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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.

What to Check Before Replacing an Original Linear Guide Block

When a CNC machine has a worn linear guide, the obvious solution is to order the same part again.

Sometimes that's the best choice.

But not always.

We've had replacement projects where the original guide was available only with a long lead time, or the cost didn't make sense for an older machine. In those cases, a compatible linear guide slider can be a practical alternative.

The problem starts when “compatible” is treated as meaning “same size.”

It isn't.

A guide block can fit the rail and still have different preload, running resistance, accuracy, or load behavior. In a precision machine, those differences can show up after installation rather than when the part is sitting on the workbench.

So if you're sourcing a THK or HIWIN compatible linear guide slider, here's what I would check before placing the order.

What I Check First When a Linear Guide Starts Causing Trouble – Based on Real Shop Floor Calls

I’ll be honest: when a service call comes in about a CNC axis “losing accuracy,” my first reaction isn’t to grab a new linear guide block.

It’s to ask a few boring questions.

Because in my experience, the guide gets blamed for things it didn’t do, and replaced for things it could have kept doing for another year. Meanwhile, the real culprit – misalignment, a dirty wiper, or even a loose bolt – sits there unnoticed.

So if you’re staring at a machine that used to hold ±5 microns and now wanders around like it’s had too much coffee, let’s walk through how I actually diagnose this. No catalogs, no generic checklists – just what I’ve learned from chasing axis problems in real workshops.

I got a call a while back from a machine builder who was convinced his linear guides had the wrong preload. The carriages had felt smooth when they arrived, but after he bolted the rails to the base, one of them felt noticeably tighter.

He wanted to return the guides.

That wasn't the first time I'd seen this. And in most cases, the preload wasn't the real problem.

But preload does matter. It just matters differently from how a lot of people think.