Precision CNC machining in Detroit: expert solutions, explained for engineers
This page covers how tolerances, setups and material behavior decide whether a part comes off the machine right the first time. Written for design engineers and sourcing staff who release parts to outside machine shops. By the end you can judge which features belong on a 5-axis center, where a 3-axis run is cheaper, and what to put in a request for quote.
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How tolerance stack-up decides the process, not the drawing alone
A print that says ±0.005 mm on every dimension is not a process plan. The number that matters is the tightest tolerance on the tightest feature, plus how many setups that feature needs before it is finished. A bore held in one setup on a 5-axis center behaves differently from the same bore reached in three separate operations. Each re-clamp adds locating error that no machine can remove later.
The second factor is feature-to-feature relationship. If a hole pattern must line up with a milled face, the two features should come from the same datum and, ideally, the same setup. When they come from different setups, the tolerance you actually hold is the sum of both locating errors, not the tighter of the two. That is where a ±0.025 mm print quietly becomes a ±0.05 mm part.
Thermal growth is the third piece. Aluminum 6061 grows about 23 μm per meter per degree Celsius. A 400 mm part that warms 5 °C between roughing and finishing moves roughly 46 μm before any cutter touches it. That is larger than the ±0.005 mm tolerance some prints call for, which is why rough and finish passes are separated and why the part is measured at 20 °C.
So the first question is never which machine. It is which features have to stay in relation to each other, and how many setups that relationship allows. Answer that and the machine choice usually makes itself.
When five-axis motion removes error instead of adding cost
Five-axis machining earns its rate when it removes setups. A housing with bores on four faces, a port at 35°, and a sealing face that must stay square to the main bore is a natural fit: one clamping, one datum, all features generated from the same zero. The alternative is four or five fixtures, each with its own locating pin and its own chance to drift.
The travel envelope sets the limit. Our simultaneous 5-axis centers run a Ø400 mm rotary table, with travels up to 4,000 × 400 × 150 mm on the large frame and 750 × 1,150 × 550 mm on the medium frame. A part that fits the table but needs a 600 mm reach past the trunnion will not be cut in one pass, no matter how the CAM is written.
Rigidity is the trade. A trunnion-mounted part hangs off a rotary axis, so long slender tools deflect more than they would on a three-axis vise. Deep pockets with a 10:1 length-to-diameter ratio are often faster on a three-axis mill with a short, stiff tool, then flipped once for the back side.
Roughly, use five-axis when the part has angular faces, features on more than three sides, or a true position callout under 0.05 mm across those features. Stay with three-axis when the part is prismatic, the features are reachable from two directions, and the tool can stay short.
What each material does to your tolerance and finish
Aluminum 6061-T6 cuts clean and holds ±0.005 mm on bores and pockets without drama. It also moves after machining: thin walls under 1.5 mm will spring when the vise releases. 7075 is stronger but gummier, so it wants sharper tools and lighter depths of cut. Both reach Ra 0.8–1.6 μm with a good finish pass, and Ra 0.2–0.8 μm after lapping or fine boring.
Stainless 303 is the free-machining grade and behaves well on a lathe. 304 and 316L work-harden, so a dwell or a feed stop turns the surface harder than the tool and the next pass chips the insert. Keep the feed constant and the depth of cut above the work-hardened layer. 17-4PH (SUS630) is dimensionally stable after aging but should be finished after heat treat, not before.
Titanium TC4 (Ti-6Al-4V) and Inconel push the other way. Both hold heat at the cutting edge, so tool life drops fast and the part grows as it warms. Use high-pressure coolant, conservative radial engagement, and expect to leave 0.3–0.5 mm for the finish pass. Magnesium AZ31B and AZ91D cut fast but require chip control and no water-based coolant.
Plastics are their own case. POM and PEEK hold tight dimensions but deflect under clamping; ABS and PC need sharp, polished tools to avoid melting. Carbon fibre eats carbide, so diamond-coated tooling is the practical choice.
Inspection evidence, and why the CMM report matters more than the number
A tolerance is a claim until it is measured. For parts with true position, profile, or concentricity callouts, a coordinate measuring machine report tied to the same datum scheme as the drawing is the only useful evidence. A caliper reading on a bore tells you the diameter. It tells you nothing about whether that bore sits where the print says.
Our flow is a raw material check on incoming stock, in-process monitoring during the run, and 100% inspection before shipment. Reports are issued on request. For first articles, we measure the full print and flag any dimension that is close to the limit, not just the ones that fail.
The measurement setup matters as much as the machine. A part measured while warm reads larger than the same part at 20 °C. Thin-walled parts measured in the fixture read round; measured free, they read oval. Agree on the clamping state and the temperature before the first article, or the two sides will argue about numbers that were never comparable.
If a feature cannot be measured with the equipment on hand, say so at the quote stage. It is cheaper to add a fixture or change a datum than to discover at goods-in that nobody can prove the part is good.
Lead time, volume and the questions that change the price
Production starts within 24 hours of a released order, and parts ship in 3–5 days for most runs. Quotation with a free DFM analysis comes back within 12 hours. Those windows assume the print is complete: material grade, temper, finish, and the datum scheme all specified. A missing temper or an undefined surface finish is the most common reason a quote stalls.
Volume changes the method, not just the price. From one prototype to 10,000+ parts there is no minimum order quantity, but the right process changes. A single part is machined from billet. A 500-part run may still be billet, or may move to die casting with machining allowance if the geometry allows. Above that, the tooling cost of casting starts to pay back.
Confidentiality is part of the quote. Uploads are handled as confidential, and an NDA is available on request before any file is shared. For defense, medical and automotive programs this is usually the first document exchanged, not the last.
Certifications to check against your own quality system: ISO 9001:2015, IATF 16949:2016 for automotive, ISO 13485:2016 for medical devices, and ISO 27001:2022 for information security. Ask which one applies to your part family, not just which ones the supplier holds.
Matching the process to the part
Use the tightest tolerance and the number of reachable faces as the starting point.
| Part condition | Process | Why it fits | Watch out for |
|---|---|---|---|
| Prismatic, features on 2 faces | 3-axis mill | Short rigid tools, fast cycle | Re-clamp error on the second side |
| Angular faces, 4+ sides | 5-axis simultaneous | One datum, no re-clamp drift | Long tools deflect in deep pockets |
| Round parts with milled flats | Mill-turn center | Turning and milling in one setup | Limited Y-axis travel on some frames |
| Wall under 1.5 mm | 3-axis with light passes | Less clamping force needed | Spring-back after vise release |
| True position under 0.05 mm | 5-axis + CMM report | Features from one zero | Must measure at 20 °C |
| Prototype, one piece | 3-axis or 5-axis billet | No tooling cost | Higher unit price than casting |
| Run above ~1,000 parts | Die casting + machining | Tooling amortized over volume | Adds weeks for tooling build |
| Titanium or Inconel | 5-axis with HP coolant | Heat and chip control | Tool life, thermal growth |
The short version
If your part has features on four or more faces or a true position callout under 0.05 mm, put it on a 5-axis center and pay for the setup once. If it is prismatic and reachable from two directions, keep it on a 3-axis mill with short tools and spend the money on inspection instead.
Questions engineers ask before releasing a PO
Can you hold ±0.005 mm on a 400 mm part?
±0.005 mm is our general machining tolerance, and it is realistic on a 400 mm part when the feature is cut in one setup and measured at 20 °C.
The constraint is thermal, not mechanical. A 5 °C temperature change moves aluminum roughly 46 μm over 400 mm, which is nine times the tolerance. For long parts we rough, let the part stabilize, then finish, and we agree on the measurement temperature with you before the first article.
Do you need a 3D model, or is a 2D print enough?
A 2D print with a clear datum scheme is enough for most turned and prismatic parts. We build the model from the print and send the DFM notes back with the quote.
For parts with organic surfaces, blended fillets or complex angular faces, a STEP file saves a round of questions and reduces the risk of a misread dimension. Send both when you have them.
How do you handle a feature that is hard to measure?
We flag it at the quote stage rather than at goods-in. Options are a dedicated checking fixture, a change of datum to something the CMM can reach, or a note on the print that the feature is reference only.
Any of those is cheaper than shipping parts nobody can prove. If your quality system needs a full dimensional report, say so when you send the print so we can plan the inspection time.
What surface finish can I expect without extra processing?
As-machined surfaces land at Ra 1.6–3.2 μm. A controlled finish pass gets Ra 0.8–1.6 μm on most aluminum and stainless parts.
Ra 0.2–0.8 μm needs a dedicated finishing operation such as fine boring, lapping or polishing, and it adds a step to the routing. Specify the finish per surface, not for the whole part, to keep the cost where it belongs.
Does an NDA slow the quote down?
No. Send the NDA with the files or request ours before uploading. The 12-hour quote window starts when we have a complete package: print, material, finish and quantity.
Uploads are handled confidentially, and we do not share drawings or part photos outside the quoting and production team.
When should I switch from machining to die casting?
Machining stays competitive up to roughly 1,000 parts for most geometries, and it is the only sensible route for one-offs and prototypes since there is no tooling cost.
Above that, die casting with a machining allowance on critical faces usually wins on unit price, but it adds weeks for tooling build and the casting tolerances are looser, so tight features still get machined after.
Send the print and get a DFM answer back
Upload your files and we return a quote with a free DFM analysis within 12 hours, plus a note on any tolerance we think is risky before the chips fly.
12-hour quote100% inspectionNDA on request
Follow the shop floor
We publish setup notes, tooling trials and inspection data from the factory floor.
