Ordering CNC Machined Parts From China: Tolerances, Materials and What the Drawing Actually Says

CNC machining is different from most of what you source in China. With a moulded or sewn product you are partly buying the factory's judgement and taste. With machined metal parts the shop will do precisely what your engineering drawing tells them, no more and no less. That is excellent when your drawing is correct and a quiet disaster when it is not. Most CNC problems I have seen came from the buyer's spec, not the factory's skill.

The drawing is the whole negotiation

Everything starts and ends with your technical drawing. If the critical dimensions, tolerances, surface finish and material are not explicit and clear, you are leaving the most expensive decisions to someone else's interpretation, and they will interpret in whatever way is cheapest for them. I have seen buyers order parts with tolerances tighter than the application needed, which silently doubles or triples the price for no benefit. I have also seen the opposite, no tolerance at all on a surface that has to mate with another part, and then a fight about who is responsible when it does not fit.

Spend the time to make the drawing unambiguous. Call out the nominal dimension, the acceptable tolerance, and the datum you are measuring from. Machinists are not mind readers. A dimension without a tolerance is a request for the cheapest interpretation that still passes inspection, and that is rarely what you actually want. If you are not an engineer, bring one in for the drawing review. It is the single best money you will spend on the whole job.

Material selection is where beginners bleed money

The metal you choose drives cost more than almost anything else. A part in 6061 aluminium is a completely different price to the same part in stainless steel, and within stainless, 303 machines far more easily than 304 or 316. If you spec a material you do not strictly need, you pay for it on every single part. Ask the factory what they recommend for your actual application rather than assuming you know which grade you want. A good shop will tell you when you are over-specifying, because a part that is easier to machine is also quicker for them to produce.

Understand the supply side too. Aluminium and common grades of stainless and mild steel are easy to find in bulk in China, which keeps quotes competitive. Anything exotic, odder alloys or specific imported brands, gets more expensive and takes longer. If your design can use a standard readily available grade, your lead time and your price both improve, and I would question a design that pins itself to an obscure alloy without a real reason.

Tolerances are priced in, per dimension

Here is the concept too few buyers grasp: tolerance is not one price for the whole part. It is a running cost across every dimension that carries it. A general tolerance of plus or minus 0.1 millimetre is cheap. Pushing a feature to plus or minus 0.01 millimetre is a different machine setup, more inspection, and a higher scrap rate, so the factory prices that feature accordingly. Tighten only the dimensions that genuinely have to be tight for the part to function, and leave the rest loose. I have cut real cost out of parts just by loosening tolerances on cosmetic surfaces and holes that did nothing structural.

Surface finish works the same way. A standard machined finish is fine for most functional parts. Mirror-polishing a surface that will never be seen is spending money on nothing. Decide which faces are structural, which have to feel and look a certain way, and which nobody will ever touch, then spec the finish to match. Be explicit, because "standard finish" means something different to every quote.

The questions to ask before you commit

• Send a clean, unambiguous drawing and have an engineer review it before it goes out.
• Confirm the exact material grade and ask whether the factory would recommend a cheaper equivalent that still works.
• Tighten tolerances only where the function demands it and leave the cosmetic stuff loose.
• Agree the inspection method, whether they send a first-article report and what measurements it covers, before the run starts.
• Understand that prototyping a part, checking the first article carefully, then authorising the full run saves far more than it costs.

China has genuinely excellent CNC shops, and many of them are faster and cheaper than their Western counterparts because the whole supporting ecosystem of material suppliers is right there. But that advantage only lands on your side if your part is specced properly. A good drawing is not protecting the factory from you. It is protecting you from ambiguity, which is where almost all the money gets lost.