A few millimetres don't sound like much until the last shelf on a sheet comes out 4 mm short. Allowing for the saw cut and planning the layout is the difference between a cutting list that works and one that sends you back to the merchant.
What saw kerf is
The kerf is the width of the cut a blade makes: the material that turns into dust. It's a little wider than the blade's body because the teeth are wider than the plate. On a track saw or panel saw it's commonly somewhere around 2 to 3.5 mm. Check your own blade: measure a test cut.
Why it adds up
Every cut between two parts costs one kerf. Cut a 2440 strip into five 480 mm pieces and the four cuts between them cost about 12 mm: five pieces need 2412 mm, not 2400. Rip a 1220 sheet into four 300 strips and the three cuts leave you 1209 mm used and only 11 mm spare.
When parts only just fit, a sheet is a cut short. Allow for the kerf in every calculation, not just the tight ones.
Trims and the first cut
Factory edges can be knocked or not quite straight. Many joiners trim a few millimetres off one edge and one end to get a clean, square starting point, and that trim comes off the usable size of the sheet too. Decide what you allow (often 5 to 10 mm) and use it every time.
Layout rules that save sheets
- Rip first, then cross-cut. Cut the sheet into strips the width of your common parts, then cut the strips to length.
- Group parts of the same width. Carcass sides, tops and bottoms are often the same depth: get them from the same strips.
- Place the longest parts first, then fill the offcuts with shorter ones.
- Keep useful offcuts. Narrow strips make plinths, fillers, face frames and battens.
- Mind the grain. On veneered or woodgrain boards, parts can't be turned to fit, so plan those first.
- Label as you cut, so the parts don't get mixed up at assembly.
CNC and other machines
On a CNC router, the gap between parts is set by the cutter's diameter rather than a blade, and parts need space for the cutter and for holding them down. The idea is the same: whatever your machine takes, allow it between every part.
Do it automatically
In JoinerKit you set your saw cut and trims once, in Settings. Every job's cutting list is then laid out on your sheets with them allowed for, with the grain respected, so the sheet count and the price are right.
Questions
Is kerf the same as the blade thickness?
Not quite. The kerf is the width of the cut, which is a little wider than the blade's body because the teeth are set or ground wider. Measure a test cut to find your real kerf.
How much should I allow for the saw cut?
Use your own blade's kerf, commonly around 2 to 3.5 mm for track saws and panel saws, and allow one kerf for every cut between two parts.
Should I trim the edges of a new sheet?
Many joiners trim a few millimetres off one edge and one end for a clean, square start, especially if the edges are damaged. If you do, take that off the sheet's usable size when planning the layout.