Before cutting linear rail, you need to understand the rail’s mounting-hole pitch and the distances from each end of the rail to the nearest mounting hole. Cutting changes the overall rail length, but it does not change the spacing between the mounting holes already machined into the rail. The correct cut length therefore needs to account for the required rail length, mounting-hole pitch, end dimensions, usable carriage travel, and mounting support.
The basic rule is:
Determine where the mounting holes will fall after the cut before deciding exactly where to cut the rail.
What Is Linear Rail Hole Spacing?
Linear rail hole spacing is the center-to-center distance between consecutive mounting holes along a profile rail.
Depending on the manufacturer and rail family, this dimension may be identified as P, F, or another designation.
For example, a particular linear rail might have mounting holes spaced:
60 mm center-to-center
Once the rail has been manufactured, that pitch is fixed. Cutting the rail shorter does not change it.
This distinction is important because a custom rail length does not automatically create a custom mounting-hole pattern.
The Three Dimensions You Need to Understand
A useful way to describe linear rail mounting-hole geometry is:
L = G1 + nP + G2
where:
L = finished rail length
G1 = distance from one rail end to the center of the first mounting hole
G2 = distance from the final mounting-hole center to the opposite rail end
P = mounting-hole pitch
n = number of pitch intervals between the first and last mounting holes
Manufacturer terminology varies. THK, for example, commonly uses F for the standard mounting-hole pitch and G/g for the end dimensions in its rail-length tables.
The letters themselves are less important than understanding the geometry.
Mounting-Hole Pitch Is Measured Center-to-Center
When measuring an existing linear rail, mounting-hole pitch should be measured from the center of one mounting hole to the center of the next.
Do not measure:
outside edge to outside edge
inside edge to inside edge
counterbore edge to counterbore edge
If the rail is difficult to measure accurately, measure across several hole intervals and divide by the number of intervals.
For example, suppose the distance from the center of the first hole to the center of the sixth hole is:
300 mm
There are five intervals between those six holes.
Therefore:
P = 300 ÷ 5 = 60 mm
Measuring several intervals can reduce the effect of a small measuring error.
Why Hole Spacing Matters Before Cutting
Suppose a rail has:
P = 60 mm
and you need to shorten it by:
25 mm
Removing 25 mm from the rail does not reposition any of the mounting holes. It simply changes the distance between the newly cut rail end and the nearest existing mounting hole.
Depending on the original geometry, the result could be:
a good end dimension
a mounting hole very close to the rail end
an unnecessarily large unsupported end section
a cut that intersects a mounting hole
a hole pattern that no longer matches the machine base
That is why the hole pattern should be calculated before the rail is cut.
Cutting Changes the End Dimension, Not the Pitch
This is one of the easiest ways to understand custom linear rail cutting.
Suppose a rail has:
P = 60 mm
G1 = 30 mm
G2 = 30 mm
If you remove 10 mm from the G2 end, the pitch remains:
P = 60 mm
but the new end dimension becomes:
G2 = 20 mm
If you remove 25 mm from that same end, the new dimension becomes:
G2 = 5 mm
The mounting holes have not moved. Only the relationship between the last mounting hole and the newly created rail end has changed.
The important principle is:
The mounting-hole pitch is fixed. The rail-end dimension changes when the rail is cut.
Why G1 and G2 Matter
The sections of rail between the first or last mounting holes and the rail ends are important because they extend beyond the outer mounting fasteners.
Manufacturer documentation reflects this consideration.
THK publishes model-specific tables containing:
standard rail lengths
standard mounting-hole pitch
recommended G/g end dimensions
maximum rail length
THK also states that for special rail lengths, values corresponding to its published G and g dimensions are recommended. As those end dimensions increase, the unsupported portion becomes less stable and accuracy can be negatively affected.
This means the rail should not simply be cut wherever the tape measure reaches the desired overall length.
The resulting end dimensions should be checked first.
Do G1 and G2 Have to Be Equal?
No.
The two rail-end dimensions do not universally have to be identical.
A new machine may be designed with:
G1 = G2
because a symmetrical mounting-hole pattern is convenient.
A replacement rail, however, might require:
G1 = 20 mm
and:
G2 = 40 mm
to match an existing mounting plate.
There is nothing inherently wrong with unequal end dimensions if they are appropriate for the selected rail and machine.
The important point is that G1 and G2 should be intentional rather than accidental.
Example: Calculating a Rail From Hole Pitch
Suppose a rail has:
mounting-hole pitch = 60 mm
G1 = 20 mm
G2 = 20 mm
20 pitch intervals
The finished rail length is:
L = G1 + nP + G2
Therefore:
L = 20 + (20 × 60) + 20
L = 1,240 mm
This is a particularly clean rail geometry because the hole pattern is centered with 20 mm at each end.
If the machine can use a 1,240 mm rail, that may be preferable to selecting an arbitrary nearby length that produces awkward end dimensions.
What Happens If You Need 1,250 mm Instead?
Suppose the same machine requires exactly:
1,250 mm
If the existing hole pattern remains unchanged, the additional 10 mm must appear somewhere in the end geometry.
One possibility might be:
G1 = 20 mm
G2 = 30 mm
because:
20 + (20 × 60) + 30 = 1,250 mm
Whether that is acceptable depends on the exact rail family and machine requirements.
This illustrates an important point:
A rail length does not necessarily need to be an exact multiple of the mounting-hole pitch.
The end dimensions account for the difference.
Rail Length Is Not Simply Hole Pitch × Number of Holes
Another common mistake is calculating rail length by multiplying the number of holes by the pitch.
That does not work because pitch describes the distance between hole centers.
If a rail contains 21 mounting holes, there are:
20 pitch intervals
between the first and last holes.
The rail also has material beyond the first and last mounting-hole centers.
Therefore:
Rail Length = first end dimension + pitch intervals + second end dimension
not:
number of holes × pitch
How to Count Pitch Intervals Correctly
If a rail has:
2 holes = 1 interval
3 holes = 2 intervals
10 holes = 9 intervals
21 holes = 20 intervals
In general:
Number of pitch intervals = Number of mounting holes − 1
This distinction becomes important when reverse-engineering an existing rail from measurements.
Example: Reverse-Engineering an Existing Rail
Suppose you have an unidentified replacement rail with:
overall length = 1,240 mm
21 mounting holes
hole pitch = 60 mm
There are:
21 − 1 = 20 pitch intervals
The distance between the first and last mounting-hole centers is therefore:
20 × 60 = 1,200 mm
That leaves:
1,240 − 1,200 = 40 mm
for the two end dimensions combined.
If the hole pattern is centered:
G1 = 20 mm
G2 = 20 mm
You have now reconstructed the basic longitudinal mounting-hole geometry.
You would still need to verify the rail width, height, hole dimensions, manufacturer, series, and carriage compatibility before treating another rail as a replacement.
Hole Pitch Varies by Rail Size and Series
There is no universal mounting-hole pitch for linear guide rail.
THK's published dimensional information provides a good example.
For one HSR-M1 family, THK publishes a standard pitch of:
60 mm for HSR15M1
60 mm for HSR20M1
60 mm for HSR25M1
80 mm for HSR30M1
80 mm for HSR35M1
Those values apply to those specific models. Other guide families use different pitches.
For example, THK's much smaller RSR family includes published standard pitches as small as 8, 10, and 15 mm depending on model.
The lesson is:
Never determine mounting-hole pitch from nominal rail size alone.
Identify the exact rail family and check its dimensional table.
Do Not Assume Two Size-25 Rails Have the Same Hole Spacing
A nominal designation such as:
size 25
does not establish mounting-hole compatibility.
Two size-25 profile rails from different manufacturers or product families may differ in:
rail width
rail height
mounting-hole pitch
mounting-hole diameter
counterbore diameter
counterbore depth
rail-end dimensions
carriage geometry
This is especially important when replacing an obsolete linear guide.
A replacement rail should not be ordered merely because its nominal size matches the original.
Check the Exact Manufacturer's Dimensional Drawing
Before cutting a known rail, locate the dimensional drawing for the exact:
manufacturer
guide family
nominal size
The drawing or dimensional table should provide the mounting-hole geometry.
For example, manufacturers such as THK publish model-specific information for their rail families rather than using one universal pitch for every guide.
This is the information that should control the cut plan.
Why the First and Last Mounting Holes Are Important
The mounting fasteners transfer rail loads into the machine structure.
The portions of rail beyond the outer fasteners therefore deserve attention.
If the cut leaves too much rail beyond the final mounting hole, the end section may not be supported as effectively as the rest of the rail.
If the cut is too close to the mounting hole, the resulting geometry may also be undesirable.
Manufacturer-recommended end dimensions provide a useful starting point.
Do not arbitrarily maximize or minimize the end dimension without checking the selected guide's dimensional requirements.
Avoid Cutting Through a Mounting Hole
Before marking the rail, determine whether the proposed cut intersects:
a mounting hole
counterbore
threaded feature
other rail feature
An accidental partial hole at the rail end is generally undesirable.
Often, the problem can be avoided by adjusting the finished rail length slightly.
For example, if:
1,505 mm
places the cut through a mounting feature but:
1,500 mm
creates a clean end and still provides sufficient travel, the nearby dimension may be the better choice.
The exact decision depends on the rail and machine.
Do Not Automatically Cut Halfway Between Holes
The midpoint between two mounting holes may look like the logical cut location, but it is not universally correct.
The appropriate cut location depends on the desired final:
G1
G2
overall rail length
stroke
mounting pattern
A centered cut may produce desirable geometry in one application and poor geometry in another.
Use the manufacturer's dimensional information rather than a generic halfway-between-holes rule.
Required Stroke Comes Before Hole Optimization
Hole spacing matters, but it cannot be considered in isolation.
The rail still needs to provide enough length for the required carriage travel.
A useful preliminary relationship is:
Rail Length ≈ Required Stroke + Carriage-Group Footprint + End Allowance
Suppose the machine requires:
1,000 mm stroke
250 mm carriage-group footprint
80 mm total end allowance
The preliminary rail length is:
1,330 mm
You would then examine the mounting-hole pitch and determine whether 1,330 mm produces acceptable end dimensions.
If not, a nearby length may be preferable, provided the machine still has sufficient travel and physical space.
Do Not Shorten the Rail at the Expense of Block Spacing
A machine using two blocks per rail requires enough rail length for:
block length
block spacing
required travel
Moving the blocks closer together simply to make a shorter rail fit can affect moment resistance.
Greater longitudinal block spacing generally improves the support footprint for pitch and yaw moments.
Therefore, the preferred sequence is:
required stroke → required block spacing → preliminary rail length → mounting-hole pattern → final cut length
not:
available rail length → squeeze everything onto it
Replacement Rail Requires Exact Hole-Pattern Verification
Hole spacing becomes particularly important when replacing rail on an existing machine.
Before ordering or cutting the replacement, record:
overall rail length
mounting-hole pitch
number of mounting holes
first-hole end dimension
last-hole end dimension
mounting-hole diameter
counterbore dimensions
rail width
rail height
Also identify the original:
manufacturer
rail series
carriage model
if possible.
A rail can be the correct overall length and still fail to fit because the mounting holes are in different positions.
A Simple Replacement-Rail Measurement Procedure
Start at one end of the existing rail.
Step 1: Measure Overall Length
Record the complete rail length.
Step 2: Count the Mounting Holes
Count every mounting hole from one end to the other.
Step 3: Measure Hole Pitch
Measure center-to-center between adjacent holes or across several intervals.
Step 4: Measure the First End Dimension
Measure from the rail end to the center of the first mounting hole.
Step 5: Measure the Last End Dimension
Repeat at the opposite end.
Step 6: Verify the Math
Check:
L = G1 + nP + G2
If the dimensions do not agree, remeasure before ordering or cutting.
Example Replacement-Rail Check
Suppose you measure:
rail length = 1,500 mm
hole pitch = 60 mm
25 pitch intervals
G1 = 20 mm
But:
20 + (25 × 60) = 1,520 mm
before G2 has even been added.
Something is wrong.
Possible causes include:
incorrect pitch measurement
incorrect interval count
incorrect overall length
wrong G1 measurement
This simple calculation can catch measurement errors before an expensive rail is ordered.
Use the Linear Rail Cut Calculator
Linear Automation USA's Resources include a Linear Rail Cut Calculator for planning custom rail dimensions.
The calculator considers information including:
brand
rail size
pitch P
buffer/end dimension
total rail length
requested G1
This can help determine how a proposed finished length interacts with the rail's existing mounting-hole pattern.
For custom-cut rail, this is much safer than selecting an arbitrary overall length and discovering the hole geometry after the cut has already been made.
Hole Spacing Matters When Designing the Machine Base
For a new machine, the best time to consider rail hole spacing is before the mounting plate is drilled.
If the rail model has already been selected, use the manufacturer's actual mounting-hole pattern when designing:
machine base
gantry
mounting plate
tapped-hole locations
Do not design a generic hole pattern and assume the selected rail will match it later.
Design the machine around the actual guide dimensions.
Hole Spacing Matters Even More With Two Parallel Rails
A two-rail system introduces another layer of installation geometry.
If both rails are identical, it is usually helpful for them to have coordinated:
finished length
mounting-hole pitch
end dimensions
hole count
The machine base should also be designed so the rails can be mounted and aligned according to the guide manufacturer's installation requirements.
Do not independently approximate the cuts on two rails that are intended to form a matched machine axis.
Hole Pitch Is Different From Rail Spacing
These terms sound similar but describe completely different dimensions.
Mounting-hole pitch is the longitudinal distance between mounting holes along one rail.
Rail spacing is the transverse distance between two parallel guide rails.
Mounting-hole pitch affects:
fastening pattern
rail-end geometry
cut length
Rail spacing affects:
machine support footprint
roll-moment resistance
carriage reactions
structural rigidity
Do not confuse the two when specifying a guide system.
Hole Pitch Is Also Different From Block Spacing
Block spacing is the longitudinal distance between carriages on a rail.
It affects the guide system's support geometry, particularly its response to pitch and yaw moments.
It is independent of the mounting-hole pitch.
A machine might therefore have:
mounting-hole pitch = 60 mm
block spacing = 300 mm
rail spacing = 500 mm
All three dimensions describe different parts of the system.
Special Rail Lengths Should Be Planned, Not Guessed
Manufacturer documentation demonstrates why custom rail lengths deserve deliberate planning.
For THK's HSR-M1 example, the published standard pitch is 60 mm for sizes 15, 20, and 25 and 80 mm for sizes 30 and 35, with published G/g end dimensions of 20 mm for those models.
Other THK families use completely different values.
This reinforces an important principle:
There is no universal “correct” linear rail hole spacing or end dimension.
Use the dimensional information for the exact rail being processed.
Long Rails Add Another Constraint
Very long applications also need to consider the manufacturer's maximum available rail length.
THK publishes model-specific maximum rail lengths and notes that maximum length can vary with accuracy grade.
If the required guideway exceeds the available one-piece rail length, the application may require a manufacturer-supported jointed rail system.
This should be determined before the machine base and mounting-hole pattern are finalized.
Can You Drill Additional Mounting Holes?
Do not assume linear rail can be drilled like ordinary structural steel.
Profile rail incorporates hardened precision raceways, and special machining may be required for nonstandard mounting features.
If a machine's existing hole pattern does not match the replacement rail, possible solutions may include:
manufacturer-supported special machining
modifying the machine mounting plate
using an engineered adapter plate
selecting a more compatible replacement guide
The appropriate solution depends on the application.
Do not casually drill additional holes into hardened profile rail without verifying that the machining method and resulting rail geometry are acceptable.
Common Hole-Spacing Mistakes Before Cutting
Measuring Edge-to-Edge Instead of Center-to-Center
Pitch is a center-to-center dimension.
Counting Holes Instead of Pitch Intervals
Twenty-one holes contain twenty intervals between the first and last hole centers.
Assuming Rail Length Must Be a Multiple of Pitch
The end dimensions account for the remaining length.
Assuming G1 and G2 Must Be Equal
They can differ when the application requires it.
Ignoring the Last Mounting Hole
The amount of rail remaining beyond that hole matters.
Cutting Through a Counterbore
Check the proposed cut location before machining.
Assuming All Rails of the Same Nominal Size Use the Same Pitch
Hole spacing is manufacturer- and series-specific.
Matching Overall Length but Not Hole Pattern
This is particularly dangerous in replacement applications.
Confusing Hole Pitch With Block Spacing
They are separate dimensions serving different purposes.
Cutting Before Checking Required Stroke
The rail still needs enough usable length for the carriage group to complete its travel.
Linear Automation USA's Perspective
At Linear Automation USA, we view mounting-hole spacing as one of the first things that should be checked before custom rail is cut.
The question should not simply be:
“How long does the rail need to be?”
It should be:
“What finished rail length gives us the required travel and the correct mounting-hole geometry?”
For custom-cut rail, we want to understand:
exact manufacturer and rail family
nominal rail size
required finished length
mounting-hole pitch
G1
G2
required stroke
number of carriages
block spacing
existing mounting pattern if it is a replacement
Linear Automation USA supplies industrial profile rail systems from SBC Linear, WON Linear, and other supported product lines, with custom-cut rail processing available for application-specific lengths.
Our rule is:
Never select a linear rail cut location from overall length alone. Calculate the mounting-hole pattern first and make sure the resulting end dimensions work for the machine.
Linear Rail Hole Spacing Checklist
Before cutting or ordering cut-to-length rail, confirm:
Manufacturer
Rail family
Nominal rail size
Finished overall length
Mounting-hole pitch
Number of mounting holes
Number of pitch intervals
G1 end dimension
G2 end dimension
Required carriage stroke
Carriage-group footprint
Block spacing
Whether the rail must match an existing machine
Whether the proposed cut intersects a mounting feature
Manufacturer maximum rail length
Whether the finished geometry has been verified before cutting
Frequently Asked Questions
What Is Linear Rail Hole Spacing?
Linear rail hole spacing is the center-to-center distance between consecutive mounting holes along the rail. The exact pitch depends on the manufacturer, guide family, and rail size.
How Do I Measure Linear Rail Hole Spacing?
Measure from the center of one mounting hole to the center of the next. For better accuracy, measure across several intervals and divide by the number of intervals.
Does Cutting Linear Rail Change the Hole Spacing?
No. Cutting changes the overall rail length and the distance from the new rail end to the nearest mounting hole. It does not change the existing mounting-hole pitch.
What Is G1 on a Linear Rail?
G1 is a useful designation for the distance between one rail end and the center of the first mounting hole. Manufacturer terminology varies.
What Is G2?
G2 represents the corresponding distance between the final mounting-hole center and the opposite rail end.
Do G1 and G2 Have to Match?
No. Equal end dimensions can provide a symmetrical hole pattern, but unequal dimensions may be appropriate for a particular machine or replacement application.
Does Rail Length Have to Be a Multiple of Hole Pitch?
No. Overall rail length also includes the two end dimensions.
How Many Pitch Intervals Are There?
For a uniformly spaced rail:
Number of pitch intervals = Number of mounting holes − 1
Can I Cut a Rail Between Mounting Holes?
Potentially, provided the resulting end dimension is appropriate and the cut does not damage a required mounting feature or functional rail surface.
Can I Cut Through a Mounting Hole?
Avoid creating an accidental partial mounting hole or counterbore. Calculate the hole pattern before choosing the cut location.
Do All Size-25 Linear Rails Have the Same Hole Spacing?
No. Nominal size alone does not establish mounting-hole compatibility. Check the exact manufacturer and rail series.
Can I Change the Hole Pattern?
Additional machining of hardened profile rail may require specialized equipment and manufacturer guidance. Do not assume the rail can be drilled like ordinary steel.
Why Does Hole Spacing Matter for Replacement Rail?
A replacement can have the correct overall length and still fail to bolt onto the machine if its hole pitch or end dimensions differ from the original.
Need Help Determining Linear Rail Hole Spacing?
For a custom-cut or replacement rail, provide:
manufacturer
rail series
nominal size
overall length
hole pitch
number of mounting holes
G1
G2
required stroke
quantity
For an unidentified replacement rail, provide photographs and measurements of both the rail and carriage.
Use the Linear Automation USA Resources to help plan rail-cut geometry, or contact Linear Automation USA for assistance specifying custom-cut profile rail.
Recommended Reading
How to Cut Linear Guide Rail to Length
Learn how to plan a custom rail cut around the required finished length, mounting-hole geometry, hardened raceways, deburring, and cleaning.
Can Linear Rail Be Cut to Any Length?
Understand why a rail can often be custom cut while mounting-hole pitch, end dimensions, required travel, and maximum manufactured length still constrain the best finished dimension.
How to Order Cut-to-Length Linear Rail
Learn what information to provide when requesting custom rail, including series, size, finished length, hole pitch, end dimensions, stroke, and quantity.
How to Select Linear Rail Length
Calculate the rail length required from usable stroke, carriage footprint, block spacing, and end allowance.
Linear Guide Selection for Long-Stroke Applications
Learn how rail length, mounting geometry, alignment, one-piece manufacturing limits, and jointed rails affect long-travel applications.
Sources & Technical References
THK — Standard Lengths and Maximum Lengths of HSR-M1 Rails
THK's dimensional documentation provides a clear example of the relationship between standard rail length, mounting-hole pitch F, G/g end dimensions, and maximum manufactured rail length. THK recommends maintaining appropriate G/g dimensions for special rail lengths because excessive end dimensions can reduce stability and adversely affect accuracy.
THK — Standard and Maximum Lengths of RSR Rails
This reference demonstrates how mounting-hole pitch and end dimensions vary significantly between different rail models. It reinforces why hole spacing must be verified for the exact guide family rather than inferred from nominal rail size.
THK — Standard Lengths of GSR-R Rails
THK's GSR-R dimensional information provides additional examples of model-specific rail lengths, mounting-hole pitches, and G/g dimensions.
THK — Notes on Ordering
THK's ordering guidance explains model-specific ordering requirements and notes that high-precision applications can impose additional limits on maximum manufactured rail length.
Linear Automation USA — Resources
Linear Automation USA provides technical resources for specifying profile rail systems, including a Linear Rail Cut Calculator for evaluating custom rail dimensions and mounting-hole geometry.
Linear Automation USA
Linear Automation USA supplies industrial profile rail systems and supports custom-cut rail, application selection, and replacement/interchange projects.
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