Cut each hub mortise as a 3 x 5 cm rectangular slot with a mortise chisel and mallet, tilted outward 5-8 degrees from perpendicular to give the wheel its dish, and make every mortise identical in size, angle and spacing. Because a spoke magnifies a hub error roughly five times at the rim, a 1 mm sideways error at the hub moves the rim end of a spoke about 5 mm, so spacing must be held to about half a millimetre.
Part of Wheelwright Craft
Mortises are the square sockets in the hub that take the spoke tenons; their angle fixes the dish of the wheel and their spacing fixes its roundness.
A Hub Error Is Multiplied By The Spoke
The parent article calls this the step where beginners struggle: the difference between a true wheel and a wobbly one is “millimeters of precision”. The numbers show why.
Tangential error. Take a hub 25 cm across, so its radius is 125 mm. A mortise placed 1 mm off its marked position is out by an angle of 1 / 125 = 0.008 rad, about 0.46°. That angle carries to the rim at a radius of 600 mm: 600 × 0.008 = 4.8 mm. The error is multiplied by 600 / 125 = 4.8, about five times.
To keep the rim round within 2-3 mm (the figure given in Spoked Wheels), the mortise spacing must be held to 3 / 4.8 = 0.6 mm or better, and about half a millimetre is a sensible working limit.
Tilt error. A 1° error in the mortise tilt moves the spoke tip sideways by L × sin 1°. For a 50 cm exposed spoke (an assumed length): 500 mm × 0.01745 = 8.7 mm. Compared with the 4.4-7.0 cm of dish worked out in Engineering Principles, a single degree is a large fraction of the whole dish.
What You Need
- A mortise chisel, sharp, and a wooden mallet
- A marking template — a board with the hole positions cut or marked at equal angles, for the circumference
- A tilt guide — a wedge-shaped block, cut to the dish angle, that you hold against the hub to sight the chisel against
- A trial tenon — a spare spoke end to test each mortise
- Awl, square and dividers
Method: Mark, Chop, Test
Step 1: Mark the positions For 12 spokes, mortises are 30° apart; for 14, about 25.7°. Divide the circumference with dividers and transfer the marks to the hub with the template. Mark the stagger on alternate positions.
Step 2: Mark each outline Square the 3 x 5 cm outline round each mark. Use the same template for every one.
Step 3: Chop Chop in with the mortise chisel, holding the tilt guide beside the chisel so the blade follows the dish angle. Work from both sides of the slot toward the middle to prevent the wood breaking out at the faces, as the parent article instructs.
Step 4: Pare the sides Pare the walls flat and square. A tenon in a rough mortise bears on the high spots only.
Step 5: Test Drive the trial tenon in. It must seat tight and square. Measure its tip position, and compare every mortise with the first one.
Common Mistakes
- Marking by eye round the hub — a small error each time adds up to a gap or overlap at the last mortise.
- Mixing tilt angles — one mortise at 5° among 8° ones puts one spoke off the dish plane by 500 × (0.1392 - 0.0872) = 26 mm on a 50 cm spoke.
- Chopping straight through to the opposite wall — leaves tearout and thin webs.
- Using a trial tenon on one mortise only — test all of them against the same tenon.
Safety
Chisel and mallet work
A chisel driven by a mallet follows the grain and can slip or split the block suddenly. Clamp the hub, keep your free hand behind the edge, and cut away from your body.
What’s Next
- Learn next: Driving & Fitting — seating the tenons you cut for
- Combine with: Spoke Shaping — making tenons to match