Wood screws

Countersunk wood screws to DIN 7997. The everyday fixture: cleats to rails, backs to carcasses, anything screwed rather than joined.

joints:
  - id: cleat_to_rail_left
    kind: screwed
    fixture: fixtures.wood_screw.4x50
    mate:
      a: slat_cleat@left:face.top
      b: side_rail@left:face.bottom
      align: { contact: true }
    fasteners:
      pattern: { along: length, count: 10, spacing: auto, margin: 80 }

That declaration puts ten screws down the cleat, and with each one: a 4.5 mm clearance hole through the cleat, an 8.4 mm × 90° countersink, and a 2.5 mm pilot 35 mm into the rail. You write none of those.

The six sizes

Id	d	Length	Drive	Head Ø
fixtures.wood_screw.3x30	3	30	Pozi 1	6.0
fixtures.wood_screw.4x30	4	30	Torx 20	8.4
fixtures.wood_screw.4x40	4	40	Torx 20	8.4
fixtures.wood_screw.4x50	4	50	Torx 20	8.4
fixtures.wood_screw.5x60	5	60	Torx 25	10.0
fixtures.wood_screw.5x80	5	80	Torx 25	10.0

What each one cuts

Through part — the board the screw passes through. Receiving part — the one it threads into. The screw enters on the mating face, the one touching the other part.

Id	Clearance	Countersink	Pilot	Pilot depth
3x30	3.4	6.0 × 90°	1.9	22
4x30	4.5	8.4 × 90°	2.5	20
4x40	4.5	8.4 × 90°	2.5	28
4x50	4.5	8.4 × 90°	2.5	35
5x60	5.5	10.0 × 90°	3.2	42
5x80	5.5	10.0 × 90°	3.2	55

All in millimetres. Pilot diameters are for hardwood; softwood tolerates the same pilot and simply holds a little less well, which the species’ withdrawal rating already captures — so the catalogue does not fork per species.

Threads are never modelled. The mesh is a smooth shank with a head, which is all a 3D preview needs and a fraction of the triangles.

The rules each one imposes

Id	Min edge	Min end	Min engagement	Min spacing	Max through
3x30	9	18	14	24	14
4x30	12	24	15	32	14
4x40	12	24	18	32	22
4x50	12	24	20	32	26
5x60	15	30	25	40	32
5x80	15	30	30	40	48

Edge, end and spacing follow the usual shop rules of 3d, 6d and 8d, resolved against the nominal diameter. Every one of these is checked, and every failure comes back with a computed suggestion rather than an observation.

Min engagement is how much thread has to be in the receiving part. This is the check that catches the classic error:

FAS-003  Screws in `cleat_to_rail_left` have 0 mm of engagement — the screw
         passes through 40 mm of a 40 mm cleat. Use
         `fixtures.wood_screw.4x50`, which gives 20 mm.

Max through thickness is the most board the screw can pass through and still reach. A 4×40 through a 25 mm cleat has 15 mm left, which is under its 18 mm minimum engagement — so the compiler tells you to go to a 4×50 before you have drilled anything.

End grain

Every size carries forbid_into_end_grain: warn. A screw into end grain holds roughly half as well as one into face grain, and every timber in the catalogue rates worse in the end than the face.

It is a warning, not an error, because it is sometimes the right answer — a screw into the end of a cleat that is also glued and carries no load is fine. When it is carrying load, the answer is usually a threaded insert or a bed bolt, which threads into steel rather than into wood.

Choosing a size

Work from the engagement, not the total length:

screw length − thickness of the through part ≥ min engagement

A 25 mm cleat onto a 38 mm rail: a 4×40 leaves 15 mm (short of 18), a 4×50 leaves 25 mm. Take the 4×50.

Then check the receiving part is thick enough that the pilot does not come out the far side. A 5×80 pilots 55 mm deep, which is more than a 38 mm rail has.

The compiler does both of these. The point of knowing the rule is being able to pick correctly the first time.

Procurement

Id	Pack sizes	Unit cost
3x30	200 · 500	0.04
4x30	100 · 200 · 500	0.05
4x40	100 · 200 · 500	0.06
4x50	100 · 200	0.07
5x60	100 · 200	0.11
5x80	50 · 100	0.15

The BOM rounds up to whole packs, so eleven screws is one box of 100.