Re: Digest Number 140

Message: 3
Date: Sat, 21 Aug 1999 13:09:03 -0400
From: "CG" <cnk@...>
Subject: BENCHTOP CNC SYSTEM


Here is a posting I made to a couple of newsgroups a week ago. Stephen
Barmash responded (thanks Stephen) and suggested I check out Onelist and
post my "proposal". Believe it or not, I hadn't even heard of Onelist and
this list before. I spent some time going through the archives (way to
long, ask my wife) and this seems like a much more appropriate forum than
what I found in the newsgroups. It's a fairly long posting, I apologize,
but I just got to writing...

To all benchtop CNC mill enthusiasts/dreamers:
I've just finishing designing my 3-axis CNC benchtop milling machine, and I
figured before I start making chips, I'd do a sanity check and try to get
some feedback.
I'm a mechanical engineer working as a product development consultant, and I
have long had a need for a means to produce prototype parts and molds for
various projects I work on. I always work in 3D solids (Solid Edge, mostly),
so I have 3D CAD data available for export to a CAM package. I've been
looking at the 3D CAM packages out there, and with my computer and machining
experience, I'm sure I'll be able to produce the G-code needed for
simultaneous 3-axis machining of complex parts.
I was hoping to buy a system, but I couldn't find anyone who made what I was
looking for. I've looked at Sherline, Taig, Unimat (haha), converted
mill-drills, Minitech, Light Machines, Denford, MHO Millright, Emco,
Defiance, converted Bridgeports, Techno-isel, MAXNC, and whoever else I
could find, and I could not find one system that met my requirements for a
small CNC mill.
Here is some of what I wanted:
* zero-backlash
* high rigidity (for limited steel-cutting, and accuracy)
* decent feedrates (120 ipm)
* generous travels (16" x-axis, 8" y- & z- axis)
* low friction slides and screws (for speed and power requirements)
* PC-based (Windows, pref.)
* affordable (under $5k minus PC)
* compact (forklift and crane not required...)
Here's why I ruled out what I found:
* The price limit ruled out Light Machines, Millright, and Defiance,
although they are definitely sweet machines and meet most of my other
requirements. Except for Millright, they are also shorter in travels than
what I hoped for.
* The Minitech machine is getting closer to what I wanted, but the
price is
still on the steep side, and the travels are too short.
* Sherline, Taig, and both the MAXNC 10 and 15 are just too small and
slow,
and I have serious questions about wear and accuracy after extended
machining. As hobbyist-oriented machines they seem wonderful, but I need a
fast, rigid, reliable machine for extended use.
* Denford and Emco have nice systems, but, again, they seem to be
targeting
deeper pockets than I have, and their travels are just too short.
* Techno-isel's are nice, just not rigid enough, and the price is a
little
too high anyway.
* A converted Bridgeport is too big and the price adds up (bigger motors
needed, high friction, leadscrew backlash, etc.)
* A converted mill-drill was about the closest to what I wanted, but it
would need to be retrofitted with ballscrews, and the quill feed would need
extensive work to maintain repeatability, let alone get 8" out of it. Five
inches on the quill could make do, however, since the headstock can be
adjusted up and down.

[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? ]

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.]


Through all my browsing, searching, and asking over the past 1-2 years, I
couldn't find what I was looking for. I would have to wait, or do it myself.
So, I decided to apply my design expertise and design my own machine in
order to get what wanted. The penalty, of course, is that time is money, and
in the end the money and hours that go into this one machine will probably
exceed what it would cost to buy a more expensive machine. Even if I was
willing to spend the extra money, I didn't find anyone that produced a rigid
enough machine with the travels and compactness that I needed.
Now, many hours later, I have finished designing my machine and have a small
stack of drawings ready for oily finger prints and pencil notes. In
designing and building my own machine, as a product designer, I could not
control the compulsion to design for manufacture and assembly, and to design
around cost-effective components. It's just a habit of mine, which finds its
way into anything I design. This is usually a good quality, but the extra
design time required is not always economical for one-time projects. So, in
the back of my mind, I hope I can impress enough people with the performance
and potential cost of my machine, and that maybe others will be interested
in one of their own.
Here's what I ended up designing:
Mechanical:
* 3 independent slideway systems (linear stages) mounted in x-y-z fashion
(c-frame) like a full-size CNC mill
* each linear stage is built up independently and precisely mounted to a
low-precision cast iron or welded steel (or...) machine base via a precision
alignment fixture and polymer chocking material
* slider/saddle of X and Y stages are mounted face to face, and rail of X
stage is used as table
* variety of spindles, routers, dremel tools, engravers, etc.
mountable to
z-axis plate (starting off with Sherline setup until I design a beefier
setup)

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

* dovetail slides with adjustable gibbs (tapered design)
* slide locks that clamp the gibbs against the dovetails, 2 per slider
* low-friction, low wear polymer bearing surfaces cast (injected) between
slider/saddle block (the shorter part) and rail/table (the longer part)

[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.]
* rail/table is anodized (Type II or Type III, depending on wear test
results)

[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?]

* 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.]

I'm packing alot of punch with the system I'm composing, and I believe I've
designed it as inexpensively as possible for what I want it to do. Is there
anything I've overlooked or that you would want to see in your own system of
this caliber?
How much will it cost?:
Well, I was willing to spend $5K for a complete system, minus PC, but I
think I will be spending more cash than that to build my own. The economy of
scale is working against me on a one-off. Fortunately, I have access to a
small machine shop. The mechanicals will require some tooling fixtures for
assembly, most of which I can jerry-rig with a granite surface plate and
some parallels, and there are other things that are hard to amortize over a
production run of one unit.
In any case, the breakdown of costs for purchased items (new), and estimated
costs for machined items (in medium production quantities), not including
tooling, assembly costs, etc., is:
* MACHINE - $1500 - 3 Linear stages (complete, inc. ballscrews, bearings,
etc.), machine base (welded 8" x 4" x .250" wall steel tube), and basic
headstock

[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.]

* DRIVE SYSTEM - $1800 - Motors, drivers, power supplies, cables &
switches
* CNC CONTROL - $1100 - software and signal generator box
* TOTAL: $4400
These costs are just about rock-bottom for a system of this type.

[Yes, I'd agree. Are these OEM prices you're quoting?]

I've been
wondering what a reasonable price would be if this system were to see
production, taking into account tooling, assembly, profit, R&D, advertising,
and all the other expenses that companies incur to produce, promote, and
sell a product. I admire Sherline's approach to offering excellent price for
the quality, while still making enough profit to stay in business with a bit
of dignity. If I were to actually sell this machine, I would definitely try
to follow their example.

[Generally, you want to "keystone" (double) your basic costs to ensure
profitability.]

Please, if anyone has already found what I'm looking for, let me know so I
don't have to "reinvent the wheel". Otherwise, if you have any comments,
suggestions, warnings, etc., they would be greatly appreciated. I'm
designing this for myself, but it would surely be nice to generate enough
interest to make it profitable to build it for others (if it hasn't already
been done) at a really good price.
If you have made it this far, thank you for your interest. If you would like
to respond personally, please email me at the address below.

Sincerely,
Carlos Guillermo
VERVE Engineering and Design
email: carlos@...
or cnk@...

[Well, using the EMC/NIST control system with the Camtronics board, like
many of us have been discussing here in the past month or so, would save
some money on the control end, and if you're just putting together a system
for yourself, you can shop around for good deals on used or surplus motors,
bearings, and screws. Putting on some sort of DRO would be helpful too,
although it would add expense. Keep us posted on what you decide to do,
okay?]

Andrew Werby

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