you do not chop with your chisel all the way through like you would with the drill.. yes- that will split your wood

chiseling for fine woodworking is a mostly a fine precision work, and not like general contracting where you poung the chisel with a hammer like there's no tomorrow.

what you want to do is after you remove most material with the drill, is to pound your SHARP chisel 1/8"-1/4" deep into the wood, and stay shy of the marked line - stay INSIDE the line as your chisel have the tendency to slip backwards.

after you have that initial 1/4" cut, pare (google it) into it from the inside to remove the material. then do the other side… rince and repeat, and slowly but surely you'll get through it.

when you are almost through - turn the piece around, and cut the end of the square hole from the OTHER side to avoid tearout and chipout at the edge of the hole.

google "chiseling mortises" also check popular woodworking for some videos on that - and you'll get the idea.

 

Good instructions - easy for me to visualize.

I've done a little chiseling on mortises but a hole is much different than a groove as half way through, the angle on my chisel will almost be flat. I've read that you should use a chisel the same size as the groove. But is this still true for holes?

(not getting much from google on chiseling holes. in fact, this post comes up near the top - lol.)

 

using a chisel as wide as your groove will make it easier as you won't have to overlap cuts, but is not a must have- just makes things easier and can get you a cleaner cut at the end.

consider the through hole as 2 half-way grooves that just so happen to meet at the middle of the board to make a complete through opening. that's also how you should cut that hole - half from each side. although it might be easier to align the 2 sides if you make one side almost through, and then just clean the edge from the other side which will make the walls of the hole aligned better.

you don't have to pare the excess material - you can also chisel it off at a 45 degree (or close to) angle into your vertical chisel line. basically take off one chunk at a time as you make your way down the hole.

 

See if you can find someone with a mortising chisel, it looks a lot cleaner.

Or alternatively to prevent tear out chisel 3/4 the way down cleaning out the pilot hole then turn the piece over and finish the job from the other side.