Selecting the router bits
As indicated, guides are generally used with straight
router bits. When routing edges, a standard straight bit
with two flutes is adequate. When routing cavities or
holes, up-cut spiral bits are preferred, as they help
clear waste from your work. The diameter of your bit has
to be small enough to allow for clearance between the
bit and the inside of the template guide. This prevents
chips and sawdust from collecting.
Making your templates
With guides, your template will be larger or smaller than
the finished size because of the offset between the cutting
edge of the router bit and the edge of the guide that
rides along the template.
You can calculate your offset by taking the guide-bushing
diameter, subtracting the bit diameter and dividing
For instance, using a 5/8" guide bushing and a
3/8" bit, the calculation is as follows:
5/8" - 3/8" = 2/8"
2/8" / 2 = 1/8"