Customers sometimes ask us which CNC machine they should buy for their own plant, often after a supplier has already shown them a brochure. Our first answer is always the same: start with the drawings of the parts you make, or expect to make, and let those decide the machine. This is how we’d work through it.

Let the parts decide the type

Mostly round parts like shafts, bushings, pins and threaded fittings are lathe work. A mill can make them, but slowly and awkwardly. Prismatic parts (plates, blocks, housings, anything with flat faces, pockets and hole patterns) belong on a mill. Large sheets of soft material such as plywood, acrylic or aluminum composite panel are router work: a big table, a fast spindle, light cuts, and much less rigidity than a mill. Routers are not for cutting steel.

Most shops that machine metal need a mill first, since prismatic parts make up most general industrial work. But if 80 percent of your product line is turned, buying a mill first is an expensive mistake.

Three axes or five

A 3-axis mill handles most industrial work: fixtures, machine parts, mold plates, brackets. A 5-axis machine is worth its much higher price if you regularly make parts that need several faces machined in one setup, or contoured surfaces like impellers. If you only see a couple of those a year, you’d be paying for capability, and programming skill, that mostly sits unused. Our shop runs 3-axis mills, and we plan around the occasional job that needs more.

Size and rigidity

The work envelope has to fit your largest part plus the vise or fixture holding it. Size the machine for the parts you make now, with some margin, not for a large part you might quote someday.

Rigidity usually isn’t on the front page of the brochure. Two machines with the same travels can perform very differently in steel. The heavier one can take deeper cuts without chatter, holds size better as it warms up, and leaves a better finish. Machine weight is a rough but useful guide. A light machine cutting hard material is limited to shallow, slow passes, and you pay for that in cycle time on every job.

Tolerances on paper and in practice

The accuracy figures in a brochure are measured with a laser on a cold machine that isn’t cutting anything. The tolerance you hold on real parts also depends on tooling, workholding, the material, temperature changes in the shop, and the operator. Expect real parts to come out looser than the brochure number. Holding very tight fits takes process control, not just the right machine.

Also check what your parts really need. Most general machine parts are fine at ±0.05 mm. Buying a machine to hold ±0.005 mm on everything is a lot of money you probably don’t need to spend.

The control and software

The control matters more than most buyers expect, mainly because of people. With a common control like Fanuc or similar, operators may already know it, local technicians can service it, and spare parts are available in the country. An unusual control on a cheap machine can leave you waiting for an overseas technician when it faults. Make sure your CAM software has a proven post-processor for that control too. Writing one yourself is not a beginner’s job.

Costs after the purchase

Over the first few years it’s common to spend about as much again as the machine price on cutting tools and holders, vises, clamps and fixtures, measuring equipment, CAM licenses, maintenance, and above all a person who can program and run it. A machine without a skilled operator produces nothing. Training someone takes months, and hiring an experienced operator means paying a higher salary. Budget for both the machine and the person.

When not to buy

If you only need a few parts a month, repairs, or occasional modifications, a manual mill or lathe and a good machinist may serve you better than a CNC machine that sits idle. If machining isn’t your core business, sending the work to a shop is usually cheaper than owning the equipment. You pay per part for the machine, tooling and experience, and nothing sits depreciating on your floor.

We’d give the same advice if we didn’t do outsourced machining ourselves. If your volumes do justify buying, the points above will help you choose well. If you want to try outsourcing first, that’s part of what we do.