Granite surface plate and base

01

Choose size and flatness grade

US Federal Specification GGG-P-463c requires all points of the work surface to lie between two parallel planes no further apart than the flatness tolerance for the grade, with Grade A and Grade B tolerances two and four times those of Grade AA. Its table gives total flatness in microinches by size, for example 50/100/200 µin (AA/A/B) for 12 × 18 in, 75/150/300 for 24 × 36 in, 200/400/800 for 36 × 48 in and 600/1,200/2,400 for 72 × 96 in plates. [1]

For sizes not listed, the specification gives Grade AA flatness as 40 + D²/25 microinches, where D is the plate diagonal or diameter in inches, rounded to the nearest 25 µin, with A and B at two and four times that. It notes that listed values follow commercial practice and do not necessarily match the formula. [1]

Tru-Stone, a US granite plate maker, says its plates meet or exceed ASME B89.3.7, that its flatness and repeat-reading tolerances are unilateral (all points between two parallel planes the stated distance apart) rather than bilateral (±, allowing twice the deviation), and that calibration certificates state total indicator reading. It lists Superior Red and Crystal Pink granite, with black diabase or grey on request. [2]

Editorial calculation: an unlisted 30 × 40 in plate has a 50 in diagonal, giving 40 + 2,500 ÷ 25 = 140 µin, rounded to 150 µin for Grade AA, 300 µin (about 7.6 µm) for Grade A and 600 µin for Grade B. [1]

Give the supplier the plate or base size, flatness grade (AA, A, B or other standard), repeat-reading tolerance, granite type, ledges or edges, inserts, T-slots and holes, support or stand, calibration certificate type and accreditation, delivery and installation conditions, and quantity.

02

Set repeat-reading tolerance, surface texture and calibration

GGG-P-463c gives repeat-reading tolerances (full indicator movement of a repeat-reading gauge) by diagonal: for Grade AA 35 µin up to 30 in, 45 µin over 30 to 60 in, 60 µin over 60 to 90 in and 100 µin over 150 in, with 25 µin for all sizes when specified; Grade A and B values are higher. It notes that a larger plate allows a larger flatness deviation over small areas, so small parts measured on large plates may need the tighter repeat-reading tolerance specified on the order. [1]

It limits surface roughness to an average of 32 µin arithmetical average for Grade AA and A plates and 64 µin for Grade B, and its acceptance tests use diagonal profile runs with the reference plane established from the ends of the profile. [1] Tru-Stone describes final inspection with an autocollimator or electronic levels and a Repeat-O-Meter, and offers T-slots, inserts and holes as modifications. [2]

A linked supplier lists granite surface plates and marble platforms in graded accuracy classes for inspection and machine bases. [3]

Editorial note: GGG-P-463c is an inch-based US federal specification; if the supplier works to a different national standard or grade system, ask for the equivalent tolerance in µm for your size and both the global and local (repeat-reading) values.

03

Mingtu's graded granite and marble plates

The listing reviewed offers granite surface plates and platforms to graded accuracy alongside marble plates; ask for the flatness grade and standard, repeat-reading tolerance, calibration method and certificate.

04

Ask how the calibration lab decides pass or fail

A 2022 IMEKO conference paper from MAGAT Tech, a Belgrade calibration laboratory accredited to ISO/IEC 17025:2017, reproduces the metric DIN 876-1 flatness tolerances for hard-rock plates in grades 0, 1 and 2, for example 6/14/28 µm at 400 mm, 7/17/33 µm at 630 mm and 11/26/52 µm at 1,600 mm. The lab picks the method by length: an optical straightness device over 1,000 mm up to 1,600 mm, an electronic level (150 mm base) over 630 mm, and an electronic level on a sine bar (100 mm base) up to 630 mm. [4]

Under its ILAC-G8:09 decision rules, simple acceptance carries up to 50% risk of a false accept; a guard band equal to the expanded uncertainty U (k=2), kept within 1/3 of the tolerance, cuts that risk below 2.5%. A grade 0, 1,600 mm granite plate (tolerance 11 µm) measured 9.5 µm with U = 1.7 µm: a Conditional Pass. A grade 0, 630 mm plate (7 µm) measured 3.5 µm with U = 1.5 µm and passed at 2.5% risk. [4]

Editorial note: ask for the expanded uncertainty and decision rule on the certificate; DIN 876 grades 0/1/2 are metric and differ from the GGG-P-463c AA/A/B grades. [1]

05

Block, lapped face, supports and inserts on a granite plate

The support points belong in the order as much as the granite type: GGG-P-463c ties sag under load to how the plate is supported. [1]

06

Flatness, roughness and room conditions to state for a plate

Flatness and roughness here are quoted in microinches from GGG-P-463c; ask for µm values when the supplier works to DIN 876 grades instead. [1] [4]

07

Send the size, grade and calibration data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Size: [length; width; thickness]
Grade: [AA; A; B; other standard]
Repeat reading: [tolerance]
Granite: [type; colour]
Modifications: [inserts; T-slots; holes; ledges]
Support: [stand; support points]
Calibration: [certificate; accreditation]
Quantity: [plates]

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