Rail Length, Hole Pitch and Joints

A rail is a length plus a hole pattern. The length sets how far the block can travel; the pattern sets where the bolts go. Both come from one formula, and getting it right before the order avoids a rail that is 20 mm too short or has its end holes in the wrong place for the drilling on the machine.

The formula

A rail with n holes, hole pitch P and end distance E has length L = (n − 1) × P + 2 × E. P and E are fixed by the rail size, not chosen per order, so the only free number is n. Work out n from the length you need, then check the end distance that results.

Example: an HGR25 rail of 1000 mm with P = 60 and E = 20 gives n = (1000 − 40)/60 + 1 = 17 holes, and the exact length is 16 × 60 + 40 = 1000 mm. If the machine needs 980 mm instead, n = 16, and the end distance becomes (980 − 15 × 60)/2 = 40 mm — which may be more than the mounting surface allows, so the length changes rather than the end distance.

Pitch and end distance by size

Note that P jumps from 60 to 80 between 25 and 30, and from 80 to 105 between 35 and 45. A design carried over from one size to the next does not keep the same hole count: the same length on a larger rail has fewer holes, and the end distance grows with it.

Joints

Where the stroke is longer than one rail, two or more lengths are joined. The joint is marked on the drawing and the ends are matched before shipping, because the running surface has to stay continuous: a step at the joint is felt as a click once per stroke and it wears the wiper at the same point. The joint should not sit where the block dwells or where the cutting force is highest, and it should be supported by the mounting surface on both sides.

When two rails are joined, the hole pattern continues across the joint with the same pitch; the two end holes are not doubled. Tell us the total length and the stroke limits and the joint position is set on the drawing rather than left to the fitter.

Cutting and drilling

Rails are cut square, deburred and drilled in one set-up, so the pitch stays accurate along the length. A rail cut on site re-opens the hole question: the cut end loses its end distance, the pattern no longer matches the drawing, and the rail has to be re-drilled or re-checked. If the length is likely to change, tell us before the order so the rail is supplied long and cut to the final length with the pattern set afterwards.

Measuring on site

Measure the stroke, not the rail. The block has a body length, and the rail has to extend beyond the block at both ends of travel, so the rail length is the stroke plus the block length plus the end margins. Measure the centre distance between the outermost existing holes if the machine is already drilled, and check that number against (n − 1) × P before ordering: a machine drilled for a different standard will not take a rail drilled to this one without re-drilling.

Sizes, blocks and rails

The rail length is chosen with the block: HGH15CA, HGH25CA, HGH35CA, HGH45CA, HGW25CC and MGN12. Rails are on linear guide rails, the block index on linear guide blocks, and the series page is HG series.

Two rails, one axis

On a two-rail axis both rails are drilled to the same n and the same P; only the length can differ if the design asks for it, and that is drawn. Ordering the two rails separately from two different drawings is how a gantry ends up with the holes a few millimetres apart, which is invisible until the blocks are fitted and the carriage will not sit square.

Send the two lengths and the hole count together, and they are cut and drilled as a pair.

FAQ