You mount a pair of profile rails on a machined bed, bolt the carriage plate on, and it slides like it’s dragging through sand. The rails are new, the carriages are preloaded, and the bolts are torqued. The problem is the rails don’t actually track parallel to each other.
Why parallelism matters more than flatness
A linear guide carriage has very little internal clearance. The recirculating balls are sized to roll with a defined preload between the rail and the block. If the two rails are even a few dozen microns out of parallel across a meter of travel, the carriage blocks fight each other. One rail pushes the blocks outward while the other pulls them in. You feel it as binding, and you see it as uneven wear.
People level each rail individually and call it done. That gets both rails flat to the bed, but doesn’t make them parallel. You have to check the gauge between the rails at several points along the stroke.
The master-and-slave method
Mount the first rail fully, bolt it down, and verify it against a straightedge. That’s the master. Mount the second rail loose, slide the carriage plate back and forth, and tighten the bolts as you go. The plate, riding on the fixed rail, pulls the second rail into alignment. This is faster than measuring and works well on welded frames that can’t hold tight tolerances.
If the frame is precision machined, measure the rail spacing with a dial indicator riding on the master rail. Tighten the second rail to the measured dimension.
Don’t over-torque the rail bolts
Profile rails distort when the mounting bolts are cranked unevenly. A rail that measures straight before bolting can bow after. Torque the bolts in a sequence from the middle outward, not end to end. That pushes any gap out to the ends instead of trapping it under the rail.
Preload is not a fix for misalignment
Bumping up to a heavier preload to make the axis feel stiffer just hides a crooked install. The extra preload adds friction, heat, and shortens life. Fix the rail alignment; don’t buy a higher preload class to mask it.
What to check when it binds
Loosen the second rail bolts, slide the plate by hand. If it moves freely loose and binds when tight, the rail is pulling out of parallel. If it binds loose too, check the carriage mounting plate for twist or a bolt pattern that’s drilled wrong.
