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Cutting Tenons

Charles D. Hepler May, 2009 revJuly 4, 2011

Once upon a time, the only way I knew to cut tenons was with a "carriage jig" (an inexpensive version of a tenon jig) that clamped the work vertically and held it steady as I slid it over the saw blade. Then I would spin the workpiece around and cut the other cheek. I had a difficult time making tenons that fit well. The difference between tight and loose for most mortise and tenon joints is less than 1/64" (based on the amount of resistance to being assembled and disassembled dry), and few adhesives are strong across a gap of more than a few thousandths of an inch. So, I had to cut tenons too thick by 1/32" or so and then use a shoulder plane to fit them. It was tedious, but so was endlessly trying to adjust the jig to exactly the right position. I did not understand this problem until I appreciated how one cuts tenons by hand. The correct way to cut tenons by hand is to lay out and mark the tenon on the workpiece. It is marked to correspond exactly to the mortise that it will mate with. Then, the tenon will compensate for any irregularities in the mortise. The tenon will fit well and the adjacent surfaces will align just the way you want them to. I want to make two points. First, unless the mortises are uniform in every respect, you can't expect uniformly cut tenons to fit correctly. I cut most mortises with a plunge router, and I use a jig to make them all uniform. The widths of the mortises vary little, but the position of each mortise relative to rthe face or reference surface may vary somewhat, as may the length. For this reason, I lay out tenons just as if I were going to cut them by hand, especially the lines for the tenon cheeks. My second point is that hand cut tenons are cut to the mark. You actually measure the tenon. Cutting tenons horizontally on a table saw with a dado stack or vertically, by flipping the board around is "cutting by subtraction." The former method is recommended in some books and magazine articles, and was shown on a popular TV woodworking show. Many woodworkers say they use it. So, obviously it can be made to work but may be unreliable. In particular, either method will produce ill-fitting tenons if the stock is not all of uniform thickness. Cut the Size You Want I know three ways to machine cut a tenon directly to the size I want. These are (a) using a tenon jig and a spacer on a table saw, (b) using two blades with a spacer between them on a table saw, and ( c) using a band saw. (Some people use a jig to cut tenons with a router, but I have never tried that.) After they have been set up for the first time, methods (a) and (b) will always cut tenons of exactly the same thickness. Method ( c) is the most flexible. It is like hand cutting except that the band saw replaces the tenon saw.

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Cutting Tenons For each method, you should lay out the lines for the tenon cheeks and shoulders as if you were going to cut them by hand. Carry the lines around to the side of the workpiece. There are two basic kinds of tenon jig: one runs along the fence of the table saw, and the workpiece is clamped to it. In effect, it is a high fence. You can find plans to make one in Kirby's The Accurate Table Saw or elsewhere on the web. The other kind is a heavy right-angle piece of iron with a clamp to hold the workpiece vertical and a bar that fits in the miter gauge slot. (See Figure 1) A carriage jig is just a much simpler (harder to adjust) version of Figure 1. Tenon Jig and Spacer

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Figure 1. Delta Universal Tenon Jig. (The rear workpiece support has been removed.)

This method takes a while to set up the first time, getting the spacer just right. After that, it cuts tenon cheeks accurately and efficiently. It is flexible to the extent that the first cut can be adjusted to one of the layout lines. It always cuts tenons very close to the same thickness. 1. Prepare a block of wood that is exactly as thick as the tenon you want, plus the width of the kerf cut by the blade you will use. For example, if you want a d" tenon and will use a 3/32" blade, the block should be 15/32" thick. (Cut some test pieces until you get the thickness of the block exactly right.) The thickness of the workpiece does not matter. Put the block between the jig and the workpiece. Set the jig so that the blade will cut on the layout line for the cheek nearest the jig. Make the cut. Remove the block and make the second cut for the other cheek.

2. 3.

Tenon Jig With Two Blades and Spacer This method requires the most initial setup time, but once the spacer and zero-clearance insert have been prepared, it is the fastest and most accurate of the three. 1. Make a spacer that will fit over the arbor of the saw, between two blades. In theory, it should be exactly the same thickness as the tenon you want to cut, e.g., d", but some fine tuning will be necessary to accommodate the set of the saw teeth. You can make it out of hard wood, fiber washers, steel washers, etc. If you make it of wood, the grain should run parallel to the saw arbor so that it will be incompressible. Fine-grained wood like maple would be OK. Put the spacer on the saw between two identical saw blades. I use the two outside cutters from my dado stack. Test the spacer by cutting grooves in a horizontal piece of scrap. Use the throat

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Cutting Tenons plate that you use for cutting dados. Do not try to make a test tenon on the end of a board because it may fall down into the throat of the saw. After you get the spacer trimmed to the desired thickness, make a zero-clearance insert with two openings in it for the two saw blades. This is mandatory for safe operation, to keep the workpiece from tilting or falling into the throat of the saw. To use this setup, put the workpiece in a tenon jig, line up the layout line and cut the tenon.

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4.

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Bandsaw Method This is the same idea as the Tenon Jig and Spacer method, except that you use a bandsaw but don't need a tenon jig. The steps are the same. You set the fence with the block in place, cut the inside cheek, remove the block, and cut the outside cheek. (The block will be a different thickness from the one used on a table saw, because the kerf cut by the bandsaw will be narrower.) Don't Cut By Subtraction Suppose you needed a 6" long piece, and you had a piece that was 10" long. Would you measure the 6" workpiece or the 4" cutoff? Most people would intuitively see that they should measure the length that they want, rather than the length that they will cut off. When you cut tenons by cutting one cheek, spinning the workpiece around, and cutting the opposite cheek, you are measuring the part you will cut off but not the part you want to use. You set the distance from the outside of the stock to the tenon cheek, the waste. The tenon is (on paper) what remains after you subtract the waste from the original thickness of the board. Likewise, when you cut tenons by running the stock horizontally over a dado cutter, you set the height of the cutter according to how much wood you want to remove. The thickness of the tenon will be what remains. That may not actually be close enough for a good fit. Setting the jig or dado accurately is necessary but may not be enough, no matter what the calipers say. You have to make test cuts on scrap, but note that the scrap must be exactly the same thickness as the actual stock. Since the tenon is what remains after you remove waste from the sides, the exact thickness of the stock is critical to the thickness of the tenon and therefore, the quality of the fit. If the boards vary a bit in thickness, the tenons will vary a bit, also. Even S4S lumber may not be close enough to produce consistent results by this method. If you have a thickness planer, it's probably best to plane each board to equal thickness all in one setup. Even then, you should probably set up to cut the tenons a bit thick and trim them to fit with a shoulder plane. Second, because you remove the same amount of waste from each side, the tenon will be cut in the center of the stock. Unless the mortise also is perfectly centered, the rail won't line up correctly.

Cutting Tenons Cutting Shoulders and Edge Cheeks Tenon shoulders are best cut on the table saw after the side cheeks are cut. Use the cheek cuts to set up the fence and blade height. That way, you can get nice crisp uniform shoulders and avoid shoulders like the one shown in the diagram. The edge cheeks are best cut on a band saw using a spacer block and a stop block. It may be necessary to adjust the fence repeatedly to cut Figure 2. The to the layout lines. If you cut the tenon directly, rather than by subtraction, you can cut shoulders of this tenon will not fit tenons that need little if any trimming to fit snugly into the mortise and firmly against the line up correctly with the rest of the joint. mortise and the A shoulder plane is extremely useful if you need to do much joint will be very trimming. It must be very sharp and the iron must cut right up against the weak. edge of the plane to avoid the little steps shown in Figure 2. If you get those steps, they can be shaved off with a sharp chisel pressed in against the bottom of the cheek. If you cut mortises with a router bit, you will now have to decide whether to square the round ends of the mortises or round over the corners of the tenons. I prefer the latter approach. I use a fine wood rasp to quickly knock off the corners. Some people use the band saw to cut the corners at 45 E References Kirby, Ian The Accurate Table Saw. Fresno CA, Linden Publishing 1998. Duginske, Mark. The New Complete Guide to the Band Saw. East Petersburg, PA Fox Chaspel Publishing 2007

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