Adjustable Scarf Jig (Proposed)

A proposed shop jig for cutting long, accurate scarf bevels in strip stock: the 4½" splice on Rod 002’s bamboo sections, and also scarfed trim, bow and paddle laminations, and anything else that needs a feathered lap joint. It’s a permanent tool, not a one-off. One knob sets any ratio from 1:8 to 1:40, and the setting doesn’t depend on the stock’s thickness.

How it works. Two hard-maple rails form a flat reference plane. A sanding beam rides on them and can’t cut below that plane. Between the rails is a tilting bed hinged so that its top surface pivots on the rail plane. Clamp the stock with its end on the hinge line and drop the far end of the bed. The stock then rises above the rails by exactly the wedge you want removed. Sand until the beam rides the rails, and the bevel feathers out precisely at the end of the stock. The only setting is the ratio: the bed’s drop at the 12″ mark is d = 12″ ÷ ratio, for any thickness of stock.

Drawings

All dimensions are in inches. The X axis runs along the jig, Y across it, and Z up.

Holding the work

A C-clamp won’t work on the bed itself. The bed sits recessed in a 3″ channel between 3″-tall rails, with the base underneath, so a clamp’s lower jaw has nowhere to go. Anything sticking up above the rail plane inside the scarf would also be hit by the beam. The hold comes in three layers instead:

1. Double-sided tape under the scarf zone (the primary hold). Use thin cloth carpet tape or turner’s tape, along the whole scarf and a few inches past it. It’s the only thing that can hold the feather, the last thin inch that no clamp can reach. On bamboo, press the section down hard for a minute. Release it afterward with a putty knife and a little naphtha.
2. The hold-down bar goes across the stock’s top flat, knobbed into the nearest insert pair past the beam’s travel. It resists the pull of each stroke. For a scarf of length L, use the first pair beyond L + 3″, and use a short beam (6″ long) on short scarfs so it stops short of the bar. For the 4½″ bamboo splice, that’s the 9″ pair. Pad the bar so it doesn’t mark the enamel side.
3. The outboard end gets the C-clamp. A rod section is 39″ long and leaves the jig sloping gently downhill (2.5° at 1:23). Support it on a block or roller stand at that height, and C-clamp it there. That’s where a C-clamp is useful: it keeps the long lever arm from rocking the stock on the bed.

Sand with light strokes toward the feather. That pulls the stock toward the hinge, against the tape and the bar, and keeps bamboo fibers from lifting at the thin end.

Settings

Drop d is measured at the 12″ setting mark, from a straightedge laid across the rails down to the bed’s top. Use a depth gauge or the depth rod of your calipers.

• Any other ratio: d = 12 ÷ r. For a scarf of length L on stock of thickness t, r = L ÷ t.
• Capacity: stock up to 2-3/4″ wide. Scarf length L up to 15″, so the maximum thickness is 15 ÷ r.
• Ratio range: at 1:8 the bed’s far end still clears the base by 0.27″, which sets the steep limit (~1:7.5). Shallow ratios are only limited by how small a d you can measure.

Parts

3D-printed parts

A 3D printer makes several of the fiddly parts easier, and repeatable. Print them in PETG, which is tougher than PLA and doesn’t creep under a clamp, at 4+ perimeters and 40% infill.

The rails, bed and base should stay wood: their flatness and stiffness are the whole tool. Printed parts only hold, locate and set.

Build

1. Mill the rails and bed from the same quartersawn board. Plane them to final thickness together so they move together.
2. Glue and screw the rails to the base’s long edges, with their outside faces flush. Put the screws in from underneath so nothing is near the rail tops. Add the feet.
3. Drill the hinge together. Shim the bed so its top is flush with the rail tops, and clamp everything. Then drill one ¼″ hole straight through rail, bed, and rail at X = 5″, 3/8″ below the top. That’s 2-5/8″ above the base’s top. Drilling it in one pass is what keeps the bed’s top surface pivoting on the rail plane.
   • The pin goes through the bed. The hole is centered in the bed’s ¾″ thickness, with ¼″ of wood above and below the pin. Leave 3/8″ of bed behind the hole and round that nose to a 3/8″ radius about it. Because the nose is centered on the pin, its highest point stays at the rail plane at every tilt, so it never sticks up into the beam’s path.
   • Fits: the pin is a press fit in the rails. Open the bed’s hole just enough for the bed to turn without play, with a ¼″ reamer or 320 grit wrapped on a dowel. A sloppy 17/64″ hole would let the feather point wander.
4. Flatten the reference plane. With the bed shimmed flush and clamped, sand the rail tops and the bed top together on a flat surface, such as PSA paper on a jointer bed, a cast-iron wing, or glass. They must be one continuous plane. Check with a straightedge: the bed flush to the rails within 0.002″ is d = 0.
5. Lift screw: install the T-nut under the base at X = 17″. Epoxy the brass pad under the bed at that spot, and thread in the thumb screw and jam nut.
6. Marks: scribe the hinge line (X = 5″) and the 12″ setting line (X = 17″) across both rail tops, and ink them.
7. Hold-down: install the two inserts in the bed, and make the bar and the fence strip.
8. Sanding beam: flatten the MDF, stick on the PSA paper, and add a handle. Wax the rail tops so the beam glides.

Using it

1. Zero: with d = 0, a straightedge should lie flat across rails and bed.
2. Set the ratio: turn the lift screw until d at the 12″ line matches the settings table, then lock the jam nut.
3. Load the stock: lay it on the bed against the fence, with its end exactly on the hinge line. Sight down the scribed line with a square, then hold it as described in Holding the work.
4. Sand: take long, light strokes along the jig, starting with 80 grit. Keep the beam flat on both rails. You’re done when the beam rides the rails along the whole scarf and the end has feathered to nothing. Finish with 120 and then 220.
5. The mating piece gets the same setting and the same procedure. Joined face to face, the two bevels restore the full section.

Accuracy. The pin sits 3/8″ below the bed’s top surface, so the pivot line moves very slightly as the bed tilts. At 1:8 the feather point shifts 0.05″ along the stock and drops 0.003″; at 1:23 it’s about a third of that. That’s well inside the material you trim off the feathered tip anyway.

Rod 002 splice, worked example

• Joint: 4½″ scarf at the 39″ joint, where the hex measures 0.193″ across the flats.
• Ratio: r = 4.5 ÷ 0.193 = 1:23.3, so d = 12 ÷ 23.3 = 0.515″.
• Tip section: lay it flat-side down on the bed with its end on the hinge line. A hex rests stably on a flat, so the bevel runs across the flats. Tape it and hold it down at the far end, then sand with light strokes. Bamboo fibers want to lift at the feather: finish with 220 and keep strokes going toward the feather.
• Butt section: same setting, same procedure.
• Mate them: rotate one section 180° about its axis so the bevels face each other. Line up the guide flats, whip the feather ends, and tape the joint, as described on the Rod 002 page.

Variations

• Router sled for thick stock. A 6″ × 12″ sled spanning the rails can carry a trim router with a flat-bottom bit set flush with the sled’s underside. Run it back and forth along the rails. It’s faster than sanding on 1″+ hardwood. Take conventional (not climb) passes, and use a full hold-down.
• Longer scarfs: lengthen the rails and bed together. Keep the 12″ setting mark, and capacity grows with the bed.
• Metric: scribe a second mark 300 mm from the hinge; d = 300 mm ÷ r there.