CNC Machined Chainrings and Spiders
A chainring looks like the easiest thing on a bike to machine, and it is not. The tooth form has to work with a chain designed around a different manufacturing process, the bolt circle has to be located as a pattern rather than as five holes, and the offset has to put the teeth on the chainline. Get the offset wrong by half a millimeter and the bike shifts badly across the whole cassette.
What this covers
- Standard and narrow wide 1x rings
- Direct mount rings, SRAM 3-bolt and Shimano
- Spiders and spider-ring assemblies
- Bash guards and chain guides
- Inner rings and granny rings
Interfaces you do not get to choose
- Common MTB BCD104 mm 4-bolt, 64 mm 4-bolt, 96 mm Shimano
- Common road BCD110 mm 5-bolt and 4-bolt, 130 mm 5-bolt
- Direct mountSRAM 3-bolt, Shimano direct mount, proprietary geometry
- Chain pitch1/2 x 3/32 in for derailleur chains
- Boost chainlineAbout 52 mm, against about 49 mm non-boost
Full tables in the standards reference.
Material choice
What each option actually buys you on this part, rather than in general.
| Material | Why it gets used here |
|---|---|
| Aluminum 7075-T6 | The standard. Teeth are thin and loaded directly by the chain, so alloy strength does real work here. |
| Aluminum 6061-T6 | Budget rings and bash guards. Wears faster at the tooth. |
| Steel | Long life rings for cargo and e-bike use, where wear matters more than weight. |
Where the tolerance belongs
These are typical working ranges, not promises. Tolerance these dimensions and leave the rest at a general tolerance, because every step tighter than you need buys machine time you will pay for.
| Feature | Typical | Why it matters |
|---|---|---|
| Tooth width, narrow wide | ±0.02 mm or tighter | The wide tooth has to fill the outer link gap. This is the tightest dimension on the part and the whole function depends on it. |
| Bolt circle position | Positional, ±0.05 mm | Dimension it as a pattern from the center bore, not as separate hole positions. Chained dimensions stack. |
| Offset to tooth centerline | ±0.1 mm | This is the chainline. Half a millimeter is a shifting complaint. |
| Center bore | ±0.05 mm | Locates the ring on the crank or spider. |
| Mounting face flatness | 0.05 mm | A dished ring runs out and wears unevenly. |
Finish
Type II anodize is normal and rings are treated as consumable. Hard anodize resists chain wear but costs fatigue life at the tooth root, so most makers stay on Type II.
Realistic quantities
Prototypes from 5 pieces. Tooth cutting dominates the cycle time, so the price curve is flatter than on a stem and volume helps less than you expect.
What goes wrong on this part
Failure modes specific to this component, collected so you can design them out rather than discover them.
- Deburring is a real line itemA 34 tooth ring has 68 flanks. A burr on any of them is felt by the rider and heard by everyone else. Put the deburr requirement on the drawing.
- Copied ramps that do not workShift ramps and pins are machined geometry, not decoration. A badly copied ramp gives a ring that shifts worse than the one it replaced.
- Color matching to a crank7075 rings will not match a 6061 crank in the same dye. Plan the matched set in one alloy or accept the difference.
- Tooth flank finishA rough flank wears the chain and makes noise. Call out the surface finish on the flanks and nowhere else.
What to send
A drawing with these on it gets quoted in a day. One without them gets quoted with padding in it, or gets four rounds of questions first.
- STEP file plus a PDF drawing
- Tooth count and whether it is narrow wide
- Both tooth widths with tolerances if narrow wide
- BCD, hole count and a positional tolerance from the center bore
- Offset from mounting face to tooth centerline
- Ramp and pin detail, fully dimensioned
- Explicit deburr requirement
Printable version: the RFQ checklist.
Go deeper
- Design
Machining Chainrings: BCD, Tooth Profile and Fits
Bolt circle position, tooth form, narrow wide profiles and chainline offset. The dimensions that decide whether a machined chainring shifts and holds.
- Materials
6061 vs 7075 Aluminum for Bike Parts: How to Choose
7075 is stronger, 6061 anodizes better and costs less. Here is how the two alloys behave in stems, cranks and mounts, and when the upgrade is worth paying for.
- Design
Tolerances on Bike Part Drawings: What to Specify
Steerer bores, bearing seats, chainline and bolt circles need tight numbers. Most of the drawing does not. How to tolerance a part without paying twice.
Other parts
Stems and faceplates
Two clamp bores against fixed standards, a faceplate that has to close evenly and a rider on top of it.
Cranks and pedal hardware
Fatigue critical, envelope fixed by the frame and the rider. The part where anodize choice changes test results.
Hub shells and freehubs
Turning plus milling in one part. Concentricity between the bearing seats decides whether it drags.
Brackets, mounts and hardware
The long tail. Low part cost, fixed interfaces, and finishing batch minimums that dominate the quote.
E-bike drive hardware
Machined like a bike part, loaded like a light vehicle part, regulated like an electrical product.