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An
Introduction to CNC Routers for the Small Shop By
B.H. Davis Purchasing
your first CNC router for a small shop operation
can be a daunting task. There are a wide range of machine types, sizes,
manufacturers and technical details from which to choose, and it
quickly
becomes necessary to narrow this field down to the specific class of
machine
that will match your needs. Once that is accomplished you can begin to
compare
features and details available from different manufacturers. Whereas
there has
been a lot written about what a CNC router will do for a small to
medium sized
woodworking business, there is less to be found in the way of technical
information describing the basic design of these machines. Hopefully
this
article will help fill that void and get you off on solid footing. To begin,
there are numerous skills required to run CNC a
router, some of which you may already have and others which will be new
to you.
Software, machine types, parts hold down, sophisticated tooling and
dust
collection are just some of the subjects with which you will have to
contend. A good
starting place is to talk with current CNC owners,
visit their shops, participate in online forums, and read everything
you can
get your hands on. If you invest some time to learn prior to visiting
dealers,
your time with them will be more productive. Software
CNC
stands for computer
numeric control, with the emphasis on “computer”. The more
familiar you are
with computers and the required software packages the easier it will be
to make
the transition to CNC. Unfortunately,
there is not just one software program that
you will have to learn, but rather three: CAD (computer-aided drawing);
CAM
(computer-aided machining) or G-code generation; and the software
control
package that actually moves the router around. I find it more
comfortable to
use the CAD and CAM software at my desktop computer, and use the router
computer for running the control software exclusively, but others like
to do
everything right at CNC control computer. If you end up using two
separate computers,
you can transfer files from one to the other by CDs, flash drives,
floppy disks,
or a network connection. There are
many CAD programs available; the most widely known
are AutoCAD and AutoCAD LT. Some CAM (G-code) programs include drawing
tools so
you might be able to combine both CAD and CAM capabilities with one
program.
You can typically spend between $100 and $3,000 for drawing software,
with the
more expensive packages having capabilities probably not needed in
general
day-to-day CNC use. A comfortable startup price range is $500 to
$1,000. As you
examine CAD packages try to get a feel for their ease
of use and the efficiency with which they operate. A good drawing
program
should allow you to draw most of your parts with about 10 to 15 basic
commands.
As you get better at drawing you will learn additional drawing aids
that can
make your work faster and easier, but initially you should start to
understand
how the program works in just a few hours. You are not going to do any
complex
drawings after such a short introduction but you should be able to
create
simple lines and arcs and begin to see how they connect together. This
will
probably be the most difficult of the three software packages to learn.
Once
you have a good handle on it the others should come along somewhat
easier. Next you
have to be able to generate router bit tool paths
for the lines and arcs you drew in your CAD program.
For example, if you draw a square tabletop,
you will want to tell your CNC router to cut around the edges of that
square.
This is where the CAM package comes in. It generates G-code, the
instructions
that tell the router where to go.
This
code can be read by most CNC routers, but you have to make sure the CAM
software
you buy has drivers for your machine. The product you purchase will be
dictated
as much by what you are cutting as by cost. Costs vary widely, but a
mid-range
price is $1,000 to $1,500. In general, you’ll get more coding power for
more
money. Once the
G-code has been created, the router control program
takes over. It should be included with your CNC machine, so there
probably
won’t be any additional cost, but you will have to learn how to use it.
The
control program reads the G-code generated by the CAM software as text
files.
Once you have some experience with your machine, you will be able to
edit these
files right at the router to tweak how parts get cut. There are
programs that allow you to draw floor plans,
design cabinets, create and print color 3D views, build parts lists and
generate code all in one package. However, with one exception, these
can cost
$10,000 or more. The exception is Thermwood’s free eCabinets Systems
software, which
can be used by anyone but can only output code to the company’s own CNC
routers. . The
Router
Routers
come in a wide range of types and sizes, from small
table top models to vertical machines that lean up against a wall to
the more
commonly seen 4x8 and larger free standing machines. You need to
consider the
materials you will be cutting and the available shop space in order to
determine the size and orientation of the machine that is best for you.
As you
look into this you also need to remember that a clear space is needed
around
the machine for material handling. Table
size is a key issue, and while 4x8 sheets may be your
most commonly cut material, you should consider the possibility of
occasionally
cutting larger parts. Many
machines come
with 5x10 tables for just this reason.
Further,
if you are purchasing a router primarily for small parts processing,
then a 2x4
or 4x4 table top model might be adequate for your needs. Some machines even come with
dual tables so
you can be cutting on one while loading material on the other. There are
two basic machine categories: CNC routers and
point-to-points. CNC
routers typically have flat table surfaces upon which to
place the material and parts are held down by one of several methods
I’ll
discuss below. You can also set up to process panel edges on these but
that is
more involved than on a point-to-point. Point-to-points
are similar to CNC routers, but rather than
a flat table surface there are a series of rails with adjustable vacuum
pods
attached. These machines are particularly useful for processing the
edges of
parts, such as hinge and lock mortise cuts on doors. They are less
useful when
processing panel stock for products like cabinet boxes. The
general discussion that follows will be directed towards
CNC routers but much of it can also be applied to point-to-points. Most
first
time small shop purchasers will be looking at CNC routers. Machine
movement systems
Once you
know what type and size router to look for, you
have to determine which of the two basic motion systems, “moving
gantry” or
“moving table” will serve you best. The basic
idea is to get the router bit moving around the
table in relation to the material being cut. The bit is held in the
spindle,
which is attached to a vertical (typically referred to as the “z-axis”)
transmission that moves up and down. This z-axis assembly is then
attached to
the gantry, which is a bridge like structure that usually (but not
always)
spans the narrower dimension of the router table. The z-axis
transmission moves
back and forth along the bridge, creating movement in the x-axis (or
y-axis
depending upon the router’s layout). You now have movement in two
directions:
up/down (z-axis) and left/right (typically the x-axis).
To accomplish movement in the third
direction, front to back, the bridge and the table surface have to move
in
relationship to one another. There are
two ways to accomplish this movement: either move
the bridge forward and back over the table surface or move the table
back and
forth under the bridge. The former is called “moving gantry” and the
latter
“moving table”. Small shops typically gravitate towards moving gantry
machines
because they take up less space and because there are few if any moving
table
machines to choose from in the light to medium duty CNC router category. Super large multi-ton
industrial machines use
either movement system but are generally too massive and expensive for
small
shop operations. Since light to medium duty moving gantry machines
generate
smaller forces during rapid direction changes then comparable movements
on the
super weights, they are easier to design and engineer, helping to keep
their
cost down. However, even though these small shop machines may not weigh
as much
as their big brothers, don’t underestimate their capacity to do the big
jobs.
Many machines in this class are capable of running 24-7 with accuracy
equal to
that of the larger routers. Drive
mechanism
So how do
you get the router to move all around, instantly
changing direction while traveling at a high rate of speed? The answer
is in
one of two basic drive systems, either “rack and pinion” or “ball nut
and lead
screw”. Rack and
pinion is used on some gantry systems and is just
what the name implies, a straight length of milled rack with teeth
machined
into its face and a mating pinion gear mounted on a motor driven shaft.
The
motor rotates the pinion, which then drives the gantry down the length
of the
rack. There are
two types of rack and pinion to consider. One has
“straight” teeth cut at 90 degrees to the length of rack while the
other, which
is referred to as “helical”, has a helical grind to the teeth that is
several
degrees off of square. The helical variety is more expensive to
manufacture but
makes for greater accuracy. The ball
nut and lead screw drive mechanism is a bit more
complex but not necessarily more accurate. It involves a long threaded
rod that
runs the length of the travel direction and a specialized nut mounted
to the
moving component. Either the threaded rod or the nut can be rotated,
resulting
in the machine component being drawn down the threaded rod. A spinning
rod is
simpler to build, but long spinning rods can develop whiplash, a
negative
feature that needs to be engineered back out of the system. As such
spinning
rods with fixed nuts are often used on short length axes while spinning
ball
nuts and fixed rods, a more difficult and expensive system to engineer,
tend to
be used on longer runs. The
result of all this in the world of mid-priced CNC
routers is a compromise. Many companies design their routers with a
rack and
pinion drive on the long axis while using a fixed nut and spinning lead
screw
on the shorter axes. When looking at these machines pay attention to
the grind
style on the rack and pinion and the size and strength of the spinning
threaded
rods. One last
issue here is backlash compensation — a mechanical
or computerized feature that takes the “slop” out of the drive
mechanism as it
changes direction. Consider your table saw blade-raising mechanism as
an
example. If the gears are clean and tight, the blade starts going up or
down
almost as soon as you reverse the handle’s rotation. If the gears are
loose,
you might turn the handle a quarter turn or so before engagement and
blade
reaction. Since neither rack and pinion nor nut and lead screw can
accomplish a
perfect mating of their interlocking surfaces, and since all parts do
wear over
time, automatic backlash compensation is designed into most systems to
compensate for play in the drive components. Steppers
vs. servos
The last
major piece of the drive system equation is the
motor that turns the pinion, ball nut or lead screw. There are two
types of
motors used here: steppers and servos. Stepper
motors are less expensive but no less accurate.
However, except for a few recent advances in stepper technology, servos
are
considered the preferred alternative as they incorporate an electrical
feedback
loop that keeps the computer continuously informed of the routers
location.
Steppers use a step counting method as they rotate, and can
theoretically miss
a step due to a physical or electronic glitch in the system. Since
there is no
information feedback loop, the computer does not know a step has been
missed
and subsequently the part can be cut wrong. This is a rare occurrence
in a
properly tuned system, so don’t feel as though you have to make the
upgrade
from steppers to servos. The
router spindle
Whereas
many different cutting devices can be mounted on a
CNC, the most common is of course a router, and it can range in size
from a
hand-held power tool to an industrial unit weighing 50 lbs. or more. Most base
level machines have a single z-axis upon which to
mount this router. However, the addition of a
second vertical
axis offers greater convenience, allowing the use of two different
tools or
router bits without having to interrupt the cutting process to change
bits. The
only downside to this is that since the spindles are mounted next to
each
other, they effectively reduce the maximum cutting width of your
machine, as
both tools have to reach all points on the router table. Also, when a
third bit
or tool is needed for an operation you still have to bring the cutting
process
to a stop to make the changeover. If a
single or double spindle machine is adequate for your
purposes, but the hand held routers sometimes installed on them lack
adequate
power for your operations, you can upgrade to industrial spindles.
Basic
spindles in this category still require bit changes like hand held
routers but
they are significantly more powerful. The trade off is that they
typically
require 3-phase power. However, the advantages are significant as they
work
with variable speed frequency drives, devices that allow you to use
different
spindle rpm speeds for cutting different materials. You should expect
an
upgrade cost on the order of $3,000 for a basic industrial spindle. If you
are going to consider a 3-phase spindle, then the
optimal router upgrade would be to add an automatic tool changer. This
single
spindle setup allows you to keep your maximum table cutting width while
gaining
the convenience of fast and easy tool changes. Tool changing routers
commonly
have four or more rack mounted tool holders that the spindle can pick
up as
needed. Once you have installed bits in the tool holders, and
registered their
lengths on the machine, all you have to do is run your cutting file.
When the
file calls for a different bit the router spindle stops, traverses to
the tool
rack to drop off the current tool holder, proceeds to the location of
the next
tool holder, picks it up, comes up to speed and continues the cutting
process. Adding an
automatic tool changing system can easily add
$8,000 to $10,000 to the price tag though. Not only is the router
spindle much
more expensive, but a whole network of air lines, relays and solenoids
have to
be designed to make the system work. Further, there is an array of
sensors and
switches required to tell the computer, among many other things, when a
tool
holder is or is not mounted in the spindle. Then finally there is the
additional computer programming required to operate this entire system.
Whether or not
you add this cost to your
purchase will depend upon the anticipated workload and how much more
you are
willing to invest up front. Tool changers can be added to machines down
the
road but it is a complex and expensive proposition, especially if the
machine
wasn’t prepared for this when manufactured. Control
system
All CNC
routers need
a computerized system to control their motion. Some use a basic PC while
others use more
elaborate devices such as Fanuc
Controllers.
PC-based machines are typically controlled with a keyboard, mouse and
sometimes
a remote hand pad. Fanuc style controls have user panels with
activation
buttons, indicator lights and assorted other features. A very rough
generalization would be that you will find PCs controlling the movement
of
routers costing under $100,000, and Fanuc style controls installed on
machines
from there up. If you
speak with people who have used one or the other of
these control systems, they will typically say that theirs is fully
functional,
easy to operate and there is no reason to use any other system. I will
add that
even though PC controls are typically found on machines in the lower
price
category they present few, if any, limitations to CNC operation. Electrical
requirements
The
electrical requirements of CNC routers vary widely based
upon the overall size, weight and power of the system. Small tabletop
units
with hand held routers for spindles can often get by with a single
110-volt
wall outlet, while the massive industrial machines can require 3-phase
power up
to 460 volts. The
typical small shop 4x8 router is somewhere in between,
with the major defining factor being the spindle involved. Steppers and
servos
will usually be powered by 110 or 220 volt line inputs and as such
don’t
dictate the overall power requirements of the system. If your
router spindle runs on single phase power then you
will only need a basic 110 or 220 volt properly sized power supply
coming to
the machine. However,
if you’ve chosen
a 3-phase spindle, and you don’t have 3-phase power in your building,
then you
will need to generate that in house.
In
discussions with people who have used rotary and static phase
converters to
generate 3-phase power for CNC routers I have heard of mixed results.
If it is
only the spindle that is being used on the 3-phase generated line there
is a
good chance the system will work. However, the computers and
electronics in the
rest of the CNC system can react erratically to the inconsistencies of
a
generated third leg, resulting in glitches in the CNC’s operation. In my own
shop I use a rotary phase converter to generate
power for most of my 3-phase equipment. Since the control system on my
CNC
router uses 110v and 220v single-phase inputs, I only need 3-phase for
the
10-hp spindle. To power this I use what is referred to as a “frequency
drive”
(or “inverter”). One of the nice things about frequency drives is that
they are
available with 3-phase and/or single-phase input options. So instead of
my
rotary phase converter generating the needed third leg, I send 220-volt
single
phase power directly to the frequency drive. It then generates the
third leg in
much the same way as a static converter. The bonus is that the
frequency drive
adds variable speed to the equation, allowing adjustment of the spindle
speed
to the optimal rpm for the job being run. The downside is that, as with
a
static converter, there is about a 30 percent loss of spindle power,
but that
can be overcome to a degree by using an oversized frequency drive. Material
hold down
No matter
how sophisticated your CNC router, it will be
brought to a standstill without adequate hold down force for your
material.
It’s pretty depressing to see a sheet of $100 veneered panel stock fly
across
the table and get ruined. There are
three basic ways to hold material on a CNC router:
mechanical fasteners; full (or true) vacuum used in conjunction with
vacuum
pods and holding fixtures; and bleed board vacuum pulling down through
a sheet
of MDF. Mechanical
fastening can be accomplished with screws, nails
(plastic nails and nail guns are available), T-posts, leverage bars and
edge
blocks — to mention just a few. You can
also buy or build a T-slot subsystem. Used
in conjunction with T-head bolts and
thread on or quick release levers, these provide edge pressure on the
material
being cut, resulting in both solid hold down and quick parts change
over. “True” vacuum systems
offer a substantially different method for holding parts. They use rotary vane or
similarly designed vacuum
pumps to pull typically 18” to 26” of mercury.
This creates enough force to hold the material in
place. A good
functional rotary vane pump size for an average CNC table would be in
the 3 to
5 hp range, although I’ve used a pump as small as 3/4 hp on our system.
It’s
not so much the volume of air being pulled as the power of the pump to
pull a
strong vacuum. The higher the horsepower, the more air this type of
pump will
pull, but it is the design of the internal vanes that determines the
strength of
the vacuum. This
system uses vacuum pods or custom built fixtures to
hold the parts. The
vacuum can get to these
devices in different ways, some of which are included with a new router
and
others which can be purchased or made in the shop. We have custom made
vacuum
manifolds running down both sides our router table with valves that
connect to
1/4” OD polyethylene tubing. The other ends of the tubes connect to our
shop
made oval vacuum pods that are screwed down to a table surface made of
MDF. This is a good
marriage of the high-tech use
of vacuum to hold the parts and the low-tech use of screws to hold the
pods. Another
adaptable system involves a milled grid work (that
you can purchase or make with the router) enclosed below a machined
tabletop
surface. All exposed faces and edges are sealed so that no air can leak
in and
a series of closeable ports are drilled or machined into the top
surface. A
vacuum is applied to the internal grid work and quick release fittings
are
inserted into open ports to access the vacuum. This type of system can
be used
with various gasket sealing configurations as well as with double sided
pods
and fixtures that use vacuum for holding the pod as well as the part.
That is
one way to take advantage of the vacuum contained within the grid work
sandwich
but setups are limited only by your creativity. The most
common hold down system you will see when you look
at CNC routers is bleed board, or through vacuum.
It uses a similar internal vacuum grid work
sandwich to that described above but with a regenerative blower instead
of a
rotary vane pump. These bleed board systems are often used for Nested
Based
Manufacturing, or more simply stated, the achievement of maximum yield
of parts
out of a sheet of panel stock. Whereas the pumps used for true vacuum
are
usually under 10 hp in size, regenerative blowers typically need to be
20 hp
and larger. These pumps pull large volumes of air over large areas, but
with
lower pulling force than true vacuum pumps. The
holding power on these systems takes place through a
spoil board, that is, a waste sheet of MDF.
The air pulled by the pump actually flows “through”
the MDF, holding
down the sheet of material placed on top of it.
The bit then cuts through the parts and just
slightly into the spoil
board. While
bleed board systems are better for cutting 4x8 panels,
true vacuum systems are typically better for cutting small or
irregularly
shaped parts. You
can easily screw
vacuum pods down on a panel cutting router, but would be a fairly
involved and
costly project to add a bleed board setup to a machine built for pods
and
fixtures. The
challenge of holding small parts Whereas
bleed board vacuum is an intriguing concept and is
definitely the best hold down method for processing 4x8 sheets, the
real
creative juices start flowing when dealing with cutting small and
irregular
shapes. Router bits can excerpt a large amount of lateral force on the
material
being cut and getting small parts to remain solidly in place can be a
challenge. One way
to deal with this is to nest a group of small parts
on of a larger piece of wood. You can then hold that large blank on the
CNC and
program the cutting path to cut shy of the bottom. If you are using
bleed board
vacuum, this is called creating an “onion skin” on the bottom of your
blank.
This very thin layer of material will keep all the parts connected so
the
regenerative blower can continue to draw down on the entire bottom
surface of
the blank. After removal from the CNC you can separate the parts by
hand and
lightly sand away any remaining onion skin. If you
are using vacuum pods, you would have to leave a
thicker bottom layer. You don’t want to risk running a bit down into
the vacuum
pods plus you need to maintain board strength. If you start with
material that
is 1/8” thicker then the final part, you can then leave a 1/8” uncut
bottom
surface, which is adequate for the application. Once off the CNC you
can run
the board through a planer or wide belt sander to remove the extra back
thickness and free up the parts. This same
method will work for cutting a single small part
out of a larger blank. At some point though you need to consider how
much extra
material is being wasted. A quick and easy alternative here is to mount
a
smaller wood blank onto a secondary spoil board that is somewhat larger
then
your part. You can screw up through the bottom of the spoil board into
the
bottom of your part, making sure the screws are clear of the router bit
cutting
path. This larger spoil board can then be held in place so the part can
be cut. Dust
collection
Dust
collection is an absolute necessity on a CNC router and
it needs to be directed as close as possible to the cutting point. Most
systems
will use a flexible 4” diameter hose attached to a brush skirt
surrounding the
bit. This hose needs to be long, flexible and mounted in such a way
that the
router head can travel around the table without interference from the
hose. In my
experience the best dust collection on a CNC router is
limited in performance. The router bit is flinging dust and chips in
all
directions while suction is occurring at a fixed point, so expect to be
cleaning chips and dust off your table after cutting. What the dust
collector
should do well though is pull in the fine dust that would otherwise end
up in
the air. As to
size and type of system, that would depend upon your
router manufacturer’s recommendations and what you might already have
in place.
I run our CNC dust collection into the same shop wide system that
collects from
our table saw and sanders. It is only powered by a 3-hp motor but seems
to work
adequately for our 5x10 router. Tooling
There is
a wide range of tooling that can be run on a CNC
but you should first get a handle on basic straight cutting bits. There
are
straight flute bits, up cut spiral, down cut spiral and up/down
compression
spiral to name just a few, and most are available in either carbide or
high-speed steel. Your CAM
software will allow you to set tool paths for
nearly any shape bit you can buy or have made. You will probably be
breaking
some bits early on though, so start with simpler projects and less
expensive
bits. A good
tooling supplier will offer free technical support
with experienced people readily available by phone. Some will even send
a
regional tech to your shop at no charge to help get you going with
their bits. And one
final word of caution — large diameter tooling needs
to be used carefully. Make sure you always understand the rpm
limitations of
bits and cutters. Spindle heads are fully exposed and a large diameter
tool
spinning beyond its rated capacity is dangerous. If your spindle is
single
speed, know what that speed is and don’t press the limits. If your
spindle is
variable speed, be sure to set it correctly for the bit being used. Taking
a look at used machines There’s a
potential bargain in buying used CNC machines.
While this can be done successfully, there are a number of caveats. • First,
if you don’t already have CNC experience, bring
along someone who does. • Second,
being mechanically inclined is somewhat more
important here as you may be doing adjustments and repairs before
becoming
fully familiar with the machine. •Third,
determine the router’s condition, a potentially
difficult proposition as many parts are hidden from view and there are
a lot of
electrical components. • Fourth,
make sure you understand what software is included
and whether it will be useful in your application. •
Finally, make contact with the manufacturer prior to
purchase to see how available they will be for service and assistance. Conclusion We have
covered a lot of ground in this article and so I’ll
try to make some concise suggestions to help bring it all together.
First, do a
thorough study of all necessary software. Be sure you end up with a
package
that will give you quick and efficient results. There are three
different
programs involved so make sure you end up with a complete package that
works
seamlessly from design to router interface. Next is
the CNC router itself. Before you shop know the size
and type of machine you will be investigating. Moving gantry, moving
table and
machine size are all options to try and have worked out in advance.
Also take a
look at your electrical situation. Know what you have in place and the
cost and
availability of a 3-phase option in case you find that is a requirement
for the
machine you choose. While
researching your purchase review all the hold down
choices. True vacuum using a rotary vane pump is less costly, but a
regenerative blower system is what you’ll want if you are going to do
panel
processing. Mechanical hold down with screws and hardware is a start up
option
that can help keep initial costs down. Dust
collection may be tied into an existing system, or if
that is not available, a relatively inexpensive stand alone two to
three
horsepower bag collector may do the job. Tooling
is usually left for last but there is no harm in
educating yourself in advance. Look into issues such as rpm vs. feed
rate, HSS
vs. carbide and spiral vs. straight flute cutters. Nothing beats hands
on
experience, but previewing tool technology before hand will be helpful
in the
long run. So which
CNC router do you purchase? The single biggest
determining factor will be cost. There are entry-level machines for
under
$10,000 that are good quality and can be used productively over the
long haul.
They can even be built up with add-ons such as vacuum hold down and
tool
changers. Then
there are the mid-priced machines in the $10,000 to
$50,000 range. There is a wide variety of choices here and you will
find
everything from rail and gantry kits to full blown turn key systems. If
you
have good mechanical ability, and some extra time, you can save quite a
bit by
purchasing a router in kit form. Things like spindles, bases and hold
down
systems are often optional, providing the opportunity for additional
savings. Next are
the mid to high end machines in the $50,000 to
$100,000 range. When looking at these you’ll be seeing complete systems
including hold down zones, servo motors and typically PC controls.
These
machines will range from the biggest and best the mid-size machine
manufacturers have to offer up to the lower end machines of the heavy
industrial lines. Then
there are the large industrial machines that can weigh
in excess of several thousand pounds. These typically cost $100,000 or
more and
need lots of floor space. Your money here is buying a lot of steel and
many
more features than can be discussed in an introductory article. If you
are
running a large operation, or have a specific requirement fulfilled by
a
machine in this class, these can be worth the investment. So,
should you go the CNC route? Since you have read through
this article you are likely considering a purchase. Remember this
though,
whereas a CNC router will do many things well, it is not the best tool
for
everything. You can rip a board to width on a CNC router but that can
be done
much more quickly on a table saw. On the other hand though, the
capabilities of
a CNC router are limited only by your creativity.
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