A guy emailed me last week asking for a price on a C5 ground ball screw for his CNC router. He was building a machine to cut wood and plastic, with a target tolerance of maybe plus or minus 0.1 mm. Nothing crazy.

I asked him why he specifically wanted C5.

He said, “Because C5 is better, right?”

That’s a very common way to think. And it’s not completely wrong. C5 does have tighter lead accuracy than C7. But on a wood router with a flexible frame, a handheld router spindle, and a workpiece that moves with humidity, that extra screw accuracy is probably going to disappear into the noise.

I’m not saying C5 is useless. I’m saying it’s only worth paying for when the rest of the machine can actually use it.

So if you’re trying to decide between C5 and C7 for a ball screw, here’s how I think about it—not from a catalog spec sheet, but from the machines I’ve worked on and the mistakes I’ve seen.