CNC Design Features That Add Cost

Most of the cost of a machined prototype is time: programming, setups and spindle hours. The block of aluminum is often the cheap part. So small design choices can move the price more than people expect. These are the usual suspects.

1. Sharp inside corners

End mills are round, so a pocket can't have a truly square inside corner. Draw one and the shop has to use a tiny cutter, add an EDM step, or call you. Give inside corners a radius a little bigger than the cutter that will make them. A common rule of thumb is a corner radius of at least a third of the pocket depth. If a mating part really needs a square corner, a dog-bone relief solves it.

2. Deep, narrow pockets

Standard cutters reach about four times their diameter before they start to chatter and need slow, light passes. Deeper than that means long-reach tools and a lot more time. Keep pocket depth within about four times the width where you can.

3. Thin walls

Thin walls vibrate under the cutter and move once they're released from the stock. As a starting point, keep metal walls above about 0.8 mm and plastic walls above about 1.5 mm. Thinner can be done. It just costs more.

4. Tight tolerances on everything

A drawing with ±0.01 mm on every dimension is slow to machine and slower to inspect. Put a general tolerance in the title block (ISO 2768-m is a common choice) and call out tight tolerances only where parts mate, seal or move.

5. Threads deeper than they need to be

Most of a thread's holding power is in the first few turns. In steel, engagement beyond about one and a half times the diameter adds very little. Softer materials like aluminum need a bit more, but not three times more. Deep tapped holes in small sizes also break taps, and digging out a broken tap is nobody's favorite job.

6. Odd hole sizes

A hole that matches a standard drill gets drilled in seconds. A 6.37 mm hole has to be interpolated or needs a special tool. Use standard sizes unless the fit demands otherwise, and keep drilled depth within about four times the diameter where possible.

7. Features on every face

Every time a part is flipped and clamped again, that's another setup, more time, and another chance for features to drift out of position relative to each other. A part that can be machined from one or two sides costs less than one with features on all six. Five-axis machining helps with complex shapes, but it isn't free either.

8. Material and finish picked out of habit

6061 aluminum cuts fast and costs little. 7075 is stronger and costs more. Stainless is slower to cut, and titanium is slower again, with more tool wear. For a fit-check prototype, a cheaper stand-in material is often fine. Finishes add up too: every surface that has to be masked before anodizing or bead blasting is hand work.

One more: quantity

Programming and setup are paid once, so the price per part falls quickly between one piece and five. If you'll want spares or a second round of testing, ordering them now is often cheaper than ordering later.

None of this means designing around the machine shop. If a feature matters, keep it, and it gets quoted. The point is to spend the money on the features that need it. Send a STEP file and drawing through the quote form, or read what to send for a fast quote. More on prototyping and CNC machining.