How is a Mortise and Tenon Joint Used?

The M&T joint is probably the oldest woodworking joint, dating back to the time of the Egyptians and possibly older. This joint has its greatest utility in joining stock at right angles to each other, usually as part of frames of some sort. It. Is particularly useful architecturally (post and beam construction, e.g.). I have read that at least some of the plinths at Stonehenge were joined with stub tenons. In woodworking, the M&T frame captures wide beveled stock for frame and panel construction to allow for seasonal expansion and contraction (doors, lids, sash).

What are the Parts of a Mortise and Tenon Joint?

The mortise has two endwalls (the endgrain parts of the hole), and two cheeks (the inside long grain parts of the hole).

The tenon has two wide long-grain cheeks, at least two endgrain shoulders (the intersection between the tenon cheeks and the faces of the tenon stock). Depending on the style of construction, the tenon edges may have shoulders cut as well, or may have a haunch on the outside edge to accommodate a groove in a frame.

How Do You Set a Mortise Gauge for a M&T Joint?

The mortise gauge generally has two conical points, one fixed and one movable. The gauge is set to the exact with of the mortise chisel you will be using. This is not done by measuring the width of the chisel, but by trial and error adjusting of the gauge and then testing the pin spacing on a piece of scrap stock or on the horn of the mortise stock, using he chisel width as a gauge for success. The very best mortise gauges (not very much more expensive) are those that have a screw adjust mechanism to adjust the pins, separate from the thumbscrew used to adjust the fence. First, adjust the pin spacing by eye to about the width of the chisel. Press the two pins into waste stock and lay the chisel between the points. Basically, you are looking for the chisel width to span the point separation, outside to outside of the prick marks. Once you have achieved this spacing, adjust the fence, by trial and error, until the prick marks are centered on the stock as well as can be expected. Do this by first pressing from one face of the stock, then reversing the gauge and pressing in from the other face. When the prick marks fall into each other or line up, the gauge is centered. Tighten the fence thumb screw. For mortise gauges where the thumbscrew controls both the fence and the movable point, this is like rubbing your tummy and patting your head at the same time—possible but awkward!

Note that one absolute rule of using a mortise gauge (or any other gauge) is to always rest the fence of the gauge on a reference edge or face, consistently.

What Saws Do I Need to Cut a Tenon? Can I Get by With One Saw Only?

Tenons are always on the faces of the stock, for strength, and you cut down the length of the stock (long grain cut), starting at the end of the stock (endgrain end). This is called a ripping cut. The saw teeth for a rip cut are shaped like chisels, filed square across, and function exactly like a chisel in cutting wood. Tenons tend to be deeper and wider cuts, thus tenon saws are often have in the range of 4” wide saw plates. Because joinery operations are generally finer work, the saw plates are thinner, and they have more teeth (or points) per inch—generally 10-12 ppi. Being thin, they are strengthened by a brass or steel back. The saws are relatively long (14” or more) to give more action on the cutting stroke. Because this sawing operation is a bit heavier, the rake on these saw teeth is more vertical (5 degrees?) and the set on the teeth is a bit wider to reduce the chances of the saw plate binding in a deep groove. For larger tenons, one might easily use a panel saw instead (having no back). These saws have a thicker plate, and more set to the teeth to give better clearance for the deep cuts. For these larger cuts, these saws may have fewer teeth per inch (5-6 ppi), and more aggressive rake (0-5 degrees?). For joinery type panel saws, a length of 22 inches is common.

One part of a tenon (sawing the shoulders) involves a cross-cutting action. In these saws, the teeth are sharpened with fleam (like knife bevels) and each tooth slices rather than chops. This is called the crosscut configuration. So, in theory, one would use a crosscut back saw for this work. The saw would be long (more than 14”), but narrow (shoulders are rarely more than 1” deep). The set can be slight since the cut is invariably shallow. The rake would be in the range of 10 degrees (relaxed), and the fleam angle would be relatively slight for hardwoods (5 degrees?). Such saws are often called sash or carcase saws.

In reality, one could use the same tenon saw (or even a dovetail saw) to cut the shoulders. To achieve a “first class cut”, however, you would want to deeply knife the shoulder scribe line, then chop into that line from the waste side to make a vee-groove. This groove allows the tenon saw rip teeth to sit below the stock surface. In this way, the sawing action leaves a clean, scribe shoulder.

What to Do If I Do Not Own a Mortise Gauge?

You can layout the joinery with a regular marking gauge. This process is a bit more finicky, and you must absolutely obey the face-side rule when doing this. In a sense, the width of your mortise chisel defines the second line of a mortise gauge. Never trust that all of our stock is exactly the same thickness! Once you have laid out a M&T joint in this manner, you will definitely be in the market for a mortise gauge!

Press the point of your chisel into a piece of scrap or joint waste stock. Try to align the chisel in the center of the stock as much as possible by eye. Registering the marking gauge on the reference edge of the stock, set the point to just scribe the close end of the chisel mark. This will constitute one edge of the mortise and the tenon. Mark all parts of the mortise and the tenon stock from the reference edges of each. Come back and reset the marking gauge, registering again from the reference face of the stock, but now just hitting the outside edge of the other prick mark. Make a short test mark on the waste to verify that your chisel does in fact just rest within this set of parallel lines. Mark all of the mortise and tenon stock from the reference edges.

My Mortise Gauge Makes Heavy Lines. How to Make It Work Better?

Marking gauges generally come in two cutting edge configurations: point or knife edges. The latter are called cutting gauges and are sometimes used for slicing veneers but are excellent as marking gauges because they slice across the grain, giving a smooth layout line. Gauges with points drag across the grain and can give fuzzy layout lines. Mortise gauges invariably come with points only. The lines that they scribe can be relatively wide (as wide as the stock of the point), thus allow for some ambiguity in placing chisel and saw work. However, the points can be converted to miniature knives easily, to great effect. Remember that for any scribe line on stock, one side of the line is in the “good” side and one side is in the “waste” side of the stock. Right down the center of the line is where you cut or chop. You want the points to be shaped so that the “good” side of the point has a vertical face, and the “waste” side of the point has an angled, beveled face (note: the good side of a point generally faces away from the fence on marking gauges and faces out from the center of the two points on mortise gauges). To shape the points, use a file with a “safe” narrow edge (no teeth). File the good side of the point vertically, halfway through the diameter of the point stock. On the opposite side of the point, hold the file tilted slightly and file in a curving manner around that side of the point. You will wind up with a rounded, bevel edged side of the point, with the cutting edge of the point sliding down each side of the point bit.

Are there Any Rules for Laying Out a M&T Joint?

Chopping a square hole through a piece of stock will undoubtedly weaken it at that point. Sawing off the two cheeks of a piece of stock to make it thinner, will also weaken the stock at that point. The goal is to balance the size of the mortise hole and the thickness of the tenon at the cheeks. For like species of wood, and for joint parts that are the same thickness, there is a rule. “More Than a Third, Less Than a Half” describes the optimal dimension of the tenon member of the joint. The mortise dimensions would then be the reciprocal of that. For example, with ¾” thick stock, “more than a third” equates to more than ¼”; while “less than a half” equates to 3/8”. Thus, in this case, a tenon should be 5/16” thick. From this calculation, all other measurements of the joint follows.

When the species of wood used for the joint differ from each other, or when the thickness of the stock is not the same for both parts of the joint (in apron and leg construction for example), you must use your best judgement and aesthetic perspective to balance these out. Good judgement comes from experience, while experience comes from bad judgement!

Should I Worry About the Unsightly Extra Long Layout Lines When I Layout the Joinery?

Residual layout lines are an unsightly concept only in the Age of CNC machines! For all of the previous history of woodworking they are a sign of careful handwork! In addition, these lines offer a look-back to verify accurate layout, to see where to chop and to saw, and to give you a place to register if you need to do additional work on a joint. In other words, a valuable reference. In the making of a frame and panel, the extra-long layout lines allow you to verify that the mortise actually does fall within the lines. When you saw the tenon cheeks, as you complete the saw cut, you obviate the layout lines, but the extra length shows how well you did the sawing and to evaluate how much tweaking you need to do. Finally, when you seat the tenon haunch in the frame groove, those extra-long mortise layout lines tell you where to place your chisel to pare to make the haunch fit in the groove. In the case of a M&T frame, the outside of the frame is planed smooth to give a good visual edge for the frame, and the inside edges of the frame are hidden by the panel that resides therein.

What is the General Process for Laying Out and Assembling a Mortise and Tenon Joint?

Use a story stick to layout the parts of joint. This will include length and the locations of the mortises on the stiles, and the length and location of the tenon shoulders on the rail stock. Identify the reference faces and edges of the stock. Draw orientation marks on the stock for assembly guide. Layout and scribe in the positions of the joinery. Chop and cut the joints. Either the tenon or the mortise can be cut in any sequence. However, the final width the tenon is measured off the width of the mortise. Do a dry fit, to allow you to dimension and fit the panel. The assembly is glued up. After the glue up, the horns and the broad ends of the tenons are sawn and/or planed off.

How Can I Chop the Mortise?

There are a number of methods or approaches to chopping a mortise. The bottom line is that “You Teach What You Learned” so we will be teaching our style! Assuming that you have laid out the shoulders of the mortise with a mortise gauge and defined the two endwalls with squared-across pencil lines and carried those marks to the opposite edge (for through mortises), then you are ready to chop. Fix the stock to the top of the bench (not in the shoulder vise) with a hold fast of some sort or a clamp. Start the chisel at one end wall, about 1/8” shy of the stock. The bevel of the chisel should be facing the opposite endwall. The chisel should be held vertical, and square to the scribe lines. Chop down with 2-3 mallet strokes until you feel the chisel bottom out. Chop across to the opposite end wall, following the bevel, stopping shy about 1/8” of the second endwall. Reverse and work back. Knock out the chips with the long edge of the chisel or knife them out. Repeat until you have gone down halfway. Reverse the stock and chop from the other side in the same way. At some point, you should break through in the middle. Knock out the waste with the chisel lightly or with a knife. Place your chisel right on an endwall pencil line, position the chisel to undercut very slightly, chop down with 1 stroke halfway. Repeat at the other 3 endwall ends. Use a 4” sliding head square to verify that there are no “bumps” in the endwalls. If there are, undercut those out.

Why Not Drill Out a Mortise, Then Make it Square with Bench Chisels?

Drilling the waste out is a good method for very large mortises (>1”) for mechanical reasons. It gets rid of a lot of waste quickly, and you do not have to manhandle a very wide chisel to do so. You do not often see mortise chisels wider that about 5/8” however. For narrower mortises, a chisel is good because the chopping is straightforward, quick, and the mortise width is a function of the width of the chisel, no additional work or calculations needed. Very much easier to get straight, square, and smooth mortises with a well-controlled chisel than trying to modify round holes.

What Is a Good Way to Clean Up a Tenon After You Have Sawn It?

The strength of the M&T joint lies in the good long grain mating of the tenon and mortise cheeks. Thus, the best way to have them mate well is to saw and chop them well! Nothing more should be done. That is our goal, not always achieved. First, hold the stock up to your eye at arm’s length. Look at the residue of the scribed layout lines and compare this to the surface of each cheek. Look from both long edges of the tenon stock. If the cheek is fat anywhere, mark this in pencil, then thin it carefully with a wide chisel used bevel up. Use slicing, circular cuts with the chisel. Stop and compare your progress frequently. If the tenon is thin somewhere, there is nothing to do but rely on the glue interface and focus more on your sawing skills in the future. You can also use a rabbet block plane to smooth out tenon cheeks. However, you do not want to smooth out the cheeks and make them too thin. A good but rough fit is excellent. Another approach is to use a router (a Stanley 71 for example) set to the depth of the tenon shoulders. Remember that the tenon is not necessarily perfectly centered on the stock, thus you will have to reset the router for each side of the tenon.

How Can I Clean Up a Mortise After I Have Chopped It?

The strength of the M&T joint lies in the good long grain mating of the tenon and mortise cheeks. Thus, the best way to have them mate well is to saw and chop them well! Nothing more should be done. That is our goal, not always achieved. First, verify that the endwalls are square to the long edges of the stock. Use a 4” sliding head metal square, with the bar extended all the way out. Insert this into the mortise and slide it up to an endwall. The square should hit the mortise shoulder lines flush. If not, there is a “bump” (overcut) in the middle of the mortise. Chop this out carefully without striking all the way through. Undercutting is just fine and safe! Do this for all shoulder lines at both side of the mortise. Now look down the mortise and check for chunks of debris. If present, those should be knocked out with a chisel. Resist widening the mortise! If you only see “fluff” in the inside of the mortise, do not worry about this, it is not enough to impede the joinery.

What is a Horn and Why Do You Need It?

The horn is that part of the mortise stock that is left longer than needed at the end of the stock. Chopping the mortise so that it ends close to the end of the stock runs the risk of breaking out that endwall. Since this is a long grain cut, the levering action of the chisel can easily break out a short piece. Adding an extra inch gives strength for the chop, and also for repeated assembly and disassembly as you dry fit. When the frame is assembled and glued up, the horns are cut off and the outside of the frame is pared smooth.

Why is My M&T Joint Not Flush Even Though I Have Chopped and Sawn Carefully?

Assuming that you have done your sawing and chopping squarely, and the pieces fit together well but not flush, then the most likely explanation is that the orientation and reference marks have not been carefully followed. First, verify that you can see all of the orientation marks and these triangles all point in the same direction. Secondly, verify that the reference marks are all on the same relative edges (outside or inside) and faces (top or bottom). Remember that the mortise and the tenon parts of each joint should be laid out from a common reference face and edge, and that these two features are not necessarily (or even likely) to be perfectly centered. If all of your marks agree, but the joint is not flush then either you sawed on the wrong side of the layout lines or you accidentally flipped one of the pieces when you were scribing it. You might try flipping one or more of the frame members to see if another fit works better. If it does, then erase and remark your reference and orientation marks! With no satisfaction, at this point, if you were too thin out the tenon or widen out the mortise to make the joint sit flush, it will also sit very loosely. The best solution will be to do your glue up, then plane the showface (upper face) smooth. You could plane the underface as well, but this is not so critical.

How Do I Fix a Tenon That is Wonky or is Loose?

The first question to ask yourself when a tenon is loose, is “how loose?” The second question is “how precious is this piece of wood?” The easiest answer is to toss the piece and begin again. Always dimension extra stock at the beginning of a project for this eventuality.

If the tenon is very loose, consider gluing the cheeks back on and starting over. You will need to clean up the cheek surfaces so that they will give a good glue surface. Pare with a chisel, flatten with a plane or a router. Reverse the offcut cheek when you glue it back on (rough side out). If you have just a portion of the tenon thin, and you deem this to be an important defect, consider gluing a piece of veneer or thinly sliced stock into that area and pare it down after gluing up. Pretty is not an issue here.

If the tenon shoulders do not sit flush (there is an even gap), there are two reasons for this. Most likely is that the shoulder cuts were not slightly undercut during sawing. Less likely is that the two opposing shoulders are not the same height. You want the scribe line edge of the shoulder to mate with the mortise edge. Clean out the inside corner of the shoulder with a sharp, thin chisel, taking care not to touch the shoulder line. Check that both tenon shoulders are the same height, using a small joinery square.

If the tenon shoulders show an uneven gap, then the shoulders are not square to the tenon stock, or your frame is wracked. You will need to square up the errant shoulder. This also means that you will need to adjust the shoulder on the other side to the same length (even if it is perfect). On top of that, you will need to adjust two shoulders on the other tenon piece of stock so that both pieces have the same shoulder-to-shoulder length. This will ensure that your frame remains square after glue up. Clearly, the best solution is to scribe, chop, and saw your shoulders square.

How Do I Fix a Mortise That is Too Wide or Too Long?

It is difficult to make a mortise too wide, if you have chopped carefully along the layout lines. The number one reason why the mortise may be wide is over-exuberant paring after chopping, attempting to remove “fluff” in the interior. Basically, do not do this. If you need to pare the cheeks of a mortise, make sure you rest your chisel in the layout scribe lines and pare straight down, halfway from either edge of the mortise stock. The easiest way to fix a mortise that is “too wide” is to modify the tenon that goes into it--you can have easy access to those surfaces you need to work on. From one perspective, if the mortise is too wide, by definition the tenon is too thin. Determine which cheek of the mortise is overcut by looking carefully at the residual layout lines of the mortise. The mating cheek of the tenon is the one you will work on. Clean that cheek up to be smooth and flat, using a bevel-up chisel, a router and/or a plane. Recover a sawn-off tenon cheek from the shop floor. Glue that cheek onto the tenon, rough face out. Using a router, carefully pare this cheek face down until the tenon fits snugly. This is trial and error, probably too fine for sawing.

Another approach would be to attempt to wedge the loose joint Use a set of very thin, long, tapered wedges. Wedge in both sides of the joint to ensure that the show face of the joint is flush. Do this during the glue up process.

If you have chopped the one or both of the endwalls beyond the penciled-in layout lines, this is an easy fix. Simply insert long, thin wedges, the thickness of the tenon, into the gaps during glue up.

Why is the Mortise Sometimes Cut all the Way Through, and Other Times Just Partway?

Through mortises (cut all the way through) are generally a stronger joint because of the extra of amount of glue surface between the mortise and tenon cheeks. This allows for extra resistance to wracking the joint. In addition, a through mortise also allows for additional strengthening features like wedging in the endgrain of the tenon. Blind (or stub) M&T joinery, where the mortise is not all the way through, gives a more finished, clean appearance of the joint interface. It also allows you to do decorative edging (molding) around the outside of the frame without having to deal with the mortise opening or the tenon endgrain. The open mortise is more easily chopped and finished out than the closed joint.

When I Chop the Mortise, I Get Tearout on the Opposite Side. How Do I Prevent This?

There are two reasons that a through mortise shows tearout on the opposite side. When you chop a mortise, the goal is to chop halfway from both sides, meeting in the middle. When you are chopping in from the second side, however, you can inadvertently allow the chisel to plunge through the whole way, striking the opposite edge of the hole, creating tearout. This is very easy to happen since there is almost no resistance to the chisel cut in the end stages of the cut. The solution is to maintain good chisel awareness and control. The other reason tearout happens is from not cleaning out the waste frequently enough during the chopping process. The waste can become tightly impacted in the bottom of the hole and the chopping action can push a bolus of waste out the bottom, carrying the outside edges of the hole with it. The solution is to remove waste from both faces of the hole frequently, so that the residual waste is just in the center of the hole and this small amount of waste should be able to be easily poked out at the end.