Saw kerf in a cut list

What the number usually is

Kerf is the width of the slot the blade cuts, which is the tooth set plus whatever the blade wanders. It is a property of your blade and your machine, not of the material.

The fixed 3.0 mm in the free tier is deliberately in the middle of that range, and the result screen states it every time, so no figure in a free plan is ever a silent assumption. Pro lets you set the width your saw actually cuts.

The two rules that matter

Rule one: the kerf comes out of the free area, never out of the part. A 600 mm shelf is 600 mm when it comes off the saw. The blade takes its 3.2 mm from what is left of the panel beside it. An app that adds the kerf to the part size is measuring the wrong thing, and every part in the plan is then slightly too big.

Rule two: no kerf at an outside edge. There is no cut there, so nothing is consumed. Charging a kerf to the last part in a row is the quiet error that throws away parts that would have fitted.

Worked through: how many 700 mm parts fit across 2800 mm?

Take a 2800 mm wide sheet, 10 mm of edge trim on each side, and a 3.2 mm blade.

• Usable width after trim: 2800 − 20 = 2780 mm
• Four parts need four widths and three cuts between them: 4 × 700 + 3 × 3.2 = 2809.6 mm
• 2809.6 is more than 2780, so four do not fit. Three do.

Ignore the kerf entirely and the arithmetic says 2780 / 700 = 3.97, which still rounds down to three, so on this sheet you get away with it. Widen the sheet a little and you do not. On a 2810 mm panel with no trim, four parts and three cuts come to 2809.6 mm and fit with 0.4 mm to spare. An app that charges a kerf to every part instead, 4 × 703.2 = 2812.8 mm, decides they do not, and sends you to buy a second sheet for a row that already fitted.

That is the whole argument for getting rule two right. It costs you a panel per job, silently, and the plan looks perfectly reasonable.

Why the arithmetic is done in integers

Every length in Sawgrid is a whole number of tenths of a millimetre. One inch is exactly 254 of them. No dimension, no kerf and no coordinate is ever a floating point number, for the same reason money is stored as integer cents: rounding drift would break the closed area balance that the whole plan rests on.

It also means imperial input is exact. A part typed as 2' 6 3/4 becomes an exact integer, not a value that is nearly right.

What the kerf costs on a whole sheet

On the ten part plan shown on the home page, at a 3.2 mm kerf, the blade accounts for 0.7 % of the panel. That is small, and it is not the point. The point is that those three rip cuts are also what decides whether the fourth strip has room for three fronts and a keepable offcut, or for two fronts and a bin liner. The kerf is small in area and large in consequence.