CNC Bicycle Parts Guide: How Machining Changes the Ride

• 1
  Where CNC bicycle parts beat forging and casting
• 2
  Material choice decides stiffness, weight and fatigue life
• 3
  Why five-axis setups suit bicycle geometry
• 4
  Wall thickness and tolerance limits you should design to
• 5
  Post-processing, fatigue and the parts that should not be machined
• 6
  Which process fits which bicycle part

Forging and casting push metal into a die shape. CNC machining starts from a solid billet and removes what is not needed with a rotating cutter. That difference is not cosmetic. A forged crank arm keeps its parting line and draft angle; a machined one can carry a wall that tapers from 8 mm at the spindle to 3.5 mm at the pedal eye, because the cutter follows the load path instead of the die.

The second difference is repeatability. Once a program is proven, the first part and the five-hundredth part sit within the same tolerance band. GreatLight holds ±0.005 mm on critical features, which matters on interfaces like bottom bracket shells, head tube bores and disc brake mounts where two parts from different suppliers have to mate without shimming.

The cost model is also different. Tooling for a forging can run into five figures before the first part exists. Machining has no die, so a single prototype and a 10,000-part run use the same program. That is why prototype suspension links and custom stems are almost always machined first.

The limit is geometry. Deep internal cavities, hollow one-piece shells and thin uniform webs are hard to cut because the tool has to reach the surface. When a part needs those features, casting or 3D printing is the better starting point.

• 1
  Machining winsLoad-following wall thickness, tight interfaces, low volume
• 2
  Casting or forging winsHollow internals, very high volume, low unit cost
• 3
  Hybrid routeCast or print near-net, then machine the interfaces

Aluminium 6061-T6 is the default for stems, seatposts, chainrings and linkage plates. It machines cleanly, takes anodizing well and costs less than 7075. Yield strength sits around 275 MPa. For most riders that is enough. Where it is not enough is a thin-wall downhill linkage or a crank spider with narrow arms, because stiffness scales with the section, not with the alloy name.

7075-T6 raises yield strength to roughly 500 MPa, so a 7075 part can run a thinner wall and still carry the same load. It costs more and anodizes to a narrower colour range. It also machines with more chatter if the setup is light, so it wants a rigid fixture and a shorter tool overhang.

Titanium Ti-6Al-4V (TC4) sits between steel and aluminium on density and has excellent fatigue behaviour, which is why it shows up in custom frame dropouts and spindle hardware. It cuts slowly, about one quarter the speed of aluminium, and the chips are abrasive. Budget accordingly.

Steel still owns the high-stress rotating parts. 4130 and 4140 are common for axles and spindle shafts; 17-4PH stainless appears in disc rotor carriers and bolts that need corrosion resistance. Carbon fibre is not machined as a structural part. It is machined as a plate or as a tube insert, and the cut edges need sealing.

• 1
  6061-T6General brackets, stems, seatposts, chainrings
• 2
  7075-T6Thin-wall linkages, high-load spiders
• 3
  Ti-6Al-4VDropouts, spindle hardware, fatigue-critical small parts
• 4
  4130 / 17-4PHAxles, rotor carriers, corrosion-resistant fasteners

A bicycle part is rarely a box. A stem has a steerer bore on one axis, a bar clamp on another and a face that has to stay square to both. On a three-axis machine each of those faces needs its own fixture, and every refixture adds stack-up error. On a five-axis center the part rotates under the tool, so the bore and the clamp face come off the same setup.

That matters most on parts with an organic outer skin. A machined crank arm or a one-piece linkage has curved surfaces that a ball nose cutter can only approximate in three axes. Tilting the tool keeps the contact point at the cutter tip, which holds the surface without a long series of passes. GreatLight runs 16 simultaneous five-axis machining centers, with a Ø400 mm rotary table for parts that need to spin through a full rotation.

Fixturing is still the hard part. Thin arms deflect under clamping force, so soft jaws, vacuum plates or sacrificial tabs are used to hold the part without crushing it. A part that measures correctly in the fixture can spring back once released, so the inspection has to happen off the machine.

The practical ceiling for size is 4,000 mm on the largest travel. Almost no bicycle part needs that. The useful numbers are the mid-range travels of 750 × 1,150 × 550 mm and the compact 500 × 500 × 450 mm, which cover frame lugs, swingarms and e-bike motor mounts.

• 1
  One setup, more featuresFewer refixtures means less stack-up error
• 2
  Surface qualityTilted tool keeps tip contact on curved skins
• 3
  Watch the clampingSoft jaws or tabs on thin arms; inspect off the machine

Aluminium walls thinner than 1.5 mm start to ring and deflect during cutting, even with light passes. Between 1.5 mm and 2 mm is workable if the part is supported, the cutter is sharp and the stepdown is small. Below 1.2 mm the scrap rate climbs fast. If the design needs a 0.8 mm wall for weight, that is a good sign the part should be reshaped rather than machined thinner.

Tolerance should follow function, not habit. A bearing bore or a brake mount earns ±0.005 mm. A cosmetic outer profile does not. Calling ±0.005 mm across a whole drawing raises inspection time and cost without improving the bike. Mark the datums and the mating features, and let the rest run at general tolerance.

Surface finish ties into the same trade-off. An as-machined finish of Ra 1.6–3.2 μm is fine for hidden structure. Ra 0.8–1.6 μm suits seal and bearing bores where friction matters. Ra 0.2–0.8 μm is reserved for sliding surfaces and spindle journals, and it adds polishing time.

Corner radii are the quiet cost driver. A cutter needs radius to clear the corner. Internal corners smaller than the tool radius have to be plunged or EDM-cut, both slower. Designing a 3 mm internal radius where a 6 mm one would work can double cycle time on a pocketed part.

• 1
  Aluminium wall1.5–2 mm supported; below 1.2 mm expect scrap
• 2
  Critical bore±0.005 mm, datum-referenced
• 3
  General profileLooser tolerance, faster inspection
• 4
  Internal radiusKeep at or above the cutter radius

Machining leaves tool marks. Those marks are stress risers. Polishing a fillet or bead blasting a linkage arm removes the sharp valleys and raises fatigue life, which is why finishing is not only about looks. Anodizing adds a hard oxide layer that resists abrasion on chainrings and pedal bodies. Hardcoat anodizing is thicker and used where the surface slides.

Plating serves a different purpose. Electroless nickel gives a uniform coating on complex geometry and resists corrosion on steel hardware. Black oxide is a thin, low-cost finish for bolts and pins that will be oiled anyway. Laser marking is how serial numbers and torque specs get onto a part, with a minimum character height of 1.5 mm so the mark stays legible after anodizing.

Not every bicycle part belongs on a mill. A one-piece hollow carbon frame cannot be machined. A cheap stamped chain guard should stay stamped. A high-volume derailleur cage with complex internal ribs is usually die cast and then machined only at the pivot bores. The engineering question is whether the part needs load-following geometry and tight interfaces. If yes, machine it. If no, the cheaper process is the correct process.

GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring and a final report on request. That matters on a batch of brake mounts where one out-of-tolerance bore becomes a warranty claim.

• 1
  Polish filletsRemoves stress risers, improves fatigue life
• 2
  AnodizeWear resistance on chainrings and pedal bodies
• 3
  Laser markMinimum 1.5 mm character height
• 4
  Do not machineHollow one-piece shells, high-volume stamped guards