An expansion bit is an adjustable boring tool. A cutter slides in a recessed guide in the bit body, is set to the required radius, and is locked by a clamp screw. The centre lead screw pulls the rotating tool axially into wood; it does not adjust the hole diameter.
Expansion Bit Interactive Calculator
Set the cutter radius and separately examine the ideal bore diameter and lead-screw advance while the complete clamped assembly rotates rigidly.
Equation Used
- Cutter edge remains at the entered fixed radius.
- Cutter, clamp and body rotate as one rigid assembly.
- Lead screw advances one pitch per turn without slip.
- No runout, wood response, cutting-force or safe-use model.
Ideal setting and feed geometry only. The cutter is assumed rigidly clamped at the selected radius, and the lead screw is assumed to advance one pitch per revolution without slip. The model does not predict chip load, torque, speed, strength, accuracy or suitability for a particular material.
Sliding cutter, clamp and feed screw
The bit body contains a recessed guide for a movable cutter. Loosen the clamp, slide the cutter until its cutting edge is at the required radius, and tighten the clamp so cutter and body rotate as one assembly.
The centre screw is a separate feature. Its thread pulls the rotating bit axially into the wood; changing its pitch changes ideal feed per turn, not bore diameter.
Adjustable boring in wood
Expansion bits provide a range of hole sizes from one tool. The cited carpentry manual directs the user to loosen the setscrew, slide the cutter to the desired graduation, tighten it, and test the setting in scrap material.
Setting and feed geometry
For cutter radius r, ideal hole diameter is D=2r and circumference is piD. For a lead screw with T threads per inch, ideal pitch is p=25.4/T millimetres per revolution and advance after N turns is A=Np.
These independent relations must not be combined into a chip-load or cutting-capacity claim.
Reading an example
A cutter set to a 50 mm radius gives a nominal 100 mm bore. A 16 TPI feed screw has an ideal pitch of 1.5875 mm per turn, so two turns correspond to 3.175 mm ideal axial advance.
Scope, limitations and sources
Graduations provide a starting setting, not proof of finished-hole accuracy. Wood species, grain, sharpness, runout, clamp security and operator technique all matter. This explanatory model does not establish a safe speed, feed, torque, material limit or structural capacity.
Primary references
Questions
Does the centre screw set hole diameter?
No. The sliding cutter position sets radius; the centre screw supplies axial feed.
Does the cutter move outward while drilling?
No. It must remain securely clamped during rotation.
Should the graduation be trusted without checking?
No. Follow the tool instructions and test the setting in scrap.
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
