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CNC Bike Parts

Bike Part Machining Glossary

58 terms that come up when designing, drawing or quoting a machined bike component, from bolt circle to wall thickness.

Last checked September 27, 2026. Free to use and to link to. Found a term missing or wrong?

A

  • Anodizing, Type IIA decorative and mildly protective aluminum oxide coating, 5 to 25 microns thick, built by electrolysis rather than added like paint. Takes dye evenly across a batch, which is why it is the default finish on machined aluminum bike parts. More on this
  • Anodizing, Type III (hard anodize)A thicker, harder aluminum oxide coating, 25 to 50 microns, used on wear faces such as pedal bodies and chain guides. Limited to dark greys and blacks, and it removes more fatigue life from the part than Type II. More on this
  • As-machinedA finish state with no coating or polishing applied. Tool marks are visible. Common for prototypes and internal parts, rare for anything sold.

B

  • BCD (bolt circle diameter)The diameter of the circle that passes through the center of a bolt pattern, most often on chainrings and direct mount interfaces. Dimensioned as a positional pattern from a center bore, not as separate hole coordinates. More on this
  • Bearing seatA bore or shaft diameter machined to a bearing manufacturer's recommended fit, usually a light press or slip fit. Taken from the bearing maker's published tolerance, not measured off a sample bearing.
  • BilletSolid bar or block stock that a part is machined from, as opposed to a forging or a casting. Bike parts cut from billet pay for the block, including the material removed as chips.
  • Boost spacingA rear hub spacing of 148 x 12 mm and front spacing of 110 x 15 mm, wider than the earlier MTB standard, used to improve wheel stiffness and tire clearance. More on this
  • Bottom bracket shellThe frame tube the bottom bracket fits into, either threaded (BSA, Italian, T47) or bored for a press-fit cup (BB86, BB92, BB30, PF30). Threaded and press-fit shells are not interchangeable. More on this
  • BurrA raised edge of material left by a cutting or drilling operation. Sharp edges and spoke holes are the usual places burrs get missed, and they cause galling or cut skin.

C

  • CAM (computer-aided manufacturing)Software that converts a 3D model into the tool paths a CNC machine runs. CAM programming is a setup cost paid once per part design, which is why short runs cost more per piece than long ones.
  • Center LockA splined disc rotor mounting interface secured with a lockring, as an alternative to the 6-bolt ISO pattern. More on this
  • ChainlineThe distance from the frame centerline to the center of the chainring teeth. Half a millimeter of error here is a common cause of shifting complaints, and the correct figure depends on the drivetrain and hub spacing. More on this
  • ChamferA small angled edge cut where two faces meet, used to remove a sharp corner or ease assembly. Spoke holes and bore edges are chamfered on both sides as standard practice.
  • CNC (computer numerical control)Machining where a computer program controls the cutting tool's path, as opposed to a machinist moving the tool by hand. Almost all modern bike component machining is CNC.

D

  • DatumA reference feature, surface, or axis that other dimensions and tolerances are measured from on a drawing. A bolt circle referenced to the wrong datum will not line up even if every individual dimension is within tolerance.
  • DFM (design for machining)Designing a part around what a cutting tool can actually reach and hold, rather than what looks correct in CAD. Internal corners, deep pockets and thin walls are the usual places a design fights the process. More on this
  • Direct mount (chainring)A chainring interface with no separate bolt circle, splined or keyed directly to the crank spider. Dimensions are proprietary to the crank maker and cannot be estimated from a bolt pattern. More on this

E

  • EN 15194The European standard covering electrically power assisted bicycles, including structural and electrical safety testing for e-bike specific components such as motor mounts and battery brackets. More on this
  • EXW (Ex Works)An Incoterm where the seller's only obligation is to make the goods available at their own premises. The buyer arranges and pays for freight, insurance and export clearance from that point.

F

  • Fatigue lifeThe number of load cycles a part survives before cracking, as distinct from its static strength. Anodizing, especially Type III, measurably reduces the fatigue life of a machined aluminum part. More on this
  • First article inspection (FAI)A detailed inspection report with measured values for a drawing's critical dimensions, produced on the first part of a batch before the rest is released. Cannot be added retroactively once a batch is finished.
  • FixtureA tool that holds a part in a fixed, repeatable position while it is machined. Fixturing is a setup cost paid once, which is why the price per part drops as order quantity rises.
  • Flat MountA disc brake caliper mounting standard using two bolts from below the caliper, used on current road and gravel bikes. More on this
  • Freehub bodyThe splined or threaded part of a hub that the cassette engages with. Shimano HG, SRAM XD/XDR and Shimano Micro Spline are separate, non-interchangeable spline geometries. More on this

G

  • GD&T (geometric dimensioning and tolerancing)A drawing standard for specifying position, flatness, parallelism and similar geometric requirements with symbols and datums, instead of describing them in words. More on this

H

  • Hollowtech IIA Shimano external bottom bracket bearing system using a 24 mm crank spindle, widely copied by other component makers. More on this

I

  • IGESA neutral 3D file format for exchanging surface and solid geometry between CAD systems, similar in purpose to STEP.
  • IS mount (International Standard)An older disc caliper mounting standard using two bolts perpendicular to the mount face. Needs an adapter for most current calipers. More on this
  • ISIS DriveAn open, multi-maker splined bottom bracket spindle interface, 22 mm diameter with 10 splines. More on this
  • ISO 4210The international safety and performance standard for complete bicycles, including fatigue and strength tests that structural components such as stems, cranks and forks are evaluated against. More on this

L

  • Lead timeThe time between placing an order and having parts in hand. Anodizing, heat treatment and other finishing steps usually add more lead time than the machining itself.
  • Low-MOQ productionA production run below a shop's normal minimum order quantity, usually 1 to 50 pieces. Priced high per part because setup and fixturing costs are spread over few units. More on this

M

  • Micro SplineA Shimano 12-speed freehub spline standard, similar in purpose to SRAM XD, allowing a smallest cog of 10 teeth. More on this
  • Mill certificateA document from the material supplier stating the actual chemical composition and mechanical properties of a specific batch of billet, used to verify the material matches the drawing callout.
  • MOQ (minimum order quantity)The smallest quantity a shop is willing to quote and run, set by how the shop wants to recover setup and fixturing cost.

N

  • NDA (non-disclosure agreement)A confidentiality agreement signed before design files are shared, used when a part or geometry is not yet public.

O

  • OctalinkA Shimano splined bottom bracket spindle interface, 22 mm diameter with 8 splines. The V1 and V2 versions differ in spline depth and are not interchangeable. More on this
  • OD / IDShorthand for outside diameter and inside diameter, the two dimensions that define a tube or bore wall thickness.
  • Over locknut dimension (OLD)The distance between the outer faces of a hub's locknuts or end caps, which sets the hub shell length and has to match the frame or fork spacing. More on this

P

  • PassivationA chemical treatment that removes free iron from a stainless steel surface to restore its corrosion resistance after machining. The finishing step for stainless parts, in place of anodizing.
  • Post MountA disc caliper mounting standard using two bolts at 74 mm spacing, the common mount on current mountain bikes. More on this
  • Powder coatingA dry pigment sprayed onto a part and cured with heat, 60 to 100 microns thick. Every bore, thread and mating face has to be masked before coating.
  • Press-fit bottom bracketA bottom bracket shell bored to a specific diameter (BB86, BB92, BB30, PF30, BB386EVO) that the bearing cups press into, rather than threaded. More on this

Q

  • Quality control (QC)The inspection steps a shop runs during and after machining to confirm a part matches its drawing, ranging from spot checks to a full first article report. More on this

R

  • Ra (surface roughness)A measured average of surface texture, in micrometers. Called out only where it matters, such as a bearing seat or a sealing face, because tighter Ra adds machine time.
  • RFQ (request for quotation)A drawing and specification package sent to a machine shop asking for a price and lead time, as distinct from a purchase order.
  • RunoutHow far a rotating feature wobbles off its true axis, measured with a dial indicator. Relevant on bearing seats, freehub bodies and rotor mounting faces.

S

  • Setup costThe one-time cost of programming and fixturing a part before the first piece is cut. Spread over more parts as order quantity rises, which is why unit price drops sharply between short and long runs. More on this
  • Spindle interfaceThe geometry where a crank meets its bottom bracket, for example square taper, Octalink, ISIS, Hollowtech II, SRAM GXP or SRAM DUB. Decides which bottom bracket the crank can run. More on this
  • STEP fileA neutral 3D file format (ISO 10303) for exchanging solid model geometry between CAD systems, the standard file to send alongside a PDF drawing.
  • Straight pull spokeA spoke with no bend at the hub end, requiring a hub flange drilled for a straight hole rather than an angled or counterbored one for a J-bend spoke. More on this
  • Super BoostA rear hub spacing of 157 x 12 mm, wider than Boost, used on some enduro and plus-tire frames. More on this

T

  • T47A threaded bottom bracket shell standard, 47 mm diameter with a 1.0 mm thread pitch, available in several shell widths. More on this
  • ToleranceThe permitted range of variation on a dimension. A drawing with three to six dimensions genuinely toleranced tight gets a narrower quote than one with a single tight default applied to every dimension. More on this
  • TumblingA mass finishing process that vibrates or rotates parts against abrasive media to deburr edges and even out surface texture before anodizing or plating.

U

  • UDH (Universal Derailleur Hanger)A standardized derailleur hanger interface, mounted with an M10 x 1.0 thread, intended to replace the frame-specific hangers that came before it. More on this

W

  • Wall thicknessThe minimum material thickness left in a machined feature, most often a pocketed or hollowed part such as a stem or crank arm. Thin walls flex under cutting force, which limits how far a design can be lightened.

X

  • XD / XDRSRAM freehub spline standards using a threaded cassette interface, allowing a smallest cog of 10 teeth. XDR is 1.85 mm longer than XD, for road cassettes. More on this

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