How Machined Bike Parts Are Made
A stem, a hub shell and a chainring go through different machines and different setups. Knowing which process makes which feature tells you what drives the price, which tolerances are easy and what the finish will do to your dimensions.
Values are typical, not a promise for a given part. Interface dimensions live in the standards tables.
Which process makes which part
Most bike parts use two or three of these. The part pages under parts by type go through each component in detail.
| Process | Typical bike parts | What it does well | Watch for |
|---|---|---|---|
| 3-axis milling | Chainrings, brake adapters, derailleur hangers, pulley cages, spacers | Lowest cost per hour, simple fixtures, fast to program | Every face with a feature is another setup; side features need a flip or a fixture |
| 3+2 and 5-axis milling | Stems, crank arms, seatpost heads, motor mounts, dropouts | Angled holes and sculpted surfaces in one or two clampings, features stay related | Higher hourly rate; worth it when it removes setups, not by default |
| Turning and mill-turn | Hub shells, BB cups, headset cups, axles, top caps, pivot hardware | Round features concentric in one clamping, single-point threads, fast cycle | Thin walls on hub shells move when clamped; flanges and spoke holes need live tools or a second op |
| Thread milling | BB shells and cups, left-hand threads, large or blind threads | One tool for many sizes and both hands, controlled thread depth, no broken taps | Slower than tapping on small threads |
| Tapping | M5 and M6 faceplate, rotor and caliper threads | Fastest route for small standard threads | A broken tap in a finished part is scrap; check engagement depth in aluminum |
| Anodizing | Almost every aluminum part | Color, wear and corrosion protection in one step | Grows dimensions; bores, threads and bearing seats must allow for it or be masked |
3-axis or 5-axis milling
The machine matters less than the number of setups. Every time a part is unclamped and turned over, the features on the new side lose some of their relation to the old one.
A chainring or brake adapter is flat: two setups on a 3-axis mill cut everything, and the hourly rate is the lowest in the shop. A stem has a steerer clamp and a bar clamp at right angles, bolt holes on three faces and a sculpted body. On a 3-axis machine that is four to six setups with dedicated fixtures; on a 5-axis machine it is often two. Fewer setups give better alignment between the two clamp bores and less handling.
3+2 machining (the rotary axes position the part, then lock while a 3-axis toolpath runs) covers most angled holes. Full simultaneous 5-axis is needed only for flowing surfaces such as the outside of a crank arm or a sculpted stem body. The detail is in 5-axis against 3-axis for bike parts.
Turning and mill-turn
Anything round starts on a lathe: hub shells, bottom bracket cups, headset cups, axles, spacers and top caps.
The value of turning is concentricity. A bearing seat, the thread beside it and the face it shoulders against are cut in one clamping, so they share an axis. A mill-turn machine adds live tools for flats, cross holes and spoke holes without moving the part. Hub shells need care: thin flanges and walls distort when clamped hard, so shops use soft jaws or mandrels and finish the bearing seats last.
Bottom bracket cups carry the shell thread from the standards: BSA / ISO threaded 1.37 in x 24 tpi, Italian threaded 36 mm x 24 tpi, T47 47 mm x 1.0 mm thread. BSA drive-side cups are left-hand. See machining T47 and BSA cups and hub shells and freehubs.
Threading standards on bike parts
Threads are an interface: the other part is already made. These are the ones that appear most often, from the standards tables. Metric threads are normally cut to ISO 6H (internal) and 6g (external) tolerance class, inch threads to UN 2B and 2A.
| Location | Thread | Notes |
|---|---|---|
| Pedal to crank | 9/16 in x 20 tpi | Right hand thread drive side, left hand thread non-drive side. One-piece cranks use 1/2 in x 20 tpi |
| Derailleur to hanger | M10 x 1.0 | Standard on modern hangers including UDH |
| Bottle cage | Two M5 at 64 mm | The universal accessory mount pattern |
| Disc rotor bolts | M5 | Six per rotor on the ISO pattern |
| Post mount caliper | M6 | Two bolts at 74 mm |
| Stem and faceplate | M5 | M6 on some single bolt steerer clamps |
Small threads (M5, M6) are tapped. Large, left-hand or blind threads are thread milled or single-pointed on a lathe. Thread engagement in aluminum wants typically 1.5 to 2 times the bolt diameter; where the wall is too thin, a helical insert carries the load. Anodize grows into a thread and can make a bolt bind, so threads are masked or chased after coating. The trade-offs are in thread milling against tapping.
Bearing seats
The tightest tolerance on most bike parts, and the one most often ruined by the finish.
Cartridge bearings press into hub shells, BB cups, pivots and pulleys. The bore is finish-bored, not drilled, and the two seats of one shaft are cut in the same setup so they stay coaxial. Name the bearing by part number on the drawing and work to the bearing maker's recommended fit rather than picking a number; common sizes are listed in the bearing table.
Type II anodize moves a diameter by roughly the full coating thickness, which on a press fit is the difference between a correct fit and a crushed bearing. Mask the seat, or machine it to a pre-anodize size from the anodize growth calculator, and say on the drawing whether dimensions apply before or after finishing. More in bearing press fits.
Lightening pockets
Every cubic centimeter of 6061 removed saves about 2.7 g. The question is where it can come out.
- Follow the load pathPockets belong in low-stress zones. Leave material around clamp bores, pedal threads and bolt bosses.
- Radius the floor and cornersThe corner radius is set by the cutter. A larger radius cuts faster and starts fewer fatigue cracks.
- Keep depth sensiblePockets deeper than about four times the cutter diameter need long tools that chatter. Two shallower pockets from both sides often work better.
- Mind the wallThin walls flex under the cutter and under load. Size them for stiffness, since a stronger alloy has the same stiffness.
- Test the finished partAnodize lowers fatigue life. A pocketed crank or stem is tested anodized, the way it ships. See weight against strength.
Anodizing and other finishes
The last process, and the one that changes your dimensions. Typical thickness from the alloy and finish data; the full comparison is in Type II against Type III.
| Finish | Typical thickness | What it means for your drawing |
|---|---|---|
| Type II, decorative | 5 to 25 microns | Full dye range. Diameter moves by roughly the full coating thickness |
| Type III, hard anodize | 25 to 50 microns | Dark greys and blacks only. Larger fatigue penalty and larger dimensional change |
| Chemical conversion | Under 1 micron | Conductive, corrosion resistant, no meaningful dimensional change |
| Powder coat | 60 to 100 microns | Every bore, thread and mating face has to be masked |
| Cerakote and similar | About 25 microns | Thin, tough, wide color range, more expensive than anodize |
Color varies between alloys and between anodize lots, so a set of parts that must match is best anodized together from the same alloy. Background on the process: anodizing.
A typical process route
The order most machined aluminum bike parts follow, from billet to box. A shop that skips a step usually shows it in the anodize.
- Saw billet or barCut stock with allowance on every face. Plate for stems and cranks, bar for round parts.
- Rough machiningRemove most of the material, leave 0.5 to 1 mm. Large pockets on a crank or stem release stress here.
- Finish machiningClamp faces, bearing seats and threads in as few setups as possible, so the features that must line up are cut together.
- Deburr and edge breakEvery sharp edge is a fatigue starter and a place anodize burns. Done before any blasting or coating.
- Bead blast (optional)Gives the matte look. Mask threads and seats; blast media removes a few microns.
- AnodizeType II for color, Type III for wear. Masking plugs in bores and threads as the drawing calls for.
- Laser mark and inspectLogo and part number through the anodize, then check the interface dimensions against the drawing.
Questions
Does my part need 5-axis machining?
Only if it has features on angled faces or sculpted surfaces that would otherwise need many setups. Stems and crank arms usually benefit. Chainrings, spacers and flat brackets are cheaper on 3-axis machines. Ask the shop to quote the setup count, not the machine.
Should BB cup threads be thread milled or single-point turned?
Both work. A lathe single-points the thread on a turned cup in the same clamping as the bearing seat, which keeps them concentric. Thread milling is used when the thread is in a frame shell or a milled part, and when one tool has to cut both the left-hand and right-hand sides.
How deep should a thread in aluminum be?
Typically 1.5 to 2 times the bolt diameter of full thread. Shorter engagement strips the aluminum before the steel bolt fails. Where the wall cannot allow it, use a helical insert or a steel threaded insert.
Can lightening pockets make a part weaker than it looks?
Yes. Aluminum has no fatigue limit, so a sharp corner at the bottom of a pocket in a highly stressed zone can start a crack long before the static strength is reached. Generous floor radii, pockets kept away from clamp zones and testing the finished, anodized part are the defenses.
Drawing ready?
Send it with alloy, finish and quantity. We match it with up to 3 shops that machine bike parts, and you hear back within 24 business hours.