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With This Jig, You Can Use Your Plunge Router to Cut Mortises


bout a year ago, faced with the need to cut hundreds of mortises, I went shopping for a machine to do the job. I discovered that I had neither the moncv nor the space for a commercially made slot mortiser. After thinking it all over and listing my requirements and desires, I realized that I could better satisfy my needs by simply designing and building my own slot mortiser. The resulting mechanism consists of a plunge router mounted horizontally on a sub-base which rides back and forth on shafts that arc fixed to a bench. This slot mortiser takes up little space and has proven to be extremely reliable, accurate and convenient. It even beats the commercial ones when mortising large, unwieldy stock, because the stock itself is fixed during mortising. Only the router moves. This article will explain how it works and how to build one yourself.

How It Works

To be effective, a slot mortiser needs to be able to move or be adjusted, as shown in Fig. 1, in three different directions: 1. L'p and down for the height of cut. 2. In and out for the depth-of-cut. and 3. Side to side for the length-of-cut. 1 designed my slot mortiser with these movements in mind. As I explain how the

This homemade slot mortiser is simply a precision-made jig with a router attached. It's easy to make and use, and it requires far less space than the average slot mortiser.

mechanism moves in these three directions, refer to Figs. 1 and 2 for clarification.

Up and Down—The mechanism can be adjusted up and down bv means of check nuts on a vertical threaded rod located at the bottom of the hearing plate. These check nuts support a slatted aluminum plate screwed to the bottom of the router sidy-base, and when turned, these check nuts adjust the height of the router. These check nuts operate in conjunction with three wriical-adjustmetti locking screws which both guide and secure the router sub-base to the bearing plate.

In and Out The in-and-out movement is performed by the plunging action of the plunge router itself. Depth is controlled by the router's own depth stops.

Side to Side—The router sub-base/bearing plate moves from side to side by means of two linear-motion bearings (mounted on the backside of the bearing plate) that slide along two precision-ground steel shafts. These shafts are mounted within a chassis that is fixed to a work surface. A threaded rod that runs horizontally through the bearing plate and the chassis controls the limits of lateral travel by means of two check mils on each end. Both the lateral-travel rod check nuts and the vertical-adjustment check nuts provide a convenient '/<n-in. adjustment per quarter turn of the nuts.

The chassis is installed under a workbench, table-saw extension, router table or any Hat work surface that allows you to clamp the stock in the necessary position. The lead photo and the photo on page 47 show the jig with and without the router and sub-base in place. Installing and removing the router takes only seconds and doesn't interfere with the adjustments you've set on the slot mortiser.

Just a word about the hardware. The linear-motion, recirculating ball bearings in pillow blocks that ride on the precision-ground, hardened-steel shafts will set you back about $150, but they're worth every penny. They run effortlessly and come equipped with efficient dust seals.

Assembling the Jig

The key to success in combining precision components such as the linear-motion bearings with a wooden chassis is to use a highly stable wood product. The best is I-in. hard maple die-board, a high-quality no-voids plywood. (See Sources.) Alternately, you could make your own plywood, as I did, by laminating thinner "Baltic" birch. Laminate with epoxy to avoid introducing moisture.

The Chassis Ends —The most critical parts that you'll be making are the chassis ends. They ensure that the shafts are absolutely parallel and that the plane of the bearing plate is perpendicular to the bench top.

To achieve the needed accuracy, cut them a bit oversize, mark the inner faces, screw them together, inner-face to inner-face, and work on them as one unit. Trim them to length and width, then lay out the holes shown in Fig. 2. Check that your drill-press table is perpendicular to the drill, and use a fence when boring the shaft holes to keep them the same distance from the edge. The shafts should fit snuggly in their holes.

The Bearing Plate—The router sub-base is mounted on the bearing plate with three locking screws screwed into three threaded inserts mounted in the bearing plate. Drill and install these inserts. If you expect to be dismounting the router frequently, you should con-

Pillow Block Arbor Assembly
Lateral movement of the router is controlled by pillow-block linear-motion bearings mounted on the bearing plate. This plate rides smoothly back and forth on the steel shafts mounted in the chassis ends.


With the router sub-base removed, you can see the basic jig assembly. The grid on the work surface makes layout a snap.



sider Quick-Acting Inserts (available from Reid Tool Supply catalog No. QA-3, see Sources for address). They allow a locking screw to slide in and out by tilting it up. Straighten the screw, and it engages the threads.

To mount the pillow blocks accurately, refer to Fig. 3 illustration. Assemble the pillow blocks on the shafts (being careful of the dust seals), and install the shafts in the chassis ends. Next, clamp the bearing plate to the pillow blocks. Use the pillow-block flange holes as a guide when drilling the Vie-in. holes for the #8 x 1 '/:-in. stove bolts. Countersink the bolts into the plywood bearing plate, and fasten them with nuts and lock washers. When assembled, the bearing plate should ride back and forth between the chassis ends casilv w and smoothlv. Be sure to install the lateral travel-

w limiting rod and plywood cover before screwing on the chassis back.

Mounting the Jig

The mounting studs must line up perfectly with the holes in the bench top to avoid distorting the chassis. One way to achieve this is to use a drilling template. Tack a scrap of plywood to the top of the chassis in the position of the bench top. and drill through this scrap into the chassis for the studs. Then, remove the scrap from the chassis, and clamp it (same-side up) under the bench top to guide the bit when drilling though the bench top.

If your epoxy is the thin, runny kind, paint the inside of the drill holes thoroughly, and leave at least a '/> in. of epoxy in the bottom of each hole. Paint the studs with epoxy. and insert them gently into the holes; don't push them down fast, forcing air into the hole and squirting out epoxy.

If your epoxy is the thick, pasty kind, drill a small vent hole into the bottom of the stud hole, and smear

Windsor Chair Dimensions


(Ajjjait length of router sub-tfase for other thicknesses.)



Set studs in epoxy.






Set rod in epoxy.



A. (2) Thomson V.-ln. die. • 12-in. cUu "L" croud ihjf,

B. (1) Thomson vingk pilow Mock V-In. bore, *SPB-12

C. (1| Thomson sopor ball-bushing twin pillow block V« in. bore, #TWN-12

F. Reid De-SU-Co togile cUmps, *TC-23S4J

G. (7| Trend I win V1.-I8 x V. threaded insert», 'RN3118

Countersink for nuts and washers.





Screw chassis back to chassi« end.




both the inside oi the stud holes and the studs \\ ith the paste, and then push them in. Wipe up the excess and set the assembly aside to cure. Whether vou use the thin epow or the thick epoxy, be sure it is fully cured before mounting the jig and tightening the nuts.

The Router Sub-Base

You will notice that Fig. 2 gives a suggested length for the router sub-base of 18'/: in. and no dimension at all for the center of the recess for the router base. This is because the best location (if the router on the sub-base depends on the thickness ot the bench top. II the bench top that you intend to use is more than 2 in. thick, increase the length of the sub-base by the difference. For example, for a 3-in. thick bench top make the sub-base 19'/: in. instead of 18'/: in.

To lay out the center of the recess for the router, proceed as follows: cut the slots, and then assemble the sub-base to the bearings plate in its lowest position. Mark the router-bit center on the sub-base at about '/j in. above the bench. Transfer the mark to the router side of the sub-base, and cut the recess for the router with a router and circle-cutting jig.

The slotted aluminum plate that straddles the height-adjustment rod is easily cut out with a carbide blade on the tablesaw. Once you've cut the slot, position the plate on the sub-base so that it straddles the threaded rod, and fasten it to the sub-base.


Once you complete the jig, vou need to check it for alignment. There are two ways it can be misaligned: 1. The axis of the rouier shaft can be out of parallel with the bench top. 2. The bearing shafts can be out ot parallel with the bench top. Any misalignment must be corrected by fitting shims between the chassis and


The mortise-and-tenon joint is a good joint because it provides relatively large, areas of flat-grain gluing surface. It is these large, flat-grain surfaces that make it superior to a dowel joint.

The great feature of a dowel joint, which is really just a specialized mortise-and-tenon joint, is that the round hole (the mortise) can be quickly and easily bored and the dowel (the tenon) can be mass-produced.

With an efficient slot-mortiser the advan-tagesof the mortise-and-tenon joint can be com -bined with the advantages of the dowel joint. The combination is a joinery uniquely suited to the small shop producing onc-ot-a-kind or limit ed ■- p rod uc t i on f u rn i t u re or h i g h -e n d ca b i ne t ry.

Traditionally, mortises were the hard part and tenons were the easy part. Today, it is more efficient (and just as strong) to cut two mortises and use a separate tenon. The mortises are quickly and accurately cut with the slot mortise r, and the tenons are easily mass-produced out of scrap or inexpensive wood in whatever sizes the woodworker needs. I usually use elm tor tenons. In my area it is readily available at reasonable cost, machines nicely, glues very well, and it's lough.

I'll make up a special size if the need arises, but I usually use tenon stock 'Ai-in. thick by 1 '/j-m. wide, or Vu-in. thick bv 2V<-in. wide, or

7:-in. thick bv 3'Ai-in. wide. When I run low on one of these sizes. I'll make up a batch.

Making up a batch ol tenon stock is straightforward: 1. Rip a length of rough stock to the rough width ot the tenon, 2. Resaw to just over the desired thickness, 3. Surface to the desired thickness. 4. Joint one edge. 5. Rip to accurate width. 6. Round over the edges on the shaper (or router).

One advantage of separate tenon joints is that parts no longer need to be sized to include an integral tenon. That not only saxes expensive cabinet wood, but also makes the sizing of pieces more straightforward and the woodworker less prone to dumb mistakes. And, once in awhile, the savings in wood makes the difference between almost enough and barely enough. Those are the occasions when I glance over at mvslot mortiscrand smile.—B.At.

Order Application Mary Kay


the underside of the bench top.

The alignments can be checked by chucking a steel dowel pin or even a router bit into the router (with the router unplugged) and measuring the height of the dowel pin above the bench top with a depth-measuring vernier or dial caliper. The height, when the router is all the way to the left, must be exactly the same as it is when all the way to the right; the height must be the same whether fully plunged in or fully withdrawn. If it isn't, mark the corners of the chassis where the measurement is high, loosen the nuts holding the chassis, and insert paper shims to shim the chassis down at the marked corners. Then, tighten it up, and check the measurements again. Repeat as necessary until all four corners measure the same.

Outfitting the Bench Top

Using the jig requires accurately positioning the workpiece on the bench and clamping it there. Reference lines permanently scribed in the bench top are useful, as shown in Fig. 2.1 adjust the lateral travel according to these lines and position the workpiece to them. I suggest that at first you simply use pencil lines. Once you know what kind of a grid works best

Secure rod with'/«-in. plywood cover screwed to bearing plate.

Vf 16 x 19-in. threaded-rod sets in groove '/•-in. wide x Vu-in. deep.

TiN coating provides longer life < in abrasive materials and helps ^ ~ protect the high-speed steel from heat if the bit is used aggressively.

Shank diameters of '/a in. are not common in the metal-working industry. You will want V«-in. and '/i-in. collets for your router to handle mills with '/-»-in. through Vj-in.cuttingdia. More than '/2-in. cutting dia. in end mills requires a collet over '/2-in. in dia. —not something one sees in woodworking routers. Prices from MSC range from $6.11 for an imported '/-»-in. dia. regular length mill without TiN coating to $23.00 for a

Conventional, straight router bits are not particularly well suited for cutting mortises. Specialized mortise bits specifically for wood are available in a variety of designs, but I have never tried them because I get such excellent results from machinist's end mills designed for cutting aluminum.

Two-flute, single end mills for aluminum in regular, long and extra-long flute length with or without TiN (Titanium Nitride) coating are the design of choice. Two-flute mills provide better chip clearance and ejection than multi-flute patterns. The pitch of mills designed for aluminum is good for wood. Long and extra-long flute lengths are more commonly available in mills for aluminum than in mills for ferrous metals.

domestic '/s-in. dia. extra-long (3-in. cut length) mill with TiN coating. Carbide enthusiasts are encouraged to look twice at these prices.

One word of caution; Sharpening any suitable bit reduces its cutting diameter, so if you intend to have bits sharpened rather than simply replace them, you may want to gauge your supply of tenon stock so that you don't wind up with lots of tenons too thick for the slots cut bv the newly sharpened bits. — B.M.

The accuracy of this jig is dependent upon the precision of the chassis ends. To make them, attach them together tempo* rarlly, and then shape and drill for the shafts and travel-limit rod.

Instead of moving the stock, with this slot mortiser you move the cutter and the stock is held stationary. Note the toggle clamps mounted to the work surface, which hold the stock tight.

for your purposes, scratch the lines in with a sharp awl. and fill the scratches with a dark wood filler.

It's important that you be able to position a clamp on the workpiece as close as possible to the mortise. 1 find the most convenient way to clamp is with toggle clamps. (See photo.) I fit them where I want them using a grid of threaded inserts in the bench top. The same grid of inserts is used for holding stops and fences for production runs and even a special 45° sloping table that I've made. Use an all-brass inser t if the bench is one where you commonK use chisels or planes.


THOMSON' INDUSTRIES INC.. Shore Rd. and Channel Dr.. Port Washington. NY I 1050. <800-645-9357): bearings and shafting.

REID TOOL SUPPLY CO.. 2265 Black Creek Rd.. Muskegon. MI 49444. (800-253-0421): knurled check nuts, locking screws, and toggle clamps.

BOULTER PLYWOOD CORP.. 24 Broadwav, Somcrvillc. MA 02145. (617-666-1340): maple die-board.

MSC INDUSTRIAL SUPPLY CO.. 151 Sumnside Blvd.. Plain view. NY 11803. (800-645-7270): end mills.

TRENDLINES, 375 Beacham. Chelsea. MA 02150. (800-343-3248): threaded inserts.

Router Modifications

Since the router operates in a horizontal position, the return springs can be a nuisance. I removed them. The spring-loaded plunge lock on some routers can also be a nuisance. I removed this too. Check the parts diagram lor your router to ligure out how to go about either ol these modifications. Removing these parts doesn't mean that you can no longer use your router separate from this jig. These parts usually are simple to remove and simple to reassemble.

Operating Techniques

Finally, a word or two about technique. I control the rate and depth-ol-plunge by keeping both thumbs against the router sub-base. I also discovered that I have smoother control applying plunging pressure with my bod> rather than with my hands. With end mills, I find it easier to plunge as I eut laterally rather than plunging first and then cutting laterally. Just be sure that you don't force a bit: a sharp bit works best without a slavemaster cracking a bullwhip.

That's it. Go lor it. And let me know if you come up with any improvements. A

Bob Xtonm is a professional woodworker. He owns and of>erates the Rnah Adonai Moods hop in Middle-town Springs. Vermont. He is an exhibiting member of \ennont State Crafts Center in both Middlebury and Windsor, Vermont.

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  • ailie
    What size toggle clamp on slot mortiser?
    7 years ago
  • Nora
    How to make a pillow block bearing?
    7 years ago

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