Is there a rule of thumb for a half lap joint ratio?

[Meatwad]

I've been building this router table I found on finewoodworking which uses a maple frame for the top support. The one thing different I'm doing than the plan is using half lap joints instead of dowels for my supporting cross beans (see my picture). I cut my half lap to be 3/4 inch from a 2.5 inch board. The first one in my picture went fine but my second one split on me. My best guess is I hit my chisel too hard on it to start a kerf on the end grain. But I have no idea if I'm using the right ratio on this thing. So any advice?

 

[wtnhighlander]

Half-laps generally work at a 50/50 ratio.

Not sure I understand why you 'started a kerf on end grain' with a chisel, rather than a saw?

[Coop]

When I hear the term half lap, this is not what comes to mind but who am I to say. I would have cut both pieces on either the table saw or maybe the bandsaw and used a chisel to clean up the inside of the “u” for lack of a better term. And I would use a 1:1 ratio unless something prevented from doing so.

[Meatwad]

23 minutes ago, Coop said:

When I hear the term half lap, this is not what comes to mind but who am I to say. I would have cut both pieces on either the table saw or maybe the bandsaw and used a chisel to clean up the inside of the “u” for lack of a better term. And I would use a 1:1 ratio unless something prevented from doing so.

I got the idea from when I built my workbench. It has "half-lapped" (for lack of a better term) joints in its cross beams as well. But I'm not sure if that's the correct term since those pieces aren't of the same size. I was attempting something similar here.

My aprons are 3.5 inches thick. I didn't think it was necessary to cut half of that so I didn't. My "U" section is good but I was just wondering if I used a 2.5 inch wide cross beam if I really needed to cut it at exactly 50%. So I just picked 3/4".

30 minutes ago, wtnhighlander said:

Half-laps generally work at a 50/50 ratio.

Not sure I understand why you 'started a kerf on end grain' with a chisel, rather than a saw?

Maybe I should've said knife wall?

[Coop]

Other than your little hiccup, everything else looks great. Nice clean joinery from aprons to legs. M&T’s?

[wtnhighlander]

Ahhh, that makes more sense. If you were striking the chisel to form the valley in front of a knife wall, the end grain could certainly split. I usually just press the chisel in toward the knife cut, more of a paring action. 

[Meatwad]

9 hours ago, Coop said:

Other than your little hiccup, everything else looks great. Nice clean joinery from aprons to legs. M&T’s?

It's actually dowels which is what the plan called for. It called for dowels on the crossbeams as well but I wasn't going to be able to use my same doweling jig on the middle of the board so I decided to attempt this which got me thinking about ratios.