Kerf, and why it matters

Kerf is the width of material the blade removes. Not the thickness of the steel, the width of the slot the cut leaves behind, which is wider than the plate because the teeth are set or carbide tipped wider than the body.

It is the single most common reason a cutting plan that works on paper does not work on the saw, and the failure is always the same shape: everything fits until the last part on the sheet, which is short by a few millimetres.

Four parts become three

Take a 2440 mm sheet and four parts at 610 mm. Four times 610 is 2440. On paper it is exact, and exact is a lovely feeling.

Put a 3.2 mm blade through it. Three cuts between four parts is 9.6 mm of material gone. You now need 2449.6 mm of sheet and you have 2440. The fourth part does not exist.

Notice what you are left with: 600.4 mm, which is nearly a whole part and completely useless for this job. That is the shape of a kerf mistake. You do not lose a few millimetres, you lose the last part and keep almost all of the material.

What number to use

Rough figures, and every one of them is worth checking against your own machine:

• Panel saw with a standard 300 mm blade: 3.2 mm
• Thin kerf blade on a site saw: 2.4 mm
• Track saw: about 2.2 mm, depending on the blade
• Bandsaw: 0.6 to 1.2 mm, which is why resawing is done on one
• CNC router: the cutter diameter, so 6 mm is common and it is not a small number
• Laser: about 0.2 mm, and it varies with material and power

The router figure catches people out. On a router the gap between parts is the tool, not a blade, and a 6 mm cutter takes out nearly twice what a panel saw does. The sign shop example uses 6 mm for exactly this reason.

Measure your own

Take a piece of scrap with a known length. Cut it in half. Measure both halves and add them together. The difference between that and the original length is your kerf.

Do it three times and take the middle figure. If the answer changes by more than about a tenth of a millimetre between cuts, the problem is not the blade, it is the fence or the feed.

A blunt blade cuts a wider kerf than a sharp one, because it wanders. If your layouts start coming up short on the last part of every sheet and nothing else has changed, the blade is telling you something.

Where it compounds

One cut costs 3.2 mm. A sheet of small parts might carry sixty cuts, which is 190 mm of sheet gone before you count a single part. On the school workshop job, 216 small parts across four sheets need a kerf allowance of 0.31 square metres, which is about a tenth of a sheet. The trim on those same four sheets costs almost exactly the same again.

That is not an argument for pretending the blade is thinner than it is. It is an argument for putting the real number in, so the layout you get is one that can actually be cut. A plan that ignores kerf is not optimistic, it is wrong, and you find out at the far end of the sheet.

The one exception

If you are laying out parts that will be cut on separate sheets from separate stock, with no part sharing an edge with another, kerf does not compound. That case is rarer than it sounds. Almost all sheet work is a sequence of cuts through one board, and every one of them costs you the blade.

In Cut Solver

Kerf is one field on the stock screen and it applies to every cut in the layout. Set it once per material and it stays with that material. If you run one saw, you set it once ever.

Related: trim allowances covers the other number on that screen, and cutting order covers the sequence the kerf actually happens in.