Saw kerf and edge trim: why your parts don't fit

A 1220 mm shelf is exactly half a sheet, so two of them should come out of a 2440 sheet. They don't: the saw eats a few millimetres at every cut, and the edge trim takes as much again at every edge. Here you will see how much they take, how to work out the sizes that do fit, and a table for the 2440 × 1220 mm sheet.

What the saw takes

Every cut turns a strip as wide as the blade into sawdust: that is the kerf. A hand-held circular saw or a table saw takes 2.5 to 3.2 mm; the panel saw at a supplier, about 4 mm. Ask yours what theirs is.

It does not depend on the size of the part: two parts cut side by side need their own sizes plus one cut between them, large or small.

What the edge trim takes

The edges of a sheet arrive from the factory knocked, chipped or not quite square. Before cutting the parts, a strip is trimmed off each edge: that is the edge trim, usually 5 to 10 mm per side. With 10 mm, a 2440 × 1220 sheet is left with 2420 × 1200 mm usable.

Why two 1220 shelves don't fit

The sums: 10 + 1220 + 3 + 1220 + 10 = 2463 mm, and the sheet is 2440. It is 23 mm short. Across the width the same happens: four 300 shelves add up to 1200, but with the three cuts in between they make 1209, and the trimmed sheet is left with 1200.

The sum that makes them fit

For several parts to fit in a row, none of them can be larger than:

(sheet size − 2 × edge trim − (parts − 1) × kerf) ÷ parts

With 10 mm of edge trim and a 4 mm kerf, on a 2440 × 1220 mm sheet, the largest size that fits:

In millimetres, rounded down. With a 3 mm kerf a millimetre or so more fits; with a different edge trim, do the sum with yours.

The example: eight shelves

With no kerf and no edge trim, eight 1220 × 300 mm shelves would come out of a single 2440 × 1220 sheet: two along the length and four across. With a 3 mm kerf and 10 mm of edge trim, only three come out of each sheet, and you need three sheets. Make them 1200 × 295 mm, 20 shorter and 5 narrower, and all eight come out of one sheet again.

When you design, think about the sheet: a few millimetres off a part can save whole sheets.

Try it with SliceLab

In SliceLab, the saw kerf and the edge trim are under Cutting settings, and every plan deducts them. The button below opens the eight 1220 × 300 shelves with a 3 mm kerf and 10 mm of trim: you will see the three sheets. Change the shelves to 1200 × 295, press Optimize cutting and they fit on one again. For a single size, the parts per sheet calculator tells you straight away.

You can also paste this into Import sheets and parts (CSV / Excel):

Sheet;Length;Width;Quantity;Grain;Material;Thickness
S1;2440;1220;0;0;White melamine;19

Part;Length;Width;Quantity;Rotate;Grain;Material;Thickness;Edge L1;Edge L2;Edge W1;Edge W2;Sheet
Shelf;1220;300;8;1;0;White melamine;19;1;0;0;0;S1

Common mistakes

• Designing parts that are exactly half or a quarter of the sheet.
• Working with the whole sheet, without the edge trim.
• Forgetting the kerf between small parts: a row of six means five cuts.
• Using your own saw's kerf when the supplier does the cutting: ask for theirs.