Hardboard has no strength. It will help with alignment but can easily
snap under stress.

If this is something you do a lot you should plane or wide belt or
drum sand a bunch of real stock to the proper thickness. The other
much more common option is to use plywood. Maybe baltic birch will
have consistent enough size or you can adjust the mortise to fit the
ply thkns.

On Jun 29, 10:06*am, "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.
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??

John