I think I made a mistake with my design initially but have ended up having to cut a mortise across the grain. I started using Paul’s method of cutting it but after two hits of the chisel noticed the wood splitting along the grain.
Would I be right in assuming that mortises are always cut with the grain, such that the chisel cuts across the grain as it is formed? Secondly, if cutting the elongated mortise across the grain is an acceptable practice what is the best way of achieving the cut. I ended up cutting down my lines with a chisel and paring out the waste material but would welcome any suggestions, even if it is not to cut mortises that way again.
Hope I have the terminology correct in the explanation
You can make a mortise that is wider accross the grain than along the grain, but I think, it is safer to keep the cutting accross the grain. Along the grain, you are seperating the fibres more than cutting and you cannot predict, how long this separation will continue. Always imagine the wood fibres as a bundle of slightly sticky straws, this helps a lot to figure out, what can happen.
Paul Sellers made a video where he compares a mortise chisel to a “normal” chisel. I think, it is the one, where he explains best, how to cut a mortise efficiently.
In Illustrated Cabinet Making (ISBN13: 978-1565233690) Bill Hyltons warns that this joint has poor gluing properties, as the mortise presents end grain. Instead, it is suggested to have several more narrow mortises cut across the grain (please see photo). This recommendation probably makes more sense for a (lower) divider in a table frame than for a frame for a wall-hung cabinet, as there will be a leverage on the divider if the table is moved over a floor, while no major such force is exerted on the cabinet.
An alternative approach to this joint is perhaps to drill out most of the waste, using a chisel only to pare out along the sides of the mortise. /soj