Most woodworking jigs fall into two categories:Those you use once and then pitch, and thoseyou use over and over again. This mortisingjig falls into the latter group. And if longevity and amountof use a jig gets is any indicator of value, consider thisjig worth its weight in gold.
More than 20 years ago I made a version of this jig asa companion to my then-new plunge router (shown inthe photo). The concept was shown in an early wood-working book by The Taunton Press by venerable wood-worker Tage Frid. Both the jig and the router continue tosee service in my shop, although I made a new jig for thisarticle and added a couple new features.
Over the years, Ive cut mortises by hand, with thisjig and using a variety of benchtop mortising machines.I find one big advantage of router-cut mortises is thesmooth wall of the mortise itself, which is certain to pro-duce a stronger glue joint than those made with a chiselor mortising machine. The smooth surface is especiallyadvantageous given the cross-grain condition of typicalmortise-and-tenon joints. The jig also sets up quickly andprovides a high degree of repeatable accuracy. Combine
the unit with an aftermarket router fence such as theMicro Fence (www.microfence.com) and set-up time iseven faster. One downside to all router-cut mortises is theround, rather than square, ends produced. The problemis overcome most easily by rounding over the square cor-ners of the tenons using a file.
Once the mortise location is drawn on a part, yousecure your work in the jig using the built-in clamps. Lowerthe router bit to the height of the part and adjust therouter fence to align with the mortise location. Theforward and backward travel of the router is quick-ly limited by the jigs adjustable stops on the top ofthe jigs side. Set the depth of plunge on the routerand youre ready to cut your mortise. Remember topull the router toward you when making cuts; thisuses the rotation of the bit to hold the router fenceto the jigs side.
Jig ConstructionBuilding the jig is simple and should take an afternoon.When done, youll be able to rout mortises for a set oftable legs, for example, in less than 15 minutes, in-cluding set-up time.
Buy your hardware before building the jig. It is eas-ier to drill the holes and install the barbed inserts in theside of the jig before assembly. These are placed in a 38"hole thats 2" deep on the top edge of the side.Take careto drill in the center of the plywood. Next, drill out a 14"hole in a length of 12" dowel that will both plug the holeand further secure the insert yet allow the bolt to ac-cess the buried insert. Now drive in the threaded insert.
by Steve Shanesy
Make quick, easy and accurate mortiseswith this jig and your plunge router.
Its a good idea to place the sidein a vise to prevent the plywoodfrom splitting during this opera-tion. When done, chase the holewith a 12" drill bit to the depthof the insert so the dowel willfit the hole. Next, glue the dowelin place. Go easy on the glue soyou dont fill the inserts threadswith glue. Apply the hardwoodedgebanding on the tops of thejigs sides.
When making the bottom,first glue up an oversized double-thickness plywood blank. Cutit to final dimension, making sureyour table saw blade is dead squareto the saw table. This will ensureyour side parts will be square tothe part you are mortising. Whenthe bottom is ready, glue and clampthe two sides in place. Keep thebottom edge of the sides in linewith the bottom surface of thebottom part.
Clamping MechanismWhile this assembly is drying,make the parts for the clampmechanism and the adjustablestops. Use hardwood and makethem to the shape and dimen-sions given in the diagrams andcutting list. Make longer lengthsthat can later be cut to final size.Its both safer and faster.
Use a router in a router tableto make the adjustable stops. First
cut them to width, then rout thegroove below so that it is cen-tered on the stock. Check the fiton the jig side. You want it tomove freely but without side-to-side play. When done, rout the14" slot. If you did-nt move
the router tables fence after rout-ing the groove, it should be cen-tered on the part. Take the jig outof the clamps, install the stops,bolts and knobs. Bore the holesfor the clamp mechanism in theclamping blocks and jig sides.Also, install the threaded insertsin the clamp blocks. Completethe jig by installing the clamp as-sembly in the jig.
Tips on UseWhen using the jig, be sure it issecurely fastened to a bench or
held in a vise. You will also need to clamp a stop block on the in-side of the jigs side to use as a gauge for placing successive parts in the exact same location.
Remember to make multi-ple passes when routing your mor-tises. Id recommend no more than a 38"-deep cut at a time when using a router bit in the 38"- to 12"-diameter range. And as for bit selection, an up-spiral straight bit is best because the spiral de-sign evacuates the waste up through the flutes in the bit.
Set-up can be a snap.With the bit set tojust clear the work piece, set the stopsfor the router, then adjust the routersedge guide to the mortise layout lines.
Rout mortises making stepped cuts forsafety. Each pass should be 14" to 38"deep. Pull the router toward you so thebit rotation forces the routers edgeguide to the side of the jig.
NO. ITEM DIMENSIONS ( INCHES) MATERIALT W L
2 Sides 34 5 26 Plywood 2 Bottoms 34 312 26 Plywood 2 Edge bands 14 34 26 Maple 2 Stops 12 114 7 Maple 2 Clamp blocks 2 2 4 Maple 1 Dowel 12 Dowel stock
3 1/2"1 1/2"
1/4" x 3/4" hardwood cap
2 layers -3/4" plywood
3/16" x 3/4" groove
1/4" w. slot
Drive-in insert for1/4" threaded rod
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