The Rail That Didn’t Sit Flat: Mounting Surface, Not Rail Defect
A profile linear rail bolted to an aluminum extrusion. After installation, the carriage didn’t move smoothly along the full length. It stuck at one point. We removed the rail and checked the extrusion surface. The extrusion was bowed 0.1 mm over 500 mm. The rail, when bolted down, conformed to the bowed surface. The carriage rode the bow — it went through a small concave section and stuck.
Why the mounting surface matters
A profile rail is straight when it leaves the factory. But when you bolt it to a surface that isn’t straight, the rail bends to match. The rail’s mounting holes are close together (every 30-60 mm). If the extrusion between the holes is bowed, the rail follows it. The carriage, riding on precision balls, encounters the bend as a binding point.
The mounting surface options
| Surface | Flatness | Use for |
|---|---|---|
| Extrusion (as drawn) | 0.1-0.3 mm/m | Non-critical guides, drawers |
| Extrusion with machined pad | 0.02 mm/m | Standard linear rails |
| Machined steel plate | 0.01 mm/m | Precision rails, measuring |
| Ground surface plate | 0.005 mm/m | Metrology, inspection |
What we did
1. Machine the mounting surface. We milled a flat pad on the extrusion where the rail mounts. The pad was 40 mm wide (the rail width plus clearance), machined flat within 0.02 mm. The rail bolted to the machined surface, not the raw extrusion. The binding disappeared.
2. Bolt in the right order. We bolted the rail from the center outward, not from one end. Starting at the center and tightening outward lets the rail conform to the machined surface without trapping stress. Bolting from one end pulls the rail along as each bolt tightens, which can bow it.
3. Check with a feeler gauge. After bolting, we checked the rail-to-surface contact with a 0.03 mm feeler gauge. If it slides under the rail anywhere, that point isn’t seated. We loosened and re-tightened. The rail sat flat along its full length.
The rail itself
A profile rail is straight to about 0.02 mm over 1 meter as supplied. But its straightness is only as good as the surface it’s mounted to. If the mounting surface is bowed, the rail bows. The rail’s accuracy comes from its mounting surface, not from its own straightness.
The carriage movement
After the fix, the carriage moved freely along the full rail. The binding was at the point where the extrusion had bowed the most. The machined pad removed the bow. The rail sat flat. The carriage rode smoothly.
The mounting rule: A linear rail is only as straight as the surface it bolts to. For precision rails, machine a flat pad on the extrusion or weldment. Bolt from the center outward. Check contact with a feeler gauge. A rail that binds at one point is mounted on a bowed surface, not a defective rail.
