Audio Manufacturing Glossary
70 terms that come up in an audio parts RFQ, defined as a buyer meets them rather than as a dictionary would. Where a figure exists, it is here.
Materials
- 6061-T6
The default aluminum alloy for milled audio parts. Strong, machines cleanly and anodizes well, though slightly greyer than 6063 because of its higher silicon and copper content. Supplied as plate and bar.
- 6063-T5
The extrusion alloy. Lower alloying content than 6061, so it anodizes clearer and brighter, which is why it is the reference for architectural anodize. Softer, so it burrs easily and marks in handling.
- 5052-H32
A formable sheet alloy used for folded covers and back panels. Anodizes to a slightly different tone from the 6000 series, which shows when a folded cover sits next to a milled panel.
- C360 brass
Free cutting brass, the benchmark for machinability. Used for knobs, feet and trim. Contains lead, so check market restrictions. Tarnishes within weeks unless lacquered or plated.
- C110 copper
Electrolytic tough pitch copper. Very gummy to machine and tarnishes faster than brass. For machined decorative parts, C145 tellurium copper cuts far better and looks close enough.
- Billet
Solid plate or bar that a part is cut out of. Gives any geometry with no tooling, at the cost of turning most of the material into chips.
- Mill lot
One production batch of metal from one mill. Two lots of the same alloy, both fully in specification, can anodize to visibly different shades. The usual explanation when a repeat order looks slightly off.
- Temper
The heat treatment and work hardening state of an alloy, written after the dash: T6, T5, H32. It changes strength, machinability and how the surface finishes, so it belongs on the drawing with the alloy.
Finishing
- Anodizing
An electrochemical process that converts the surface of aluminum into a porous oxide layer. Dye sits in the pores. It is not a coating applied on top, which is why it cannot chip.
- Type II anodize
Conventional sulphuric acid anodize per MIL-A-8625, typically 10 to 20 microns. Class 1 is undyed, Class 2 is dyed. The normal choice for any audio part that has to match a colour.
- Type III hard anodize
A thicker, harder layer, typically 25 to 50 microns. Darker and much less predictable in colour. Right for a wear surface, wrong for a front panel that has to match anything.
- Rack mark
The contact point where a part was clamped to the anodizing rack to carry current. Every anodized part has at least one. If you do not choose where it goes, the finisher does.
- Dye lot
One batch of dye bath. Dye depletes as it is used, so a fresh bath and a tired bath give different results with the same specification. One reason to anodize a whole product family in one load.
- Sealing
The step that closes the pores after dyeing, by hot water, nickel acetate or a cold seal. It locks the dye in and slightly shifts the final colour, so it is part of the colour specification.
- Bead blasting
Propelling media at a surface to produce an even matte texture. Glass bead peens and gives a soft satin. Aluminum oxide cuts and gives a harsher, flatter grey rarely right for hi-fi.
- Media mesh size
The size range of blast media, written as a mesh range such as #80 to #120. A smaller number means a larger bead and a coarser finish. Specify it or you get whatever is in the cabinet.
- Brushed finish
Controlled directional abrasion that leaves fine parallel scratches, so light reflects along the grain and reads as satin. Directional, so it must be specified with a grit and an arrow.
- Grain direction
Which way a brushed finish runs. Turn a brushed part ninety degrees and it looks like a different colour. If two parts sit side by side, the grain relationship is a design decision that belongs on both drawings.
- Lacquer
A clear coat over brass or copper to stop it tarnishing. Two part polyurethane is the durable option. The critical step is degreasing, because anything left underneath shows as a permanent cloudy patch.
- Electroplating
Depositing nickel, black nickel, chrome or gold over a base metal. The plating reproduces the surface underneath, so the part has to be polished or blasted first exactly as if the plating were not there.
- Laser marking
Burning through the dye in an anodize layer to leave a light grey or near white mark. Happens after finishing, so artwork can change late. High contrast on black, weak on clear.
- Engraving
Cutting a character into the metal with a small cutter, before finishing. The anodize then covers the groove, so the mark reads as a shadow and is effectively permanent. Inside corners carry the cutter radius.
- Silkscreen
Ink pushed through a mesh screen onto a finished surface. Any colour including opaque white, but it sits on top of the finish and wears where fingers rub. A screen per colour makes it expensive below about fifty pieces.
- Pad printing
Ink lifted from an etched plate by a silicone pad and pressed onto the part. Better than silkscreen on curved surfaces, which makes it the usual choice for knobs.
- Passivation
A chemical treatment that removes free iron from a stainless surface and restores its corrosion resistance. Not a cosmetic process, though it slightly evens out appearance.
- Powder coating
Electrostatically applied powder, cured in an oven. Thick and durable, and it softens sharp detail. Good on a steel cover, rarely right on a machined front panel.
Machining and forming
- Extrusion
Pushing heated aluminum through a steel die to produce a continuous profile. Cheap per length once the die exists, with a die cost of a few thousand dollars and six to ten weeks lead time.
- Die cost
The one-off tooling charge for a custom extrusion profile. Typically pays back against billet machining somewhere between 200 and 500 units, though the real trigger is usually whether the shape is possible in billet at all.
- Sheet metal forming
Cutting flat sheet by laser or punch, then bending it on a press brake. The cheapest way to enclose a volume. Bend radius is visible and tolerances accumulate across a folded part.
- Setup
Everything paid once per order regardless of quantity: programming, fixtures, tool setting, first article. On a run of 20 to 50 parts it is often the largest single line on the quote.
- Deburring
Removing the raised edge left by cutting. On a cosmetic part it is bench work done by hand, one part at a time, and on small parts like knurled knobs it can cost more than the machining.
- Knurling
A textured grip pattern. Rolled knurling displaces material so the diameter grows and the pattern can look uneven where the roll starts. Cut knurling is cleaner, slower and more expensive.
- Countersink and counterbore
Two ways to recess a fastener. A countersink is a cone for a flat head screw, a counterbore is a flat bottomed pocket for a cap head. On a visible panel the recess is a cosmetic feature and needs its own finish note.
- Witness mark
A visible line where two toolpaths or two setups meet. Invisible on a functional part and a defect on a cosmetic one. Brushing does not hide a deep one.
- Chamfer
An angled edge break, written as for example 0.5 x 45 degrees. On audio parts it is both a safety feature and a tolerance strategy, because a chamfer at a joint creates a shadow line that hides misalignment.
- Fixture
The tooling that holds a part while it is machined or finished. Refixturing between batches is a common and invisible cause of batch two looking different, so ask the shop to keep and label yours.
Tolerance and inspection
- Bow and twist
Deviation along the length of a long part. Extrusion tolerances often allow 1 mm per metre or worse, and a thin rack panel can bow when its corner screws are tightened. Neither shows on a flatness check of a short section.
- Tolerance stack
The accumulation of individual part tolerances across an assembly. A visible gap is a stack, not a dimension, which is why every part can measure correctly while the product looks wrong.
- Panel gap
The visible space where two parts meet. Uniformity matters far more than size: a consistent 1.2 mm gap reads as precise, a gap varying from 0.4 to 0.9 mm reads as broken.
- Datum
The reference feature everything else is measured from. On a turntable platter it is the bearing bore. Choosing the datum badly is how a drawing that is fully dimensioned still produces the wrong part.
- TIR
Total indicated runout. How much a surface moves relative to a datum through one full rotation. The callout that separates a platter drawing from a drawing of a disc.
- Flatness
A form tolerance: how far a surface may deviate from a perfect plane. Not the same as a thickness tolerance. A part machined to thickness within 0.05 mm can still be visibly dished.
- Ra
Arithmetic average surface roughness in micrometres. Lower is smoother. Useful on a sealing or mounting face. On a cosmetic face it does not capture what the eye sees, so it is a poor substitute for a reference sample.
- Cosmetic zone
A labelled region of a part with its own appearance requirement. A three zone scheme, primary, secondary and non-cosmetic, tells a shop where a defect matters and where a rack mark is acceptable.
- Reference sample
A physical approved part, dated and held by both buyer and supplier, that later batches are judged against. The only thing that closes the gap between a written specification and an appearance.
- Retained sample
The copy of the reference sample kept for the life of the product. Judging batch four against a part in a drawer beats judging it against a memory or a photograph.
- First article
A small number of parts made on production fixtures and through the real finish, inspected and signed off before the batch runs. On cosmetic work, three pieces tell you about consistency in a way one cannot.
- D65
A standard daylight illuminant used as the reference light for judging colour. Specifying "D65 diffuse light at 300 mm" turns an opinion about appearance into a repeatable test.
Audio specific
- Rack unit (U)
The vertical pitch of a 19 inch rack: 44.45 mm. A panel is made slightly shorter so a stack does not bind, by the formula h = 44.45n minus 0.79 mm, giving 43.66 mm for 1U.
- EIA-310
The standard governing 19 inch rack dimensions: 482.6 mm nominal panel width, 465.1 mm horizontal hole spacing and a 7.14 mm clearance hole. It does not cover the clearances that actually cause panels not to fit.
- Faceplate
The front panel of an audio component, usually milled from 6061 billet between 3 and 10 mm thick. The one part where the surface is the product rather than a property of it.
- Fin pitch
The spacing between heatsink fins. For natural convection, 8 to 12 mm is the usual optimum. Below about 5 mm a natural convection sink starts losing performance as fins are added, because the airflow chokes.
- Thermal resistance
Degrees Celsius of temperature rise per watt dissipated. Extruder figures assume a 75 mm section, vertical fins, free air and a 75 degree rise. Change any of those and correction factors apply.
- Rotational inertia
What resists speed change in a turntable platter. It scales with the square of the radius, so mass at the rim counts far more than the same mass near the centre. Heavier is not the point, distribution is.
- Residual unbalance
How far a rotating part is still out of balance after balancing, stated in gram millimetres. Specify a figure rather than writing "balanced", and say where the shop may remove material.
- Matched pair
Two parts inspected and shipped as one unit, such as left and right headphone cups. Both halves from one material lot, finished in one load and judged side by side. The pair fails, not the part.
- Acoustic chamber
The internal volume of a headphone cup or enclosure that sets its low frequency behaviour. Depth, diameter and any vent are tuned features and belong under controlled tolerances, not a general tolerance block.
- Spike and cone
A spike has a genuine point meant to penetrate carpet, and needs a hard tip such as stainless. A cone has a small flat or radius and is safe on hard floors. Aluminum points round over on the first move.
- Thread engagement
How much thread actually carries load. At least 1.5 times the diameter in aluminum, 2 times if the part is adjusted often. A 5 mm deep M8 in a levelling foot strips on the third adjustment.
- Galling
Cold welding between two like metals under load, most commonly aluminum on aluminum. A galled levelling foot is permanently stuck. Use a steel stud or a threaded insert.
Buying and sourcing
- RFQ
Request for quotation. On cosmetic work it should include a STEP file, a marked up PDF, the finish specification, a colour reference and both this order quantity and the annual figure.
- STEP file
The neutral 3D format every shop can open. Send it with a PDF drawing rather than instead of one, because a STEP file carries geometry but not tolerances, finish or which faces are cosmetic.
- MOQ
Minimum order quantity. In finishing it is usually a minimum charge covering one rack cycle rather than a piece count, which is why 30 parts can cost nearly what 300 do.
- Rack cycle
One pass of a loaded rack through the finishing line. The real unit of cost in anodizing, since tank times are fixed whether the rack holds 30 parts or 300.
- VCI
Volatile corrosion inhibitor, a treated paper or bag that stops bare brass, copper and steel tarnishing in transit. Standard practice for any uncoated part that travels.
- Protective film
Low tack film applied to a finished flat face for transit. Applied after finishing only, never before, because adhesive on a pre-anodize surface leaves residue that shows through.
- EXW
Ex Works. The price covers the goods at the supplier’s door and nothing else. Freight, insurance, export clearance and duty are all yours. The most common basis for a machining quote.
- FOB
Free On Board. The supplier delivers to the named port and clears export. Compare quotes on the same Incoterm or you are comparing different things.
- NDA
Non-disclosure agreement. On an RFQ it should be signed before files are released, not after. A preview showing part type, material and quantity does not need one.
- Lead time
Calendar time from order to dispatch, not machine hours. On a cosmetic part it includes the finishing queue and the sample approval round, which together are often longer than the machining.
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