500,
> sign me up for one.]

The linear bearings are replaced with the low-friction polymer and extruded
dovetail slide system. An excellent source for cheap high-precision
ballscrews is Thomson. They have rolled, .001/ft accuracy ballscrews for
$1.50 per inch.

[Does this mean you lose a thousandth in accuracy for each foot of
length? So if I had an 8-foot screw I could count on being out +/- .008? Is
this because of torsion on the screws while in use, or just manufacturing
imprecision adding up?]

You have to call them direct. You can get the standard
accuracy class (.004/ft) for $1.00/inch through McMaster-Carr, which also
carries the ballnuts for $18 each. the trick, though, is to go with the
.631 dia. x .200 pitch "value-priced" series. That size, because of volume,
is way, way, cheaper than any other size, even the smaller ones. You would
need to make an adjustable block to mount the ballnuts, and use two ballnuts
per screw to dial in the preload for no backlash. The .631 dia size is way
to small for Bridgeports, but it's nice for mill/drills and the like.

Carlos

[Thanks for running this down for me, Carlos. I think the .631 screws would
work fine for my router. But do you think they're sufficient for your
steel-eater? The screws on the mill I've got are about 3/4". By the way,
these things have to be lubed constantly- we've been fixing the oil system
in our old mill, which is something of a chore. You need to add the oiler
and (expensive) oil-lines to the cost estimate you're working up.]

Andrew Werby





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Andrew Werby - United Artworks
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