Linear Guide Accuracy Grades Explained
Accuracy is how close the assembled guide runs to a straight reference. It is measured on the pair, not on the block alone: the catalogue gives a height tolerance and a running parallelism for each grade, and those two numbers decide what the machine can hold. Grade is independent of preload — a guide can be ZA preload and P accuracy, or ZB preload and H accuracy.
What the grade controls
Two measurements matter in practice. The first is the height of the block above the rail mounting surface: if two rails on the same bed carry one gantry, their blocks have to sit at the same height, or the gantry is tilted before it moves. The second is running parallelism: as the block travels, its top face should stay parallel to the rail datum within the tolerance of the grade.
Both are printed per size in the catalogue, because the tolerance scales with the size — a 15 mm guide and a 45 mm guide are not held to the same number. When a machine is designed around one grade, the two rails are normally ordered together so the height tolerance is set as a pair.
The five grades
| Grade | What it is for | Sold as |
|---|---|---|
| C | General automation, transfer slides, packaging axes | Interchangeable block or matched set |
| H | Machine tools with a machined or hand-scraped bed | Interchangeable block or matched set |
| P | Machine tools where the axis is measured against a straight edge | Interchangeable block or matched set |
| SP | Grinding, measuring machines, high-precision spindles | Matched block-and-rail set only |
| UP | Highest grade in the range, for the most demanding axes | Matched block-and-rail set only |
The break is between P and SP. Up to P, a block can be bought separately and fitted to an existing rail of the same size. SP and UP are lapped as a pair, so the block and the rail are ordered together and cannot be swapped afterwards. That is a supply rule, not a quality statement: a P block is not “worse made” than an SP block, it is simply graded as a single part rather than as a pair.
Choosing a grade from the machine
Work backwards from what the machine has to hold. A packaging axis that repeats to a tenth of a millimetre does not need P; a grinder that has to hold microns does, and it will not get there on a frame that is not straight. The guide can only follow the surface it is bolted to. If the bed is machined flat and the rails are aligned to a straight edge, a P guide will hold its tolerance; on a welded frame with no stress relief, an SP guide will simply follow the movement of the frame and the extra money is wasted.
The second question is whether the grade is needed on both rails. On a gantry, the two rails are usually ordered as a pair so the height tolerance is set together. On a single-rail axis the pair question does not arise, and the grade is chosen on running parallelism alone.
Accuracy and preload are separate
Grade controls position; preload controls clearance and rigidity. A Z0 block in P accuracy is a precise block with no preload, which is correct for a vertical axis where the load keeps it seated. A ZB block in C accuracy is a stiff block with a wide tolerance, which suits a heavy cut where the machine structure is the limiting factor. Ordering UP with ZB to “be safe” is the most common way to spend money on a grade the frame cannot hold. The preload page covers the other half of this choice.
Ordering mistakes worth avoiding
Three mistakes come up again and again. The first is ordering SP for a machine whose bed has never been measured, where the guide will hold the frame's error rather than correct it. The second is mixing grades on one gantry: two rails at different heights tilt the gantry before it moves, and no amount of alignment at the ends fixes a height difference in the middle. The third is replacing one block from an SP set and expecting the grade to survive; a matched pair is matched for a reason, and a single replacement block is only interchangeable up to P.
How to read the datasheet
Each size page lists the height tolerance of the block and the running parallelism over the rail length for the grades we supply. The figure is quoted for the pair, so when a replacement block is ordered for an existing rail, the height tolerance is re-checked against that rail rather than assumed from the catalogue. Send us the rail size and the grade you need and we return the current sheet for that size before you commit.
Sizes and models this applies to
The rules above are the same at every size; the numbers differ. These pages carry the current figures for the sizes that come up most: HGH15CA, HGH25CA, HGW25CC, HGL30CA and MGN12. Series pages give the full range: HG series and miniature series; the block index is on linear guide blocks.
FAQ
| Can I buy an SP block on its own? | No. SP and UP are lapped as a block-and-rail set, so the pair is ordered together. |
| Is a higher grade more rigid? | No. Grade is position; rigidity comes from preload and from the size of the guide. |
| Do both rails on a gantry need the same grade? | Yes, and they are normally ordered as a pair so the height tolerance is matched. |
| My axis holds tolerance cold but not hot — is that the grade? | Usually not. Thermal growth of the frame and the screw shows up the same way. Check the structure before changing grade. |
Example sizes: HGH25CA, MGN12 (miniature, C / H / P only). All blocks · Technical reference.