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SPINDLES AND SPEED
WHAT IS A MILLING MACHINE?
Mills were invented in the early 1800s. Origi-
nally they were large, heavy, rigid, high-
precision, motorized (but not computer-
controlled) machine tools made to cut very
hard materials like metal. The spindle that
rotates a rotational cutter capable of lateral
cutting called an end mill is stationary, while
the material is clamped in a vice to an x-y mov-
ing table. 1 Most mill spindles have a speed
range of 3,000–7,000 RPM. Anything higher
than 7,000 RPM is considered, routing or high-
speed machining (see “High-Speed Spindles”
on page 143).
WHAT IS A ROUTER?
In 1915, Oscar Onsrud and his son Rudy repur-
posed a jet motor, creating a hand-held spindle
that turned much faster than a mill spindle and
ran on compressed air—founding Onsrud
Machine Works. In order to cut at higher RPM,
Onsrud created a new type of cutting tool that
could spin at 30,000 RPM, far faster than any
end mill at the time could cut (http://
www.crcncrouters.com/History.html). Called
router bits, or router cutters, these tools had
modified end-mill geometry that could cut
wood at high rotational speeds and lateral
feeds.
Modern routers are woodworking power tools
that use router bits to carve joinery grooves,
decorative moldings, engraving, inlays, or sim-
ply to cut out parts. This is usually accom-
plished by tracing a template, sometimes with
a bearing-guided router bit. Routers come in
many types and sizes and are usually handheld,
but can also be mounted in a table—or
attached to a machine frame. Some routers are
single speed, but most have adjustable, desig-
nated, spindle speeds somewhere between
10,000–25,000 RPM, much faster than mill
spindles.
For example, a Porter-Cable 7518 3-1/4 HP vari-
able speed router used on some ShopBots can
be adjusted to run at any one of five different
speeds: 10,000, 13,000, 16,000, 19,000, or
21,000 RPM.
When working with a CNC router, never use a
router bit with a guide bearing.
Routers can cut much more than wood; plas-
tics and soft metals like aluminum cut well at
high RPM.
HIGH-SPEED SPINDLES
Some CNC routers have actual router power
tools attached, while others have variable high-
speed spindles, so-called to differentiate them
from traditional mill spindles, which operate at
a much lower RPM.
Overall, high-speed spindles (Figure 6-2) are
powerful and much quieter than routers. On
large-format CNC routers, like the ShopBot
PRSstandard and PRSalpha, 2 high-speed spin-
dles are industrial-quality components with
precision bearings that enable them to cut bet-
ter and last longer than a router. They also have
less runout, or rotational wobble, can operate
at full torque (twisting force) at lower RPM
without bogging down, and have minutely
adjustable speeds from around 5,000–25,000
RPM.
There are also modern milling machines that
are capable of high-speed machining, or HSM.
These mills move quickly at high RPM, making
light passes to achieve high metal removal
rates.
1. A Google image search
for “Bridgeport mill” will
give you an idea of what an
old-school, knee-and-
column mill looks like.
2. PRS or Personal Robotic
System
06/MACHINING FOR DESIGNERS
143
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