Micro-Adjustable Box-Joint Jig

Jim Harrold Posted on Jun 1, 2011 6:26 PM

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This article is from Issue 41 of Woodcraft Magazine.

Make projects with a choice of four finger sizes.

Designer: Dick Reese
Builder: Bill Sands

Overall dimensions:18 1⁄4"w × 12"d × 4 1⁄4"h

Box joints provide a strong, attractive way to assemble drawers, chests, and boxes of all types. But to cut clean, consistent fingers and notches you need a jig that adjusts in fine increments. Unlike the simple tablesaw jig in the Oct/Nov 2008 issue that cuts only one size finger, this model, with its switchable finger keys, lets you cut precision box joints having 1⁄8", 1⁄4", 3⁄8", or 1⁄2" fingers. After micro-adjusting the setting with the end knob, you can lock it in with the two five-star knobs. Also, the replaceable hardboard backers eliminate tear-out, resulting in splinter-free cuts.

First, make the jig

1 From 3⁄4" stock, cut the fixed fence (A) and adjustable fence (B) to the sizes in the Cut List. Mill stock to 1⁄2" thick, and cut the fence end (C) and blade cover (D) to size. Mill stock to 1⁄4" thick, and cut the fixed base (E), four key bases (F), and the miter slot runner (G). The runner should fit your saw’s slot with no slop and be 1⁄16" above the tabletop.

2 Drill 1⁄4" blade start holes and rout the 11⁄2" slots in the fixed fence (A) where shown in Figure 1.

3 Chuck a 1⁄2" × 14° dovetail bit in a table-mounted router. Now raise it for a 1⁄4"-deep cut for the 21⁄4"-wide dovetailed recess in the adjustable fence (B) in Figure 1. Adjust the router-table fence, and make a cut along one cutline. Adjust the fence again, and cut along the second cutline as shown in Photo A. Continue adjusting the fence and remove the waste between these two cuts. Finally, angle-cut the chamfers at the ends of the fence.

4 Measure the width of the dovetailed recess in Step 3 in the adjustable fence (B), and cut a 24" strip of 1⁄4" hardboard to this width. Rout 14° bevels along the edges of the strip with the same dovetail bit. Test-fit the strip in the recess. If it fits, crosscut the backers (H) to final length. (Consider cutting a batch of backers for later use.)

5 Switch to a straight bit sized to your miter slot runner (G), or, using a dado set, cut the 1⁄16"-deep dado in the bottom face of the fixed base (E). Test-fit the runner in the dado. Glue it in place.

6 Lay out and bandsaw the blade cover (D) to shape, sanding the edges smooth.

7 Lay out the screw and threaded insert hole locations, referencing Figure 1. Now bore the holes, countersinking where indicated.

8 Carefully cut 1⁄8"-, 1⁄4"-, 3⁄8"-, and 1⁄2"-square 12"-long strips for keys (I), (J), (K), and (L) and the spacer stock, measuring the strips’ thickness as shown in Photo B. Also check the stock for square. Now cut the keys to length, and glue them one key width from the inside end of the adjustable key bases (F). Finally, cut 3"-long pieces from each strip of key stock to serve as setup spacers and label them.

9 With the hardware shown in Figure 1, assemble the jig parts. Widen the hole in the wooden knob for the all-thread rod if needed, and epoxy the knob to the rod. You’re now ready to put the jig to work.

Clamp on a sacrificial pushblock to rout the dovetailed recess in the adjustable fence to avoid tear-out.

Establish dead-on key and spacer stock with a caliper; reduce the thickness with sanding or planing.

Use the spacer to fix the distance between the key and dado set, adjusting the key with the knob.

Holding the workpiece snug to the adjusted key, move the jig forward and make the first cut.

Straddling the key with the workpiece, cut the remaining fingers and notches.

Use the spacer to determine the corner or beginning notch on the mating workpiece.

With the spacing of the mating workpiece established, cut the corner notch.

Place the corner notch on the key and cut the next finger and notch. Cut out the remaining notches.

Put the jig to work

1 Based on the desired finger and notch widths (I chose 1⁄2"), install a matching dado set and key base (F). Make a test cut in scrap, and fit the appropriate spacer in the notch. Add shims to the dado set to ensure a perfect fit. Now raise the dado set to a height equal to the combined thickness of your workpiece and fixed base. (For 1⁄2" stock, I raised the dado set to 3⁄4".)

2 Set the jig against the front edge of the dado set. With the correct spacer (M) held against

the key on the key base (F), turn the wooden adjustment knob

until the spacer just grazes the dado set (Photo C). Tighten the five-star lock knobs.

3 Using stock the same thickness as the project parts, set one “test” workpiece against the key and make the cut (Photo D).

4 Place the first test workpiece notch over the key, and make the second notch cut (Photo E). Continue cutting the remaining fingers and notches across the width.

5 Place the spacer (M) against the key and the mating test workpiece against it to establish the corner notch location (Photo F).

6 Clasp or clamp the

workpiece firmly to the fence so it doesn’t move, and remove the spacer. Cut the notched corner on the mating test piece (Photo G).

7 Place the corner notch over the key, and cut the fingers and remaining notches (Photo H).

Fit the mating test pieces together. To fine-tune the joint, see Figure 2 and cut another pair of mating sides. For design help, see Box-Joint Pointers. Now cut your project parts.

Box-Joint Pointers

For good-looking corners, adjust the width of the sides to be an increment of the finger.
Joints that terminate with full fingers are preferred over joints that terminate with one notch and one finger.
While you can cut box side widths when cutting the lengths, some woodworkers cut widths proud by 1⁄8" to 1⁄2", trimming the assembled box to final width.
Cut the notches at both ends of one workpiece first and those on same-length workpieces before cutting the neighboring notches.
For box bottoms, rout stopped grooves instead of through grooves to avoid creating unsightly joints.