Metal Gear Cutting

Part of Gear Making

Cutting gear teeth in metal using hand tools, formed cutters, and simple machine setups.

Why This Matters

Metal gears are the backbone of durable machinery. Wooden gears serve in lightly loaded, slow-speed applications and are easily made, but they wear rapidly under load and cannot handle the forces required by serious mill drives, metalworking machinery, or precision mechanisms. Metal gears — initially cast iron or bronze, later steel — extend the range of what machinery can do and dramatically increase service life.

Metal gear cutting by hand is demanding but achievable. Before milling machines and gear hobbers existed, craftsmen cut gears by marking out tooth spaces, drilling holes to define the root of each space, and filing the flanks to shape. The method is slow — cutting a 30-tooth iron gear by hand takes two to three days — but produces a functional gear with proper tooth profile.

Understanding the progression from fully hand-cut to semi-machine-assisted gear cutting allows a rebuilding community to produce increasingly precise gears as its tool-making capability improves. Each step in this progression produces better gears more efficiently.

Step 1: Preparing the Blank

The gear blank is the prepared disc of metal that will be machined into the gear. It must be:

Material: Cast iron is the most accessible early metal for gear blanks. It is brittle (care needed against impact loads) but casts easily to near-net shape and machines well. Bronze is better mechanically (tougher, less brittle) and meshes well against iron, but requires more copper and tin. Wrought iron can be forged and machined but is more labor-intensive to produce in flat disc form.

Dimensional accuracy: The blank must be cylindrical within 0.3–0.5 mm. Face must be flat and perpendicular to the shaft axis within 0.2 mm per 100 mm diameter. Achieve this by:

• Turning on a lathe to final diameter (best)
• Filing to roundness against a scribed circle (slower but workable)
• Boring or drilling the center bore concentrically with the outside diameter

Finish: Turn or file the outside diameter to a smooth finish — this will become the top circle of the gear teeth. Mark the pitch circle (at a radius less than the full OD by one addendum) with a scribed line.

Step 2: Dividing the Blank

Using a dividing plate or compass-arm method, mark all tooth positions on the pitch circle. Transfer these marks to the tooth tip circle (OD) and the tooth root circle (smaller diameter). Connect marks with scribed lines defining each tooth space boundary.

Scribe the full depth of each tooth space from the tip to root circle. A pointed metal scriber and a steel rule are sufficient for this.

Step 3: Drilling Out the Root

Each tooth space has a semicircular root. Drill a hole at the bottom of each tooth space at the root circle diameter. Drill diameter should equal the root fillet diameter (typically 0.2–0.3 × module). This removes most of the metal from the root of each space and defines the minimum diameter cleanly.

The drill must enter perpendicular to the face and centered on the scribed line for each tooth space. A drill press or carefully supported brace achieves this. Mark each hole center with a center punch before drilling.

After drilling, the holes appear as a series of scallops around the inside of the gear space pattern. These define the roots of the future teeth.

Step 4: Hacksawing Out Tooth Spaces

Use a fine-tooth hacksaw (or hand bow saw with a metal blade) to cut down each side of each tooth space from the OD to the drilled root hole. Two cuts per space, angled to follow the tooth flank lines.

The saw cuts need not be precise at this stage — leave 0.5–1.0 mm of material for filing. Cutting through the full metal depth takes time: for a 20 mm deep tooth in cast iron, expect 5–10 minutes per cut. A 30-tooth gear requires 60 saw cuts — a full day’s work.

Use the hacksaw cuts to remove the bulk of the waste metal from each space. The remaining rough tooth flanks are cleaned up by filing.

Step 5: Filing Tooth Flanks

This is the critical step that determines gear quality. File each tooth flank to the correct involute profile using flat files and half-round files.

Profile template: Make a tooth form template from thin sheet metal (a saw-blade blank or thin iron strip) filed to the correct involute profile at the gear’s module. Use this template to check each tooth flank during filing — place the template on the flank and check for gaps that indicate high spots.

Filing sequence: Work one flank at a time, keeping the file perpendicular to the tooth face (not angled). Use a flat file for the working flank (straight or slightly involute portion), and a half-round file near the root fillet where curvature is needed.

Check with the template every few strokes. Remove material only where the template shows high spots. Stop when the template fits the full flank without visible light gaps.

Pitch line check: After filing several teeth, hold the gear against a known-good template or mesh it lightly with a pre-made gear. Check for smooth, even engagement.

Step 6: Formed Cutter Methods

When a lathe or simple milling frame is available, a formed rotary cutter dramatically speeds the process. The cutter is a disc of hardened steel with the tooth space profile ground into its edge.

Making the cutter: File a blank disc of tool steel to the negative profile of the gear tooth space. Harden and temper the filed profile. Mount on an arbor. This is advanced tool-making, but once made, the cutter can cut hundreds of teeth.

Using the cutter: With the gear blank indexed (using the dividing plate), pass the cutter through each tooth space in succession. Each pass cuts one space to nearly finished form. Light cleanup filing may still be needed for the final profile.

With a formed cutter and dividing plate, a skilled operator can cut a 30-tooth gear in 4–6 hours rather than 2–3 days. This is the critical capability improvement that enables a community to produce gears at useful rates for machinery building programs.

Quality Acceptance Criteria

A finished metal gear is acceptable for service when:

• All tooth spaces are complete with no remaining drill marks or saw marks
• Root fillets are smooth curves, not sharp corners
• Tooth flanks show consistent file marks with no obvious high spots
• Adjacent tooth thicknesses vary by less than 0.3 mm (check with calipers)
• The gear runs smoothly by hand mesh with its mating gear with acceptable backlash
• No burrs remain on any tooth edge