Print or machine? How to decide in five minutes and without arguing

Metal 3D printing doesn't replace machining, nor does it compete with it in general. It competes part by part, and it wins in five fairly identifiable situations. Outside of those five, it almost always loses, and saying so openly saves everyone from failed projects.

The quick way to decide is this: if your part is not in any of the five cases below, machine it.

The five cases in which it wins

1. The geometry cannot be machined. Internal channels that follow a surface, curved ducts, internal lattices, closed cavities. It's not that it's more expensive with numerical control: it's that there's no way to fit the tool. There's no comparison here; there's either one alternative or none.

2. It replaces a multi-part assembly. A manifold welded from nine parts, or a bolted hydraulic body with its gaskets, becomes a single unit. And then the numbers change: welding, assembly tooling, gaskets, bolts, joint inspection, and a list of part numbers in the warehouse all disappear. Many cases that seemed expensive cease to be so when the entire assembly is compared to the individual parts.

3. The amount of material used matters a lot. If you start with a 30 kg block of titanium to produce a 2 kg part, you're paying for 28 kg of titanium to turn it into shavings. In expensive alloys and heavily hollowed-out parts, the balance is reversed.

4. Time is of the essence. A machine is down, a spare part is discontinued, tooling is needed on Thursday. Without a mold, tooling, or specific equipment, the part can be produced in days. In a real emergency, the price per part ceases to be the deciding factor.

5. The series is short and the geometry changes. Functional prototypes, series of five to five hundred, variations of the same component. Anything that amortizes tooling works against additive manufacturing; when there is no tooling to amortize, it works in its favor.

There is a sixth case that deserves special mention because it is the most profitable of all when it appears: conformal cooling mold inserts, where the savings are recouped with each injection-molded part for years. We explain it in full in its own article.

Signs that no

For the sake of honesty, and because these are more useful than the previous ones:

The part is a solid block with no internal geometry. There's nothing additive manufacturing can offer, and a lot of material to melt slowly.
All surfaces have tight tolerances. They'll end up being machined anyway, so you're paying for two processes.
The production run is long and stable. After a few thousand identical parts, almost any conventional process wins.
The part is large and simple. If it's half a meter long and a support, build volume is the enemy.
The material isn't weldable. If it cracks when welded, it will crack when printed. Brass, for example, can't be welded: the zinc evaporates.
No one has measured the current cost. Without the starting data, no improvement can be demonstrated, and the investment ends up being justified on faith.

How to compare without cheating

The most common mistake is comparing the price of a printed part with the price of a machined part, and additive manufacturing almost always loses in that regard. It's a flawed comparison, because the process changes, but not the design.

Honest comparison has three rules:

Compare the whole, not the part. If the printed part replaces five, the comparison term is the five plus their assembly.

Compare the total cost of having it. Price of the part, tooling costs, storage, lead time, transport, and the cost of not having it on time.

Redesign before comparing. Additively manufacturing the same geometry designed for milling is a waste of money. The advantage arises when the part is redesigned for the process, and then it usually weighs less, has fewer joints, and fewer part numbers.

The Short Method

With the part in front of me, five questions and one decision:

Can it be machined as is? If not, additive manufacturing is the way to go, and that's the end of the discussion.
Is it an assembly of several parts? If so, consider the entire assembly before comparing.
How many per year? Above a few thousand, machining or casting is the way to go.
How much material is currently being scrapped, and what material is it? High rates of expensive alloys push towards additive manufacturing.
What is the cost per day of delay? If it's significant, lead time dictates the price.

If all five answers point to machining, then machine it. Our work in additive manufacturing doesn't obligate us to recommend it where it's not suitable, and recommending it incorrectly is the surest way to lose a customer.

And very often, both

The real answer in most industrial projects is neither one nor the other: it's additive manufacturing followed by machining. The part comes off the machine with its complex geometry and excess thickness on the four surfaces that need to fit together, and those four surfaces are machined afterward.

It's normal, it's not a failure of the technology, and it's advisable to include it in the budget from the beginning.

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If you already know your part fits the bill, the quote calculator gives you a cost estimate without you having to call anyone. Learn how that price is calculated and what influences it in how much does a metal-printed part really cost. And find out which parts you shouldn't manufacture this way, even if it's possible, in five parts you shouldn't print.

The other frontier, against foundry and MIM, in print, cast or sinter.