Why wooden toys break

Our first prototype broke in my hand. Once we understood why, we started seeing the same mistake in most of the toys on the shelves. It is easy to spot.

First reason: grain direction

Wood is very strong along the grain and very weak across it. The gap is not small. In beech, bending strength along the grain is around 110 MPa. Tensile strength across the grain is roughly a tenth of that.

Woodworkers call the failure mode short grain. If the fibres in a thin, long part do not run along its long axis, that part is up to fifteen times weaker than it looks.

Where it shows up:

• Animal legs cut sideways out of a board
• The ends of a curved part cut from a single flat board
• Thin projections like comb teeth
• CNC cut parts where orientation was left to the nesting software

How to check in a shop: look at a thin, long part. Do the grain lines run along its length, or do they cross it? If they cross it, that part will break the first time it falls.

Second reason: thickness

This one is less known and the number is striking. The breaking strength of a rod rises with the cube of its diameter.

So increasing diameter by a third more than doubles strength:

• A 6 mm beech rod with 25 mm of free length breaks at roughly 9.5 kg of force
• At 8 mm it takes about 22.5 kg
• At 10 mm, about 44 kg

Those are theoretical failure figures; a safe working limit is around a third of them. Since a child can apply somewhere between 5 and 15 kg with one hand, free standing rods of 6 mm or less carry real risk.

Our rule: at least 8 mm for any axle or rod that projects freely. Between 5 and 6 mm is acceptable for an axle supported at both ends and running through a body. We never use solid beech rod under 4 mm in a toy for under threes.

What happens when it breaks

This part is about safety. A broken wooden piece creates two new hazards: a sharp point and a loose fragment.

The European standard checks the second one with a concrete measure. There is a cylinder with an internal diameter of 31.7 mm and a depth of 57.1 mm, roughly the size of an average three year old’s throat. If a piece that comes off during testing fits entirely inside it, that toy is not legal for under 36 months.

We keep one of these gauges in the workshop and test every prototype with it.

Third reason: moisture

This one works more slowly. Beech grows and shrinks noticeably with humidity. An axle fitted tight will loosen in a dry winter and bind in a humid summer. When it binds, the part under stress cracks.

A well designed toy does not use a press fit axle. The axle is glued and the wheel spins free, with 0.2 to 0.3 mm of clearance. We wrote about this separately.

The short version, toy in hand

• On thin parts, does the grain run along the length?
• Are the free standing rods and axles thinner than a pencil? If so, they are weak.
• Does the wheel turn freely by hand, or does it bind?
• Are the edges rounded? A sharp edge both hurts and lifts a splinter on first impact.