Porter Cable 42045 template guides are a great way to guide a router for a very controlled cut using a pattern or template. The guide attaches to the router base to keep the bit a specific distance from the template's edge while allowing the bit to cut in where needed.
Porter Cable 42046 5/8 Template Guide
Porter Cable 42045 Template Guide for use with stanley T-3 Template
Porter Cable Canada
POWERTEC 10-Piece Router Template Guide Set (Short Shank), Fits
Powertec 71051 Router Template Guide Set, Fits Porter Cable Style
Porter Cable 42045 5/8 In. Router Template Guide
POWERTEC Brass Router Bushing Set with Case 71051 - The Home Depot
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Porter Cable 42037 Router Template Guide