Circular Saw

Part of Mill Construction

Adding circular saw capability to a mill — design, blade production, and safe operation.

Why This Matters

The circular saw fundamentally changed the economics of wood processing. A reciprocating (up-and-down) sawmill, whether water-powered or hand-operated, cuts only on one stroke, runs at low speed, and produces a relatively rough cut. A circular saw cuts continuously on every revolution, can run at much higher speeds, and produces a smoother, more uniform cut. The introduction of circular saws in the early 19th century transformed lumber production — a water-powered circular sawmill could cut 10-20 times as much lumber per hour as an equivalent reciprocating mill.

In a rebuilding scenario, a circular sawmill at a watermill site transforms timber availability. Buildings, flooring, furniture, and machine components all require sawn lumber. Producing this lumber manually is among the most exhausting tasks imaginable — a circular saw attachment to an existing watermill makes it practical.

Circular Saw Blade Design

The circular saw blade must be perfectly flat, properly tensioned, sharp, and correctly set (alternating tooth bevel for clearance). Requirements:

Blade material: High-carbon or medium-carbon steel, 2-3mm thick for blades under 400mm diameter, 3-5mm thick for larger blades. The steel must be resilient — tempered to spring hardness, not maximum hardness. A blade that is too hard shatters; a blade that is too soft deflects under cutting load.

Tooth geometry: Saw teeth for rip cutting (along the grain) have steep rake angles (15-20 degrees forward lean) and minimal set (0.3-0.5mm per side). Crosscut teeth have a more negative rake and are filed at an angle to produce a scoring action on the wood fibers. For general mill use, a combination tooth works adequately.

Tensioning: A circular saw blade must be tensioned — stressed internally so the center is under slight compression and the rim is under slight tension. This stiffens the blade against the aerodynamic and cutting forces that would otherwise cause it to flutter. Test tensioning by striking the blade (suspended horizontally) with a hammer — a correctly tensioned blade rings with a clear, even note; an improperly tensioned one rattles or buzzes.

Traditional smith tensioning: heat the center of the blade with a torch or over a forge, then allow to cool, shrinking the center slightly. This prestresses the rim in tension. Check and correct by tapping lightly with a planishing hammer while spinning slowly.

Making a blade from steel plate: Cut a disc of the required diameter with a cold chisel or hacksaw from flat bar stock. File or grind to a true circle, checking with dividers. File teeth by dividing the circumference into equal spaces (mark with a scratch awl and dividers), then filing each tooth to the correct profile. Set alternate teeth left and right with a saw setter (a small anvil-like tool that bends each tooth tip the required amount without cracking the tooth root).

Mounting the Circular Saw

The saw blade mounts on a horizontal shaft (arbor) on flanges that clamp the blade firmly. Critical requirements:

Arbor runout: The arbor must run true with minimal wobble. Any runout is amplified at the blade rim — 0.1mm arbor runout becomes 0.1mm of blade wobble (acceptable), but combined with a slightly unbalanced blade, the total can cause chatter and rough cutting. Mount the arbor in the best available bearings and check runout with a dial indicator before mounting the blade.

Flange design: Two flat flanges, each 1/4 to 1/3 the blade diameter, clamp the blade on the arbor. The inner flange is fixed; the outer is threaded (or keyed and nut-clamped). The flanges must be perfectly parallel and their clamping faces flat. Any taper in the flanges cocks the blade off its rotation plane.

Guard: Every circular saw must have a blade guard covering all of the blade not engaged in the cut. A wooden box guard is simple and effective. The blade emerges from a slot in the bottom and the guard covers the top, back, and sides. A splitter (a thin plate behind the blade, in line with the blade, slightly thicker than the kerf) prevents the cut closing on the blade and kickback.

Feed Mechanism and Table

Unlike a grain mill (where the product falls away under gravity), a sawmill requires controlled feeding of the log or plank against the saw. The feed mechanism must push the work at a consistent rate and resist the kickback tendency of the saw.

Manual feed table: A flat table with a fence parallel to the blade guides the workpiece. The operator pushes the workpiece steadily. Simple but requires attention and skill to maintain consistent feed rate.

Mechanical feed: A pair of power-driven feed rollers (rubber-tired wooden rollers pressed against the top of the workpiece by springs) grip the work and feed it at consistent speed regardless of operator effort. This is the more productive and safer system for production sawing. Drive the rollers from the same shaft as the saw, geared down to an appropriate feed rate (feed rate in mm/sec should match blade RPM times tooth spacing divided by 1000, roughly).

Safe Operation

Circular saw accidents are severe and fast. Non-negotiable safety rules:

Never stand directly behind the blade — kickback throws work violently toward the operator position. Stand to the side.

Never reach over or around the spinning blade to retrieve work or clear chips. Stop the saw first.

Keep the saw guard in place except when absolutely necessary to remove it. Never operate without a splitter in softwood cutting.

Keep hands away from the blade by more than 150mm at all times. Use a push stick for finishing cuts.

Inspect the blade before each use: check for missing teeth, cracks, and secure mounting.