Alloy CNC Machining: How to Choose a Supplier
This guide is for engineers and sourcing staff who need alloy parts cut to tight tolerances and want to know which shops can actually hold them. It covers how common alloys behave on the machine, which tolerance and finish numbers are realistic, what the four certificates mean for your program, and the questions that separate a capable supplier from a broker.
In this article
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Key takeaways
Which alloy shop fits your part
Match the material and program stage to the shop profile before you send an RFQ.
| Scenario | What matters most | Shop profile to pick | Red flag |
|---|---|---|---|
| One-off prototype, 6061 or 1045 | Speed and price | 3-axis mill plus turning, fast quote | Week-long quote turnaround |
| Aerospace bracket, Ti-6Al-4V | 5-axis and surface integrity | 16 simultaneous 5-axis centers | No in-process inspection plan |
| Automotive bracket, 4140 | IATF 16949 and PPAP paperwork | IATF-certified plant, material certs | Cert says ISO 9001 only |
| Surgical instrument, 316L | ISO 13485 and Ra 0.2–0.8 μm | Medical-certified clean handling | No finish measurement report |
| Hardened tool steel insert | EDM and grinding after heat treat | Wire and sinker EDM in house | Sends hardened parts to a third party |
| 10,000-part aluminum run | Fixture repeatability and per-piece cost | Mill-turn cells, 4,000 mm envelope | No first-article report |
| Confidential new design | NDA and data controls | ISO 27001, secure upload portal | Asks you to email STEP files |
Qualify the shop before you qualify the part
Send one hard-alloy part, read the DFM notes, and confirm the certificate matches your industry. If those three pass, the production run is a scheduling problem, not a quality gamble.
What changes when the part is alloy, not mild steel
Alloys are not a single category. Aluminum 6061 cuts fast and forgives a light depth of cut. Ti-6Al-4V and Inconel do the opposite: they hold heat at the cutting edge, work-harden if the tool rubs, and wear out carbide in minutes if the speed is wrong. A quote that treats 7075 and Inconel as the same line item tells you the shop has not looked at the drawing.
The practical difference shows up in three places. Cutting speed drops, sometimes by a factor of four or five. Tool engagement has to stay constant, which is why trochoidal or adaptive clearing paths matter on hard alloys. And coolant has to reach the edge, not just flood the part, because titanium and nickel alloys transfer heat poorly into the chip.
At GreatLight we run 6061, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12 aluminum, plus 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH stainless. Titanium grades TA1, TA2 and TC4, Inconel, and magnesium AZ31B and AZ91D are also in scope. Each group gets its own speeds, feeds and tool grades rather than a shared recipe.
- 1Aluminum and brassHigh speed, low cutting force, easy to hit Ra 0.8–1.6 μm as machined.
- 2Stainless 17-4PH and 316LModerate speed, watch for work hardening on light finishing passes.
- 3Titanium and InconelLow speed, rigid setup, sharp tools, generous coolant, no dwell.
- 4Magnesium AZ31B / AZ91DCuts easily but needs chip control and fire-safe handling rules.
Tolerance, finish and part size: what is realistic
Tolerance is a per-feature conversation. On a 500 mm aluminum housing, ±0.005 mm (about ±0.0002 in) is achievable on a bored bearing seat when the shop controls temperature and uses a warm-up cycle. The same callout on a 4,000 mm weldment is a different problem, because thermal drift and fixture compliance dominate. Split the drawing into critical and non-critical features and let the shop quote them differently.
Surface finish follows the same logic. Ra 0.2–0.8 μm usually means a fine finishing pass or a grinding step, and it costs time. Ra 0.8–1.6 μm is a normal fine-milled or turned surface. Ra 1.6–3.2 μm is as-machined and is fine for brackets and housings that will be painted or anodized. Specifying Ra 0.2–0.8 μm across every face of a part that only needs a cosmetic finish is the most common way to inflate a quote for no reason.
Size sets the machine class. GreatLight runs 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. The largest travel is 4,000 × 400 × 150 mm. Medium work sits in the 750 × 1,150 × 550 mm and 600 × 600 × 600 mm envelopes, and compact parts run on 500 × 500 × 450 mm or 500 × 310 × 200 mm machines. A Ø400 mm rotary table covers most round work that needs angular features in one setup.
- 1Hold tight only where it mattersMark datum and fit features; leave cosmetic faces at general tolerance.
- 2Finish callouts cost moneyEvery step down in Ra adds a pass, a tool change or a grinding operation.
- 3Size picks the machineConfirm the shop's envelope covers your largest single dimension, not the diagonal.
Certificates, inspection and lead time to check
Certificates are a filter, not a score. ISO 9001:2015 covers general quality management and fits most industrial work. IATF 16949:2016 is the automotive standard and matters if your part feeds a vehicle program with PPAP expectations. ISO 13485:2016 is the medical device standard and belongs on the shortlist for surgical or diagnostic hardware. ISO 27001:2022 covers information security, which is what protects your CAD files during the quoting stage. GreatLight holds all four.
Inspection depth is the next question. A 100% inspection before shipment sounds strong until you ask what is measured and how. Ask for the inspection plan: raw material check on incoming stock, in-process monitoring on critical features, final dimensional report. Reports are available on request, so build the requirement into the PO rather than asking after the parts ship.
Lead time has two halves. The first is quoting: a shop that reads the model, flags thin walls and suggests a datum change is doing free DFM work. GreatLight returns a quotation and DFM analysis within 12 hours, and production can start within 24 hours. The second half is delivery: parts ship in 3–5 days on typical jobs, and the historical late-delivery probability is below 2%. Treat any shop that quotes a delivery date before it has read your tolerance stack as a risk.
- 1General industrial workISO 9001 plus a material certificate is usually enough.
- 2Automotive and EVLook for IATF 16949 and ask how first-article reports are handled.
- 3Medical devicesISO 13485, plus a documented cleaning and handling routine.
- 4Confidential designsISO 27001 controls and an NDA before you upload STEP files.
Why secondary operations belong in the same quote
An alloy part is rarely finished when the spindle stops. Anodizing changes dimensions slightly, plating can add thickness on threads, and bead blasting can round a sharp edge you needed. If the machine shop quotes only the cutting, you own the coordination risk between vendors, and tolerance stack-ups get lost in the handoff.
GreatLight keeps finishing in house: anodizing in clear, color, hardcoat and conductive types, electroless nickel, zinc, silver and gold plating, powder coating and black oxide, plus bead blasting, tumbling, brushing and polishing. Laser marking and engraving are available with a minimum character height of 1.5 mm. That last number matters: if your part number or UDI string is smaller than 1.5 mm, it will not be legible after marking.
The engineering point is simple. When cutting and finishing sit under one quality system, the drawing revision you approved at quote time is the revision that reaches the anodize line. That is the difference between a supplier and a broker.
- 1Anodize and platingCall out masked threads and bores so coating does not close them.
- 2Bead blastingAsk for edge protection if the drawing has sharp functional corners.
- 3Laser markingKeep character height at 1.5 mm or larger for a clean mark.
Step by step: qualifying an alloy supplier
Work through these in order. Each step is designed to fail fast if the shop is not a fit.
- 1Send one part, not tenStart with a single representative part in the hardest alloy you actually use. No minimum order quantity means a one-piece trial is a valid test of speed, finish and communication.
- 2Check the DFM responseWithin 12 hours you should get a quote plus notes on thin walls, deep pockets, tool reach and datum choice. A price with no engineering comments is a warning sign.
- 3Ask for the inspection planRequest raw material check, in-process monitoring points and a final report before the PO. Specify which features need a measured value and which only need a pass or fail.
- 4Confirm the machine envelopeState your largest dimension and the alloy. The 4,000 × 400 × 150 mm travel covers long parts; a 600 × 600 × 600 mm envelope covers most boxy housings.
- 5Verify tolerance on one featurePick the tightest functional feature and ask how it will be held. On a 500 mm part, ±0.005 mm is realistic for a bored seat with temperature control, not for every face.
- 6Match the certificate to the industryISO 9001 for general work, IATF 16949 for automotive, ISO 13485 for medical, ISO 27001 for data. Ask to see the scope, not just the logo.
- 7Lock finishing and paperworkPut anodize or plating, laser marking and the inspection report in the same PO. This avoids the gap where parts sit between two vendors and nobody owns the tolerance.
Questions buyers ask before releasing a PO
Can one shop handle both my aluminum prototype and my titanium production part?
Yes, if the shop runs both material groups as separate processes. Aluminum moves fast with high spindle speeds; titanium needs lower surface speed, rigid tooling and flood coolant to the cutting edge.
Ask how tool life is tracked per alloy. A shop that logs tool changes by material will hold tolerance across the run. One that uses a single tool library for everything will drift.
Is ±0.005 mm realistic on a large part?
It depends on the feature and the size. On compact and medium parts, a bored or ground feature at ±0.005 mm is routine with temperature control and a warm-up cycle.
On the 4,000 mm travel class, thermal growth and fixture compliance dominate. Expect to split the drawing: tight tolerance on functional seats, general tolerance elsewhere.
What is the smallest order you will take?
There is no minimum order quantity. Runs range from a single prototype to 10,000 or more parts.
That means a one-piece trial is a reasonable first step before you commit to tooling and fixturing for a larger batch.
How do you protect our design files?
Uploads are handled as secure and confidential, and an NDA is available on request. Information security is covered under ISO 27001:2022.
If your program requires it, ask for the NDA before you send the STEP file, not after the quote.
What do you need to quote an alloy part quickly?
A 3D model or 2D drawing with tolerances, the alloy grade, the quantity, and any finishing requirement such as anodize type or laser marking.
With those four items, a quotation and DFM analysis come back within 12 hours, and production can start within 24 hours.
Do you machine hardened alloys after heat treatment?
Hardened and difficult alloys are handled with wire and sinker EDM for sharp internal corners and features that cutting tools cannot reach, and with precision grinding for final dimensions and finish.
Tell the shop the heat-treat state at quote time. Machining before hardening and finishing after it is a different process plan from cutting everything in one setup.
Send your alloy part for a 12-hour quote
Upload the model and drawing, tell us the alloy and quantity, and we will return pricing plus a DFM review. No minimum order quantity.
12-hour quoteNo MOQ100% inspectionNDA on request
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