Why Linear Rail Mounting Hole Position Matters

Nov 4

Linear rail mounting hole position matters because the rail must not only be the correct overall length—it must also place each mounting hole where the machine base, support structure, and cut geometry require it. A rail with the correct length but the wrong first-hole position, last-hole position, or mounting-hole pitch can fail to fit an existing machine, create poor end support, complicate installation, or force unnecessary modification of the mounting base.
A useful way to describe the longitudinal 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 center of the last mounting hole to the opposite rail end
P = mounting-hole pitch
n = number of pitch intervals between the first and last mounting holes
The important lesson is:
Overall rail length alone does not define where the mounting holes will land.

What Is Linear Rail Mounting Hole Position?

Mounting hole position describes where each rail mounting hole is located relative to the ends of the rail and to the other mounting holes.
Three dimensions establish most of the longitudinal hole pattern:

first-hole end distance
mounting-hole pitch
last-hole end distance
If the rail has:
G1 = 20 mm
P = 60 mm
then the first hole is centered 20 mm from the rail end and each following hole center occurs another 60 mm along the rail.
If the opposite end dimension is:
G2 = 20 mm
the pattern is symmetrical.
If:
G2 = 35 mm
the pattern is asymmetrical, even though the pitch remains unchanged.

Why Overall Rail Length Is Not Enough

Suppose two rails are both:
1,250 mm long
They may appear interchangeable based on overall length.
But consider:
Rail A:

G1 = 20 mm
G2 = 30 mm
P = 60 mm
Rail B:
G1 = 30 mm
G2 = 20 mm
P = 60 mm
Both rails have the same:
overall length
mounting-hole pitch
number of pitch intervals
But their mounting-hole patterns are shifted by 10 mm relative to the rail ends.
If the machine base already contains tapped holes, one rail may fit while the other does not.
This is why:
A correct rail length does not guarantee a correct mounting pattern.

The First Mounting Hole Establishes the Pattern

Once the mounting-hole pitch is fixed, the position of the first mounting hole determines where the rest of the pattern falls.
Suppose:
G1 = 20 mm
and:
P = 60 mm
The hole centers occur at:

20 mm
80 mm
140 mm
200 mm
260 mm
and so on from the selected rail end.
If G1 changes to:
30 mm
the complete pattern shifts:
30 mm
90 mm
150 mm
210 mm
270 mm
The pitch has not changed.
The entire pattern has simply moved 10 mm relative to the rail end.
That difference can determine whether the rail bolts onto an existing machine.

Why the Last Mounting Hole Matters Too

The first-hole position establishes the beginning of the pattern, but the final mounting hole determines the geometry at the opposite end.
The distance between the last hole center and the rail end affects:

finished rail length
rail-end support
cut location
compatibility with existing mounting holes
symmetry of the rail
A rail with an excessively large end section may leave more unsupported rail beyond the final fastener than desired.
A cut made too close to a mounting feature can create another problem.
Manufacturer-specific dimensional guidance should therefore be checked before finalizing the rail-end geometry.

How Mounting Hole Position Relates to Rail Length

The basic relationship is:
L = G1 + nP + G2
Suppose:

G1 = 20 mm
G2 = 20 mm
P = 60 mm
n = 20
Then:
L = 20 + (20 × 60) + 20
L = 1,240 mm
If the desired length changes to:
1,250 mm
but the hole pitch and number of intervals remain the same, the extra 10 mm must appear in the end dimensions.
For example:
G1 = 20 mm
G2 = 30 mm
gives:
20 + 1,200 + 30 = 1,250 mm
The mounting-hole position and overall rail length are therefore mathematically connected.

Mounting Hole Position Matters When Cutting Linear Rail

Cutting a linear rail changes the relationship between the nearest mounting hole and the new rail end.
It does not move the mounting holes.
Suppose:
G2 = 40 mm
If 10 mm is removed from that end:
G2 becomes 30 mm
If 25 mm is removed:
G2 becomes 15 mm
The pitch remains exactly the same.
Therefore, before cutting a rail, you should determine:

where the first mounting hole will remain
where the final mounting hole will remain
what the new end distance will be
whether the cut intersects any mounting feature
whether the finished pattern still works with the machine

Why You Should Never Cut by Overall Length Alone

A common mistake is marking the desired finished length on a rail and cutting without first locating the mounting holes.
That approach can produce:

a partial mounting hole
a cut through a counterbore
an undesirable end distance
a shifted replacement pattern
insufficiently supported rail ends
A better sequence is:

Identify the exact rail.
Confirm the mounting-hole pitch.
Determine the required finished length.
Locate the first and last mounting holes.
Calculate the resulting end dimensions.
Verify the cut location.
Cut only after the mounting geometry is confirmed.

Mounting Hole Position Is Critical for Replacement Rail

Replacement applications are especially sensitive to mounting-hole position because the machine base may already be drilled and tapped.
When replacing an existing rail, record:

overall length
mounting-hole pitch
number of mounting holes
G1
G2
mounting-hole diameter
counterbore diameter
counterbore depth
rail width
rail height
Also identify:
manufacturer
rail family
carriage model
if possible.
Two rails can have similar dimensions and still have incompatible mounting patterns.

Nominal Rail Size Does Not Guarantee Hole Compatibility

A designation such as:
size 25
does not tell you:

exact mounting-hole pitch
first-hole end distance
last-hole end distance
hole diameter
counterbore geometry
rail width
rail height
Different manufacturers and product families can use different dimensions even when the nominal size appears similar.
That means:
nominal rail size alone does not establish interchangeability.
Use the exact dimensional drawing for the selected rail family.

Manufacturer Drawings Should Control the Hole Layout

The safest source for mounting-hole position is the exact manufacturer's dimensional data.
THK, for example, publishes rail tables showing:

standard rail length
mounting-hole pitch F
G/g rail-end dimensions
maximum rail length
Those values are model-specific rather than universal. THK's RSR rail data, for example, shows different standard pitches and G/g values depending on the model.
Schaeffler likewise treats mounting-hole position as an explicit rail-design parameter. Its current rail configurator allows users to define the distance from the rail start and end to the nearest fixing holes or use symmetrical values calculated by the system.

Hole Pitch and Hole Position Are Different

Mounting-hole pitch describes the center-to-center spacing between consecutive holes.
Hole position describes where that repeating pattern begins and ends relative to the rail.
For example:
P = 60 mm
does not tell you whether:
G1 = 20 mm
or:
G1 = 30 mm
Both rails could use 60 mm pitch but have different mounting-hole positions.
This distinction is extremely important when specifying custom-cut rail.

Why Mounting Hole Position Matters for Machine Base Design

On a new machine, the base should ideally be designed around the actual selected rail.
Once the exact rail family has been chosen:

obtain the manufacturer drawing
determine the required finished rail length
identify the mounting-hole pitch
establish the first-hole position
establish the final-hole position
lay out the machine mounting holes accordingly
Designing the base first and assuming a generic rail will fit later can create avoidable rework.

Mounting Hole Position Matters for Two-Rail Systems

Most profile-rail axes use two parallel guide rails.
When the two rails are intended to be identical, coordinate:

rail length
mounting-hole pitch
G1
G2
hole count
The machine base should be drilled and machined to match the intended rail pair.
If one rail is cut with a different first-hole location from the other, the mounting pattern can become unnecessarily complicated.
This is particularly important when ordering custom rails in matched quantities.

Mounting Hole Position Affects Support Near the Rail Ends

The outer mounting fasteners help transfer rail loads into the machine structure.
The portion of rail extending beyond those fasteners is less directly supported.
Manufacturer dimensional guidance reflects this relationship.
THK's published rail tables use model-specific G/g end dimensions and standard pitch values rather than arbitrary end locations. Its dimensional documentation also distinguishes maximum rail length by model.
Schaeffler's calculation methodology similarly uses a minimum end-to-hole distance when determining mounting-hole count and symmetrical end geometry for profile rail guides.
The practical lesson is:
Do not treat the leftover material beyond the last mounting hole as irrelevant.

What Happens If the First Hole Is Too Far From the End?

A large first-hole end distance leaves a longer rail section beyond the first mounting fastener.
Depending on the guide and application, this can:

increase unsupported rail length
move the usable mounting pattern
interfere with the machine envelope
shift every remaining mounting hole
That does not mean large end dimensions are universally prohibited.
It means the result should be intentional and checked against the selected rail's dimensional requirements.

What Happens If the First Hole Is Too Close to the End?

A very small end distance can also be undesirable.
The cut may:

approach the mounting-hole counterbore
intersect a mounting feature
leave an unsuitable amount of rail material beyond the hole
Schaeffler's linear-guide calculation method explicitly includes a minimum distance from the rail end to the first mounting hole when determining a valid rail-hole pattern.
This is why the correct end distance should be derived from the exact rail data rather than a universal rule.

Symmetrical vs. Asymmetrical Mounting Hole Position

A symmetrical rail has:
G1 = G2
For example:
G1 = 20 mm
G2 = 20 mm
An asymmetrical rail might have:
G1 = 20 mm
G2 = 35 mm
Neither arrangement is automatically right or wrong.
Symmetry can be convenient when:

designing a new machine
cutting identical rails
creating a clean mounting layout
Asymmetrical geometry may be necessary when:
matching an existing machine
accommodating a fixed rail length
working around existing mounting features
reproducing an original rail
Schaeffler's current configurator explicitly allows separate left and right nearest-hole distances or the use of calculated symmetrical values, confirming that unequal end positions can be an intentional configuration rather than an error.

Example: Same Rail Length, Shifted Hole Pattern

Suppose:
L = 1,250 mm
P = 60 mm
n = 20
The pitch intervals occupy:
20 × 60 = 1,200 mm
That leaves:
50 mm
for G1 and G2 combined.
Configuration A:
G1 = 20 mm
G2 = 30 mm
Configuration B:
G1 = 30 mm
G2 = 20 mm
Both satisfy:
L = 1,250 mm
But the entire mounting-hole pattern is positioned differently.
This is exactly why mounting-hole position needs to be specified rather than inferred from total rail length.

Example: Matching an Existing Machine

Suppose an existing rail measures:

overall length = 1,240 mm
hole pitch = 60 mm
21 mounting holes
G1 = 20 mm
G2 = 20 mm
There are:
21 − 1 = 20 pitch intervals
Therefore:
20 + (20 × 60) + 20 = 1,240 mm
If a replacement rail is also 1,240 mm long and has 60 mm pitch but:
G1 = 30 mm
the complete mounting pattern would be shifted 10 mm from the original.
The replacement would therefore not automatically fit the existing holes.

How to Measure Mounting Hole Position on an Existing Rail

For an unidentified replacement rail:

Step 1: Measure Overall Length

Measure the entire finished rail from end to end.

Step 2: Measure G1

Measure from one rail end to the center of the first mounting hole.

Step 3: Measure the Hole Pitch

Measure center-to-center between adjacent holes.
For improved accuracy, measure across several intervals and divide by the number of intervals.

Step 4: Count the Mounting Holes

Count all mounting holes along the rail.

Step 5: Measure G2

Measure from the center of the final mounting hole to the opposite rail end.

Step 6: Verify the Geometry

Check:
L = G1 + nP + G2
If the numbers do not reconcile, remeasure before ordering.

Measuring Across Several Holes Improves Confidence

Suppose you measure from the center of hole 1 to the center of hole 6:
300 mm
There are five pitch intervals.
Therefore:
P = 300 ÷ 5
P = 60 mm
This method is often more reliable than trying to identify the exact centers of two adjacent holes over a short distance.

Mounting Hole Position Can Affect Cut-Length Decisions

Suppose the desired rail length is approximately:
1,500 mm
and:
P = 60 mm
With 24 pitch intervals:
24 × 60 = 1,440 mm
At:
L = 1,500 mm
the combined end distance is:
60 mm
For symmetrical ends:
G1 = G2 = 30 mm
If the exact rail's recommended geometry is closer to another end dimension, a slightly different finished length may provide a better hole pattern.
For example, if the machine envelope allows a small adjustment, changing the rail length can sometimes produce more appropriate end geometry without sacrificing required travel.

Required Stroke Still Comes First

Mounting-hole position is important, but it should not drive the design at the expense of required motion.
A preliminary rail-length relationship is:
Rail Length ≈ Required Stroke + Carriage-Group Footprint + End Allowance
After establishing the necessary rail length range, the designer can optimize the finished length around:

mounting-hole pitch
G1
G2
machine envelope
mounting-hole pattern
Do not reduce necessary block spacing or required stroke merely to create a prettier hole pattern.

Mounting Hole Position Is Not the Same as Block Position

Mounting holes fasten the rail to the machine.
Guide blocks or carriages move along the rail.
The mounting-hole position does not directly define carriage spacing.
A system might have:

G1 = 20 mm
G2 = 30 mm
P = 60 mm
block spacing = 350 mm
These are separate dimensions.
Block spacing influences support geometry and moment response.
Mounting-hole position determines how the rail is attached to the machine.

Mounting Hole Position Is Not Rail Spacing

Rail spacing is the transverse distance between two parallel rails.
Mounting-hole position is longitudinal along an individual rail.
Rail spacing primarily affects:

support footprint
roll-moment resistance
carriage reactions
machine rigidity
Mounting-hole position primarily affects:
fastening layout
cut geometry
replacement fit
end support

Long Rails Add Another Constraint

For long-stroke applications, mounting-hole position must also be coordinated with the manufacturer's maximum available rail length.
THK publishes maximum rail lengths for individual models, and those limits vary across rail families and sizes.
If the required guideway exceeds the available one-piece length, a manufacturer-supported jointed rail solution may be required.
The joint locations, mounting-hole pattern, and machine base should then be planned together.

Can You Redrill the Machine Base Instead?

Sometimes modifying the machine base may be possible, but it should not be the default assumption.
Potential options for an incompatible replacement include:

selecting a rail with a closer mounting pattern
modifying the machine base
using an engineered adapter plate
specifying special rail machining where supported
The preferred solution depends on:
machine structure
required accuracy
available space
replacement urgency
engineering cost
The mounting pattern should therefore be evaluated before the replacement rail is purchased.

Can You Add Different Holes to the Rail?

Do not assume profile rail can simply be drilled like ordinary steel.
Linear guide rail is a precision component with hardened raceway surfaces and controlled geometry.
If a special mounting pattern is required, verify whether:

the manufacturer offers special machining
the rail supplier can provide the required configuration
an adapter solution would be preferable
Avoid improvising additional holes in precision rail without an engineering review.

Common Mounting Hole Position Mistakes

Specifying Only Overall Length

Overall length does not define the hole pattern.

Specifying Only Nominal Rail Size

Nominal size does not establish mounting-hole pitch or end dimensions.

Measuring Hole Pitch Edge-to-Edge

Pitch should be measured center-to-center.

Assuming G1 and G2 Must Be Equal

They can be unequal when required by the application.

Cutting Before Locating the Holes

This can create poor end geometry or intersect mounting features.

Ignoring Counterbore Size

A cut may miss the hole center but still intersect the counterbore.

Matching Pitch but Ignoring First-Hole Position

The same pitch can still produce a completely shifted mounting pattern.

Assuming a Replacement Will Fit Because the Rail Length Matches

Replacement compatibility requires much more than overall length.

Linear Automation USA's Perspective

At Linear Automation USA, we treat mounting-hole position as part of the complete rail specification.
When evaluating custom-cut or replacement rail, we want to know:

manufacturer
rail series
nominal size
required finished length
mounting-hole pitch
G1
G2
mounting-hole count
required stroke
carriage configuration
whether the rail must match an existing machine
For replacement work, photographs and measurements of the original rail and carriage can also help establish the required geometry.
Our rule is:
Never approve a custom rail based on overall length alone. Confirm where the mounting holes will actually land.
Linear Automation USA's Resources page includes a Linear Rail Cut Calculator with inputs for brand, rail size, pitch, buffer/end geometry, total rail length, and requested G1.
For a custom-cut or replacement application, you can also contact Linear Automation USA for assistance.

Linear Rail Mounting Hole Position Checklist

Before ordering or cutting rail, confirm:

Exact manufacturer
Rail family
Nominal rail size
Finished rail length
Mounting-hole pitch
G1
G2
Number of mounting holes
Number of pitch intervals
Mounting-hole diameter
Counterbore diameter
Counterbore depth
Required stroke
Carriage quantity
Block spacing
Existing machine-hole pattern
Maximum rail length
Whether the proposed cut intersects a mounting feature

Frequently Asked Questions

Why Does Linear Rail Mounting Hole Position Matter?

Because the hole locations determine whether the rail fits the machine base, how the ends are supported, and whether a custom cut produces usable geometry.

Is Overall Rail Length Enough to Order a Replacement?

No. You should also verify mounting-hole pitch, first-hole position, last-hole position, hole dimensions, rail section, and exact guide family.

What Determines the First Mounting Hole Position?

The first-hole end dimension, often described generically as G1, establishes where the repeating mounting-hole pattern begins relative to the rail end.

Can Two Rails Have the Same Length and Different Hole Positions?

Yes. Different G1 and G2 values can shift the mounting pattern even when overall length and pitch are identical.

Does Cutting the Rail Move the Mounting Holes?

No. Cutting changes the rail-end distance relative to the existing holes.

Does Mounting-Hole Pitch Determine G1?

No. Pitch determines spacing between holes. G1 determines where that pattern begins relative to the rail end.

Do G1 and G2 Have to Be Equal?

No. Symmetrical values can be convenient, but unequal end dimensions are possible and may be necessary for replacement applications.

Why Is the Last Mounting Hole Important?

It establishes the amount of rail extending beyond the outermost fastener and affects the final cut geometry.

Can I Match a Replacement Rail by Nominal Size?

Not reliably. Nominal size alone does not prove mounting-hole or carriage compatibility.

Should I Use Manufacturer Dimensional Drawings?

Yes. The exact manufacturer's drawing or configurator should be the primary reference for the rail's mounting pattern.

Need Help Matching a Linear Rail Mounting Pattern?

For a new or custom-cut rail, provide:

manufacturer
rail family
size
required finished length
mounting-hole pitch
desired G1
desired G2
required stroke
quantity
For a replacement application, also provide:
existing overall length
hole count
mounting-hole diameter
counterbore dimensions
rail width
rail height
photographs of the rail and carriage
Use the Linear Rail Cut Calculator to help evaluate rail-hole geometry, or contact Linear Automation USA for help selecting and specifying the rail.

Sources & Technical References

Schaeffler — Linear Guide Calculator Help

Schaeffler explains that the number of mounting holes depends on rail length, standard mounting-hole distance, and the minimum distance from the rail end to the first mounting hole. Its calculator uses those dependencies to calculate a symmetrical end dimension.

Schaeffler — Linear Single Rail Configurator Help

Schaeffler's current configurator allows separate left and right distances from the guideway ends to the nearest fixing holes or allows the user to accept calculated symmetrical values, illustrating that mounting-hole end positions are explicit configuration parameters.