Granite Tri-Squares

BS 939:2007 | Hand Lapped | UKAS Certified

Eley’s tri‑squares are hand‑finished in our laboratory to the precision and accuracy aligned with BS 939:2007. Featuring two master faces finished flat, square and perpendicular to tight tolerances.

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  • Two master faces finished flat, square and perpendicular, enabling multi‑face squareness and perpendicularity checks from a single artefact - improving efficiency and confidence in your measurement results.

     

  • Repeatable reference geometry: when used with granite surface plates or tables. Ideal for tooling setup, alignment and inspection tasks.

     

  • Granite construction: offering superior vibration damping and long‑term dimensional stability compared with steel or cast-iron alternatives.
  • Squareness and perpendicularity inspection in precision engineering and machine tool manufacturing.
  • CMM and assembly verification practices, often found in calibration laboratories and quality departments.
  • Geometry checks during installation of CMMs, machine tools and fabricated assemblies.

Granite Tri-Squares to BS 939 (2007) Standard Sizes & Overall Flatness

GRADE
SIZE (mm)
SQUARENESS (µm)
AA
250 x 160 x 25
2
AA
400 x 250 x 40
4
AA
630 x 400 x 63
7
A
250 x 160 x 25
4
A
400 x 250 x 40
8
A
630 x 400 x 63
14

Related Measurement Solutions

Digital Height Gauges

Pair tri‑squares with digital height gauges to verify angular and linear features in one setup.

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Granite surface plates

Deploy granite tri‑squares on granite surface plates when checking compound angles and complex geometries.

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Industries Used In - Granite Tri-Squares -

Three industrial technicians in dark blue overalls wearing protective gloves and inspecting cylindrical metal components.

Metrology & Quality Inspection

Verifying squareness, perpendicularity and angular relationships between multiple faces, and establishing right‑angle datums for dimensional inspection.


Metrology and inspection labs rely on granite tri squares as master artefacts for squareness verification, layout work, and calibration of other angle and square tools.

A coordinate measuring machine (CMM) probe performing precise dimensional inspection inside the bore of a machined aluminium component.

Precision Engineering

Checking squareness and perpendicularity of small precision components, verifying 90° angles between faces, and inspecting geometric accuracy on complex parts.


Precision engineering companies use granite tri squares as stable reference standards for multi‑face squareness checks and high‑accuracy inspection on surface plates.

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

Aerospace

Checking squareness and perpendicularity on structural components, precision brackets, frames and complex machined parts with multiple reference faces.


Aerospace manufacturers employ granite tri squares where tight geometric tolerances and reliable multi‑face squareness verification are required for critical airframe and engine components.

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

Automotive

Inspecting engine blocks, cylinder heads, gearbox housings and fixtures for squareness between mounting faces, bores and machined reference surfaces.


Automotive manufacturers use granite tri squares on surface plates and in toolrooms to verify right‑angle relationships on safety‑critical and performance‑critical components.

Side view of a factory worker in blue uniform and safety gear setting up raw material on a metal milling machine.

Machine Tool Manufacturing

Checking squareness of machine columns, tables and saddles, assessing perpendicularity between axes, and inspecting right‑angle relationships on machine structures.


Machine tool manufacturers use granite tri squares during build and alignment to confirm that critical machine elements meet geometric tolerances for squareness and perpendicularity.

FAQs

What size granite tri-square will I need?

Size selection is determined by the working envelope of the machine or component being inspected, the measuring faces should be tall and wide enough to fully engage the axis or feature being checked.

 

Eley granite tri-squares are available in a range of standard sizes in both Grade AA and Grade A, with squareness tolerances defined in accordance with BS 939:2007. For CMM and large machining centre qualification, larger sizes are typically specified to ensure full-length axis coverage. For bench-level inspection and smaller machine tools, compact tri-squares offer the same accuracy in a more manageable form factor. Bespoke sizes are available where standard configurations do not meet specific application requirements.

 

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

A granite tri-square has three finished faces — one large flat face and two narrow perpendicular edges — allowing X, Y and Z axis squareness to be checked from a single lightweight artefact.

 

A master square has six fully lapped faces, providing squareness, flatness and parallelism references across all axes simultaneously and with greater versatility for more complex measurement setups. The tri-square is a more compact and cost-effective solution where the primary requirement is multi-axis squareness verification; the master square is preferred where a full six-face reference standard is required for laboratory-grade calibration work.

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

 

Eley’s engineers hand-lap the measuring faces back to BS 939:2007 specification before UKAS traceable re-verification. This is a cost-effective alternative to replacement and can be performed at our Derby laboratory or on-site where appropriate.

 

Reach out to us for resurfacing support.
Email: sales@eleymet.com
Phone: 01332 367475

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

 

As a practical guide, tri-squares in high-use production or shop-floor environments benefit from calibration every 6–12 months; those in controlled laboratory conditions may suit an annual schedule. In regulated sectors such as aerospace, medical device or nuclear manufacturing, more frequent calibration intervals are typically specified to ensure measurement drift does not compromise product conformance decisions. Calibration should also be triggered immediately following any impact, suspected damage or unexplained measurement inconsistency.

 

All Eley granite tri-squares can be supplied with UKAS traceable calibration and we offer ongoing recalibration to BS 939:2007.

Contact our speialists for a calibration quote.
Email: sales@eleymet.com
Phone: 01332 367475

Granite tri-squares require minimal but consistent care to retain their accuracy:

  • Cleaning: Wipe all 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 the lightening holes for lifting and handling larger tri-squares. Always set down carefully on a clean, flat surface to avoid edge impacts or point loading.
  • Storage: Cover when not in use and store in a stable, temperature-controlled environment to minimise thermal cycling effects on the granite faces.
  • Inspection: Regularly check measuring faces for chips, contamination or signs of wear. Any suspected deviation from squareness tolerance should be referred for professional recalibration.
  • During use: Ensure the surface plate or reference surface is clean before positioning the tri-square, and verify the artefact is fully stable before taking measurements.

Eley offers resurfacing and recalibration to restore granite tri-squares back to BS 939:2007 specification.

Reach out to one of our specialists to discuss your maintenance queries.
Email: sales@eleymet.com
Phone: 01332 367475

Grade AA (Laboratory Grade) is manufactured to the tighter squareness tolerances of BS 939:2007 and is the correct choice for UKAS-accredited calibration laboratories, primary reference standard use, and high-precision CMM and CNC machine qualification tasks.

 

Grade A (Inspection Grade) carries slightly wider tolerances and is well-suited to quality department inspection, production verification and on-site machine setup work. The standard traceability hierarchy applies — using a Grade AA tri-square to periodically verify any Grade A working standards within your calibration system.

Granite tri-squares offer several material advantages over steel squares in precision metrology environments. Granite’s low coefficient of thermal expansion ensures the geometry remains stable across temperature fluctuations common in workshop and laboratory settings. Granite is non-magnetic and rust-free, eliminating interference with electronic gauging and removing the corrosion risk associated with steel artefacts.

 

The triangular form factor also provides inherent structural rigidity without additional mass, reducing the handling fatigue associated with heavy master squares while maintaining excellent measurement stability. When damage occurs, granite chips rather than raising a burr, making surface degradation visible and easier to detect.

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