I may be a power tool kind o' guy, but nevertheless, I favor traditional joints, those proven through centuries of use. The mortise-and-tenon is one. It has been in use not simply for hundreds of years, but for thousands of years, with examples found in ancient Egyptian furniture.
Woodworkers have come up with many ways of cutting both the mortise and the tenon, ranging from chisels, mallets and handsaws to single-purpose machines costing thousands of dollars. I've used a number of different methods over the years, but the one I come back to again and again is router-oriented.
Instead of cutting a mortise in one part and a tenon on the other, I rout identical mortises in both parts to be joined, and connect them with a fitted strip of wood called a loose tenon. (You may know it as a floating tenon or a slip tenon.)
Here's the nub of it: fitting a tenon that's integral to, for example, a rail to a mortise routed in a stile is jamming a square peg in a round hole. Just to get to the nub, you've set up two different power tools. One cuts the mortises, the other cuts the tenons. And now, at the nub itself, you're fiddling with a chisel or a file to round off crisp corners. Bah! It's too much monkey business.
Loose tenon joinery is versatile. I've used to it construct tables, a chair or two, post-and-rail casework, a workbench, cabinet doors, even architectural doors.
Loose tenon joinery is surprisingly easy. No special-purpose tools are required. A commonplace plunge router equipped with an edge guide and a plunge-cutting straight bit does the heavy cutting. The key is a fixture called a mortising block, which holds the workpiece and guides and constrains the router's movements.
You build your own mortising block. (I swear you can do it in a weekend.)
The piece to be mortised is clamped to the face of the block. The router sits on top of of the block with the bit centered over the work. (Interchangeable work-holders -- one horizontal for edge mortising, the other vertical for end mortising -- simplify orienting and clamping workpieces.) The edge-guide fence slides in a track along the block's back edge, ensuring the router moves in a straight line. Adjustable stops atop the block limit the router's travel, thus controlling the length of the mortise.
One setup serves for both edge and end mortises. You switch work-holders to change over from edge to end, and you can do that without impacting the router stops or the edge-guide setting.
The upshot is that the system ensures you get uniformly sized mortises.
Only after all the mortises are cut do you make tenons. You mill strips of stock to just the right thickness, rip it to just the right width, round the edges to perfectly match the mortise ends, and then crosscut them to length.
Mortising Setup, Step-by-Step
A mortise for a conventional right-angle joint is set up this way:
- Width of the mortise is determined (usually) by the diameter of the bit used
- Depth of the mortise is controlled by the router's plunge depth setting
- Position and alignment of the mortise is controlled by the edge guide
- Length of the mortise is governed by router stops on the block
1. Lay out a mortise on the working stock. It's most efficient to start with an edge mortise rather than an end mortise. The layout need not be elaborate: just lines marking the ends and a centerline between them. What's essential is a setup line (the red line) across the stock midway between the mortise ends. You align this line with the registration line on the mortising block. (Once the setup is done, the setup line is the only layout needed for every edge mortise.)
2. Set the laid-out workpiece on the work-holder, and adjust the holder so the top edge of the work is flush with the block's top. Loosen the mounting bolts and move the work-holder up or down. The holder's keys, riding in the dadoes in the block, ensure the holder stays parallel to the block's top surface when you adjust it. Retighten the bolts to lock the holder in position.
3. Line up the registration line on the workpiece with the one on the block. Adjust the spindles on the toggle clamps to hold the work securely. Snap the toggle clamps to secure the setup piece.
4. Pick a suitable bit. Up-spiral bits are popular for mortising (I favor them), but they are costly. A plunge-cutting two-flute straight bit will do the job. Bear in mind that the shank size impacts the depth to which you can rout in some cases. Any diameter cutter (smaller than 1/2") on a 1/2" shank is going to have a shoulder that limits the depth to which you can cut. Conversely, using a 1/4"-shank cutter to rout a 1/2"-wide mortise is problematic. If you are doing deep 1/4"- or 5/16"-wide mortises, use a 1/4"-shank bit (left) because you can actually plunge deeper than the bit's specified cutting depth. For 3/8"- to 7/16"-wide mortises, maximize your reach without unduly stressing the bit by using a 3/8"-shank bit with a collet adapter (center). At 1/2" and beyond, of course, use a 1/2"-shank bit. You'll find dramatically longer cutters are available in that diameter.
5. Chuck the bit you are using in the router. Here, I am using a 3/8" up-spiral bit with a collet adapter. I'll seat the rim of the adapter against the collet, of course.
6. Fit the edge guide to the track. You can use virtually any edge guide with the mortising block, but to move accurately and freely, the guide must have a facing that fits snugly in the track. Fine-tune the fit before you mount it to the router.
7. Bottom the bit against the work, and set the router's plunge depth for the desired mortise depth. Rather than trust the accuracy of the router's adjustment scale, I usually use gauge blocks between the stop rod and the stop turret to set the depth. The three gauges here add up to 1-1/4", the depth of the mortises I want.
8. Loosen the edge guide and move the router fore and aft to center the bit on the mortise centerline. Look closely and turn the bit as necessary as you move the router; when the tips of both cutting edges (assuming you're using a two-flute bit) are on the line, lock down the edge guide. You want the most precise setting you can achieve.
9. Slide the router to one side, positioning the bit just inside the end of the mortise. Butt the router stop against the base, and tighten the stop's mounting bolt.
10. Slide the router to the other end of the mortise, again positioning the bit inside the end mark. Set the stop against the router and lock it.
11. Make a shallow test cut to check the accuracy of your setup. With the router in place, edge guide in the track and bit clear of the work surface, plug it in and switch it on. Move the router against a stop (either one) and plunge the bit about 1/8" into the work. Simultaneously, feed the router toward the other stop. As soon as it hits the stop, retract the bit, switch off the router and, after it has wound down, lift the router off the block and take a good look at the cut. Is it centered? Does it touch the end-lines you laid out? This is the time to shift the edge guide or move the stops.
That's All It Takes
All the mortises routed with the setup will be the same, regardless of where they fall on the workpiece. Just scribe a setup line for each mortise, and align it with the registration line on the block.
You also must mark a reference face for each part, and consistently orient that face, so it is always against the block. Or always exposed. The choice is yours, but be consistent.
The problem this procedure addresses is misalignment of faces. You assemble the joint with a perfectly crafted tenon and discover the faces aren't flush. There's a step, a ledge where the parts meet (see photo below). At that point, the fix is to sand the assembly until its faces (both front and back) are flush. Better is to avoid the problem, and you do that by keeping track of a reference as you rout the mortises.
The truth is that centering a mortise on the edge is darn hard. If you have a precision edge guide that allows adjustments in thousandths of an inch, you can use machinist's calipers to compare the shoulder widths of a test mortise and make them equal by adjusting the edge guide. Micro Fence makes the only such edge guide that I know of.
Absent such an edge guide, the workaround is to mark a reference face, as I explained. Orient that face consistently, and the mortises will be uniformly positioned. If they're off-center, as the photo below, all will be off-center toward the same face. When you assemble the joints, the faces of the parts will be flush with one another.
The horizontal workholder has three toggle clamps on it, one aligned with the registration line, the others flanking it. As you can see in the photos below, this enables you to rout two (or more) mortises in a workpiece without turning it around; instead, you slide the piece, thus keeping the all-important reference face against the block.
The one hitch in the workaround affects the end mortises. If the mortise isn't intended to be equidistant from the edges (as in the top photo below), you will have to rout one mortise in each part, then shift the workholder before routing the second mortise in those parts. Accurately marking a setup line for every mortise will alert you that adjustment is needed if it doesn't align with the block's registration line (bottom photo below).
Now, before we swap workholders, rout all the edge mortises your project requires.
Routing, Step-by-Step
You want to excavate the mortise quickly, making a series of light cuts. Ideally, you want to begin with the router against one stop, stab the bit into the work, feed quickly to the opposite stop, then retract the bit from the cut as you return the router to your starting position.
You don't want the bit to dwell in the cut, since that generates heat. Heat breaks down the cobalt binder holding the carbide particles, and as the particles separate from the binder, the bit dulls. Some woods -- maple and cherry come quickly to mind -- scorch easily, and scorched surfaces don't glue well. So you want to cut, then get that bit out of there.
Using a router with dust extraction is good in this regard (among others). Chips packed in the cut around the bit contain the heat. Pulling out those chips as they're generated helps lower the temperature. An up-spiral bit helps pull chips out of the cut, but a router with dust extraction does a far superior job.
Also, it's a whole lot easier to see what you're doing without the chips, as the comparison photos above show. Moreover, when chips are flying, they can collect between the router base and the stops, screwing up the accuracy of your cuts. And when you complete a mortise, you don't need to brush away chips so you accuractely position the next workpiece. Capturing the chips as they are created is altogether wonderful.
(Just as an aside, I want to explain that, while I generally use a router with dust extraction capability for mortising -- didn't I just say it is wonderful -- I didn't use it during the photo session for this article because the shrouding around the bit obscures what's going on (see the photo above). When you're in a production mode, routing mortise after mortise, what's going on is controlled by the setup; you don't need to see it up close. But to understand the process, you do need to see.)
If there is difficulty in routing mortises, it's choreographing the moves. Mortises generally aren't very long --2", 2-1/2", maybe 3". The feed stroke between the plunge and the retraction is short, and until you acclimate yourself to the router controls and the dance steps, you can feel awfully clumsy.
You can use the router's adjustable turret to stage your cut depths, but that probably will feel like an intrusion in the dance. (It does to me, anyway.) I guesstimate the depth of each pass. If feed resistance seems high and bit chatter particularly resonant (even with muffs on), I'll back off.
In the end, just remember the steps: plunge -- feed -- retract -- return, plunge – feed – retract -- return.
Here's a typical routine, step-by-step:
1. Set workpiece on the work-holder, aligning the registration line on it with the mortising block's setup line. Snap the toggle clamps closed.
2. Set the router on the mortising block, its edge guide in the guide track and its bit over the workpiece. Slide it against the left-hand router stop.
3. Switch on the router. Plunge the bit about 1/8" into the work and feed the router to the right.
4. When the router hits the right-hand router stop, retract the bit from the cut. Return the router to its starting position against the left-hand stop.
5. Replunge the bit, deeper, to about 1/4", and again feed the router to the right-hand stop. Retract the bit and return to the starting position.
6. Repeat the routine, cutting fractionally deeper with each pass, until the plunge bottoms, meaning you've cut the mortise to the desired depth.
Regardless of the mortise -- either edge or end -- this is the routing routine. The trick is to develop a smooth rhythm of movement, so you keep the bit moving while it cuts. You don't want to overheat the bit -- and scorch the cut surfaces -- by allowing the bit to "dwell" in the cut. Take a break if need be; retract the bit and just take a breath. Relax. Then continue routing.
End Mortise Changeover, Step-by-Step
A loose tenon joint requires a mortise in each half of the joint. So, having completed the edge mortises means you've completed one-third of the job. Now you must rout the end mortises.
Switching from edge mortising to end mortising requires only a workholder swap. Nothing in the setup changes, so your end mortise will duplicate the edge mortises.
1. Mark the mortise setup line on the end of a workpiece. With the horizontal work-holder removed from the mortising block, align the setup line with the registration line on the mortising block, and clamp the workpiece to the block.
2. Hang the vertical work-holder on the block, slide it up against the workpiece, and tighten the mounting bolts. With the workholder's key caught in the groove for it in the block, the holder will be perpendicular to the block's top surface.
3. Snap the toggle clamps onto the workpiece. Remove the clamp. Now you are set for the end mortises.
4. Set the router on the block and rout away. Since the workpiece's setup line is aligned with the block's registration line, and because neither the router adjustments or the stop settings were changed, the mortise matches all your edge mortises. Moreover, if the end mortises are uniformly positioned on the workpieces, you don't even need the setup lines on them.
Making Tenons
The common way to produce the loose tenons is to mill strips of stock to the approriate thickness. Rip the strips to width, then round the edges with a round-over or small half-round bit. Crosscut the individual tenons. Some craftsmen rout grooves in their tenons to provide a reservoir for excess glue; I don't bother with this extra step.
I usually use scraps of the working stock to produce the tenons. But the tenons don't have to be the same species as the parts being joined; they won't be visible after assembly.
It's worth pointing out that it's important to get the fit right. No matter how well-proportioned the joint is, if it's too loose, it won't glue well. Conversely, if it's too tight, you may not be able to seat it once glue is applied.
How close does the fit have to be? A glue technician may tell you that, for glue to bond well, the gap between tenon cheek and mortise cheek can't exceed 0.005". That's five thousandths of an inch, a machinist's sort of tolerance.
A machinist's dial caliper may have some value in zeroing in on a tenon thickness. But you can get the right fit by feel. You want to be able to press the tenon into a dry mortise (no hammering), yet have to work to separate the parts. Just that will yield the tolerance needed for a good glue bond.
But there's a little more. To glue well, the mating surfaces must be flat, smooth and clean. Glue must penetrate the surface of the wood if it is to cure to its maximum strength. A burnished surface -- the sort you get with a dull cutter or a too-slow feed rate -- doesn't allow the glue to penetrate.
Finally, if you want to be really compulsive about achieving the best fit and the best glue bond, you should assemble the joints the same day you cut them. Joints that fit well dry one weekend may fit poorly the next, thanks to weather changes and wood movement.
1. Plane tenon material to fit the mortises you've routed. Select sticks that are at least as wide as the mortises; otherwise, the width of the pieces is unimportant. Joint a face to ensure the material will be flat, then plane, fitting the stock to a mortise to monitor you process. When a corner fits snugly into the mortise, you're done planing.
2. Measure the maximum width of the mortise, and rip a trial piece to width. I tend to cut it a skosh wide. It is so much easier to plane off a shaving or two to get the perfect fit than it is to rectify looseness.
3. Compare the tenon width to the mortise. Here the tenon is about a sixteenth overwide, which is fine for a trial tenon.
4. Round over the tenon edges on the router table. You can use a round-over bit, a bullnose bit, or, as here, an edge-beading bit. Adjust the fence carefully so you don't inadvertently trim the tenon's width as you rout.
5. Check the tenon against the mortise. This still doesn't fit; it's too wide. But I am happy that the routing operation didn't impact the original width of this sample. Thus, I know my router table setup is spot-on. (Remember that this is a trial tenon. I have thicknessed stock, and my table saw is still set for the baseline rip that produced this sample. I'll tweak its fence position before ripping the stock for the rest of the job's tenons.)
6. Tweak the sample with a block plane. Clamp it in a vise and take a few swipes. Check it against the mortise once again. When it appears to be the exact width, re-rout the planed edge.
7. It fits! The tenon enters the mortise readily and can be bottomed with little effort.
8. After crosscutting the sample to about 1/8" less than the combined depth of the mortises, the joint goes together. The 1/8" total depth clearance provides a modest reservoir for excess glue. I apply a thin coating of glue -- emphasis on thin -- to both mortise walls and tenon cheeks; insert the tenon in one mortise, seating it; scrape squeeze-out from the shoulders; then fit the mating part on the tenon. And clamp, of course.
Mortising Block Construction
And now you want the details of the mortising block. What's it made of? What are its dimensions? How's it constructed? The answers to these questions follow.
Click here for the Mortising Block Elevation Drawings (front elevation, top view, back view, end view, router stop, horizontal work-holder, vertical work-holder)
1. Mill the stock for the parts to the specified thicknesses; exceptions -- the guide-track bottom and the keys -- are noted on the Cut List. Rip and crosscut them to final dimensions; again, exceptions are noted on the Cut List. The body and the horizontal work-holder must be exactly the same length. You reference the ends in routing the keyways, and if one piece is slightly longer than the other, the keyways won't align.
2. Rout keyways in the body and the horizontal work-holder. These are through dadoes, 3/4"-wide by 1/4"-deep, located 3" in from each end of both the body and the work-holder.
3. Rout a keyway in the body and in the vertical work-holder. These through cuts also are 3/4" wide by 1/4" deep; they are located 5 1/8" below the body's top edge and the work-holder's top end.
4. Using a drill press (to ensure the holes are perpendicular to the work-holder surfaces), drill 7/16" dia. holes through the work-holders, delineating the ends of the mounting-bolt slots. The slots are centered 4-1/2" from the ends of the horizontal work-holder, and 3-1/2" and 6-1/2" from the top end of the vertical one. Use a plunge router and edge guide to cut the slots between each pair of drilled holes. Rout about halfway through the stock, then turn the stock over and complete the slots from the second face. Capture the workpiece between scraps of the same-thickness material to provide stable support for the router as you work.
5. Thickness stock to match the width of the keyways in the work-holders and the body. Rip 1/2"-thick strips of it and crosscut pieces to match the work-holder widths. Attach the keys to the dadoes in the work-holders with screws.
6. Place the vertical work-holder on the body and clamp it. Use a transfer punch (or just a drill bit) to transfer each slot location to the body. Remove the work-holder. Line up a square with one mark, and scribe a horizontal line across the body. Repeat at the second mark. Measure 3/4" to each side of both vertical dadoes and, with the square, scribe lines crossing the horizontal lines.
7. Drill 5/16" dia. holes at the eight spots where the horizontal and vertical lines cross. Do this on the drill press. Next, use a conventional 3/8"-16 tap to cut threads in the holes. No cutting fluid is needed.
8. Glue and clamp the top support to the mortising block body. After the glue sets, scrape off any squeeze-out and joint the top surface with the body's face against the jointer fence.
9. Cut an overlong blank for the two router stops. Rather than work the small router stops individually, make the necessary cuts in a single long piece of 5/8" by 3" stock. In the same way you slotted the two work-holders, make slots for the stop mounting bolts. That is, drill 5/16" dia. holes through the blank delineating the ends of the slots. Then use a plunge router and edge guide to open the slots between the pairs of holes.
10. Cut the router stop keyways, making them 1/2" wide and 1/8" deep. Use the same setup to groove both the mortising block and the stops so the cuts align. Separate the stops.
11. Make keys and attach them to the stops.
12. Lay out and drill mounting bolt holes in the top support. Put a stop in place and use a transfer punch in the mounting slot to mark the body's top. Scribe a line through the mark along top. Measure and mark six spots on this line 2" apart, beginning 1" from each end (see Top View Drawing). Drill a 13/64" dia. hole, at least 1-1/4" deep, at each mark. When all six are completed, use a 1/4"-20 tap to cut threads in each hole.
13. Mill and crosscut the two guide track parts. The bottom must be only a few thousandths thicker than your edge-guide's wooden facing; the facing must track smoothly without play or chatter. Clamp these parts to the back of the top support, drill pilot holes, and fasten them with screws.
14. Glue the clamp bar to the mortising block.
15. Measure and mark the center of the mortising block. Guided by a square, scratch a line down the face of the block and across the top. Carefully highlight the scratched line with a fine-tip pen. This is the registration line.
16. Ordinary hex-head bolts are the inexpensive means of mounting and holding the work-holder and the router stops to the block body. Using bolts means you must use a wrench to adjust or swap holders and the stops. For tool-free adjustment, make studded knobs to use instead of bolts. Buy a pair of plastic knobs with the appropriate size stopped inserts and a length of all-thread. Cut pieces of all-thread and turn a knob onto each, using thread locker. Whether you use bolts or studded knobs, I recommend using a large diameter fender washer and a regular flat washer on each bolt.
17. Toggle clamps are not essential, but good ones are not inexpensive. You can use common woodworking clamps to secure workpieces to the mortising block. I'd suggest you forego the toggle clamps until you've gotten some experience with the setup and are really sold on it. (On the other hand, these clamps are handy for all sorts of jigs and fixtures, and you can easily move them from one jig to another as the need arises.)
Mortising Block Cut List
Piece |
Number
|
Thickness
|
Width
|
Length
|
Material
|
|
Body
|
1
|
1-1/2"
|
9"
|
16"
|
8/4 hardwood
|
|
Horizontal work-holder
|
1
|
1-1/2"
|
6"
|
16"
|
8/4 hardwood
|
|
Work-holder keys*
|
2
|
1/2"
|
3/4"
|
6"
|
4/4 hardwood
|
|
Vertical work-holder
|
1
|
1-1/2"
|
4-3/4"
|
12"
|
8/4 hardwood
|
|
Work-holder key*
|
1
|
1/2"
|
3/4"
|
4-3/4"
|
4/4 hardwood
|
|
Top support
|
1
|
1-1/2"
|
2-5/8"
|
16"
|
8/4 hardwood
|
|
Router stops**
|
2
|
5/8"
|
3"
|
4"
|
4/4 hardwood
|
|
Router stop keys*
|
2
|
1/4"
|
1/2"
|
4"
|
4/4 hardwood
|
|
Guide track bottom†
|
1
|
3/8"
|
1"
|
16"
|
4/4 hardwood
|
|
Guide track side
|
1
|
3/4"
|
2"
|
16"
|
4/4 hardwood
|
|
Clamp pad
|
1
|
1-1/2"
|
3 3/8"
|
22"
|
8/4 hardwood
|
*Thickness these parts to fit after grooves for them are cut.
**Cut the necessary groove and slots in an overlong strip of material; crosscut to length last.
†Make this part 3 to 5 thousandths of an inch thicker than your edge-guide's wooden facing.
Hardware for mortising block
| hex head bolts w/washers |
2
|
3/8" x 2-1/2"
|
mount work-holder to block
|
|
fender washers
|
2
|
3/8" i.d.
|
|
hex head bolts w/washers
|
2
|
1/4" x 1-1/2"
|
mount stops to block
|
|
fender washers
|
2
|
1/4" i.d.
|
|
Flathead wood screws
|
4
|
2" #8
|
mount track to block
|
Optional
| toggle clamp |
5
|
DeStaCo 225-U*
|
for work-holders
|
|
spindle
|
5
|
DeStaCo 225208*
|
longer (2+") spindle for above clamp
|
|
check nut
|
5
|
Reid Supply V-3*
|
tool-free spindle adjustments
|
|
wing-nut
|
5
|
5/16"
|
tool-free spindle adjustments
|
|
panhead screws
|
20
|
1" # 1
|
mount clamps on work-holders (4 per clamp)
|
*Available from Reid Supply Co.
800.253.0421
www.reidsupply.com