Stamping vs CNC machining: choosing for flat parts

Two different ways to make a flat part

Stamping starts with a sheet or coil at the finished thickness. The die cuts the outline, pierces the holes and forms any bends. The material that is not part of the component becomes scrap, but the part itself is made by moving material rather than removing it.

CNC machining starts with a plate or a block and removes material with a rotating cutter. A flat part can be milled from plate, with the outline, holes, pockets and chamfers all cut by the tool. The process is programmed rather than tooled, so the setup is quick and the design can change between jobs. The trade-off is time per part and the material that becomes chips.

When stamping wins

Stamping wins when the part is thin, flat or shallow, and needed in repeatable volume. The die cost is spread across the parts, the cycle time is short and the piece price falls with volume. A terminal, a bracket, a washer or a shield with a stable design is a natural stamping part.

It also wins when the part needs a formed feature that machining cannot make economically. A bend, a rib, an emboss or a lance is quick in a die and slow or impossible in a mill. If the part has a bend and a few holes, stamping can make it in one tool, while machining would need a separate bending step.

When machining wins

Machining wins when the part is thick, three-dimensional or needed in small numbers. A part with a pocket, a step, a thread or a tight bore is a machining job. A part that is 10 mm thick and has a complex profile may be cheaper to mill than to stamp, because the die would be heavy and the press tonnage high.

It also wins when the design is not stable. A prototype or a first build can be machined from a program in days, while a die takes weeks. If the part is likely to change, the money spent on a die may be wasted. Machining also wins when the tolerance is tighter than a die can hold, or when the flatness call is severe.

A machined part can also carry features that a die cannot: a counterbore, a tapped hole, a chamfer, an engraved mark or a pocket with a flat floor. Those features can be added in the same setup, which removes a secondary operation and the handling that goes with it. If the part is an assembly interface, that can matter more than the piece price.

Capability comparison

The table below compares the two routes on the features that usually decide the choice. The values are practical starting points, not guarantees. A supplier should confirm the capability for the specific part and material.

Tolerance and feature limits

A stamped part holds its tolerance through the die. The repeatability is high, but the absolute band is limited by the clearance, the material thickness and the wear. A hole position can be held well, but a tight bore or a flatness call across a large part is harder. A stamped edge also has a burr and a shear zone, which may need a secondary operation.

A machined part holds a tighter band because the tool is positioned by the machine and the cut is measured. A bored hole can be held to a fine tolerance, and a pocket depth can be controlled. The limiting factors are the rigidity of the setup, the tool wear and the heat. A thin part can deflect under the cutter, so a flat part may need a fixture or a support.

The datum scheme should match the process. A machined part is usually set from a face and two edges, because that is how the vice or fixture holds it. A stamped part is located by the strip and the pilots. If the drawing uses a datum that the process cannot hold, the supplier will either add a fixture or accept a wider tolerance. Neither is free.

Material waste and the cost model

Machining removes material, so the blank has to be larger than the part. For a part with a large outline and a few holes, most of the plate can become chips. The chips have scrap value, but the recovery is less than the cost of the material. Stamping also produces scrap, but the strip layout can be designed to use the material well, and the scrap is a predictable part of the quote.

The cost model should compare total cost at the same annual volume. For machining, add the program, the fixture, the machine time, the tool wear and the material. For stamping, add the die, the trial parts, the maintenance reserve, the material and the press time. Divide by the good parts. A low piece price that ignores the die or the fixture is not a comparison.

Machine time is not just the cut. It includes the setup, the probe or touch-off, the tool change and the inspection. A part with a tight tolerance may need a first-off check and a periodic check through the run. Those hours belong in the piece price. On a stamping route, the equivalent is the die trial, the first article and the in-process check. Compare the complete route, not the cutting time alone.

Surface finish, burr and secondary work

A machined surface can be left with a controlled roughness, and a fine finish is a matter of a finishing pass. The edge can be chamfered or deburred in the same setup. That is useful for a part that slides, seals or carries current through a contact face.

A stamped edge has a burr and a work-hardened shear zone. Deburring is a separate step, and a tight bend radius can crack if the edge is not clean. If the part needs a fine surface on a face, machining has the advantage. If the part only needs a clean outline and a few holes, stamping is usually enough.

A practical decision path

Start with the thickness and the feature list. If the part is under about 3 mm, flat or shallow, and has no pockets, stamping is the likely route. If it is thick, has a pocket, a thread or a tight bore, machining is the likely route. Then check the volume: below a few hundred pieces a year, machining or laser cutting is usually cheaper. Above a few thousand, the die can pay back.

Finally, check whether the design is stable. A part that will change in the next six months should not have a die. A part that is frozen and will run for years is a candidate for stamping. If the programme needs both, use machining for the prototype and a die for production, and design the part for the die from the start.

It also helps to look at the inspection plan. A stamped part can be checked with a go/no-go gauge, a calliper and a simple fixture, which keeps the inspection cost low. A machined part with a tight bore may need a CMM or a dedicated gauge. That difference is small per part but adds up over a programme, and it should sit in the comparison next to the piece price.

• Thickness and feature depth
• Presence of pockets, threads or tight bores
• Annual volume and programme life
• Tolerance and flatness calls
• Whether the design is frozen