There are several ways to move a CNC axis. This can be
done with belts, chains, ball screws, ACME screws, or a rack
and pinion drive. Ball screws,
ACME screws, and rack and
pinion are the most prevalent.
Rack and Pinion Drives
Rack and pinion drives
(Figure 12) utilize a geared
down pinion that runs along
a matching rack that runs the
length and width of the
— while a little more
complicated — allow for
very quick movement of the CNC.
Screw systems use a large screw that runs the length of
the CNC. Normally, the screw
is fixed on the CNC machine.
A carriage is attached to a nut
that moves as the screw
An ACME screw like in
Figure 13 is not the same as
standard screw threads found
on a threaded rod. They are
much more efficient and can
include multiple threads to
62 SERVO 04.2014
There are two basic
types of CNC machines
designed for rotary tool
cutting: the CNC router and
the CNC mill.
CNC routers utilize a
high speed router or spindle.
They ride on rails or rods
utilizing bearings, much like
the machine shown in
Figure 8. CNC routers excel
at cutting wood, wood
products, and plastics. Rigid
routers can also cut
aluminum. The KRMx02
shown in Figure 9 has a
dust evacuation system and
a large clamping table. This
makes it well suited for
cutting sheet goods.
CNC mills utilize a low
speed spindle. Unlike the
high speed spindle, the low
speed spindle can also utilize twist drill bits. This allows
them to perform drilling operations. The CNC router can
only use very small drill bits due to the speed. Being able to
drill larger holes allows the CNC mill to produce metal parts
much easier and quicker than a CNC router.
The part shown in Figure 10 has several 1/4” holes.
This part takes less than 30 minutes to make using a drill bit
on a CNC mill. Milling the holes on a CNC router would
take 30 minutes every couple of holes.
CNC mills excel at cutting metals, but with the correct
end mills can cut and drill other materials, as well. CNC
mills only move the head in the Z (up and down) direction.
Instead the table moves in the X and Y directions. The table
does not use a bearing and rail system like the CNC router
does. Instead, the table uses a set of greased or oiled dove
tails like those shown in Figure 11. This setup creates a
very rigid machine which is required for milling metal.
Unlike CNC routers, DIY CNC mills are rarely built from
scratch. They are usually created by converting milling