Microadjustable Mortising Jig
Here's a start with my 14 x 32 drill press table. It consists of a 3/4 plywood ladder frame with a 3/4 particleboard cover edged with oak, all glued & screwed. Underside center section is doubled (glued) particleboard for strength and mounting of T-nuts.
In later photos with the fence removed, an oak reinforcement bar is visible at the back of the table, made necessary because of carving the ladder frame apron to fit around cast iron drill press table. Wood table was made flat while bolted in place. The whole is varnished, sanded, and paste-waxed (varnish helps to control movement and stabilizes the particleboard surface.) Knobs and tracks are Rockler.
I anticipate a litle further work in installing lever-actuated clamps and/or through-holes and T-nuts to mount cam-type workpiece hold downs when usage defines their need.
- The high fence (#1) allows square locating and clamping of tall workpieces. Not shown is low fence, just straight 1" stock held via the same T-tracks.
- The upper oak crossbar (#2) is fitted with T-track for attachments, and can be rotated 900 to locate the T-track in vertical plane. Attached to fence with three 1/4" FHWS.
- The end stops (#3) are on overlength bars to allow their extension beyond the net table length. The stops have alternate mounting holes (visible in the photo) so they can be exchanged for very small offsets from table center. The mount bars themselves are a "stubby T" shape whose underside leg closely fits in the T-track channel.
- Not shown are table extensions for left/right support and remote end stops.
A point of weakness:
In seeking stable material, I foolishly made the fence of 3/4 plywood (the end blocks are 2x2 oak on end for through-passage of the clamp bolts.) Without the upper oak bar with the T-track, there is just a tad of deflection possible at the center of the fence. It's an L-shape so as to fit over the rear oak reinforcing bar on the table, and, since it's not a box, the plywood has just enough give to wiggle a little under stress. Were I do to do it over, I'd probably use luan for stability and added rigidity.
Here are the mortising jig components (table fence removed.) Pay no attention to off-center alignments - they're an accommodation for my overly tight workspace. In a normal situation, the entire rig would be built on-center.
(#1) is the mortising table. Oak frame with double-laminated 3/4 particleboard surface. Detail pix are here.
(#2) is the oak guide block which bolts to the drill press table and keeps the mortising table square with the mortising chisel (the chisel is set parallel only once, then the guide ensures that square alignment is maintained as fore-aft distance for the workpiece is adjusted.) Raised rub surfaces are sections of 3/8 HDPE. 1/4-20 bolts to T-nuts in underside of drill press table.
(#3) is the fore/aft microadjust attachment. More on this below.
(#4) is the group of end stops to set ends of workpieces (left and right units were made to allow machining with same side of workpiece toward fence for all mortises.) The sculpted inner ones were simply marked with a pencil to approximately follow the hold-down contours and then cut with a coping saw. The rectangular outer stops can be set so that their distance from the contoured stop is exactly the length of the mortise. (In practice, you'd plunge all of the endmost mortise first cuts on all workpieces, then move the contoured stop and complete remaining cuts to establish identical length and location for that mortise on all workpieces. The advantage of stops lies in being able to mark one mortise, then cut 4 or 100 just like it without another measurement or layout mark.)
On the loose stop lying on table at left, the "stubby T" conformation is clearly visible. Its width barely slides freely in the T-track channel. I made a long piece of stock with the "stubby T" as its cross-section, then just cut off shorties as needed for individual stops. They'll fit any similar T-track on other jigs.
There's a closeup photo here, which also more clearly shows the mounting bolts for the microadjuster (in shadow on the table's front apron.)
Mortise table and guide assembled. Knobs in rear are 4" long to project above mortise fence. All knobs are threaded 5/16. Mortising table clearly is riding against the guide blocks at right.
Rockler's 5/16 through-hole star knobs are nicely designed, as their topside holes provide a bare interference fit for the head of a 5/16 bolt inserted through the knob from the top. The bolt can be well-tightened and expected not to come loose when the knob is turned.
Note inset 5/16 T-nut for microadjuster attachment, located at front center, bottom edge of mortising table. Countersunk with Forstner bit and secured with brass RHWS (typical.)
These last two photos show the microadjuster attached and functional. Construction details are here. It is not used to achieve a specific position or measurement, but to allow for fast and accurate change of fore/aft position of the workpiece. Total available travel is just under 2", which will center a mortise in a net 4" thick workpiece.
One turn on 5/16-18 bolt yields about 0.056" movement, or, 0.001" movement will occur for each 6.50 of rotation. The 3.5" diameter protractor is widely available - try Google for C-Thru 255 Protractor.
