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.