Custom Machined Spur Gears

A Gear Is Math, Not a Tracing Job

The reason a worn gear is one of the most reproducible parts we make is that its geometry is defined by a handful of standard parameters and a mathematical involute tooth form. We don't copy the damaged teeth — we derive the correct gear from its numbers: tooth count, module, pressure angle, face width and bore. Get those right and the new gear meshes exactly with its partner, even if the sample was chipped or missing teeth.

Custom Spur Gear Specifications

Attribute	Capability
Module / pitch	~0.5–8 module (DP equivalents)
Pressure angle	14.5° / 20° / 25°
Materials	4140 / 8620 / 4340 steel, brass, bronze, Delrin/nylon
Hardening	Case-harden, through-harden, nitride (as required)
Bore & keyway	To shaft fit; keyway to DIN 6885 / ISO 773
Quality grade	To AGMA 8–11 / DIN 7–9 (ISO) as the application needs
Quantity	One replacement to batch of spares

Spur Gear Dimensions & Formulas — How to Read a Sample Gear

A spur gear is fully defined by a few numbers, all tied to the module (m) and tooth count (N). The same formulas let you derive a gear's spec from a worn sample — count the teeth, measure the outside diameter, and the rest follows:

Dimension	Formula (metric module)
Module (from a sample)	m = OD ÷ (N + 2)
Pitch diameter (PD)	PD = m × N
Outside diameter (OD)	OD = m × (N + 2)
Root diameter	≈ m × (N − 2.5)
Addendum / dedendum	1 × m / 1.25 × m
Circular pitch	p = π × m
Centre distance (two gears)	m × (N₁ + N₂) ÷ 2

So to reproduce a gear, we really only need tooth count, outside diameter and pressure angle (20° unless it's clearly fine-pitch 14.5° or heavy-duty 25°), plus the bore and keyway. Send the worn gear and we read all of it off; send the mating gear too and we cross-check the centre distance. This is why a gear — unlike most parts — can be perfectly reproduced even when the teeth are damaged.

Gear Quality — Spec It by Application

"Custom gear" buyers usually need to state a quality grade (AGMA class, or DIN/ISO) — it sets the tooth-form accuracy, runout and backlash. Higher grades cost more, so match the grade to the job rather than over-specifying. A practical guide:

Application	Typical module	Quality grade	Material
General machinery / handling	M3–8	AGMA 6–8	Case-hardened steel
Pumps & drives	M4–10	AGMA 7–9	Alloy steel (4140)
Machine tools	M2–5	AGMA 8–10	Ground case-hardened
Precision instruments	M1–3	AGMA 9–11	Low-backlash, fine pitch

Not sure what grade you need? Tell us what the gear drives and how fast — or just send the worn gear — and we'll spec it.

Gear Variants We Also Make

• Pinions — small high-ratio gears
• Cluster gears — multiple gears on one blank
• Gear-shaft combinations — gear integral with its shaft
• Internal gears and racks (see replacing a discontinued gear)

Material & Hardening

For power transmission we usually machine gears in 4140 or 8620 steel and harden them so the new gear matches — or outlasts — the original's wear life. Surface hardening (case-hardening or nitriding) can raise the allowable contact stress roughly 2–3× over a through-hardened gear, so a discontinued gear is a chance to make it last longer. Brass or bronze suits quiet light-duty drives; Delrin and nylon make low-noise plastic gears — we machine them, or mold them for higher quantities as part of being a full custom parts maker. Tooth form and bore are verified on inspection.

Frequently Asked Questions