Vertical Saw

Part of Mill Construction

The vertical (up-and-down) sash saw — the simplest and most buildable water-powered sawmill blade mechanism — converts rotary wheel motion into reciprocating cutting strokes.

Why This Matters

The vertical sash saw is the fundamental mechanism that made water-powered sawmills practical for over five centuries. Unlike circular saws (which require high-speed bearings and very precise blades), the vertical saw requires nothing more exotic than a crank, a connecting rod, and a guided sliding frame. The entire mechanism can be built from wood and basic iron fittings by a competent carpenter and blacksmith working together.

Understanding the specific geometry and construction details of the vertical saw — not just the general concept — is what allows a community to actually build one rather than simply know that such things exist. The difference between a sash saw that runs smoothly for years and one that vibrates itself to pieces in weeks is in the specific proportions, the guide tolerances, and the blade tensioning. These details are what this article addresses.

The Sash Frame

The sash (the wooden frame that holds the blade and slides up and down in the guides) must be:

• Stiff enough not to flex as the blade cuts through resistance
• Light enough not to require excessive energy to reverse on each stroke
• Well-guided enough to keep the blade perfectly vertical throughout the stroke

Typical sash dimensions for a medium sawmill:

• Overall height: 1.5–1.8m (this must exceed the stroke length plus blade length)
• Width: 150–250mm (matching blade width plus some clearance)
• Cross-section of the sash timbers: 100mm × 75mm

The sash is built as a rectangular frame with two vertical members (the “stiles”) connected by horizontal cross-members (the “rails”). The top rail receives the connecting rod from the crank; the bottom rail carries the blade tensioning hardware. Both joints (top rail to stile) must be extremely strong — these take the full reversing load of each stroke.

Use mortise-and-tenon joints with through-bolts or iron strap reinforcement at the top rail. Doweled or pinned joints alone will loosen under the repeated reversing loads.

Guide Design

The guides are the grooves or channels that the sash runs in. They must keep the sash aligned in three dimensions:

• Vertical alignment: guides on both sides prevent the sash from swinging sideways
• Fore-aft alignment: guides prevent the sash from tilting toward or away from the log
• Twist alignment: guides at top and bottom prevent the sash from rotating about the vertical axis

Traditional guide designs:

Timber groove guides: Two heavy posts with deep grooves cut into their inner faces. The sash stiles run in these grooves. Simple to build but requires precise cutting of the grooves. The groove faces should be lined with hardwood (oak or hornbeam) for wear resistance, and lubricated with tallow.

Iron rail guides: Iron straps or bars nailed along the guide posts, forming a channel that the sash slides in. More durable than wood guides but requires more ironwork. The best traditional design uses iron strap both on the contact face and on the back of the sash to form a captive sliding joint.

Rolling guides: Small wooden wheels or iron rollers mounted on the guide posts, against which the sash faces run. Reduces friction dramatically but requires good bearings and regular maintenance. Best for high-speed applications.

Guide clearance (the gap between sash face and guide face) should be 1–2mm — enough to move freely when dry but not so loose that the blade wanders. Account for wood swelling in wet weather; what is 1mm clearance in dry summer may be zero clearance after autumn rains. Either use dried and sealed wood, or use iron-lined guides.

Blade Mounting and Tensioning

The blade mounts at the bottom of the sash between two metal straps bolted across the sash width. The blade must be:

• Vertical: any tilt causes the cut to angle
• Tensioned: the blade must be in tension (being stretched rather than compressed) to run straight

Blade tensioning is achieved by bolts at the top and bottom of the blade mounting:

1. Mount the blade with hand tension only
2. Tighten the top mounting bolts to press the blade against the lower stop
3. Use tensioning bolts at the bottom mounting to pull the blade taut — typically 1/4 to 1/2 turn beyond hand-tight
4. The blade should ring at a consistent pitch when plucked (like a guitar string) across its full width

A properly tensioned blade is very stiff across its width and resists sideways flex during cutting. An under-tensioned blade bows in the cut and produces wavy cuts; an over-tensioned blade may crack or crack the blade mounting hardware.

Crank and Connecting Rod Proportions

The stroke length (how far the sash moves up and down) is twice the crank throw (the offset of the crank pin from the shaft center). Typical strokes for a sash saw: 300–450mm. Larger strokes allow longer blades for deeper cuts; shorter strokes allow faster cycling for thinner stock.

The connecting rod length must be at least 3 times the stroke to minimize the side force on the sash guides. For a 400mm stroke (200mm crank throw), the connecting rod should be at least 600mm pin-to-pin — 800–1,000mm is better.

Short connecting rods transfer more side force to the sash guides (because the connecting rod angle is steeper at mid-stroke), increasing guide wear and causing the blade to be pushed sideways at mid-stroke, reducing cut quality. The length investment is worth making.

The Cutting Sequence

A correctly operating vertical sash saw:

1. Downstroke: The blade cuts through the wood. The log carriage advances by the feed distance. Resistance is highest here — this is when the most power is demanded from the water wheel.

2. Bottom reversal: The crank passes through bottom dead center. The blade stops momentarily, then begins rising. The carriage does not advance during reversal.

3. Upstroke: The blade lifts clear of the cut. Ideally, the blade should be designed to cut on the downstroke only (teeth pointing down-and-back) so that the upstroke is a free return. In practice, the blade often cuts slightly on the upstroke too, which reduces efficiency but produces a cleaner cut surface.

4. Top reversal: The crank passes through top dead center. The blade stops and begins descending for the next cut.

The crank shaft speed controls the strokes per minute. At 50 strokes per minute with a 5mm feed per stroke, the carriage advances 250mm per minute — a 3-meter log takes 12 minutes to cut. At this rate, a productive sawmill might cut 40–50 planks in a 10-hour day.

Feed Mechanism

The log carriage must advance by a fixed amount on each downstroke. The classical mechanism: a ratchet wheel mounted on the carriage rack gear shaft. A pawl driven by a rod connected to the sash engages the ratchet on the downstroke (advancing the carriage) and lifts clear on the upstroke.

The feed per stroke is adjusted by changing the pawl engagement — either by a lever that changes how far the pawl swings, or by using a different ratchet with more or fewer teeth. Adjust feed rate for the wood species and plank thickness:

• Softwood, thin planks: 6–8mm per stroke
• Hardwood, medium planks: 3–5mm per stroke
• Large hardwood logs: 2–3mm per stroke

Too fast a feed rate causes the blade to bind and may stall the water wheel. Too slow wastes operating time. The correct rate produces a steady cutting sound with no binding and no hesitation.