I’m working on all of the mortising for my shoji garden gates. Because I love my 6mm mortising chisel I’m using it for both the kumiko mortises and the rail/stile connections, which are twin 6mm mortises. But Japanese mortise chisels have very little taper tip to handle for clearance when cutting deep mortises.
In comes the aburatsubo, a small vessel with silk cotton or a rolled cloth soaked with oil. The difference that a little oil makes on the chisel while you work is amazing, especially the deeper you are cutting. Not only does it lubricate the sides of the chisel, the swarf seems to come free more easily as well. I had my reservations about getting a little oil on the important glue surfaces of the mortise, but I’m going to follow in the wisdom of Japanese tategu shokunin of the past and give this a try. Smooth is fast, and oil makes things much, much smoother.
The tools from left to right: 1/4″ dovetail chisel for cutting down the sides of the mortise, sokozarai-nomi for cleaning the bottom of mortise, flat-tipped screwdriver for loosening chips in mortise, 6mm Japanese mortise chisel, hammer.
I find with very small and deep shoji mortises that the chips come free much more easily if I cut down the cheek of the mortise. When I started with chisel mortising I used various wood mallets, which are great, but I’ve come to prefer the density of a steel hammer when chopping. I’ve become extremely fastidious about the placement of my tools when working. Lets face it, mortising by hand is slow compared to a power hollow chisel mortiser or a spiral up-cut bit on a plunge router. I own neither, so I seek to work as quickly as possible, and every little repeated movement adds up.
Part of my quest for faster mortising involves the final trimming of the end grain surface. Tategu use a slight inward taper to the mortise to compresss the tenon as it is driven together. Its great because shoji can assembled without the use of any clamps, but judging the cut can be difficult. Depending on the size of the mortise I try to tilt the cuts in for a total of 1mm to .5mm less width at the bottom. My method at the moment uses a small wooden square that I watch as I cut to the line. This way I cut it once right as I want it and don’t fuss around wasting time carefully paring the endgrain. It teaches you how important hitting the chisel in the right spot is to cutting a straight line. If the cut is too heavy the chisel wants to travel back in under your cut line, making for a weaker assembly.
Here are the twin 6mm mortises for a rail/stile connection. I mortise in the traditional Japanese manner. Starting in the middle cuts are made inward, producing a ‘V’ notch as close to your final depth as you can manage. Chips are continuously pushed up and out as each side of the ‘V’ is cut down. Then vertical cuts are made back to the line. Its really important in terms of speed to cut as close to your depth with the ‘V’ as possible. The aburatsubo made these deep mortises possible, and kept me from stabbing myself in the thigh while trying to yank a stuck chisel from the mortise…
The aburatsubo is traditionally made with a small section of bamboo, 1-1/2″ – 2″ diameter, but anything will work really. A small glass pint size mason jar with a bit of rolled up cotton t-shirt would be easy enough. Or maybe you live in Hawaii where bamboo abounds. I have a lathe, so a nicely turned object is what I gravitate towards. I started by boring out my square blank 1-1/4″ wide by about 2″ deep.
I then turned down a lightly tapered mandrel in my lathe chuck to fit the bored blank.
If the taper is too tight it can split the blank, so I leave things barely snug on the mandrel and bring the tail stock up to keep some pressure on the blank while it is roughed to round. Once the major shaping is done the tailstock can be backed off and the bottom finished slightly concave so that it sits properly on a flat surface.
I used a couple of coats of paste wax to seal the grain and keep the oil from soaking through the bottom. The silk I used for the ‘cotton’ filler is prepared sliver for spinning into yarn. I’m not convinced that silk is somehow better than some cotton balls or a bit of old t-shirt, but I have it so I’ll try and find out.
This is the first time I’ve used aburatsubo for mortising. To quote Toshio Odate, “When mortising, the tategu-shi continually stabs his mortise chisel into an aburatsubo filled with cotton and clear vegetable oil to lubricate the chisel.” When I try this the silk ends up packing down into the oil pot, only lubricating the first half inch. Maybe I need a bigger pot that I can form more of a “nest”. For now I just pulled a bit of silk out the top and use the aburatsubo to wipe the sides down periodically. The amount of oil it uses is miniscule, so I’m using camellia oil, though expensive, because I love the smell.
Does anyone else use an oil pot while mortising? This is one more thing I’m surprised not to have tried sooner. Its already made my mortising twenty-five percent faster, at least.