CNC Metal Machined Service: How to Pick One That Holds Your Tolerance

• 1
  Key takeaways
• 2
  What to compare before you award the job
• 3
  Tolerance: where the real difference hides
• 4
  Lead time: separate the clocks
• 5
  MOQ, volume, and the prototype-to-production handoff
• 6
  Certifications and inspection: what actually protects you
• 7
  Material range tells you how deep the shop is
• 8
  Seven steps to evaluate a CNC metal machined service

Every supplier will tell you they machine to tight tolerance. Very few will tell you which machine holds it. When a drawing calls for ±0.005 mm, the machine, the fixture, the tool, and the thermal state of the shop all matter. A three-axis mill in an uncontrolled bay behaves differently from a simultaneous 5-axis center with a temperature-stable spindle. Ask which machine will run your part and why that machine was chosen.

The second question is verification. A tolerance claim without measurement is a guess. A capable CNC metal machined service measures the features that matter and records the numbers. GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers, and the inspection lab checks parts against the print before shipment. That is the difference between a promise and a record.

Not every feature needs the tightest number. If a bore sits in a bearing seat, it earns ±0.005 mm. If it is a clearance hole for a cable, it does not. Marking which dimensions are functional and which are reference helps the shop spend time where it pays. It also shortens the quote, because the estimator stops guessing.

• 1
  Bring the GD&T, not just the modelDatums and position tolerances change the setup plan more than the nominal geometry does.
• 2
  Flag functional featuresTwo or three critical dimensions are easier to hold repeatably than twenty.
• 3
  Ask for the inspection planKnowing what will be measured tells you whether the tight callout is covered.

A single lead-time number hides the risk. Your part waits in four places: the quote desk, the DFM review, the machine queue, and the inspection bench. A supplier who quotes five days but takes three to answer the RFQ has already spent most of the budget. Ask for each date separately and you will see where the buffer actually sits.

Material is the quiet delay. Aluminum 6061 and 304 stainless usually sit on the shelf. Inconel, Ti-6Al-4V, and 17-4PH may not, and a two-week mill delivery will not show up in a machining lead time. If your part uses a specialty alloy, ask whether the stock is on hand before you approve the schedule.

GreatLight quotes and returns free DFM analysis within 12 hours, starts production within 24 hours in most cases, and ships parts in 3–5 days. Historical late-delivery probability is below 2%. Those numbers are useful only if the supplier can tell you which one applies to your job. A vague 'about a week' is a warning sign, not a courtesy.

• 1
  Ask who owns the DFM reviewA real engineer review catches thin walls and unreachable features before the first cut.
• 2
  Confirm material on handExotic alloys and large plate stock are the most common hidden delay.
• 3
  Do not compress inspectionRushing the final check moves the cost to your assembly line.

Minimum order quantity is where small projects die. A supplier with a 100-piece floor forces you to buy inventory you may never use, or to split the work between two shops that do not share a process. Neither is cheap. A shop that runs one piece and 10,000 pieces on the same floor keeps the same fixtures, the same CAM strategy, and the same inspection standards from first article to last.

Volume also changes the process choice. Below a few hundred parts, CNC machining usually wins on total cost because there is no tooling. Between 1,000 and 10,000 parts, die casting starts to compete on unit price, but the tooling lead time and the minimum run may not suit a product still in flux. Above that, casting or forging plus finish machining is often the rational route.

Ask how the shop handles engineering changes between the prototype and the production run. If the answer is a new quote and a new setup every time, your revision cycle will be slow. If the shop keeps the CAM file and fixture notes, a change to one pocket is a same-week task.

• 1
  No minimum order quantity removes a real constraintOne prototype and 10,000+ part runs are both available here.
• 2
  Match the process to the quantityMachining for low volume, casting for high volume, machining again for critical faces.
• 3
  Ask about revision handlingKept CAM files and fixture records make engineering changes faster.

Certificates are a filter, not a guarantee. ISO 9001:2015 tells you the supplier has a documented quality system. IATF 16949:2016 adds the automotive discipline around traceability and change control. ISO 13485:2016 covers medical device manufacturing. ISO 27001:2022 covers information security, which matters when your CAD files are the product. GreatLight holds all four.

The certificate only helps if the system touches your order. Ask what happens when a dimension is out of tolerance on the first article. The answer should be a nonconformance report, a root-cause note, and a corrected setup, not a phone call asking whether the part will 'still work.' That response is the system doing its job.

Inspection is the last line. A shop that inspects 100% before shipment and will send reports on request gives you something to file. Raw material checks, in-process monitoring, and final inspection each catch different failure modes. If a supplier skips the in-process check, the final bench becomes the only gate, and scrap found at the end is scrap you paid for.

• 1
  Match the certificate to the industryAutomotive, medical, and ITAR-adjacent work each need their own coverage.
• 2
  Ask for a sample reportA blank template tells you what will be measured before the part exists.
• 3
  Confirm the qualification rate99.99% on shipped parts is a claim worth holding a supplier to.

A shop that only runs aluminum will quote your 17-4PH job anyway, then struggle. Material range is a useful proxy for process depth. Aluminum 6061, 7075, 2024, 5052, and 6082 behave very differently from 316 stainless or Inconel, and the feeds, speeds, and coolant strategy change with them.

Hardened stainless and nickel alloys work-harden if the tool dwells. Titanium Ti-6Al-4V needs lower surface speed and more coolant, and it moves when you cut it. Copper and brass cut freely but grab the tool and throw chips. A shop that knows these differences will tell you before you ask, and will adjust the quote because the cycle time is genuinely longer.

Plastics belong in the same conversation. POM and PEEK machine cleanly with sharp tooling and light passes. ABS and PC can melt and smear if the spindle runs hot. If your assembly mixes metal and plastic parts, one supplier who understands both reduces the fit problems that appear at assembly.

• 1
  Ask about the specific alloy, not the family6061-T6 and 7075-T6 are not interchangeable for strength or finish.
• 2
  Confirm heat treatment and stress reliefThin walls move after machining if the blank was not relieved.
• 3
  Check finish compatibilityHardcoat anodizing changes dimensions; plan the allowance before the first cut.