Alloy CNC Processing Service: How to Choose One That Holds Tolerance
Picking an alloy CNC processing service comes down to seven checks: tolerance capability, alloy experience, machine travel, finishing, inspection, MOQ and lead time. This guide lists the numbers to ask for and the answers that should make you walk away.
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Key takeaways
What to ask a supplier, and what a good answer sounds like
Use this as a phone script. The left column is the question, the right two columns separate a shop that machines your alloy daily from one that will learn on your job.
| Question | Strong answer | Walk-away answer |
|---|---|---|
| What tolerance do you hold on a ±0.005 mm feature? | Named machine, alloy and report | We can hold anything you draw |
| Which alloys do you run every week? | Titanium, 17-4PH, 7075, Inconel | Whatever the customer sends |
| Max part size you can machine? | 4,000 × 400 × 150 mm travel | Send the file, we will see |
| Do you quote surface finish per feature? | Ra range per face, mask noted | Ra 0.8 μm on the whole part |
| What inspection data ships with the parts? | CMM report, raw values, on request | Visual check, trust us |
| Minimum order quantity? | One prototype, up to 10,000+ | Tooling fee before we talk |
| Lead time from PO to shipment? | Quote in 12 hours, ship in 3–5 days | Six weeks, maybe |
| Can you sign an NDA before the file? | NDA on request, secure uploads | Email us the STEP file |
The verdict: choose on evidence, not on the tolerance on the homepage
If a supplier can name the machine, the alloy history, the workholding, the finish per face and the inspection report, they can probably hold your tolerance. If they answer with one number and a promise, keep looking.
Tolerance: what the number really covers
A shop advertising ±0.005 mm is quoting its best case, on a stable alloy, on a short part, with a warm machine. Your part may be a 900 mm 17-4PH housing with thin walls and two datums. Those are different jobs. Before you accept the tolerance, ask which features fall inside it and which ones are called out separately on the drawing.
The useful question is not 'what is your tolerance' but 'which of my features are you pricing at ±0.005 mm, which at ±0.05 mm, and why'. A good shop answers in minutes because it already looked at the geometry. A weak shop answers with a single number and no feature list.
Also ask what happens at the limit. If a bore comes in at 0.004 mm over nominal and the band is 0.005 mm, does it ship or does it get reworked? The answer tells you whether the shop manages the process or just measures the result. Statistical process control on a critical diameter is worth more than a tight number on the brochure.
- 1Pin down the featureA datum-referenced bore and a cosmetic edge are not the same tolerance class.
- 2Ask for the instrumentMicrometer, bore gauge, CMM or optical comparator. The tool sets the believable limit.
- 3Watch thermal driftA 500 mm aluminum part grows with the shop floor. Ask when it is measured.
Alloy machinability and what it does to your quote
Alloy choice drives tool life, cycle time and risk more than any other single decision. 6061 aluminum cuts fast and forgives a light fixture. 7075 is stronger and more brittle, so it chips well but cracks at sharp internal corners. 304 stainless work-hardens the moment the tool rubs instead of cuts, which is why feed and speed matter more than the spindle.
Titanium TC4 (Ti-6Al-4V) is the classic trap. It conducts heat poorly, so the cutting edge keeps the heat and wears quickly. Inconel is worse. If a supplier quotes titanium at the same rate as aluminum, one of two things happened: they misread the drawing, or they plan to run it slowly and absorb the loss. Neither is good for your schedule.
Magnesium AZ31B and AZ91D machine beautifully but need chip control and fire precautions. Beryllium copper machines well and needs dust extraction. These are not reasons to avoid the alloy. They are reasons to ask whether the shop has run it before and how it handles the chips.
- 1Free-machining grades303 stainless, 6061 aluminum, C36000 brass cut fast and hold finish easily.
- 2Work-hardening grades304, 316 and 17-4PH need constant feed. Stopping mid-cut hardens the surface.
- 3Heat-bound gradesTC4, Inconel and tool steel need coolant through the tool and conservative depth of cut.
Machine travel, axis count and part size
Axis count is not a quality ranking. A 3-axis mill with a good fixture beats a 5-axis machine with a lazy setup. What matters is whether the shop can reach your features in the fewest setups, because every refixture adds error and time. Five-axis work earns its cost when the part has compound angles, deep pockets or features on five faces.
Size is the harder limit. GreatLight runs a 4,000 × 400 × 150 mm travel machine for long parts, plus 750 × 1,150 × 550 mm and 600 × 600 × 600 mm envelopes for mid-size work, and 500 × 500 × 450 mm and 500 × 310 × 200 mm for compact parts. A Ø400 mm rotary table covers round work that needs milling and turning in one setup.
Ask how the part is held, not just how big the machine is. Thin-wall aluminum rings need soft jaws or a vacuum plate. Long shafts need a steady rest. If the quote does not mention workholding, the shop is assuming a vise and a prayer. That assumption shows up as chatter marks on the first article.
- 1Count the setupsEach refixture adds stack-up. Fewer setups usually means tighter true position.
- 2Match the envelopeA 4,000 mm part needs the long-travel machine, not a sub-spindle workaround.
- 3Ask about workholdingSoft jaws, vacuum plates and fixtures are part of the process, not an extra.
Finishing, masking and the microns they move
Finishing is where alloy parts quietly go out of tolerance. Type II anodizing builds roughly 5–10 μm per surface. Hardcoat builds more. Electroless nickel adds a uniform layer that can close a thread if nobody planned for it. If your drawing calls a ±0.01 mm fit and the part gets hardcoated after machining, the coating consumes most of the band.
The fix is to state the finish before the quote, not after. Masking also has to be drawn. Laser marking needs a minimum character height of 1.5 mm to stay legible after coating. Bead blasting changes surface texture and can round a sharp edge you wanted crisp.
As-machined finish sits around Ra 1.6–3.2 μm. A high-quality machined finish lands at Ra 0.8–1.6 μm. Fine finishing reaches Ra 0.2–0.8 μm, usually on sealing faces, bearing bores or optical mounts. Ask which faces need which band. One flat Ra callout across a whole part is a sign the quote was not read carefully.
- 1Anodize buildsPlan the pre-plate dimension or the fit closes after coating.
- 2Mask before, not afterThreads, bores and electrical contact pads need masking notes on the drawing.
- 3Match the finish to the functionSealing faces need fine Ra. Cosmetic panels do not.
Inspection, documentation and certifications that fit your industry
Every shop says it inspects. The difference is what ships with the parts. Ask whether you get a first article inspection report, an in-process record and a final CMM report with raw values on request. If the answer is a generic certificate of conformance with no data, you are buying trust, not evidence.
Certifications should match your sector. ISO 9001:2015 covers general quality management. IATF 16949:2016 matters for automotive and EV work. ISO 13485:2016 is the one medical device buyers look for. ISO 27001:2022 covers information security, which matters if you are sending CAD files and BOMs to an outside shop.
GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring and final inspection. Reports are available on request. That structure is more useful than a certificate on a wall, because it tells you when each check happens and who signs it.
- 1Name the reportFAI, in-process record, final CMM. Ask which one you get by default.
- 2Match the standardIATF for auto, ISO 13485 for medical, ISO 27001 for file security.
- 3Check the sampling plan100% inspection on critical features, not a random sample of the lot.
MOQ, quote speed and what the lead time actually means
Minimum order quantity reveals who the shop is built for. A shop that wants 500 pieces minimum is a production shop. A shop with no minimum, running from one prototype to 10,000+ part runs, is set up for both. If your project starts as a prototype and grows, a low floor now saves a supplier change later.
Quote speed is a proxy for engineering depth. A quotation and free DFM analysis within 12 hours means someone read the drawing, flagged the thin wall, and suggested a fillet. A three-day quote with no comments usually means a salesperson forwarded your file to a scheduler.
Lead time needs a start point. Ask when the clock starts: at PO, at material arrival, or at first chip. At GreatLight production can start within 24 hours once the order and material are clear, and parts ship in 3–5 days. For planning, ask about the historical late-delivery rate. A shop that tracks it and reports a figure under 2% is managing the schedule, not guessing it.
- 1MOQ floorNo minimum means the prototype and the production run share a process.
- 2Quote contentA DFM note in the quote is worth more than a fast number with no comment.
- 3Lead time startDefine whether the clock starts at PO, material or first cut.
Step by step: vetting an alloy CNC processing service in one week
This sequence works for a new supplier or a re-quote on an existing part. Keep it to one page and you will compare shops on the same facts.
- 1Send one representative part, not tenPick the part with the tightest tolerance and the hardest alloy. Add the STEP file, the 2D drawing, the finish callout and the annual volume. A shop that reads the whole package will comment on it.
- 2Ask for the feature-level tolerance planRequest which features are priced at ±0.005 mm, which at ±0.05 mm, and which need a separate operation. Compare shops on the same feature list.
- 3Confirm the alloy has run beforeAsk for the last three jobs in that alloy, the tooling used, and the cycle time range. If titanium or Inconel is on your drawing, this step filters most shops.
- 4Check the machine envelope and workholdingMatch part size against travel, then ask how the part is held. Soft jaws, vacuum plates and steady rests should appear in the process plan.
- 5Lock the finish before the quoteState the Ra band per face, the masking zones and the coating thickness. Confirm the pre-plate dimension so the fit survives anodizing or hardcoat.
- 6Define the inspection packageName the reports you want: FAI, in-process record, final CMM with raw values. Confirm the sampling plan on critical features.
- 7Test the first responseTime the reply. A quote plus DFM notes within 12 hours is the baseline. Then ask whether production can start within 24 hours and what the historical late rate is.
Questions buyers ask before the first PO
Is 5-axis always better than 3-axis for alloy parts?
No. Five-axis earns its cost when the part has compound angles, deep pockets or features reachable from five directions in one setup. For a simple plate with holes on one face, a 3-axis machine with a solid fixture is faster and cheaper.
The real question is setup count. If five-axis removes two refixtures from a part with a tight true-position callout, it usually pays for itself in scrap avoided.
What is the hardest alloy you can machine?
GreatLight machines titanium TA1, TA2 and TC4 (Ti-6Al-4V), Inconel, 17-4PH stainless, 4340 steel and magnesium AZ31B and AZ91D, alongside the common 6061, 7075, 304 and 316 grades.
Harder alloys need slower feeds, more coolant and more frequent tool changes. Expect cycle times several times longer than the same geometry in 6061.
Do you charge for DFM analysis?
No. Quotation and free DFM analysis come back within 12 hours. The notes cover wall thickness, tool access, tolerance stacking and finish risks.
Use those notes as a comparison tool. Two shops quoting the same part with different DFM comments tells you which one actually read the drawing.
How do you handle confidentiality on CAD files?
Uploads are secure and confidential, and an NDA is available on request before you send files. Information security is covered by ISO 27001:2022.
If your program needs a signed agreement first, request it before the quote rather than after. It costs nothing and removes the awkward conversation later.
Can you run both a prototype and a 10,000-piece production order?
Yes. There is no minimum order quantity, so the same process can start with one prototype and scale to 10,000+ part runs.
Keep the drawing revision identical between the prototype and the production order. Most prototype-to-production problems come from an undocumented change, not from the machining.
What surface finishes can ship with the parts?
Anodizing (clear, color, hardcoat, conductive), electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing, plus laser marking with a minimum character height of 1.5 mm.
Specify the finish per face on the drawing. A single blanket callout across the whole part usually means someone will guess.
Send one part. Get a quote and a DFM review in 12 hours.
Upload the STEP file and the 2D drawing. We return a price, a feature-level tolerance plan and free DFM notes, then start production within 24 hours once the order is clear.
12-hour quoteFree DFM analysis100% inspection before shipmentNo minimum order quantity
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