Kerf Spacing Calculator

Kerf Spacing Calculator for bending wood. Enter the stock thickness, bend radius, angle, skin left uncut and blade kerf to see the cut count, the spacing and where each kerf lands.

Kerf Spacing Calculator for Bending Wood to a Curve

Kerf bending lets a straight board wrap a curve by sawing a row of cuts across it and leaving a thin skin on the outside. The Kerf Spacing Calculator finds how far apart those cuts go, how many a bend needs and where to mark each one. It suits curved aprons, bow-front cabinets and curved panels, where steam bending or bent lamination would be more work than the part deserves.

Enter the stock thickness, the bend radius, the bend angle and the skin you want to leave uncut, all in inches. Add your blade's kerf, and set in the accordion whether the radius is measured to the inside or outside of the curve. The tool assumes the kerfs sit on the inside of the bend and close as it curves.

Why the Cut Count Ignores the Radius

Each kerf closes by a fixed angle, set only by its width and how deep it goes. When its two faces touch, the board has turned by that angle and no more.

$$\varphi =2\arctan \left(\frac{k}{2d}\right)$$

Here k is the kerf and d is the cut depth, stock thickness minus the skin. The number of cuts is the bend angle divided by that closing angle, rounded up. Those cuts are then spread along the arc at the bottom of the cuts, where the skin keeps its length.

$$s=\frac{(R+d)\theta }{N}$$

R is the inside radius, θ is the bend angle in radians and N is the number of cuts. With 3/4 inch stock, a 1/8 inch skin and a 1/8 inch blade, each kerf closes about 11.4°. A 90° bend at a 12 inch radius takes 8 cuts spaced 2.479 inches apart over a 19.83 inch zone.

Change only the radius and the cut count stays at 8, as the table shows, while the spacing grows. Change the blade or the skin and the count moves instead, because each cut now closes a different angle.

Rounding the count up means the kerfs close a little less than fully. At the defaults each one closes about 98.5%, which leaves a hair of slack rather than a kerf forced past closed. The Kerf Closes row on the first card shows that figure for your setup.

Checking It Against the Lift Test

The shop method for finding kerf spacing is older than any calculator. A reprinted bending guide on the Lost Art Press blog describes it. Make one saw cut in scrap, mark off the bend radius from it, lift the free end until the kerf closes and measure the lift. That distance is the spacing.

The lift test and the tool measure the same thing. Lifting over a distance R closes the kerf when the lift equals about R times k divided by d. That gives 2.4 inches at the default 12 inch inside radius, or 2.525 inches measured at the cut bottom, against the tool's 2.479 inches once the count is rounded to whole cuts.

One mistake to watch for in quick formulas is dividing by the full stock thickness instead of the cut depth. At the defaults that gives 2.0 inches, so the cuts land about 20% too close and the kerfs never fully close.

The same guide adds a test worth running. Note whether the bend stops because the kerf closes or because the wood near the kerf starts to split. If it splits, cut deeper. If the kerf closes first, a wider kerf lets you space the cuts farther apart.

Cut Depth Is Still a Judgment Call

No geometry settles how thin the skin can be. The author of the blocklayer kerf calculator said as much when he introduced it on Sawmill Creek. Cut depth depends on the wood and the dimensions, so it comes down to trial and error.

A reply on the Fine Homebuilding forum suggests cutting about two thirds of the way through as a starting point. The default here goes deeper, leaving 1/8 inch of 3/4 inch stock. Try both on scrap from the same board before cutting the real part.

Tight radii put the most strain on the ribs between cuts. A 180° bend at a 3 inch radius needs 16 cuts only 0.712 inch apart, leaving ribs of 0.587 inch. The third card turns amber when the ribs get thinner than the kerf itself.

The Lost Art Press guide also suggests bending acute curves in stages, first to about twice the final radius. Sponging the wood with warm water helps prevent splitting.

Once bent, the curve needs holding. Fine Woodworking's write-up on calculated kerfs describes gluing veneers or splines over the kerfs to lock the shape.

The tool assumes the kerfs face the inside of the curve. WoodWorkers Guild of America notes that kerfs cut on the outside of a curve can be spaced farther apart. The kerfs open rather than close there, so this closing-angle math does not apply.

Counting Pieces and Kerf Loss

Switch What Are You Doing to Cutting Pieces and the tool counts how many parts a board yields once each cut eats one kerf. Enter the board length, piece length, number of boards and end trim.

The defaults cut 14 inch pieces from five 8 foot boards with a 1/2 inch end trim and a 1/8 inch blade. Each board gives 6 pieces, 30 in all, and leaves a 10.75 inch offcut. A piece no longer than about 13.536 inches would squeeze a seventh from each board.

The Blade Comparison table runs the same job with four common blades. Here every blade still gives 30 pieces, because the offcut is far wider than any kerf saving. For kerf width on its own, see the Saw Kerf Calculator, and for sheet parts use the Plywood Cut List Calculator.