I keep seeing the same question on forums and in emails:

“Should I use SFU1605 or SFU2005 for my CNC?”

Usually the person asking already has a machine design in mind. They’ve read that the 20 mm screw is “stronger” or “more rigid,” so they’re leaning toward the bigger one just to be safe.

That instinct isn’t wrong. But it also isn’t the whole story.

SFU1605 and SFU2005 are both 5 mm lead ball screws. The only major difference is the screw diameter—16 mm versus 20 mm. That 4 mm changes how the screw behaves mechanically, but it doesn’t automatically make the bigger screw the right choice.

I’ve worked on small CNC routers, compact linear stages, and a few heavier gantry machines. In almost every case, the final decision came down to what the axis actually had to do, not just which screw looked more substantial on paper.

So instead of giving you a catalog-style comparison, let me walk through how I actually think about this choice.