Confirmat screw

Overview

A confirmat screw, also called a cabinet-connecting screw, is a fastener designed to hold in particleboard, medium-density fibreboard and similar materials. It is common in furniture assembly. It has a blunt tip, a large shank, and often a shoulder: a broad length of unthreaded shank just below the head that helps hold the screw in position. Its coarse thread grips the board's fibres instead of a hole's wall. It is the screw behind most ready-to-assemble cabinets.

Types and sizes

• Standard: 7 mm (about 9/32 in) diameter, 50 mm (2 in) long, with a hex or Pozidriv head.
• Flat or barrel heads: flat sits in a countersink; barrel sits proud in a counterbore.
• Cap: plastic covers hide the head on finished cabinets.
• Drill bit: a stepped drill bit makes the pilot, shank and countersink in one pass.

Where it is used

Knock-down cabinet carcases in plywood or particleboard; the example library's plywood bookcase uses dowelled knock-down joints in the same family. Memphis and Art Deco furniture built from laminated board used glue, dowels and screws; mid-century American case furniture in plywood and particleboard did too. It fits any carcase designed to be assembled, disassembled and moved.

Choosing and sizing

Choose a length that puts about two-thirds of the screw in the end-grain board: a 50 mm (2 in) confirmat fits 19 mm (3/4 in) board into the edge of 19 mm (3/4 in) board. Space screws 50 to 100 mm (2 to 4 in) from the board end and no more than 200 mm (8 in) apart, with a minimum of two per joint. A shelf in a carcase needs one confirmat at each end of the front edge plus a dowel at the back.

Installation

1. Mark the face board for a 7 mm (9/32 in) clearance hole and the edge board for a 5 mm (3/16 in) pilot.
2. Drill the clearance hole through the face board; drill the pilot 45 mm (1-3/4 in) deep into the edge of the other board.
3. Clamp the boards flush and drive the screw until the shoulder seats.
4. Use a drill stop on the pilot: running through the face is a visible mistake.

Wood movement and species

Particleboard, MDF and plywood move very little in width, which is why confirmats hold well. In solid wood, a 7 mm screw across a wide board will split it. Edges of MDF (medium-density fibreboard) and Particleboard are weak in withdrawal; Baltic birch plywood holds better because its plies are all hardwood and run in alternating directions. Do not use a confirmat in solid wood end grain.

Common mistakes

• Pilot too large: the screw spins.
• Pilot too small: the board bulges.
• Too close to a corner: the edge blows out.
• Over-tightening: the head crushes the face.

Related

• Wood screw: the general fixing.
• Pocket-hole screw: the hidden alternative.
• Threaded insert, Cross dowel and bolt: other knock-down fasteners.