On Jun 29, 1:06*pm, "John" wrote:
I built an adjustable fixture that allows me to make very accurate loose
mortise and tenon joints. I have been using tenons fabricated from hardwood,
either Oak or Poplar. Needless to say, it requires some work to achieve the
1/4 inch thickness required for a tight joint.

Biesemeyer fence, featherboards to run the stock straight.
Vernier calipers to mike the test thickness, dial indicator
with magnetic base to help tap the fence over as needed.
I shoot for an easy 0.005" max slack. Most glues have
ample shear strength to make the joint indestructible.

I am now considering using 1/4 inch tempered hardboard. It is stable, strong
and has a uniform thickness. Any have experience with this technique??

Birch veneer ply will be uniform and a heckuvalot stronger.