An easy-to-use, durably constructed and precisely machined jig for routing perfect mortises up to 3" long.
Mortise and tenon joinery is a fundamental technique that is indispensable in furniture and carcass construction. It provides exceptional strength and resists racking and twisting forces better than most other joints, while providing an ample glue surface. When you only have a few to make it’s time to get out chisels, mallet and saw. But, when you have dozens to make, especially when doing production work, a mortising jig is the way to go.
Unfortunately, there aren’t many mortising jigs to be had. The Mortise Pal, a very effective jig that I’ve been using for years is no longer in production. And the Leigh FMT Pro, which enables you to mill both mortises and tenons, is close to $2 grand – out of reach for most hobbyist woodworkers and a good many professional woodworkers.
You can, of course, make your own mortising jig – I’ve made my share. However I think that a commercial jig is the better choice as it offers a higher degree of precision, versatility and durability, especially if you’re doing production runs.
Needless to say I was very eager to try the new Mortise Master. And I’ve not been disappointed. I’ve milled several dozen mortises with the Mortise Master and every one has been spot on. The jig is intuitive to use and durably constructed with precisely machined parts to give years of reliable service.
With the Mortise Master you only cut mortises, not tenons, which means you’ll use this jig for loose tenon joinery. There really is no difference in joint strength between conventional tenon joinery and loose tenon joinery – provided the tenon stock you use is a precise match for the mortises. You can make your own tenon stock, or you can purchase it in various thicknesses from Lee Valley. I find that it’s better to mill your mortises first and then mill the loose tenons to fit.
You can use any plunge router with the Mortise Master. I’ve been using a compact 1-1/4 hp DEWALT (#DWP611PK) which is sufficient for the 1/4″ and 5/16″ router bits that I use. For larger diameter bits I’d probably switch to my 2-1/4 hp Bosch (1618EVS).
The width of the mortise is determined by the size of router bit you use, typically 1/4″ to 1/2″. I’ve been using an up-cut spiral bit that I get from KJP. You can mill mortises up to 3″ long, which easily covers the range of mortises most commonly used by woodworkers. Mortise depth is determined by the length of router bit that you use. And, you can rout stock up to 3-1/2″ wide (and unlimited length).
The Mortise Master comes with two hardwood clamp plates (made from Sapele) with integrated aluminum tracks, a pair of stops with adjustment knobs, a round slide plate with two glide blocks, and a guide bushing.
You’ll likely find, as I did, that the Mortise Master is easy to assemble. You can almost use this jig straight out of the box. However, I would recommend that you first look though the online user guide, which shouldn’t take more than ten minutes – particularly if you are new to mortise and tenon joinery. I found the text on the printed guide too small to read comfortably.
In the photo above I’ve shown the guide bushing sitting on the slide plate – the bushing will, of course, be installed on your router.
One of the clamp plates has a registration mark scribed on it – you’ll always align your workpiece to this registration mark. The mark is hard to see so you might want to darken it using a Sharpie.
The key to this jig is the slide plate, which is predrilled with a series of 11 holes. By attaching two small glide blocks into a pair of holes you establish the offset from the edge of your workpiece to the center of the the mortise you’ll be milling. In the photo below the glide blocks are inserted in holes 1 and 7. This will ensure that, regardless of the width of your workpiece, the mortise will be centered precisely in the middle.
Changing the glide blocks to, for example, positions 7 and 10, will offset the center of the mortise 1/2″ from the edge of the workpiece. A note of caution here – don’t screw the glide blocks tightly to the slide plate. The blocks need to be able to shift slightly as you insert the plate onto the jig. The user guide lists 10 combinations that provide offsets from 3/8″ to 1-1/2″.
Installing a workpiece in the jig is pretty easy. Begin by marking the center of your mortise on the workpiece and then insert the workpiece between the two clamp plates, aligning the center mark with the registration mark on the jig.
What you need to remember is that there are levelling lips just below the top of the two clamping plates – your stock needs to be positioned flush below the lips.
The easiest way I found to do this is to position the jig on it’s side, insert the workpiece and then tighten the plastic stop locking knobs. The locking knobs don’t apply sufficient pressure to keep the workpiece in place – you’ll need to use a couple of clamps. However, if you put the jig with workpiece into a bench vise you won’t need to use clamps.
To set width of a mortise you need to mill two setup blocks, using this formula to calculate the width of the blocks: “mortise width” – “bit diameter” ÷ “2”.
For example, to rout a mortise that’s 3/8″ by 1-1/2″ the formula would be 1-1/2 – 3/8 ÷ 2 = 9/16. The next step is to mill two 9/16″ blocks. Then chuck a 3/8″ bit in the router, set the mortise depth on the router, place the 9/16″ block on either side of the slide plate, push the stops up against the setup blocks and lock them in place, and then remove the setup blocks. You’re now ready to rout. I use MDF for the setup blocks and label them for future use. I have six pairs for the various mortise widths I use.
As a quick reference I created a template that shows the ten offsets. It makes it easy to determine which pair of holes to use on the glide plate.
I’ve very pleased with the performance of the Mortise Master. It’s a practical, efficient shop jig – straightforward in design, superbly machined, easy to use, and the results are consistently perfect. When you have a lot of mortises to rout it’s significantly quicker using the Mortise Master.