-- $160, 4 hour glulam workbench build -- part 3
to the top...
Roubo benches have the legs flush with the front but have overhangs on the sides. My first reference mark on the bottom of the table was how far away from the sides I wanted the legs to be. Make a 90* mark from the front of your top all the way across to the back on both sides. Then using the same mortising gauge you used on the tenons mark out the mortises front and back. Personally I don't worry about marking too far because the bottom won't show so I'll run the gauge along a 6"-8" run. Because yellow pine is so light I then run a pencil along that mark to darken it.
Now it's important to match each mortise to a specific tenon. So I marked the mortises A-D to correspond with the appropriate legs. The reason it's important is because you'll be using the tenon on the leg to mark the length of the tenon and position of the draw bore. Mixing and matching will result in poorly fitting legs. Simply lay the legs tenon on it's side. Register the shoulder to the front. Align the outside edge of the leg with the mark running across the table. Then mark your mortises width directly from the tenon.
There are several ways of cutting the tenon. Lot's of people just have at it with the chisel and mallet. I'm more the type who bores out as much waste as possible then clean it up with the chisel. This is the time you set the depth of your mortise. I use tape on the screw to be consistent. First bore four small holes in each corner and then use a large bit to hog out most of the waste.
Then using the chisel you used to set the mortise side clean up the sides. You'll know it's perfect when you can lever the chisel front and back. I always leave a millimeter or so at the edges so the last thing I do is pound it down in order to get a clean edge in the end grain.
Remember me saying I didn't cut the length of the tenon because I'm lazy? This is why. I never know how long it'll be until I get tired of cutting mortises so at a certain depth... I stop. So at this point I use a stick or tape measure and figure out what is the highest spot in the mortise. This now becomes the height of that corresponding tenon.
Once you've got the tenon cut to length use that to measure the shape of the mortise on the face of your table. Use the same steps used to measure the width of the tenon earlier. This'll help with a later step, draw boring.
You should now be able to drop each leg into it's appropriate mortise and have it register against the projects main reference point, the bottom of the table. It is at this point we should measure, mark and cut the legs to length. Why did I wait until now... because I screw up a lot. If I had cut them at the beginning I might have screwed up cutting the shoulders thus changing the leg length. This way I eliminate a chance for screwing up. I also suggest that you cut corresponding side legs together. This'll increase the reference line. (Remember me saying I find it easier to cut a straight line when I cut a longer distance?) Also if you're a little off at least they'll be off the same amount on each end so it won't rock (so much).
Draw boring is what will correct your crazy loose mortise and tenon and save you money on clamps. The concept is simple. Drill an offset hole in the tenon that corresponds to one on the table that'll suck the legs shoulder tight when you drive a peg thru it. I always have to sit and think for a while to figure out which direction to offset the tenon's hole. On this project I only used one peg instead of two because I wanted to see how well a slightly larger peg held compared to two smaller ones.
Steps to draw boring:
- Drill a hole from the face, thru the mortise and into the opposite wall at least an inch. Use the mark on the face of the shape of the tenon to center one or two holes.
- Drop the tenon into the mortise so it fully seats.
- Shove the same drill bit back thru the hole marking the tenon with the center point of drill.
- Remove tenon.
- Mark where you'll drill a horizontal hole in the tenon a little ways closer to the shoulder.
- Slobber glue all along the long grain of the mortise.
- Drop the tenon back into the mortise and drive a long peg thru the tenon into the back wall of the mortise. It helps to sharpen the peg so it doesn't get stuck on the tenon or back wall.
- Trim off the excess peg.
- If you just saw or chisel it off there might be a gap. To avoid that I normally cut the peg off a few millimeters proud. Drip some water on it and then pound it with a hammer. This'll spread it out to fill the gap. At that point cut it flush with a saw or chisel.
Guess what... you're done. Dad and I spent about 4 hours assembling this table after getting all the supplies. A perfect base to attach whatever kinds of tools or vises to make into the bench of your dreams.
I think you'll find you have one incredibly sturdy table now. Personally I instal stretchers along the bottom to increase long term durability. You can mortise them into the legs, or lap them so that they are flush. Dad just wanted a shelf so we nailed a stretcher along the length and the shelves as a stretcher on the width.