CNC processing subcontract: how the scope split works
A CNC processing subcontract is not a purchase order for parts. It is a split of manufacturing scope between your plant and an outside machine shop. This page explains what changes when you move that scope out, what stays your responsibility, and how to judge whether a shop can actually hold it.
In this article
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What a CNC processing subcontract actually transfers
A CNC processing subcontract is an agreement where you own the design and the finished product, and a machine shop owns the cutting. You keep the drawing, the revision control, the assembly, and the customer. The shop keeps the spindles, the fixtures, the tooling, and the operators who run them.
That split sounds simple until a dimension goes out. The part is still your part. If a bore comes back at Ø20.03 mm instead of Ø20.00 mm and the mating bearing will not seat, the drawing is what decides who pays. A subcontract with no stated inspection method and no datum callout leaves that argument open.
The transfer is narrower than most buyers assume. You are not outsourcing the requirement, only the material removal. For a housing with a 4,000 mm maximum processing size, the shop controls stock prep, workholding, cutting parameters, in-process checks, and the final report. You control the tolerance the report must meet.
One more thing moves across: part of your process knowledge. If a feature needs a specific insert or a rough-then-finish sequence to avoid chatter, say so. Written notes beat a phone call nobody recorded.
- 1You keepDrawing, revision, GD&T scheme, assembly, source inspection criteria
- 2Shop keepsMachines, fixtures, cutting data, operator skill, in-process monitoring
- 3SharedMaterial spec, finish callout, inspection report format
Why the scope split changes tolerance and cost
Tolerance is not a number you attach to a drawing. It is an output of a machine, a fixture, and a thermal state. When the work moves to another building, the machine stays the same but the fixture changes, the ambient temperature changes, and the operator changes. Each of those shifts the achievable band.
A shop running 16 simultaneous 5-axis machining centers can hold ±0.005 mm on a reamed bore because the part is cut in one setup. Move the same feature to a three-axis machine with two setups and the stack-up roughly doubles. The drawing did not change. The process did.
Surface finish follows the same logic. Ra 0.8–1.6 μm is a normal as-machined target for aluminum on a rigid setup. Chasing Ra 0.2–0.8 μm means slower feeds, sharper tooling, and a finishing pass that adds cycle time. If the drawing asks for that finish on an internal pocket, expect the quote to say so.
This is the practical reason a CNC processing subcontract costs more per part than in-house turning on an idle lathe. You are paying for the machine that fits the tolerance, not for a machine that happens to be free.
- 1Setup countEach extra setup adds to the tolerance stack, not just the price
- 2Machine class5-axis holds position in one setup; 3-axis needs re-datuming
- 3Finish targetRa 0.2–0.8 μm is a separate operation, not a parameter tweak
When a CNC processing subcontract is the wrong call
If your shop has an idle three-axis machine and the part is a simple bracket held to ±0.1 mm, subcontracting adds a purchase order and a shipping leg for no gain. Keep it in-house. The overhead of a subcontract only pays back when the outside process does something your floor cannot.
Volume is the other boundary. A subcontract makes sense for one prototype and for a 10,000+ run, for different reasons. One prototype buys you a machine setup you do not own. A large run buys you cycle time and a second shift. The awkward zone is 50 to 300 parts where the per-part price is still dominated by setup.
Do not subcontract a part whose acceptance depends on feel or on a fixture you already built. If your assembly line gauges a shaft by sliding it into a housing, the subcontractor cannot see that gauge. Write the numeric limit or the gauge travels with the job.
Finally, skip subcontracting when the material is the risk. If you are cutting a rare titanium forging that took eight weeks to source, a first-time subcontractor with no history on that alloy is a bad bet. Send a coupon first.
How to judge a subcontractor before you send a drawing
Ask what machine will run the part, not what machines the shop owns. A quote backed by a specific machine class and a stated setup count tells you the shop read the drawing. A quote that only lists a price and a lead time tells you nothing about capability.
Then ask how the first article is checked. A shop that inspects 100% before shipment and can produce a dimensional report on request has a process. A shop that says it will check it has a habit. Both may deliver, but only one gives you something to attach to a corrective action request.
Certification matters when your customer audits you for it. 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 your drawings leave your network.
Read the DFM feedback as a test. A useful shop will tell you that a 0.5 mm wall on a 60 mm deep pocket will deflect, or that a Ø2 mm hole 12 mm deep needs a pilot drill. That feedback is free engineering. A shop that quotes without comment is quoting blind.
- 1Machine, not fleetName the machine class and setup count in the quote
- 2Inspection methodFirst article plus 100% inspection before shipment, reports on request
- 3Certificates that fitMatch the certificate to your industry, not to the biggest logo
- 4DFM commentsWritten feedback within 12 hours is a good sign, silence is not
Step by step: writing a subcontract that holds up
- 11. Fix the datum scheme firstDefine A, B, C on the drawing before any quote. A datum that moves between revisions moves the tolerance with it.
- 22. State the inspection method per featureCMM, micrometer, pin gauge, or functional gauge. A feature checked by two different methods is checked twice.
- 33. Send a 3D model plus a 2D drawingThe model defines the surface, the drawing defines the tolerance. Contradictions between them are the most common source of scrap.
- 44. Ask for a first article before the runOn a 10,000+ order, hold the first article for approval. It costs one part and prevents a full-run rework.
- 55. Agree on the finish callout in numbersAnodize type, thickness range, and Ra value. "Anodized" alone leaves the thickness open.
- 66. Confirm the report format up frontDimensional report, material cert, and finish cert. Ask which arrive by default and which cost extra.
- 77. Lock the NDA before the uploadSign the NDA before drawings leave your network. Confidentiality terms signed after the fact cover nothing.
In-house machining vs a CNC processing subcontract
Match the row to your part before you choose a route.
| Condition | Keep in-house | Use a subcontract |
|---|---|---|
| Tolerance band | ±0.1 mm or looser | ±0.005 mm on critical features |
| Setup count | One setup on an existing fixture | Three or more setups, or one 5-axis setup |
| Quantity | 50 to 300 parts on a free spindle | 1 prototype or 10,000+ parts |
| Geometry | Prismatic, open faces | Contoured, undercut, 5-side access |
| Material | Free-machining stock you already run | Titanium, Inconel, hardened tool steel |
| Deadline | Two weeks is fine | Parts ship in 3–5 days |
| Drawing release | Internal only | NDA and ISO 27001 needed |
The short version
Keep the part in-house when the tolerance is loose and the fixture already exists. Use a CNC processing subcontract when the tolerance needs a machine you do not own, when the geometry needs five sides in one setup, or when the run is a single prototype or past 10,000 pieces. In between, do the math on setup time before you do anything else.
Common questions
Does a CNC processing subcontract cover the material too?
It can, but decide which way you want it. If the shop buys the stock, the shop owns the material cert and the risk on a bad heat. If you buy it, you keep control of the alloy and the traceability, and the shop invoices cutting time only.
For aerospace and medical parts, most buyers supply the material or name the mill and heat number. For general industrial work, letting the shop source it is simpler and usually faster.
What tolerance can a subcontractor realistically hold?
On a rigid setup with a reamed or bored feature, ±0.005 mm is achievable. That number depends on feature type, not on the shop's marketing. A long thin wall will not hold it. A Ø8 mm bore in a 20 mm plate will.
Ask for the tolerance per feature, not per part. One tight bore and twenty loose holes should be quoted as two different operations.
How fast can a subcontract start?
Quotation and DFM analysis come back within 12 hours. Production can start within 24 hours of approval. Standard parts ship in 3–5 days.
Those windows assume the drawing is frozen. If the model is still moving, the clock restarts on every revision.
Is there a minimum order quantity?
No minimum order quantity. The range runs from one prototype to 10,000+ part runs.
That matters for a CNC processing subcontract because the first article and the production run can sit on the same order. You approve a single part, then release the rest.
How are drawings kept confidential?
Uploads are handled as secure and confidential, and an NDA is available on request. Information security is covered by ISO 27001:2022.
Sign the NDA before the first upload. A confidentiality clause added after drawings are on someone's server does not undo the exposure.
Which finishes can be included in the subcontract?
Anodizing in clear, colour, hardcoat, or conductive; electroless nickel, zinc, silver, and gold plating; powder coating and black oxide; bead blasting, tumbling, brushing, and polishing; laser marking with a minimum character height of 1.5 mm.
Each finish is a separate operation with its own tolerance effect. Hardcoat anodize adds thickness and can close a tight bore, so call it out before the bore is sized.
Send a drawing and get a machinable answer
Upload your model and 2D drawing. You get a quotation and a free DFM analysis within 12 hours, with comments on the features we would change before cutting.
12-hour quote1 pc to 10,000+100% inspectionNDA on request
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