Printing or machining: when to choose which

What is the difference between printing and machining?

Machining starts from a block, bar or plate and removes whatever is not the part. Printing builds it up. ISO/ASTM 52900 defines additive manufacturing as joining materials to make parts from 3D model data, usually layer upon layer, as opposed to subtractive and formative methods.

The standard names seven process categories. For metal precision parts two matter most: powder bed fusion, where thermal energy fuses regions of a powder bed, and directed energy deposition, where material melts as it is laid down.

That moves the limits of the manufacturing route. In machining the setup decides what you can reach; in printing the build orientation decides which faces come out smooth and where the supports go.

Which shapes favour printing, and which do not?

Printing wins where a cutter cannot go: a cooling channel that curves through an insert, a manifold with crossing passages and no plugged drillings, a lattice, five parts that become one. There the shape itself is the argument.

A prismatic block with pockets, bores and threads is the opposite. Machining wins on price, accuracy and lead time. TWI, the British research and technology organisation, still calls conventional manufacturing the better choice for simple parts that are easy to make.

Printing and machining are often sold as two camps, with the technology picked up front. That is too early: the choice is made face by face. Whoever decides needs to know how a freeform part goes into a machine and where its work origin sits. That knowledge lives in the shop, not in a price that counts only the volume of a model.

Does a printed part come off the machine accurate and smooth?

Not on the faces that matter. A literature review from Luleå University of Technology describes metal parts from laser powder bed fusion as rough in their as-built state. In one example it cites, a printed heat exchanger measured Sa 24.0 µm against 0.316 µm for the conventional design.

The review calls that surface roughness too high for many end products, mainly because of fatigue: in one study it cites, the high-cycle fatigue limit was up to 40 per cent lower than on machined specimens. Horizontal faces come out best; the shallower an overhanging face, the rougher it gets.

Residual stress and distortion come from the heat of the build: the stress stays locked in the part and pulls it out of shape once it comes off the build plate. A dimensional tolerance that machining holds in one setup is not a given in printing. Any face the drawing gives a fit, a roughness requirement or a geometric tolerance gets a machining operation.

Six criteria side by side

The choice rarely turns on one criterion. These are the six that settle it in a quote. When in doubt, pick the route in which the fewest faces are printed only to be machined afterwards.

What does machining a printed part involve?

To a machinist a printed part is a blank without a straight edge, on a build plate and often with supports; ISO/ASTM 52900 notes that removing supports and cleaning are often necessary. Metal parts usually get a heat treatment, such as stress relieving, before cutting.

A freeform shape has no face to locate against and no datum to measure from. Agree before printing where the part will be held: a clamping lug or flat foot printed on and cut away later, plus stock on every face that gets an operation. The print then becomes stock you can plan a route from, working back from the last setup.

Allow for dedicated workholding and a probe to align the part: a printed shape does not sit exactly where the model puts it. An internal channel is out of the probe's reach. It stays as printed and is checked another way, industrial CT for instance; that check belongs in the agreement and the price too.

How does quantity change the choice?

Printing needs no tooling per shape, which makes it attractive for small quantities. A literature review by NIST, the US National Institute of Standards and Technology, finds additive manufacturing costlier than traditional methods in many instances, and cost-effective for small batches. In a cost example for a stainless steel part that the report shows in several variants, the machine is the largest item in all but one variant: on average 62.9 per cent of the cost.

In machining, setup and programming are one-off costs spread over the batch, as why a small batch costs so much per part shows. A hybrid route carries both: a printed blank priced per piece and a machining step with its own program and workholding.

Of the €1,500 for a single part, €700 is one-off; at ten parts that drops to €70 each, while print and heat treatment stay put. At one part the machining preparation is the largest item, at ten it is the print. Quote a hybrid part at two quantity breaks at least, so it shows which item drives the cost price at which quantity.

Hybrid routes: one part, two processes

Hybrid manufacturing can happen in one machine: a review from the University of Bath describes workstations that combine additive and subtractive processing; many pair directed energy deposition with CNC machining on a multi-axis machine. Building, measuring and reworking in one place also opens up repair of damaged high-value parts.

More often two firms share the route, and then the agreement sets the quality. ISO/ASTM 52901 lists what customer and part provider fix when ordering a printed part: part definition, feedstock requirements, final properties, inspection and acceptance, up to a first production part. What you want to prove at delivery, you agree when you place the order.

Put a hybrid route in the quote as separate lines: printed blank, heat treatment as a subcontract line, programming, workholding, each setup, machining and inspection. If the print supplier's price is missing, leave the line open, as with a finishing step without a price. An open line is a question for the supplier; a guessed line is a risk to your margin.