Tuesday, May 11, 2010

Tech Tip Tuesday

One of the more traditional woodworking projects that we do when building the canoe is the fabrication of the seat frame. Because commercially made seat frames are available so cheaply, many people skip this part of the process and just spend the $40-$50 that a seat costs and call it a day.

For many of my students this isn't always a good option as they want to use a specific kind of wood that isn't commonly available as a seat frame. The other problem that I have with commercially made seat frames is that they are often made from machine woven cane that is pressed into the frame with splines. I have found that it tends to pull out more than hand-woven cane that we install. This process does take a fair amount of time, but it is important to remember that my students are building the boats - not just assembling kits.

This joinery would be familiar to anyone who has done any furniture or cabinet work. Basically, the seat is an "H"-shaped frame made from hardwood and held together using a mortise-and-tenon joint. For those not familiar with a mortise-and-tenon joint, it is clearly shown in one of the photos of last Saturday's class:

In the long rail sitting on the saw table is a rectangular hole - this is the mortise. On the end of the stretcher that is pointing straight up we see the tenon - a relieved "tongue" that fits into the mortise. Once glued together, this creates a very, very strong joint. For our Wee Lassie and Wee Lassie II, we make the same seat frame. It has an 11" square opening and is 28" wide before we cut it to fit the boat.

To get started on the seat, we first "dress" the rough-cut stock from the lumberyard. This is done to achieve flat stock with smooth parallel faces on the top and bottom and a smooth perpendicular edge so that we can then cut the wood on a table-saw without difficulty.

To do this, we flatten the board (if necessary - if the board is warped, twisted or cupped) on at least one face using a jointer and then plane the stock on both faces using a thickness planer to get flat, parallel surfaces. The flattening process on the jointer can be very important because if we just tried to plane the stock down, the planer can apply enough force to press the stock flat while it is being cut, but let it reflex after it leaves the planer resulting in parallel, but warped, twisted or cupped stock.

After the wood has been planed to the appropriate thickness - between 3/4" and 7/8" thick depending on the type of wood and the paddler's, um, gravitational attraction. We then use the jointer to mill one edge of the board so it is straight along the length of the board and the edge is both smooth and square to the faces of the board. We are very fortunate to have a 12" wide jointer and a 24" wide planer in the wood shop which help to make this job easy.

After the wood has been "dressed", it should look like this:

We now have to cut the board to the proper width and length for the rails and stretchers for our seat. Using the table saw, we rip stock to just over 1-1/2" wide, and at least 30" long. We'll need three pieces per seat. These cut pieces are then cleaned up using the planer to get rid of the saw marks and we use a cabinet scraper to assure perfectly smooth surfaces on the four sides of these three pieces. It is easier to do this before we assemble the frame.

Nothing like a nice pile of shavings from scraping...

We then use the "chop saw" - a dual bevel miter saw - to square one end of each piece. We then cut two of the pieces down to 28" long for the rails. To get pieces that are exactly the same length, we cut both pieces at the same time. We cut the third piece in half and then the two remaining pieces are cut to be 14" long to become the stretchers - again making the cut at the same time.

The general location of the mortise is then marked on the rails. As I mentioned before, the distance between the stretchers will be 11". I make marks on the rail to indicate the inside and outside edge of the stretchers. The inside edge of the stretchers are 11" apart, centered on the rail. When considering the amount of material to be remove for the mortise, I want to leave a minimum of 3/16" of material all around. I mark the size of the mortise opening and put an "x" through the area to remove.

While this next step can be done with a mallet and chisel, we use a mortising machine. This machine is basically a drill press with a hollow, square chisel that surrounds the drill, allowing us to cut square holes. We use a 3/8" square mortising chisel and set the mortiser up so that the depth is 1/4" less than the width of the rail. The width of the mortises is just a smidgen (That's the technical term for it...) wider than it needs to be - about 1/32" - 1/16" on both sides of the four mortises. This will still be hidden by the shoulder on the stretchers. We then flip the rails and repeat the mortising cut. This is to assure that the mortise slots are centered in the rails.

At this point, we have two rails with four mortise slots in them.

The tenons are then marked out on both ends of both of the 14" long stretchers. The tenons will be 1-1/2" long, yielding an 11" space between the rails on the finished seat. What I do is simply to make a cut that goes all the way around the rails 1-1/2" from the ends using a utility knife. When the tenon is finally cut, this keeps the surface from tearing out beyond the cut.

The table saw is then set up with a dado blade. A dado blade is usually a stack of several blades. End blades which look like typical saw blades and "chipper" blades which go between the two outer blades. Dado blades let you cut rabbets (rebates), dados (slots) and tenons. We're using it to cut tenons. In our case, I raise the height of the blade to be the same as the thickness of material between the mortise and the outer face of the rail. This will remove the right amount of material all around the stretchers to form the tenons.

I then attach a block to the fence of the table saw. It is imperative that the block stops before the point where the saw blade protrudes from the table. This is to prevent jamming the stock and causing a dangerous kick-back. This block will be used as a stop to set the tenon depth. The surface of the block closest to the saw is set 1-1/2" from the opposite side of the blade. (Use the scribe mark for the tenon on a stretcher as a guide. Here's the set-up shown with a standard table saw blade and a finished tenon for clarity:

Using the miter gauge for the table saw, cross-cut the stretcher starting at the shoulder and then pulling back from the block and making more passes to remove all the material. Rotate the stretcher and repeat to remove material from all four surfaces. The completed tenon should look like this:

Once all the parts have been milled, we dry fit the whole thing together and check if any tune-ups to the tenon are needed. When the fit is perfect, we apply glue to the mortise and the tenon and glue the frame together making sure that everything is square. The seat needs to sit and let the glue cure before further work happens.

Once the frame is assembled and the glue is cured, we use a palm router with a 1/4" or 3/8" radius round-over bit with a follower to break the sharp edges on the seat frame. This will make the frame more comfortable to sit on when complete. It will also avoid thin spots in the finish on sharp outside edges.

We then drill the holes for the cane. For the common cane (a specific size of rattan cane) that we use to weave the seats, a 5/16" diameter hole on 7/8" center-to-center spacing is required. These holes are drilled 7/8" away from the inner edge of the frame. When drilling, be sure not to break out the back surface of the frame. We do this by using a brad-point bit and setting the depth on a drill press to let just the center point come through . We then flip the seat over and use these marks as a guide to complete the hole from the opposite side. We then use a counter-sinking bit in the drill press to make a 3/32" chamfer on the holes on both sides of the frame. This relieves the edge so that the cane doesn't get cut on the sharp edge of the drilled hole.

Once this is complete, your seat frame is ready for sanding, varnish and weaving of the cane.

A little bit of work, but worth it!

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