If you’re planning to build a projectwith mortise and tenon joinery,you may want to consider thisquick and easy variation

instead — a mortise and tenon jointwith a “loose” tenon. It has all thestrength of a traditional mortise andtenon joint without the fussing aroundthat’s involved in getting a good fit.

As you can see in the Photo atleft, a mortise and loose tenon jointdiffers from its familiar namesake ina couple of ways. First, there are twomortises — one in each matingpiece. And second, the tenon is

machined as a separate part (hencethe term “loose”) and then gluedinto the mortises. This creates a largeglue surface, which makes for anexceptionally strong joint.

Besides its strength, though, thereal beauty of this joint is it makesaligning the two pieces virtuallyfoolproof.That’s because both mor-tises are cut using a single setup.Specifically, a plunge router with aspiral upcut bit and a simple jig.Thejig, which is explained on the nextpage, ensures identical mortises, andas a result, perfectly aligned pieces.

router workshopMORTISE & LOOSE TENONS

{ TTTo make a mortiseand “loose” tenon joint,cut matching mortises intwo pieces. The “loose”tenon is machined sepa-rately to fit the mortises.

60

build a mortising jig

NOTE: Locate clamp blockso opening is centeredon thickness of stock

#8 x 3" FhWoodscrew

End Block( " x 1 " x 6")#/4 #/4

%/32" hole,countersunk

Clamp Block( " x 3" x 4")#/4

#8 x 1 "Fh Woodscrew

!/2

Center Strip( " x " x 1 ")#/4 %/8 !#/16

End Block( " x 1 "

x 6")#/4 #/4

Centerline

4"

3"

&/64

%/8

" hole," deep

%/32" hole

%/8" GuideBushing

!/2" SpiralUpcut Bit

FORMULA: Jig opening equalslength of mortise, minusdiameter of bit, plus "%/8

Resultingmortise size

2!/4"

2#/8"

CenterStrip

End Block

End Block

Side DetailJig Opening Detail

A plunge router makes quick workof routing mortises.All you need isa stable platform to prevent therouter from tipping and an accurateway to guide the router bit.

This T-shaped jig accomplishesboth things.The top of the jig pro-vides a solid platform for the router.And an opening in the jig serves asa template for the mortise. In use, aguide bushing that’s mounted in thebase of the router “follows” this tem-plate, ensuring identical mortisesevery time.

The thing to keep in mind is it’sthe guide bushing,not the router bit,that rides against the sides of theopening. This means the openingmust be slightly larger than thedesired size of the mortise.

There are several factors to con-sider when sizing the opening: thewidth and length of the mortise, thesize of the bit, and the outside diam-eter of the guide bushing. Use theformula in the Jig Opening Detailbelow to determine the correct sizeopening.Note:This opening is sized

to create a 1/2" x 21/4" mortise whenusing a 1/2" bit and a 5/8" bushing (thesame size as the mortises in theCraftsman table shown on page 52).

Jig Basics — The jig itself isquite simple.The top consists of fourpieces: two end blocks and twocenter strips that form the opening(see Box below for more on this).A cen-terline marked on one end block isused to align the workpiece fromside to side. And a clamp blockscrewed to the top positions theworkpiece from front to back.

on bits & bushingsCONSTRUCTION VIEW

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1

Once the jig is assembled, routing the mortises goes likeclockwork.You simply tighten the workpiece in a vise,clamp the jig against it,and rout back and forth (Photo, left).

Keep in mind that the opening in this jig (Inset Photo)is sized to make the mortises in the Craftsman table. Ifyou plan to rout a different-sized mortise, it requiresanother jig — with a different-sized opening.And as Imentioned earlier, before making the jig, you’ll need todetermine the size of the mortise.A few guidelines fordoing that are explained in the Illustration below.

Lay Out Mortises — Regardless of their size,though, the procedure for laying out the mortises is thesame.The idea is to mark the center of the mortises onthe mating pieces (Figs. 1 and 2) and then align thatmark with the centerline on the jig (Fig.3).Matching upthe centerlines like this ensures that the mortises willalign with each other.

Routing the Mortises — With the layout complete,it’s time to rout the mortises. Install the guide bushingin the base of the router and mount the spiral upcut bit.(The Sidebar on page 63 shows why this type of bit isspecially suited to routing mortises.) Then set the turreton the router to make a series of progressively deeper cuts.

Next, set the router on top of the jig, carefully fittingthe guide bushing down into the opening. Now graspthe router, flip the switch, and plunge the bit into thewood until the stop rod contacts the turret.Rout to oneside until the guide bushing contacts the end of theopening, and then rout back to the other end. Repeatthis process for the remaining depth settings, plungingand routing until the mortise is completed.

Workpiece

Mark CenterlinesTo lay out the mortises, dry assemble thepieces and mark centerlines across the jointline. Mark mating joints “A-A,” “B-B,” etc.,so you can reassemble the pieces.

Transfer the MarkNow use a square to transfer the markfrom the face to the end (or edge) of theworkpiece. This line will be used toposition the jig, as shown in Fig. 3.

Align the JigTo ensure accuracy, align the centerlineon the jig with the centerline on the work-piece. Then clamp the jig onto the work-piece, and you’re ready to rout.

21 3

lay out, then routMORTISES

{ TTWith the T-shaped mortising jig clamped tothe workpiece, make a series of progressivelydeeper passes, routing from one end of theopening to the other to cut the mortise (Inset).

LENGTH: Two-thirdswidth of workpiece

FrameRail

FrameStile

FrameStile

Length

Depth

LooseTenonWidth

WIDTH: Roughly half thethickness of the workpiece

DEPTH: !/32" deeper thanhalf the length of the tenon

SIZING MORTISES

lessons forLOOSE TENONS

!/2"

!/4"round-over

bit

End Detail

Once the mortises are cut, you canturn your attention to the loosetenons.The tenons are cut, one at atime, from a long blank of “tenonstock” (Photo, right).That part is easyenough, and I’ll get back to it in aminute.The trick is making the tenonstock so it fits into the mortises.

Rip Stock to Width — Toaccomplish that, the first step is to ripa piece of hardwood to width.Theidea is to rip the piece 1/16" narrowerthan the length of the mortise.Thisprovides a “fudge factor” that will letyou make minor adjustments whenit’s time to glue the pieces together.

Thickness Stock — The nextstep is to plane the tenon stock tothickness to match the width of themortise (1/2", in my case).The goal isto have the tenon slip into the mor-tise with a friction fit. So take sometime to “dial in” the planer to get apiece that will fit just right. I use anarrow scrap piece as a test piece,planing it along with the tenon stockand using it to check the fit.

Of course, even if the thickness isdead-on, the square edges of the

tenon stock won’t fit the roundedends of the mortise.

Round the Edges —The solu-tion is to round the edges of thetenon stock to match the radius atthe ends of the mortise.A roundoverbit mounted in a router table makesquick work of that (see Illustrationbelow).Use a bit that’s half the size ofthe thickness of the tenons. (Forexample,my tenons were 1/2" thick,so I used a 1/4" round-over bit.)

Trim Tenons to Length — Nowyou’re ready to cut the tenons tolength.You’ll want to make them 1/16"shorter than the combined depth ofthe mortises.This allows for excessglue at the bottom of the mortises.

To ensure identical length tenons,clamp a block to the rip fence sev-eral inches in front of the saw blade(Photo, above).Then set the fence sothe distance from block to bladematches the desired length of thetenon. Butt the end of the tenonstock against the block, and using amiter gauge,push the workpiece pastthe blade.Repeat this process to trimeach tenon to length.

{ TTFor identical-length tenons, butt the tenon stockagainst a block clamped to the rip fence. Then usethe miter gauge to guide the stock past the blade.

Spiral Upcut BitsWhen it comes to routing mortises, there are three bigadvantages to using a spiral upcut bit (Photo, below).

TThhee TTiipp ooff tthhee BBiitt CCuuttss —— First of all, its cuttingedge extends across the tip of the bit, which allowsit to “drill” into the workpiece.

SSmmooootthh CClleeaann CCuuttss —— Another plus is the spiralcutting edges of the bit shearthe wood fibers, producinga smooth, clean cut.

CChhiipp EExxttrraaccttiioonn —— Finally,the flutes of the bit pull chips upout of the cut (toward therouter), so they don’t clog themortise. This reduces heatbuild-up, which candamage thebit.