Auxiliary planer bed
Thickness planers are wonderful tools; but like most machines, they have their limitations. Many of them will not thickness stock thinner than 1⁄4 ", and none of them will plane a bevel. However, outfitted with an auxiliary bed, any planer can perform both of these tasks. We designed our bed to fit a Delta portable planer, but you can change the dimensions, and the shape of the bottom-side cleats, to fit any planer. (The cleats prevent the jig from sliding on the table.)
The 3⁄4 " thickness of the auxiliary bed raises the height of the workpiece so that you can plane stock less than 1⁄4 " thick. There's no danger of damaging the knives because at the worst they will only cut slightly into the plywood surface.
By adding a spacer block under one side of the auxiliary bed, as we did in the photo, above, and the End View drawing, below, you can raise that side so the planer cuts a bevel. You'll find this handy for making such things as siding and thresholds. Note in the photo that we clamped down both sides of the jig. Additionally, we added a fence spacer on the low side of the jig to prevent the planer knives from cutting into the bed before it cuts the full depth of the bevel.
Smooth hardwood plywoods, such as birch or maple, work well for the auxiliary bed. If you use a lesser grade, sand it as smooth as possible and apply paraffin wax to lubricate its surface.
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