Morticing With a Router
Why bother with a mortice? Without it, furniture joinery would be compromised. The mortice and tenon Is the connection of choice in framed ware of all sorts, especially tables and seating. The joint resists tension (when glued) and twist, and enjoys itself in compression and shear.
Appreciate that with a mortice, you’re only half way there. Most joints are asymmetric; they have gender (glue joints excepted). A mortice requires a tenon, a tongue a groove, a dovetail requires a socket (way) and so on, a complication to be sure. Notwithstanding, this complexity lends itself well to flexibility. There are a lot of ways to make a mortice. A chisel & hammer, a power chain morticer, a drill press with morticing attachments, a router, and there are assembly tricks, (glue ups e.g., can be such that space can be left vacant for a mortice).
Router Morticing has 2 requirements, one a layout and location function, the other fixturing. Locating the mortice is insensitive to the mortice making procedure. It makes no difference how the damn thing is cut; you always have to locate & layout the excavation. And there are always engineering, practical and esthetic variables to consider. The walls of the mortice cannot be too close to the edge or end of work for example. You must to decide whether you want it to poke through or stop blind. Is hardware part of the connection? Cross dowels and screws, for example, can change the entire design of the joint.
The fixturing demand presents several challenges. The complexity of the fixturing varies substantially depending on whether the work is hand or table routed. Each method has its advantages and risks. Plunging the work straight down on a hot spinning cutter (the most common table approach) is risky business, complicates stage routing, & can break a cutter. The ends of the mortices always burn, the chip is trapped, and the cutter gets beat to hell. Work piece control is precarious, and the dimensionality and quality of the excavation is invariably compromised. I would not recommend this method of mortice routing.
A more sophisticated approach has the work fixtured on the router table & the router spindle horizontal. The work clamped & indexed on a platform, moves in 2 or 3 planes, with or without templets & stops, and usually with levers. This method is common in semi-production, safe and accurate. It is complicated, however, with setup and hardware. As a rule, this approach is expensive, or the worker has to be damn good jig maker. The woodworker with resources can also buy ready-made tools for this, (Joint-Matic, Multi-Router, et.al.).
A third approach, & one I am a student of, has the work fixtured in a jig whilst the router zips around to plow out the diggin’s. I like this method but it too has its complexities. You have to be a jig maker (or you can buy from me) and you have to be able to create the components of the jig precisely. The work is registered to the jig and the router and edge guides (2 opposing) are too. And, as such, your jig not only has to be made precisely, but the utmost in parallelism is essential,. The fence, the work is toggled to, has to be parallel to the edges of the platform. Screw this up just a little bit and your mortices will not be parallel to the edges of the work. Nevertheless, there is a payoff for this design and your careful execution of it.
The beauty of this system is its simplicity. The skills required for cutting and fixturing have been transferred to the jig. The platform manages the router surface problem; there is always plenty of real estate for the work & the router. It matters not what the shape of the work is; skinny or fat, long or short, there is always support for the router and the work is handily secured to the fence with over-center toggle clamps. The routing is done through a window where the chip is easy to collect. There are no collars or cutter bearings to roll over the chip and spoil the cut. You can clamp the jig to the work if the stick is big, say 3.5″ square, or can clamp the jig to the bench for sticks you can easily handle.
Adjustable end stops limit the router travel north & south (6″) and define the length of the mortice. The width of the mortice is arrived at x the cutter diameter or the cutter diameter and the slop in the 2 edge guides. If the cutter diameter is 1/4″ and the slop in the edge guides is 1/4″, the mortice will be 1/2″ wide. With this flexibility any mortice width is possible; you can tune the mortice width to match the thickness of the tenon. Use these concepts in your next jig and save time, improve yield, and your accuracy.
Cutters for Morticing
The gold standard for morticing is the up-spiral solid carbide garden variety straight bit. They are ground on the end, pull the chips out of the mortice but might make a mess exiting through mortices. They also can spoil the edge at entry. For my money an ordinary straight but solid carbide bit is the better choice. It also cuts on the bottom, but it enters and exits with far less insult to the work. Moreover they’re cheaper and the chip collection on this jig is not much of a problem . The chip is in a well and exhaust tubes from DeWalt (& similar) routers, handily collect the mess. A third choice and cheaper yet is the double fluted carbide tipped straight (no shear) bit.
The main advantage of this cutter is inventory variety. You can find this cutter in diameters from 3/16″ to more than 1″. Expect the diameters of solid carbide not to exceed 1/2″. This bit, often claimed to be a plunge bit, will not plunge if the router is not moving, (Solid carbide drills into stock whether the router is moving or not.) So if you choose this design, anticipate a short learning curve to plunge and sweep simultaneously.
A new option, an experimental cutter I’ve been working on, is a short single flute, long shank bit for mortices up to 2″ deep. The price range for these cutters will be in the 25 – 35$ dollar area. We’ve cut the diameter to just over the shank (~.505″) so whence the cut length of the flute has been exceeded the shank will not rub nor burn the walls of the mortice. The single flute plunges nicely tho not as efficiently as solid carbide. 1/4″ & 3/8″ diameter bits will be in the morticing bit ensemble.