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Chopping Mortises – A Quick Tutorial

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  • Chopping Mortises – A Quick Tutorial

    In a small response to years of subtle but predatory marketing on PBS that has newcomers asking about mortising machines for their early projects, I prepared a short primer this morning in the shop to answer a younger man’s question. The entire session, including stock preparation and photography, took 30 minutes. The actual mortise chopping took 4 minutes, and I wrote this up on my lunch break.

    First, you need mortise, not paring or firmer chisels. These Japanese ones below I bought from Highland Hardware when they first opened more than 2 decades ago to replace the badly worn family ones.

    Note the flat bevels in the photo above; not hollow ground and no secondary bevel. These are laminated blades designed for striking, and they come in the exact width of your intended mortise, ¼, 3/8, and ½ inches.

    Also note that the backs are hollow ground to facilitate easy flattening as you hone them during their life. You can see the hollow above the edge.

    Their bevels should be touched up on the hone every time you use them; your stones ready for use should be a permanent fixture on a corner of your bench. Honing these is easy; just index the flat bevel on the stone. You also need to hone the back dead flat, and I also hone the sides lightly on the fine stone to remove any burrs. I use a set of 4 Arkansas stones all the way to the finest “black” grade, but you can use what you normally sharpen with. I wiped the oil off the stone and chisel for clarity; don’t hone them dry or the stone’s pores will clog.

    I do a final stropping on the stitched muslin wheel with Knifemaker’s Green Rouge. Hard felt wheels are the best for this, but they are expensive.

    Then I lay out the mortises on the prepared stock. First, the mortise gage double tines are set to the width of this half-inch chisel. Then the mortise gage fence is set for mortise location on the stock, and lines scratched. Want to have the mortises dead center? Simple, just run the fence down the opposite face of the stock, compare the marks, and adjust the fence until the marks are identical.

    It’s a simple matter next to mark your mortise width with try square and marking knife. I’ll cut a simple blind double mortise and have used a pencil to make the lines clear in the photograph. You need to leave your pencils in the drawer for this and other joinery marking, as they are insufficiently precise.

    Index your mortise chisel plumb in the end knife cut, and strike with a wood mallet. Do all four ends.

    Continued on Part II

  • #2

    The next chopping sequence is from the center of the mortise, and the waste merely levered out…splitting rather than cutting the wood along its long grain. My training aid today is a 2 X 2 of sopping wet Douglas Fir from the scrap pile out in the weather…. hard, tough and splintery. Your dry cabinet hardwoods will be easier. When working with highly figured wood such as Birdseye Maple you are afraid to split, then simply strike the outline of your mortise lightly with a paring chisel, first.

    I repeat chopping from end and center, levering out the waste until I reach the desired depth; then I reverse the chisel and use it bevel-down to clean up the mortise bottom.

    The final result is a little furry because of the wet wood, but entirely satisfactory. There is a small chip-out at the top of the right mortise, but this will be completely hidden by the tenon shoulder when assembled.

    Frankly, even working in large, commercial, 3-phase shops, I never found many mortising machines that were entirely satisfactory. The bits are difficult to sharpen so you need two sets of them for production work, and the over arm design likes to flex under pressure, especially when the bit is getting dull, which also tends to pull your work piece out of alignment with the fence.

    Why bother? These few simple, inexpensive tools and a shop-made mallet are all you need to do mortise and tenon joinery on one-off projects for the whole of your lifetime.


    • #3
      Great posts! I wish you were my next door neighbor.
      Thanks for the information.


      • #4
        Thanks for this post, only one question: So drilling or routing to full depth first is a waste of time?


        • #5
          Great job! You made it look as simple as it realy is.


          • #6
            Originally posted by Robo:
            Thanks for this post, only one question: So drilling or routing to full depth first is a waste of time?
            Not always...sometimes the wood is too brittle or too highly figured to get a clean mortise by chopping and Forstner bits and firmer chisels are a better choice.

            And for 2-shouldered tenons where the ends of the mortise have to be clean, simple strike about a 16th in from the ends and pare it later, which will remove all the levering marks.

            In a large, commercial shop, my standard for whether I set up the big 3ph mortising machine used to be 12 mortises...I can chop 12 by hand about as fast as fussing with the machine...and doing it by hand is more pleasant.

            Besides...all hand work is an investment in training...and always a worthy investment. There will be a project one day...a boat pieces too big to move or at remote sites...where you'll have no choice as to whether you'll use hand might as well learn now.

            For personal use...and I do woodworking every single day...a mortising machine will be my last priority for purchase...if ever.