Thread: How do I make sure the sides and bottom of a mortise are perfectly flat/square?

Hi There,

For flat, I would just hold something up to the wall of the mortise on each side (such as the flat of a chisel or a straight edge), shine a torch into the mortise and see if you can see any gaps between the straight edge and the wall.

Square is a bit trickier. You could:
1) make a long tenon to fit the mortise, put it in, and then hold a square against the protruding part of the tenon;
2) if you have an adjustable square, then simply adjust the height of the ruler edge to fit the length of the mortise and angle it in to the mortise, referencing the square edge off the top of the mortise stock to check whether your walls are straight and square.

For the bottom of the mortise, just adjust the ruler on your square (or if you don't have one, just mark a ruler at the top of the stock when you insert it into the mortise) and run it across the bottom - if it stays the same height all the way along, then you're fine.

I'm not sure I described those tasks very well, but hopefully they made sense.

To be honest, I'm not usually that worried about checking the mortise for square. If you have set up your production properly (holding the chisel square etc when chopping the mortise) then it will usually be fine.

Having said that, I appreciate that you want to check it's OK before you continue on.

Cheerio,
sCORCH