Guide
How Flat Does a Machined Base Need to Be for Linear Guide Rails?
A machined base is the plate, frame or machine bed whose top face carries one or more profile rails, and the rail follows that face closely once the screws are tight, so its flatness, the parallelism between rail seats and the position of a reference edge all end up in the motion of the carriage.
By Saola Manufacturing engineering team · Published · Updated
Short answer: there is no single number. Rail makers publish a mounting-surface tolerance for each rail series, accuracy class and preload, and one maker says the surface should be finished to at least the parallelism of the rail itself. Take the value from the catalog of the rail you chose, put flatness, parallelism and the reference edge on the plate drawing, and agree how the plate will be measured.
On this page
Why does the flatness of the base matter for a profile rail?
Because the rail is not rigid enough to straighten the surface under it. Once the rail is bolted down it follows the plate, and any hump or hollow in the plate becomes a hump or hollow in the carriage path.
THK, in its design notes for cross-roller and ball guides, tells the user to mount the rail so that it closely contacts the mounting surface, and says that to reach a high level of running accuracy the mounting surface needs a certain level of accuracy in parallelism and straightness (THK, mounting surface accuracy). That page covers one product family, not every guide, but the principle is the same for a recirculating-ball profile rail: the rail is the straightedge only if the surface under it lets it be.
The second effect is on the carriage. A preloaded block has no clearance to absorb error. As a matter of mechanics rather than a quoted figure: if the rail is pulled into a wave by the plate, the blocks ride up and down that wave and the preload changes with position, which shows as drag, uneven wear or noise. That is why rail makers tie the allowed surface error to the preload class, and why a looser plate sometimes needs a lighter preload instead of a better plate. Confirm the relationship for your rail series with the manufacturer.
None of this is a reason to chase micrometer flatness everywhere. It is a reason to know which tolerance you are buying, because the cost of a ground or lapped plate is much higher than a milled one, and a base that is better than the rail needs is money spent twice.
How flat do rail makers say the mounting surface must be?
It depends on the rail series, the accuracy class and the preload, so the honest answer is to read the catalog. As a rule published by one maker, finish the surface by grinding or similar machining to at least the parallelism of the rail itself.
The THK page quoted above says the parallelism and flatness of the rail-mounting surface should preferably be finished by grinding or similar machining, to at least the same degree as the parallelism of the rail, and points to the accuracy standard of each model for the numbers. Read that as a ceiling on how much worse than the rail the plate may be, not as a target for the whole plate.
A published example with numbers comes from Rollon, a rail maker that compares its compact rail with conventional profile rails. For a setup with several rails in parallel it says such rails typically require approximately 30 µm of mounting-surface flatness and 130 µm of rail-to-rail parallelism. Those are one vendor's figures for a type of rail, not a standard, and the page ties them to multi-rail machines. They show the order of magnitude, tens of micrometers over the mounting area, but they are no substitute for the table in the catalog of the rail you actually buy.
The same Rollon page points out that some rail types allow mounting surfaces that do not require machining. If your plate is a welded frame, ask whether such a rail would meet the accuracy of the machine before you commit to machining and grinding the frame.
Which callouts go on the plate drawing: flatness, parallelism or a reference edge?
All three, each with a purpose. Flatness limits the height error of the seat, parallelism limits how far two rail seats drift from each other, and the reference edge locates the rail sideways.
Write the callouts with the symbols and datum rules of ASME Y14.5-2018. Flatness applies to each rail seat as its own surface, so call it out per seat rather than once for the whole top face; a single flatness zone on a long plate allows a bend that is too large for any one rail. Parallelism between rail seats, or between a seat and the reference edge, needs a datum: choose one seat or the machined base face as datum A and measure the others against it. The reference edge is a machined shoulder, a straightedge or a row of dowels that the rail is clamped against so that two rails end up parallel without trial-and-error.
| Feature | What to call out | Where the number comes from | Example from a published source |
|---|---|---|---|
| Rail seat flatness | Flatness zone on each seat, with seat width and length shown | Rail maker's table for the series and preload | approx. 30 µm (Rollon, multi-rail) |
| Parallelism between rail seats | Parallelism to datum A over the rail length | Rail maker's table; ties to accuracy class | 130 µm rail-to-rail (Rollon) |
| Reference edge | Straightness and perpendicularity to the seat; relief in the corner | Rail maker's drawing for shoulder height and corner radius | Not quoted here |
| Surface texture of the seat | Roughness value and lay, if the rail maker asks for it | Rail maker's installation notes | Not quoted here |
| Mounting hole position | Position tolerance to the datum, with thread depth | Rail screw size and pitch from the catalog | Not quoted here |
Can a CNC-milled plate hold the tolerance, or does it need grinding?
A milled plate can often meet a moderate flatness requirement when the plate is stable. Where the rail maker asks for much tighter values, finish the seats by grinding, and plan the process so the last operation does not release stress.
Whether a milled plate holds depends less on the cutter than on the plate. Material removed from one side of a stressed block lets the block spring, so a plate that measured flat on the machine can bow when it is unclamped. Clamping itself bends a thin plate the other way. A sequence that rough-cuts both faces, stress-relieves the plate, then finish-mills the seats with light clamping gives the part a chance to stay flat; the guide on stress relieving before machining explains the steel side of that.
Thermal effects matter at this scale. Dimensional measurement is specified at a reference temperature, which the NIST summary of ISO 1:2016 confirms as 20 °C, and a long plate held in a warm hand or measured next to a window will not read the same as one that has soaked in the inspection room.
For production, check the fit of the supplier first. Saola’s CNC milling at the lead CNC plant is set up mainly for machining castings, and that page says a part with no casting or forging step is often a better fit for a dedicated machining house, so say early whether the base is cut entirely from plate. Whoever machines it, the rail seats, the reference edge and the hole pattern should be machined in one setup so they share a datum, and if the rail maker’s value calls for ground seats, the milled blank needs a grinding allowance and a final measurement after grinding.
How do you check supplier flatness reports on a rail plate?
Ask for the method, the datum, the point pattern and the temperature, not only a pass or fail. A flatness number without those four cannot be compared against the rail maker's table.
A report should say which instrument was used (a CMM, a granite surface plate with an indicator, or a laser or autocollimator for long beds), which seat or face was datum A, how many points were taken and where, and the room temperature. Flatness from a handful of random points can miss a bow; for a rail seat, a line of points along its length plus a few across is the minimum a rail maker would expect to see. Ask for the same report on the first article and then on a sample from each later lot.
Before a plate is released for volume, fit one rail to the first article and run a carriage over it by hand. Drag that changes with position, or a block that tightens at the same spot on every stroke, means the surface needs another look before the next lot is machined. Record the torque sequence you used, because the screw order changes how the rail settles.
For a yearly volume in the thousands, the interesting question is repeatability: the first plate may pass, and the question is whether the hundredth does. Agree a sampling plan and what happens to a plate that falls near the limit, before the first lot ships.
Frequently asked
Is there a general flatness number for linear guide mounting surfaces?
No. Rail makers publish values per series, accuracy class and preload. Rollon's typical figures for multi-rail setups, approximately 30 µm flatness and 130 µm parallelism, are one example and are not a standard.
Do I need to grind the mounting surface?
THK's design notes recommend finishing the mounting surface by grinding or similar machining to at least the parallelism of the rail. Whether a milled finish is enough depends on the tolerance your rail maker lists; ask for the value before choosing the process.
Does the rail need a machined shoulder?
Not always, but a reference edge makes two rails parallel without trial and error. A straightedge or a row of dowels can do the same job. Follow the shoulder height and corner relief in the rail maker's drawing.
Why not just use a lighter preload on a poor plate?
Sometimes that is the right trade, since a lighter preload tolerates more surface error. It also lowers stiffness, so check the load case with the rail maker before relying on it.
Should the plate be stress relieved before the final milling pass?
For a large or heavily machined steel plate, yes, because stored stress can move a surface after it is cut. Do the stress relief before the finish pass that creates the rail seats.
Sources
- Accuracy of the mounting surface (cross-roller guide and ball guide), THK, accessed 2026
- Rollon, mounting-surface requirements for multi-rail setups, Rollon, accessed 2026
- ASME Y14.5-2018 (reaffirmed 2024), Dimensioning and Tolerancing, ASME, 2018
- The 2016 revision of ISO 1: standard reference temperature for the specification of geometrical and dimensional properties, NIST, 2016