Walk into any inspection department and you'll usually find a handful of granite tools sitting around a surface plate - squares, parallels, straightedges, V-blocks - and it's not always obvious to someone newer to the field why there are so many different shapes doing what seems like a similar job. Each one is actually solving a slightly different measurement problem, and picking the wrong tool for the task is a more common mistake than people expect.
Granite surface plates: the reference foundation
The surface plate is the starting point for almost everything else on an inspection bench. It provides a flat reference plane against which other measurements - height, parallelism, squareness - are taken. Surface plates are typically specified by grade (00, 0, or 1, per DIN 876 or equivalent JIS/GB standards) based on how demanding the application is, with Grade 00 reserved for calibration lab and reference-standard work and Grade 1 suited to general workshop inspection.
What often gets overlooked is that a surface plate's usefulness depends heavily on how it's supported and maintained after installation - improper support points, uneven floor loading, or careless handling of gauge blocks and workpieces sliding across the surface can degrade a plate's calibrated flatness well before its rated recalibration interval. A high-grade plate on a poorly leveled stand performs worse than a lower-grade plate correctly installed.
Granite squares: checking perpendicularity
A granite square (sometimes called a try square or master square) is used to check that two surfaces or features are at a true 90-degree angle to each other. Unlike a surface plate, which establishes a single flat plane, a square establishes a perpendicular relationship between two planes - critical for checking machine tool column alignment, fixture squareness, and assembly verification where an out-of-square condition would cascade into downstream assembly errors.
Granite squares come in different configurations - flat squares, cylindrical squares, and beveled-edge squares - each suited to slightly different checking geometries. A cylindrical square, for instance, allows checking squareness from any radial direction, which is useful for round or rotationally symmetric fixtures where a flat square's single reference face would require repositioning.
Granite parallels: elevating and aligning workpieces
Parallels are precision blocks, typically supplied in matched pairs, used to elevate a workpiece off the surface plate while keeping it perfectly level and parallel to the reference plane. They're essential when a workpiece has features on its underside that need clearance, or when a part needs to be held at a consistent, known height during height-gauge or comparator measurement.
The key spec that matters for parallels is the matched tolerance between the pair - two parallels that are each individually flat but differ from each other by even a few microns in height will introduce a tilt into whatever is resting on them. This is why quality parallels are sold and calibrated as matched sets rather than individual pieces.
Granite V-blocks: holding cylindrical parts
V-blocks are used to cradle round or cylindrical workpieces - shafts, pins, rollers - securely and concentrically for inspection or marking-out work. The V-groove angle (commonly 90 degrees, though other angles exist for specific applications) needs to be machined with enough precision that it doesn't introduce its own error into whatever measurement is being taken against the part resting in it. Granite V-blocks are particularly useful in non-magnetic or clean environments where steel V-blocks would be undesirable, and where long-term dimensional stability matters more than the tool's resistance to impact (granite is harder and more brittle than steel, so it's the better choice for stable, low-impact inspection use rather than heavy-duty marking-out work involving hammering or high force).
Granite straightedges: checking linear flatness
A straightedge checks flatness or straightness along a single line rather than across an entire plane, making it useful for checking machine ways, guide rails, and long linear features where a full surface plate check isn't practical or where the feature being checked is too long to fit on an available plate. Straightedges are typically specified by length and grade, with longer straightedges requiring more careful handling and support (often at defined Airy or Bessel points) to avoid the straightedge itself sagging under its own weight and introducing a false reading.
How to actually choose between them
The practical decision usually comes down to what geometric relationship you're actually checking:
Checking overall flatness of a single surface → surface plate
Checking perpendicularity between two surfaces or a fixture's squareness → square
Elevating a part to a known parallel height → parallels
Holding or checking a round part → V-block
Checking straightness along a single long line or edge → straightedge
A common mistake is trying to make one tool do a job it wasn't designed for - using a surface plate alone to infer squareness, for instance, when a dedicated square would give a far more direct and reliable reading. Another common mistake is under-speccing the grade for calibration-critical work while over-speccing it for general workshop use, which wastes budget without buying any real benefit.
Verification matters more than the tool itself
Whichever tool you're specifying, the calibration certificate behind it matters as much as the granite quality. Flatness, squareness, and parallelism should be verified using traceable instruments - electronic levels, laser interferometers, or high-precision comparators - with certificates traceable back to a national metrology institute, not just a manufacturer's internal claim. A well-made granite tool with a weak or non-traceable calibration certificate is a harder thing to trust in a formal quality system than a slightly lower grade tool with solid, traceable documentation behind it.
If you're putting together an inspection bench or upgrading your metrology tooling and aren't sure which combination of surface plates, squares, parallels, or V-blocks fits your application, we're happy to help you work through the specifics based on what you're actually measuring.