Guides / Sourcing / Choosing a Supplier

How to Choose a Machine Shop for Cosmetic Audio Parts

September 6, 2026

Machining a front panel is not difficult. Almost any shop with a mill and a decent programmer can hold your dimensions. That is why price comparison between shops is so misleading on audio work: you are comparing quotes for a job that only one or two of them understand.

The filter is not machining. It is everything that happens after the part is cut.

The four questions that do the filtering

1. “Who controls the finish?”

There are three answers and only one of them is bad.

• “We anodize in house.” Good. Fast feedback, one throat to choke, usually the best colour consistency on repeat orders.
• “We use a finisher we have worked with for years and we inspect on return.” Also good, and more common. Most machine shops are not chemical plants.
• “We send it out, that is the finisher’s part.” Bad. This shop will hand you a batch with a bad black and tell you to talk to somebody you have no contract with.

Follow up with: what happens when a batch comes back wrong? A shop with a real process will describe what it does. A shop without one will tell you it has never happened.

2. “How do parts move between operations?”

This is the question nobody asks and everybody regrets. A perfect faceplate can be ruined in the five seconds between the mill and the deburr bench.

Good answers sound boring: parts go into foam lined trays, finished faces are covered with protective film before the second operation, operators wear gloves after finishing, nothing is stacked face to face. Bad answers involve the word “bin”.

3. “Show me a cosmetic part you have made.”

Not an audio part necessarily. Any part where appearance was the specification: a watch case, an instrument panel, an architectural fitting, a camera housing. You are looking for evidence that somebody there has been judged on appearance before.

If everything in the portfolio is brackets and fixtures, the shop may still do a good job, but you are paying to teach them.

4. “What is your minimum and how does the price move with quantity?”

You will reorder. A shop whose price barely improves between 30 and 300 pieces is telling you its cost is all in handling and finishing, which is honest. A shop whose 30 piece price is the same as its 300 piece price is either not thinking or not interested in your volume.

What to send so the quotes are comparable

Most quote spread is caused by ambiguity, not by the market. A shop that cannot tell which faces are seen will assume all of them and quote high, or assume none and quote low and then argue.

Send:

• A STEP file for geometry
• A PDF drawing with tolerances, and with the cosmetic faces marked and named
• Finish spec: alloy, process, class, thickness range, surface preparation, grain direction
• Colour reference: a physical part, or a numbered standard. Not a word and not a screenshot
• Quantity for this order and a realistic annual figure
• First article expectation: real finish, how many pieces, how long you need to approve
• Packing requirement, if you ship to customers directly from the shop

That packet takes an afternoon to put together once and it pays for itself on the first order.

Reading the first article

When the first article lands, do these in order.

1. Look at it before you measure it. Under a bright, diffuse light, at arm’s length, then close. Rotate it. Anodize defects and brush inconsistencies show at particular angles and disappear at others.
2. Find the rack marks. Every anodized part has contact points. Are they where you specified? Are they on a face the customer sees?
3. Check the grain. On a brushed part, does the grain run the direction you asked for, and does it run the same direction across every part in the set?
4. Compare against your reference in the same light, side by side, touching. Not from memory.
5. Then measure. Panel cutouts, hole positions, thickness, flatness.
6. Fit it to the real assembly. Gaps are a system property, not a part property. A panel that measures perfectly can still show a wedge shaped gap against a cover that is also perfect.

Write down what you accepted and why. That note becomes the standard for batch two.

Protecting the repeat order

A first batch that goes well means nothing if the second one is made differently. Three cheap protections:

• Retained samples. One approved part kept at your office, one at the shop, dated and labelled with the order number. Every later batch is judged against the physical part.
• A locked process. Written into the PO: same alloy supplier, same finisher, same rack fixture, same thickness range. No substitution without written approval.
• Tooling and fixtures stay put. Ask the shop to keep and label your fixtures rather than break them down. Refixturing is a common and invisible cause of batch two looking different.

Red flags in a quote

• No questions asked about the drawing. Either they understood everything or they read nothing.
• A finish line item of zero, or finish rolled into the machining price with no breakdown.
• A price that barely changes between 20 and 200 pieces with no explanation.
• An offer to skip the first article to save time.
• Certifications quoted in the email signature but not on the quote.
• A promise to match your previous supplier’s colour exactly, sight unseen.

The last one matters most. Colour matching without a physical reference in hand is not something an honest finisher will promise.

Frequently asked questions

Keep reading

• Telling a Machine Shop Which Faces the Customer Will See
• Running a First Article on a Part Where Appearance Is the Spec
• How Finished Audio Parts Get Damaged Between the Shop and You

This guide is part of Sourcing and Quality Control for Audio Parts, one of four sections in the guide library.