UK CNC Gear Processing: How Accuracy Is Actually Held

• 1
  What UK CNC gear processing actually removes
• 2
  Milling, turning and shaping: which one fits the part
• 3
  Why the setup decides the runout number
• 4
  What the numbers mean on a gear drawing
• 5
  Material and hardness decide the finishing route
• 6
  How we check a gear before it ships
• 7
  Matching the gear to the process

A gear is not a disc with slots in it. It is a set of coordinated surfaces: the involute flanks, the root fillet, the tip circle, the bore, and the face that locates it on a shaft. Every one of those surfaces is measured against the others, so an error in any single one shows up as noise, wear or heat.

UK CNC gear processing removes material with a rotating cutter following a programmed path. The flank is not generated by a cutter that matches the tooth shape, as it is on a hob or a shaper. It is approximated by many passes of a small tool. That difference sets everything else: what you can make, what you cannot, and what you have to measure afterwards.

This matters most at the transition points. The root fillet controls fatigue life, because that is where bending stress concentrates. A sharp internal corner from a small end mill becomes a crack starter. So the tool radius you choose is not a finishing detail, it is a design decision. Write the fillet radius on the drawing before quoting, not after.

The bore and the pitch circle have to agree. If the bore is offset by 0.02 mm, every tooth runs at a slightly different depth of mesh, and the gear will whine at a frequency you can calculate but not fix by tightening the housing.

• 1
  FlankInvolute surface that carries the load
• 2
  Root filletSets fatigue life, not a cosmetic radius
• 3
  BoreLocates the gear; offset shows up as noise
• 4
  FaceControls how the gear sits on the shaft

Three routes cover most gear work we see. Milling cuts the tooth space with an end mill or a form cutter on a 3-axis or 4-axis machine. Turning and mill-turn work is for gears where the blank, the bore and the face are one continuous setup. Shaping and hobbing generate the flank properly but need a dedicated machine.

Milling wins when the gear is large, when the tooth count is low, or when the gear is not really a gear. Sprockets, ratchets, sector segments, timing pulleys and one-off replacement wheels all fall here. Setup is fast, the drawing can change on Tuesday, and you are not paying for a cutter that only fits one module.

Shaping and hobbing win when the same gear repeats in volume and the flank has to be a true involute within a tight total profile tolerance. A hob generates the whole tooth in one continuous motion, so the flank is smoother than a milled approximation. If the gear runs at 8,000 rpm in a gearbox, that difference is audible.

Turning matters more than people expect. On a mill-turn center, we can hold the bore, the two faces and the tooth circle in one setup, so concentricity between the bore and the pitch circle is a machine capability, not a stack of fixturing errors. That is often the deciding factor on a small gear with a long hub.

• 1
  Choose millingPrototypes, low tooth counts, non-standard forms
• 2
  Choose hobbing or shapingRepeating gears, tight profile tolerance, high speed
• 3
  Choose mill-turnBore-to-pitch-circle concentricity is critical

A gear is only as round as the way it was held. Most runout problems we are asked to solve did not come from the cutter path. They came from a three-jaw chuck that squeezed a thin rim, or a vise that lifted the blank 0.03 mm off its seat when the jaws tightened.

Thin-rimmed gears are the classic case. A 2 mm rim on a Ø80 mm gear will deflect under normal clamping pressure and spring back when released. The teeth are then cut in a shape that only exists while the part is in the chuck. Soft jaws bored to the actual blank diameter, or a fixture that clamps on the bore, removes the problem entirely.

For any gear that will be measured for runout, plan the datum before the first cut. If the drawing calls out the bore as datum A, the first operation should establish that bore and every later operation should reference it. Cutting teeth first and boring afterwards means the runout depends on how well two separate setups agreed with each other.

Temperature is part of the setup too. A blank that comes off a saw and goes straight into the machine is not at room temperature. On a 200 mm steel gear, a 5 °C difference is roughly 0.012 mm of diameter, which is more than the tolerance we are trying to hold.

• 1
  Soft jawsBored to the real blank diameter, not nominal
• 2
  Clamp on boreFor thin rims and ring gears
• 3
  One datumBore first, everything else references it

Total profile tolerance and total lead tolerance are the two numbers that decide whether a gear will run quietly. Profile is the error across the flank from root to tip. Lead is the error along the tooth width. A gear with good profile and poor lead will still be noisy, because contact is concentrated at one end of the tooth.

Runout is a different measurement again. It describes how far the tooth flanks move in and out relative to the axis over one revolution. It is measured with a pin or a ball in the tooth space, and it is the number most often missing from the drawing. If the gear meshes with a fixed center distance, runout directly changes backlash.

Surface roughness on the flank sits behind all of these. Ra 0.8–1.6 μm is a normal machined flank. Ra 0.2–0.8 μm needs a finishing pass or a ground flank and costs time. Below that, you are into lapping territory, and the price stops being about machine time.

Backlash is a system property, not a gear property. It comes from the center distance, the tooth thickness and the runout of both gears together. Chasing backlash by tightening the tooth thickness tolerance alone usually makes the parts expensive without making the assembly quiet.

• 1
  ProfileError across the flank, root to tip
• 2
  LeadError along the tooth width
• 3
  RunoutChanges backlash directly
• 4
  BacklashA property of the pair, not one gear

Soft steel is easy to cut and hard to keep. A 1045 or 4140 gear can be milled, then heat treated, but heat treatment moves the part. Distortion after quenching is normal, and if the teeth were finished before hardening, the profile after hardening is not the profile you measured.

The usual answer is to cut soft, harden, then finish. That means leaving 0.15–0.3 mm of stock on the flanks for a finishing operation after heat treatment. Hard turning and grinding both work here. Neither is free, so decide the sequence early, because it changes the blank size.

Aluminium gears are a different story. 6061-T6 and 7075 machine well and hold a good flank finish, but they wear quickly against steel. They belong in low-load applications: instrumentation, prototype drivetrains, timing references. If the gear carries real torque, anodizing helps wear resistance only slightly.

Stainless grades matter for UK food and medical equipment. 303 machines cleanly but has lower corrosion resistance than 316L. 17-4PH gives you hardness after aging, which is useful for small gears that must stay dimensionally stable. Tell us the environment, not just the grade name.

• 1
  Cut soft, then hardenLeave 0.15–0.3 mm for finishing
• 2
  AluminiumGood finish, poor wear against steel
• 3
  17-4PHHardness after aging, stable dimensions

A gear that is not measured is a guess. We check the raw material certificate first, because a grade substitution at the mill is not something you can see after machining. Hardness is verified on the blank before cutting when the drawing calls for a heat-treated condition.

In process, the operator checks the bore, the face and the tooth circle while the part is still fixtured. That is the cheapest moment to catch a drift, because the setup is already correct. After the part comes off, final inspection covers the drawing dimensions, runout, and surface roughness on the flank.

We do 100% inspection before shipment, with reports available on request. For gears that go into a regulated assembly, we can hold the part to the inspection plan you supply. Our quality system is certified to ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022.

What we cannot do is inspect a specification that was never written. If the drawing says "gear, 24 teeth" and nothing else, we can make a gear that matches the drawing and still not match what you needed. Send the mating gear details and the center distance.

• 1
  Material firstCertificate check before any cutting
• 2
  In processBore, face and tooth circle while fixtured
• 3
  FinalDrawing dimensions, runout, flank roughness