Two rails, one gantry: the alignment errors that show up at the ends
2026-11-06
# Two rails, one gantry: the alignment errors that show up at the ends
A single rail is forgiving. A gantry is not. When two rails carry one moving frame, every geometric error is doubled and then multiplied by the distance between them — and the symptom usually appears at one end of the stroke, which sends people looking in the wrong place.
Four faults produce four different symptoms. Learn to tell them apart before touching a bolt.
1. Binds at one end, free at the other
Cause: the two rails are not parallel in plan. The centre distance narrows toward one end. The blocks are being squeezed as the gantry approaches the narrow end.
Test: measure the centre distance at both ends of the stroke with a caliper or a laser. The difference should be within the rail's specified tolerance — commonly 0.02–0.05 mm over the measuring length, depending on size.
Fix: loosen one rail, bring it into line, torque down from the middle outward. This is the most common gantry fault and the easiest to blame on the guides.
2. Binds in the middle, free at both ends
Cause: rails are bowed, not straight. Both rails curve in the same direction, so the centre distance is correct only at the ends.
Test: a straightedge or a dial indicator along the side face, checked at three points. Sagging is common on long rails that were not supported during handling.
Fix: check the mounting surface flatness first. A rail bolted to a warped bed takes the shape of the bed — the rail is not defective.
3. The gantry tilts as it travels
Cause: the two rails differ in height. The blocks are at different elevations, so the moving frame rolls progressively along the stroke.
Test: a dial indicator on the top face of each rail, same reference. Height difference between rails should be within the specified tolerance — typically 0.02–0.05 mm for a gantry.
Fix: shim the lower rail. Do not try to correct it by preload — preload cannot compensate a height error, it only adds friction to it.
4. A step you can feel, once per pass
Cause: rail joints. On long gantries, both rails ship in sections. If the sections are not joined at matched positions and heights, the block lifts at every joint.
Two rules:
- Both rails must have the same number of sections and the same joint positions. Otherwise the frame bridges a joint on one side and not the other, and the gantry skews for the length of one block.
- Never park a block on a joint. A stationary block on a joint takes continuous load on the step and is the first place to fatigue.
Joint position is a drawing item. It has to be specified when ordering, not corrected on site.
A note on preload as a cure
When a gantry binds, the instinct is to raise preload to "take out the play". It does the opposite: higher preload makes the block less tolerant of misalignment, so a rail that is 0.05 mm out will bind harder, not softer. If the machine binds because of geometry, fix the geometry.
Order of checks
- Centre distance at both ends — parallelism
- Rail straightness along the side face — bow
- Rail height with a dial indicator — level difference
- Joint positions and whether a block parks on one
- Only then: alignment of the mounting surfaces and torque sequence
Torque the mounting bolts from the centre outward, in the documented sequence, and re-check after the first few hours of running. New machines settle.
What to send with an enquiry
For a gantry application, five things let us specify the pair correctly:
- Distance between the two rails, and total stroke
- Machine type and what the moving frame carries
- Required parallelism and height tolerance
- Whether rails must ship in sections, and the maximum section length you can handle
- Whether either rail is vertical
The fifth one catches people out: a vertical pair carries load very differently from a horizontal pair, and the preload choice changes with it.