CNC machining vs 3D printing: when should a metal part be machined instead of printed?

How do you recognize that a metal part is in the wrong process?

A part is usually in the wrong process when most of its cost goes into fighting the process instead of making the shape. A printed part that needs machining on nearly every surface, or a machined part that needs a long series of setups, plugs and cross-drills to fake an internal path, is a candidate to switch.

Signs that a printed metal part should probably be machined: the quote shows a large post-processing line for support removal, heat treatment and finish machining; every functional face, bore and thread on the drawing carries a note to machine after printing; the part is a simple bracket, shaft, spacer or plate that a lathe or mill reaches from outside; or the quantity has grown from a few prototypes to a recurring order. The ASTM specifications for powder bed fusion parts state openly that such parts are often post processed by machining, grinding, EDM or polishing to reach the desired surface finish and critical dimensions (ASTM F3184). If that post processing touches almost every surface, you are paying for two processes to make one part.

Signs that a machined part may be better printed: the part is assembled from several machined pieces only because a cutter could not reach inside; it carries plugged cross-drilled holes that exist only to connect internal passages; it is a lightweight structure whose stock removal is most of the bar; or the design is still changing every few weeks and each change scraps a fixture. Those are the cases where building the shape directly can remove parts, joints and leak paths.

How do you test a specific part against both processes?

Walk the drawing feature by feature and ask, for each one, whether a standard cutter can reach it from outside and whether its tolerance and finish could be met as printed. The answers tell you whether to machine from solid, print, or print a near-net blank and machine the critical features.

Use this checklist on one part at a time, with the drawing and the 3D model open:

• Reach. Can every surface be reached by a turning or milling tool from outside, in a reasonable number of setups? If yes, machining is available. If a passage, cavity or lattice is fully enclosed, only printing (or casting with cores, or an assembly) can make it in one piece.
• Functional features. List the bores, threads, sealing faces, bearing seats and datum surfaces. These are almost always machined, whatever process makes the rest of the part. If they are most of the part, machine from solid.
• Tolerance on the rest. Check whether non-critical dimensions are covered by a general tolerance such as a class in ISO 2768-1, and whether that class was chosen for this part or inherited from a template. ISO 2768-1 is written for parts made by metal removal or formed from sheet metal, so it does not by itself tell a supplier what to hold on an as-printed surface; if some faces stay as built, give them their own general tolerance note.
• Material and condition. Confirm the alloy exists in the form you need, as wrought bar or plate for machining or as a printable powder with a published specification, and that the required properties apply to that form.
• Quantity and repeat. Estimate the annual quantity and whether the design is frozen. A frozen design with recurring orders favors machining, or casting plus machining once volumes justify tooling.
• Solid volume. Printing time and cost rise with the volume of metal fused, the build height and the supports, while machining cost rises with the volume removed and the number of setups. A chunky part with little removal favors machining; a thin, hollow part cut from a large block favors printing.
• Like-for-like quotes. Before deciding on price, ask for each route to be quoted to the same finished-part definition: the same machined features, thread checks, stress relief or heat treatment, finish and inspection report. A print quote that stops at the as-built part and a machining quote that delivers a finished part are not comparable.

If the checklist splits, with an enclosed internal feature plus several precision faces, the usual answer is a hybrid: print the near-net shape, then machine the functional features. That is not a compromise; it is how the published specifications for powder bed fusion parts expect finished parts to reach their critical dimensions.

What should the drawing and RFQ say so the part goes to the right process?

State the process route you want, or state that the supplier may propose one, and make every critical feature and its tolerance explicit. A drawing that relies on a general tolerance block lets each supplier quote a different part.

• Process freedom. “Machine from solid”, “additively manufactured blank, finish machine noted features”, or “process at supplier option, subject to approval”. The last one invites alternatives, which is useful when you are not sure.
• Material and condition. Name the alloy, its form and its specification, for example a wrought bar standard for machining, or the AM part specification such as ASTM F3184 for printed 316L, and say which property basis applies.
• Critical features. Identify the faces, bores and threads that must be machined, with their own tolerances and finish callouts, and leave the general tolerance to cover the rest. For printed parts, ASME publishes a product definition standard for additive manufacturing, ASME Y14.46, which a drawing can cite alongside the usual dimensioning and tolerancing standard.
• Heat treatment and stress relief. Say what is required and in which order relative to machining, so a printed blank is not finish-machined before it moves in a later thermal step.
• Inspection and acceptance. Name the inspection method for each critical feature and how hidden internal features will be accepted.
• Quantity profile. Give the prototype quantity and the expected annual quantity on the same RFQ, so the supplier can say where the route should change.

Model and drawing should agree. If the 3D model is the master for printed geometry and the 2D drawing is the master for machined features, say so in a note; the STEP file vs 2D drawing guide covers how to split that authority without leaving gaps.