pattern question

[alex adkison]

I want to make some patterns for a chair. I have a sheet of 3/8 hardboard laying around. Any problem with a guide bearing on a flush trim bit riding something this size, or should I double it up?

Thanks

[Sam Yerardi]

Unless you have a very, very small base diameter you should be good to go.

[Peter Quadarella]

I've used 1/4" plywood for most of my early patterns.

[David DeCristoforo]

3/8" should be fine. That is more than the thickness of a standard bit bearing. I use hardboard ("Masonite") for patterns because plywood can have voids and thin MDF can crumble. For shaper patterns, I use 3/4" MDF because the shaper guide bearings are much thicker and larger so the extra bearing surface makes sense.

YM

[Tom Veatch]

... I have a sheet of 3/8 hardboard laying around. Any problem with a guide bearing on a flush trim bit riding something this size...

I don't think I've ever used anything that thick. 1/4" or even 3/16" hardboard works fine for me (or whatever the actual thickness is for that durn millimeter stuff that is neither 3/16 nor 1/4)

[Peter Quinn]

On a router your good, for a shaper you need min 1/2". I like to make prototype patterns out of 1/4" MDF or birch ply, easier to sand and fit, then transfer these to my 'working pattern' a la David Marks.

Hard board makes a better template for repeated use, MDF seems to take an imprint from the bearing after too many passes, not always good if parts have to be joined precisely.