Granite Squares

Hand Lapped | UKAS Certified | BS 939:2007

Hand‑lapped with diamond abrasives to tight tolerances, Eley’s granite squares provide reliable squareness over the full measuring face even in demanding production environments. 

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  • Superior material: Granite offers superior vibration damping and is far less susceptible to burrs, scratches or rust - helping maintain precision and accuracy over many years of use.

  • Six fully lapped faces on master squares: enable comprehensive checks of squareness, straightness and parallelism across multiple planes from a single, stable reference artefact.

  • Great for Inspecting and calibrating X, Y and Z axes: Eley's squares are designed for inspecting and calibrating machine tools, CMMs and other precision equipment; helping reduce measurement uncertainty during setup and inspection.

  • Standard and custom sizes: are available in both Grade AA (laboratory) and Grade A (inspection), with squareness tolerances defined in line with BS 939:2007 and Eley Metrology’s internal quality controls.

  • Optional lifting or lightening holes: improve handling and safety for larger sizes without compromising accuracy, making them suitable for shop‑floor inspection, toolroom use and metrology laboratories where traceable results are essential.
  • Machine tool geometry checks.
  • CNC and CMM installation and verification.
  • Jig and fixture inspection.
  • General squareness checks in high-precision manufacturing. 

Eley master granite squares are manufactured with six lapped faces to facilitate multi‑plane inspection and calibration of machine axes and inspection equipment. 

 

Available with handling features such as lifting handles or lightening holes, they are built for repeatable, traceable use across shop floors and laboratories. 

Granite Squares to BS 939 (2009) Standard Sizes & Overall Flatness

GRADE
SIZE (mm)
SQUARENESS (µm)
WEIGHT (kg)
AA
200 x 200 x 25
2
3
AA
250 x 250 x 30
4
5
AA
315 x 315 x 30
4
7
AA
400 x 400 x 40
5
14
AA
500 x 500 x 50
7
33
A
200 x 200 x 25
4
3
A
250 x 250 x 30
8
5
A
315 x 315 x 30
8
7
A
400 x 400 x 40
10
14
A
500 x 500 x 50
14
33

Related Measurement Solutions

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Combine granite standards with digital height gauges for quick verification of key dimensions on the shopfloor.

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Use our granite surface plates alongside metrology standards to create a complete calibration and checking station.

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Industries Used In - Granite Squares -

An experienced engineer and an apprentice inspecting a machined metal part on a workshop table next to a CNC machine control panel.

Precision Engineering

Precision engineering companies use granite squares as reference standards for squareness and geometric checks on high‑accuracy parts and assemblies.

 

This includes, checking squareness of small precision components, verifying 90° angles on machined features, and inspecting perpendicularity of mating surfaces with a granite square. 

A manual machinist with safety glasses and an apron adjusting components on a blue industrial lathe machine in a machine shop.

Machine Tool Manufacturing

Machine tool manufacturers and service engineers use granite squares for machine geometry checks and alignment during build, installation, and calibration.

 

Inspecting machine tool geometry, checking spindle-to-table squareness, verifying column alignment, and assessing perpendicularity of axes on CNC and conventional machines.

A female aerospace technician in blue overalls inspecting a complex aircraft jet engine component.

Aerospace & Defence

Aerospace and defence manufacturers employ granite squares for high‑accuracy squareness checks where tight geometric tolerances and traceable inspection are required.

 

Inspecting structural components, brackets, frames, and precision machined parts for squareness and perpendicularity; verifying 90° relationships in assemblies and fixtures.

A Renishaw motorised CMM probe head performing high-precision inspection on a grooved metal component.

Metrology & Calibration Labs

Serving as a master reference for squareness calibration, perpendicularity verification, and geometric error mapping of CMMs and CNC machine tools.


Metrology and calibration labs rely on granite squares as high‑stability artefacts for calibrating squareness and perpendicularity across inspection and production equipment.

Unfinished car bodies moving along an automated assembly line track inside an automotive manufacturing facility.

Automotive

Checking squareness of engine blocks, cylinder heads, gearbox housings, and precision fixtures; verifying right‑angle relationships on critical mounting faces.

 

Automotive manufacturers use granite squares in quality and toolroom environments for squareness inspection of safety‑critical and performance‑critical machined components.

FAQs

What size granite square will I need?

Size selection is driven by the workpiece envelope and required squareness tolerance. Eley granite squares are available from 200 × 200 × 25 mm up to 500 × 500 × 50 mm, with squareness tolerances from 2 µm (Grade AA, 200 mm) to 14 µm (Grade A, 500 mm) per BS 939:2007. 

 

As a rule, the measuring face should be at least as tall as the feature being inspected. For large vertical axis checks on machine tools or CMMs, a 400–500 mm square is typically specified; for bench-level and laboratory inspection, 200–315 mm is most common. 

Custom sizes are available on request.


Contact a specialist today to discuss your requirements.

Email: sales@eleymet.com
Phone: 01332 367475

Yes. Granite squares that have moved out of tolerance can be professionally resurfaced and recalibrated rather than replaced.

 

Eley’s skilled engineers hand-lap the measuring faces back to BS 939:2007 specification, followed by verification and UKAS-traceable recertification. This is a cost-effective alternative to replacement, particularly for larger master squares and can be carried out either at our temperature-controlled Derby laboratory or, where appropriate, on-site.

 

Contact one of our specialists today to discuss your requirements.
Email: sales@eleymet.com
Phone: 01332 367475

Grade AA (Laboratory Grade) is manufactured to the tighter tolerances of BS 939:2007 and is suited to UKAS-accredited environments and primary reference standard applications. 

 

Grade A (Inspection Grade) carries slightly wider tolerances and is appropriate for quality department inspection and machine installation tasks. The standard practice is to maintain a clear traceability hierarchy – using a Grade AA square to verify a Grade A working square.

Calibration frequency should be determined by usage intensity, application criticality and your QMS requirements. 

 

As a guide: high-throughput or shop-floor environments typically warrant calibration every 6–12 months; controlled laboratory use may suit an annual cycle. In regulated sectors such as aerospace or medical, quarterly or semi-annual intervals are common where single-micrometre tolerances are critical. Calibration should also be triggered immediately following any impact or suspected damage. All Eley granite squares can be supplied with UKAS traceable calibration and we offer ongoing recalibration to BS 939:2007.

 

Get in touch with one of our specialists today to discuss calibration requirements.
Email: sales@eleymet.com
Phone: 01332 367475

Granite offers a combination of properties that metallic alternatives struggle to match. Its low coefficient of thermal expansion (~8 × 10⁻⁶/°C vs cast iron at ~12 × 10⁻⁶/°C) ensures dimensional stability across varying temperatures. It is also non-magnetic, non-conductive, rust-free, and resistant to burring, meaning surface integrity is maintained far longer than comparable steel squares. 

 

 

When damaged, granite chips rather than raising a burr, reducing the risk of a compromised reference surface going undetected. Combined with excellent vibration damping, these properties make granite the benchmark material for squareness references in precision engineering.

Granite squares are precision reference standards used to verify squareness across a wide range of inspection and measurement tasks, including machine tool geometry checks, CMM and CNC axis alignment, jig and fixture inspection, and component verification. Master squares, with six fully lapped faces, extend this further by enabling squareness, straightness and parallelism to be checked across all three axes from a single stable artefact. 

 

They are widely specified in aerospace, automotive, medical device manufacturing, toolrooms and UKAS-accredited calibration laboratories.

Granite squares require minimal but consistent care:

  • Cleaning: Wipe measuring faces before and after use with a lint-free cloth and a specialist stone cleaner or denatured alcohol. Avoid water-based or aggressive solvents.
  • Handling: Use lifting holes on larger master squares; always set down on a clean, flat surface to avoid edge impacts.
  • Storage: Cover when not in use and store in a stable, temperature-controlled environment.
  • Inspection: Check faces regularly for chips or contamination. Any suspected deviation from tolerance should be referred for professional recalibration.
  • Avoid misuse: Never use the square as a structural support — localised loading and impact are the most common causes of damage.

With proper care, a granite square will maintain its specified accuracy for many years. Eley also offers resurfacing and recalibration to restore artefacts back to BS 939:2007 specification.

Contact us today to discuss your requirements.
Email: sales@eleymet.com
Phone: 01332 367475

A granite square typically has two lapped measuring faces set at 90° and is used for squareness checks against a single reference plane. A master square extends this with six fully lapped faces — two large faces and four edges — all flat, square and parallel to one another. This allows squareness, straightness and parallelism to be verified across X, Y and Z axes simultaneously from a single artefact, making master squares the preferred reference standard for CMM installation, machine tool geometry verification and laboratory calibration environments.

Eley granite squares are manufactured to BS 939:2007, the British Standard governing the form and accuracy of precision squares. Each square is individually hand-lapped and verified against these tolerances before leaving our facility.

Yes. All Eley granite squares can be supplied with a UKAS traceable calibration certificate issued from our UKAS-accredited laboratory (No. 30213). This confirms the square’s measured squareness deviation, measurement uncertainty, and traceability to national measurement standards, providing the documented evidence required for QMS frameworks.