Buyer guide

Lexington CNC Processing Service Guide

A practical buying guide for engineers sourcing machined parts. It covers how to compare quotes, which tolerances and finishes hold up in production, and when a supplier is the wrong fit.

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±0.005 mm toleranceNo MOQQuote in 12 hoursISO 9001 / IATF 16949

Lexington CNC processing service guide for precision machined parts

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Key takeaways

Tolerance must be per featureA blanket ±0.005 mm on every dimension adds cost with no benefit. Call it out only where the fit needs it.

Quote speed is not capabilityA 12-hour quote tells you the shop answers fast. Ask what they will inspect before you read the price.

Prototype and production differThe same part can be machined two ways. Ask which route the quote is based on.

Certificates matter by industryAerospace, medical and automotive buyers need different paperwork. ISO 9001 alone may not clear your audit.

Freight is part of lead timeMachining days plus transit days equal your real delivery window. Ask for both numbers.

Judging criteria

What to compare before you place a PO

Pull these five numbers from every quote and the comparison gets much shorter.

Check	Good sign	Warning sign
Tolerance stated per feature	Critical fits marked on the drawing	One blanket ±0.005 mm callout
Quote turnaround	12-hour quote with DFM notes	Price only, no manufacturability feedback
Order range	One prototype to 10,000+ parts	Setup charge that only pays off at volume
Inspection	100% inspection before shipment	Sample check only, reports on request
Certifications	ISO 9001, IATF 16949, ISO 13485	Certificate expired or scope unclear
Lead time	3–5 day shipment on standard parts	Vague window with no machining days listed
Confidentiality	NDA on request, secure uploads	Drawings sent by plain email only

The short version

Send a model, mark your critical features, and ask for DFM notes with the price. If the quote has no manufacturability feedback and no inspection plan, keep looking.

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Tolerances and geometry

Tolerance, geometry and how a shop proves both

Most disputes on a machined part come from tolerance, not from the machine. A shop that can hold ±0.005 mm on a small feature may not hold it across a 4,000 mm part, because thermal growth and fixture deflection grow with length. Before you accept a quote, ask which features carry the tight number and how those features are fixtured.

Geometry matters just as much. A pocket with a sharp internal corner cannot be milled with a round cutter. A deep bore with a length-to-diameter ratio above roughly 6:1 needs a boring bar that can reach without chatter. Send the 3D model and let the shop return DFM notes instead of discovering these limits after the first article.

Surface finish is a separate line item. As-machined aluminum lands around Ra 1.6–3.2 μm. A sealing face usually needs Ra 0.8–1.6 μm, and optical or bearing surfaces run Ra 0.2–0.8 μm. Those steps mean slower feed, a different tool and sometimes a second operation. That is real cost and real time, so specify finish only on the faces that need it.

1
Call out critical fitsMark the two or three dimensions that control assembly and leave the rest general.
2
Check corner radiiMatch the tool radius to the smallest internal radius on the model.
3
Name the finish per faceRa values on sealing or sliding surfaces only.

Process choice

Which machining route fits your part

Axis count is the first decision. Three-axis work handles plates, brackets and housings that can be reached from one direction. Four-axis adds rotation around one axis, which suits shafts and cylindrical parts with cross features. Five-axis reaches compound angles in a single setup, which removes repositioning error on impellers, medical housings and engine components.

Mill-turn centers cut round and prismatic features in one program. If your part starts as bar stock and ends with milled flats, cross holes and a turned bore, mill-turn usually beats two separate operations because the concentricity stays inside one setup. Ask whether the shop quotes it that way or splits it.

Material changes the plan too. Aluminum 6061 and 7075 machine fast and hold tolerance well. Stainless 316L and 17-4PH work-harden, so light passes and rigid tooling matter. Titanium Ti-6Al-4V and Inconel need slower speeds and more tool changes, which shows up in price rather than in tolerance.

1
3-axisPlates, covers, simple housings, one-direction access.
2
4-axisShafts, bushings and parts with radial holes.
3
5-axisCompound angles, deep pockets, contoured surfaces.
4
Mill-turnRound parts with milled features, tight concentricity.

Cost and lead time

Cost drivers and lead time you can actually plan around

Setup dominates small orders. For a one-off prototype, programming and fixturing can be most of the price, so the part geometry matters less than how many setups it needs. That is why a quote can drop sharply between 1 piece and 50 pieces from the same shop.

At higher volume the balance flips. Cycle time, tool wear and inspection time take over. A part with 40 tapped holes and three tight bores will cost more per piece than a simple bracket of the same size, even when the material is identical. Ask for the price at 1, 100 and 1,000 pieces so you can see where the curve flattens.

Lead time is two numbers, not one. Machining days come first, then transit. Standard parts ship in 3–5 days once production starts, and production can start within 24 hours of a released order. Add freight to your schedule and keep a buffer for the first article if the geometry is new.

1
Setup costFalls per piece as quantity rises.
2
Cycle and inspectionDominate at volume, not at prototype.
3
Plan two clocksMachining days plus shipping days.

Finishing and paperwork

Finishing, inspection and documents that clear an audit

Finishing is where schedules slip if it is planned last. Anodizing, plating and powder coating are outside processes with their own queues. If your drawing calls for hardcoat anodizing or electroless nickel, raise it when you request the quote, not after the parts are machined. Bead blasting, tumbling and brushing are faster and often enough for cosmetic parts.

Inspection should be described, not assumed. A workable default is raw material check, in-process monitoring and final inspection, with 100% inspection before shipment and reports on request. If your assembly needs first article inspection or a full dimensional report, say so up front so the shop plans the CMM time.

Certificates follow the industry. ISO 9001:2015 covers general quality systems. IATF 16949:2016 is what automotive and EV programs expect. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when you send proprietary CAD files. List the ones your audit actually requires.

1
Name finishes earlyOutside processes add their own queue.
2
Define inspectionSay whether you need reports or FAI.
3
Match certs to industryAutomotive, medical and IT security differ.

From RFQ to first article

Step by step

Seven steps that keep a machining order from stalling.

1
Send a 3D model and a 2D drawingSTEP for geometry, PDF for tolerances, finishes and material. A model alone leaves every tolerance open to guesswork.
2
Mark the critical featuresFlag the two or three dimensions that control fit. For everything else, use general tolerances and save cost.
3
Read the DFM notes before the priceGood feedback names thin walls, deep pockets and sharp corners that will need a second setup or an EDM pass.
4
Confirm the process routeAsk how many setups, which machine, and whether turning and milling happen in one operation or two.
5
Lock the material gradeWrite 6061-T6, 316L or 17-4PH, not just aluminum or stainless. Grade changes hardness and finish.
6
Settle inspection and paperworkAgree on what gets measured, whether reports ship with the parts, and which certificates you need on file.
7
Approve the first article, then release volumeCheck the first article against the drawing before the run continues. Fixing a fixture is cheap early and expensive late.

FAQs

Frequently asked questions

How tight a tolerance can a machine shop hold?

For most metal parts, ±0.005 mm is achievable on features with a short dimension and a rigid setup.

Long parts, thin walls and deep bores are harder. Temperature, fixture stiffness and tool deflection all push the number wider. Tell the shop which features need the tight value and let them confirm it.

Do I need a minimum order quantity?

Not for machining. A shop can run one prototype and then scale to a 10,000+ part production order from the same program.

Per-piece price drops as quantity rises because setup and programming are spread across more parts. Ask for pricing at a few quantities to see the curve.

What files should I send for a quote?

Send a STEP or similar 3D file plus a 2D PDF with tolerances, material, finish and any inspection requirements.

If you only have a drawing, say so. A model can be built, but it adds a step and a day or two before machining starts.

Which certifications should I ask for?

Start with ISO 9001:2015 as the baseline quality system. Add IATF 16949:2016 for automotive and EV work, ISO 13485:2016 for medical devices, and ISO 27001:2022 if your drawings are confidential.

Ask for the certificate scope, not just the logo. A certificate that does not cover your process or plant tells you little.

How do I keep my design confidential?

Uploads through a supplier portal are secure and confidential, and an NDA can be signed before files are shared.

Keep the paper trail clean too. Send files through the portal rather than as plain email attachments, and ask who inside the shop can open them.

When is a machine shop the wrong choice?

If your part is a thin sheet enclosure with no machined features, sheet metal fabrication is faster and cheaper.

If you need a hollow shell with internal geometry and low volume, vacuum casting or 3D printing may beat machining. Machining wins when you need metal, tight tolerance and a real surface finish.