Porter-Cable 7539 Instruction Manual page 9

SELECTING THE BIT
Models 7538 and 7539 accommodate bits with 1/2" diameter shanks that install
directly into the power unit collet. Collets are available that will allow the use of
bits having 1/4" or 3/8" diameter shanks.
RPM. Router bits with a diameter up to 3" may be used with the 7539 motor
operating in the 10,000 or 13,000 RPM speeds.
DISCONNECT IT FROM THE POWER SOURCE.
DO NOT USE router bits with a diameter in excess of 2-1/2",
except when using Model 7539, set for either 10,000 or 13,000
While preparing the router for use, while making
adjustments, and when router is not in use, ALWAYS
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