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MACHINING VARIABLES
MACHINING VARIABLES
Each material has a range of workable spindle
speeds. For example, spindle speed for routing
plywood is somewhere between 11,000–18,000
RPM. But speed is only one of several variables
in the feeds and speeds equation that deter-
mines how the tool, material, spindle speed,
and tool movement through the material, or
feed, work together to make a cut.
Knowing the optimal spindle speed range for
plywood is helpful, but how do you determine
the exact spindle speed to use?
Speed
Spindle speed, measured in revolutions per
minute (RPM) is calculated from a combination
of specific aspects of cutter geometry, feed
rate, and chip load:
RPM =
FeedRate
NumberOfFlutes × ChipLoad
Feed Rate
Feed rate is the velocity at which the tool moves
laterally through material. Feed rate is meas-
ured in distance per time, or inches per minute,
and abbreviated IPM 1:
RELATING EVERYTHING TO
EVERYTHING
Feeds, speeds, and chip load are machining’s
trinity. They can be defined separately, but
when cutting, all three work together simulta-
neously to produce chips.
When trying to grasp a concept, always look at
how it is measured or calculated. That’s the key
to understanding exactly what the thing is,
what variables are in play, and what relation-
ships are important.
FeedRate = RPM×NumberOfFlutes × ChipLoad
Chip Load
Chip load is the amount of material removed by
each flute’s tooth as it makes one revolution
while being pushed along at a feed rate and
spindle speed. Chip load is measured in feed
per tooth, or inches per revolution, abbreviated
as IPR:
ChipLoad =
FeedRate
RPM × NumberOfFlutes
1. Under the hood, Shop-
Bots are configured in
inches per second. Either
way, feed is still length
traveled per time elapsed.
ROUGH CHIP LOADS BY MATERIAL
The chip load, feed, and speed formulas all take individ-
ual tool geometry into account by including the number
of flutes (cutting edges) into the equation.
While actual chip load is different for each specific tool
geometry, chip load generally increases gradually with
bit diameter. Some rough chip loads for sheet materials
are shown in the following table. 2
Onsrud provides chip load data specific to
their tooling. For more information, see http://
www.onsrud.com/xdoc/Feed-Speeds.
CHIPLOAD
Softwood
Hardwood
MDF
Plywood
Laminated Chipboard
1/4 inch
.002-.020
.005-.020
.005-.030
.005-.020
.008-.020
1/2 inch
.003-.030
.005-.030
.005-.020
.005-.030
.010-.030
2. Onsrud data compiled
by ShopBot and Centurion
Tools
06/MACHINING FOR DESIGNERS
145
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