Trim allowances

A 2440 by 1220 sheet is not 2440 by 1220. It is somewhere near it, with one corner knocked in transit, an edge that has been sitting on a damp concrete floor, and at least one side that is not quite square to the others.

A trim allowance is how much of that you agree to throw away before you start. It is the second number on the stock screen and the one people leave at zero because it looks like waste.

Why the edges are not usable

Three separate problems, and they arrive together:

• Handling damage. Sheets are moved by forklift and by hand, and the corners take it. On melamine and veneered board a bruised edge is visible after finishing.
• Squareness. A sheet is trimmed at the factory to a tolerance, not to a guarantee. Two or three millimetres of out of square across 2440 mm is normal.
• Edge condition. Chipboard and MDF absorb moisture at the cut edge first, and a sheet stood on end in a damp unit for a fortnight has a swollen bottom edge.

How much to give away

Ordinary numbers, for a 2440 by 1220 sheet:

• 5 mm: sheets you have handled yourself, stored flat, cutting parts that will be edged
• 10 mm: the sensible default for most panel work
• 12 to 15 mm: veneered board, marine ply, anything expensive where a bruised edge is a remake
• 0 mm: finished stock that is already the right size, like the boards in the flooring job

What it costs

Less than it feels like, because you are removing a thin band and the area of a thin band is small.

Three per cent to protect every exposed edge on a veneered job is not a bad trade. It becomes a bad trade only when the trim pushes you over a boundary, which is the thing to watch for.

The boundary problem

Trim does not usually cost you three per cent. It costs you nothing at all, or it costs you a whole part.

If your parts leave 40 mm spare across the sheet, a 10 mm trim costs nothing, because the spare absorbs it. If your parts fit with 6 mm spare, a 10 mm trim costs you an entire column of parts and you will not see why.

This is the same failure as kerf, and it has the same cure: put the real number in and let the layout tell you what fits, rather than finding out at the saw.

Trimming two edges instead of four

Common and sensible. You need one straight reference edge and one square to it. Trim those two, work from them, and let the far edges fall where they fall, because parts positioned from a good reference are square regardless of what the far edge is doing.

Cut Solver takes a single trim value applied to all four edges. If you work from two references, halve the number you would otherwise use, or set it to the amount you actually cut off the two reference edges and accept that the layout is slightly conservative at the far end. Both approaches are used by people who know what they are doing.

The one thing not to do

Do not set trim to zero because a sheet looks fine. The damage you are trimming off is the damage you cannot see until the part is finished, which is the worst time to find it.

Set it to 10, cut a job, and look at what came off the edges. If it was clean board every time, go to 5. If a bruise came off on the second sheet, you have just paid two per cent for a part you did not have to make twice.