A spoon bit is a bit whose working end has the shape of a spoon or half tube. In building the term covers two different tools: a soil auger for ground investigation, and a wood bit for hollowing out timber.
The spoon bit as a soil auger
In ground investigation the spoon auger is a hand tool for establishing the soil profile on the spot. You turn the auger into the ground; when you withdraw it, the soil stays in the spoon-shaped section. That sample can be looked at, felt and identified: sand, clay, peat, loam, rubble.
The same family includes the screw auger and the Edelman auger; which one you use depends on the soil type. In dry, loose sand a closed spoon or piston auger works better than an open one, because otherwise the sample simply runs out.
What is it used for?
- For an extension or garden building: establishing at what depth load-bearing ground lies and whether there is made ground or rubble.
- For damp problems: determining whether a clay layer or a groundwater table sits just below the foundation.
- For soakaways: checking whether the ground is permeable enough for an infiltration crate or gravel soakaway.
Important: a hand-augered hole gives a picture of the soil profile but not a bearing capacity calculation. For a foundation, a cone penetration test with its borehole log remains the basis; hand augering is the cheap first reconnaissance.
The spoon bit as a wood bit
In woodworking a spoon bit is a bit with a spoon-shaped blade that cuts along its side. It does not cut from a centre point the way a modern twist bit does, but pares and scrapes from the edge. As a result it can:
- be started at an angle without wandering, which historically made it indispensable in chairmaking, where legs enter a seat obliquely;
- shift or enlarge a hole afterwards, something a twist bit cannot do;
- hollow out timber, as in making clogs, troughs and hollowed logs.
Practical tips
- Auger: turn, do not wrench. Wind the auger in steadily and pull it out straight; levering widens the hole and makes the sample unreliable.
- Record the hole. Note the depth and soil type of each layer — that is your borehole log. Without a record the hole is worthless afterwards.
- Watch for buried services. Even a hand auger can strike a cable or pipe. Check service drawings or request a utility search before boring.
- Wood bit: turn by hand or run slowly. A spoon bit is made for a brace or a low machine speed; at high revolutions it burns and wanders.
- Keep the cutting edge sharp. Hone on the inside of the spoon with a curved slipstone; grinding the outside changes the shape and therefore the behaviour of the bit.
Related terms
- Soil auger
- Edelman auger
- Increment borer
- Hand drill
- Borehole log
- Gimlet
- Chisel
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