RE: Digest Number 140

Andrew -

>
> [While I applaud your efforts, and don't want to discourage you,
> it doesn't
> sound like the device you're thinking about building will end up costing
> much less than the high-end systems you eliminated because their
> price was
> too high, although making it yourself will doubtless save you
> money, if not
> time. But it's difficult to make money manufacturing things on a small
> scale here in America- or are you someplace else? ]

Thanks for the tactfulness. You're probably right. I added up just bare
material and componenet costs of what I'm doing, and there are always
unanticipated costs. However, from what I'm learning about EMC and the
Camtronics 5A drivers, my overall estimate might come down. I'm keeping
production in mind mostly as an excercise right now and during construction.
That way, when I'm done building one for myself, I can look back and have a
realistic idea of what multiples might cost.

>
> To buy or not to buy?:
>
> [That depends on whether you want to spend your time building
> your machine,
> or using a machine you bought to make other things. If you decide to buy
> one, I'd agree that the converted mill-drill would come the closest to the
> criteria you laid out, and would fit in your budget. Have you
> looked at the
> CNC Jr? The specs on it are as follows:]
>
> The CNC Jr. Milling Machine:
> Drilling capacity .........................................1-1/4" 32mm
> End mill capacity ...........................................3/4" 20mm
> Swing ......................................................15-7/8" 405mm
> Spindle nose to table (maximum.)...................18" 480mm
> Spindle
> taper............................................................ R-8
> Diameter of Spindle sleeve................................ 3" 75mm
> Head Swivel................................................ 360 degrees
> Diameter of column.................................. 4-1/2" 115mm
> Overall height (w/o stand)..................... 43-1/2" 1100mm
> Overall length....................................... 42-1/2" 1080mm
> Overall width ........................................39-3/4" 1010mm
> Motor power.......................................................... 2 HP
> Spindle speed........................... 12 speed (120-2500rpm)
> Forward/back table travel................................ 7" 175mm
> Left/right table travel ................................17-1/2" 444mm
> Spindle up/down travel.................................... 5" 127mm
> Working area of table............................... 28-3/4 x 8-1/4"
> Net/gross weight....................................... 661 lb. 300kgs
>
> The CNC Jr. Control Unit:
> Microcomputer based micro-stepper motor driver with 4D interpolation
> capabilities. Accepts motion commands from a host computer through a
> parallel channel. The control can drive up to 4 motors. Other optional
> outputs are the coolant, oiling, and spindle controls. The power
> requirement is 5 amperes at 115VAC. The X, Y, and Z axis motors
> are size 34
> with 650 in/oz. of torque. The minimum possible motion is .00025" with a
> minimum speed of 0.5" per minute.The maximun speed is 50" per minute.
>
> [I've come to an agreement with the manufacturer to provide my customers
> with a 5% discount off their already reasonable prices ($4650.00 including
> software for their basic package). It comes equipped with ball-screws on
> the x and y axes, but I'm not sure if its quill would meet your
> requirements as is or would need to be fitted with one as well. There have
> been some suggestions on this list for reducing quill backlash without
> resorting to ball-screws- you might check the archives.]
>

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

>
>
>
> [Forget about the Dremel- the plastic body will lose its rigidity when it
> gets hot.]

Thanks.

>
> [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.

> [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. 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. 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.

> * 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

>
>
> [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. 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