Inspecting prismatic parts: block and box check

What counts as a prismatic part

Blocks, boxes, plates and beds belong to the prismatic family: their working surfaces are planes, and the drawing asks for lengths, widths and heights between those planes, steps from one plane to another, and holes with positions measured from them. Turned parts ask mainly “how big is each diameter”; prismatic parts ask “where is each plane, and how square is it to the others”. The measuring follows the drawing: shafts, bushings and flanges read the diameters; this page reads the planes.

Two kinds of requirements appear on such a drawing. Size tolerances — 120 ±0.20, Ø20 +0.033/0 — are measured with a caliper or micrometer and judged against the limits. Geometrical tolerances — flatness of a mounting face, parallelism of a slideway, perpendicularity of a flange face — are marked with the symbols of GB/T 1182-2018 (a modified adoption of ISO 1101:2017) and verified as GB/T 1958-2017 describes.

Pick the reference face first

Everything on a block is measured from one face. Choose the face machined with the best finish and the tightest tolerance — usually the one that sits against something else in the assembly — and call it the reference. Measure the other faces, the steps and the hole positions from it, never from a burr or a chipped corner.

Before the first reading: deburr and wipe the part and the instrument, and let both sit at the workshop temperature (sizes refer to 20 °C; see measurement conditions). A burr under a jaw adds its own thickness to the part.

Outside dimensions, steps and holes

Length, width and height: close the outside jaws on the two faces, square to them — the jaw faces flat on both surfaces, the beam roughly across the part. A caliper held slanted across a width reads the slanted diagonal (true size ÷ cos θ), always on the big side; square it up and read again. Choose the caliper by the tolerance: a dimension whose band is tighter than the caliper can judge repeatably belongs to a micrometer or a comparison setup (choosing the instrument).

Steps take the depth rod with the beam end seated on the upper face; bores take the inside jaws rocked for the true diameter (measuring a bore); the distance from a hole to the reference face is an outside-jaw measurement from the near hole edge, plus the radius — or read centre distances directly (steps and centre distances). Measure each outside dimension at more than one spot: the spread between the largest and smallest readings is the part’s taper and bow speaking.

Flatness, parallelism, perpendicularity

Flatness bounds how much one plane may deviate from perfectly flat; it has no datum — the face is compared against an ideal plane of its own. Parallelism and perpendicularity compare two real elements — the slideway against the mounting face, the flange face against the bore axis — and always carry a datum letter on the drawing.

The workshop instrument for all three is a dial indicator on a stand, sweeping the face while the part rests on a surface plate. Set the plunger square to the surface, preload it, and move the indicator across the face in a grid; each reading is the face height at that point relative to the plate. The difference between the largest and the smallest reading — the range — is how shops judge the face in production; GB/T 1958-2017 evaluates the geometrical error on the minimum-zone principle, and where the two disagree the standard’s evaluation settles it.

Measure parallelism by sweeping the upper face with the part resting on its datum face on the plate; measure perpendicularity with the part stood on the datum face and the indicator sweeping the side face, or with a square for a quick shop check. Geometrical checks come after the size check, on a clean plate, with the same 20 °C patience.

A worked example: one box, five lines

A gearbox-style housing calls for length 120 ±0.20, width 80 −0.10/−0.20, a step 30 +0.10/0 above the mounting face, a bore Ø20 +0.033/0, and flatness 0.05 on the mounting face. The inspector checks each and fills one line per characteristic:

The flatness line comes from five indicator points on the mounting face: −0.010, +0.015, +0.005, −0.005, +0.020 mm. Largest +0.020, smallest −0.010, range 0.030 mm against a 0.05 tolerance. Note the record keeps the actual readings, not just “pass” — a later inspection record or a first-article package (FAI) reuses these numbers.

Running the full check

Start from the characteristic list: every ballooned dimension and geometrical symbol becomes one row. Work in a fixed order — outside sizes first, then steps and holes, then the geometrical tolerances on the plate — so no row is skipped and nothing is measured twice. Judge each value against its own limits the moment it is written down; a fail stops the line for that characteristic and goes into the record as a nonconformity, not into memory.

For a batch, check the first part completely before the rest (that is the first-article idea); for one-offs, the same flow is simply “the inspection”.

Standards

Drawing symbols and tolerance items: GB/T 1182-2018 (current; a modified adoption of ISO 1101:2017, confirmed 2022). Inspection and verification of geometrical tolerances: GB/T 1958-2017 (current) — the 2017 edition changed the title from 检测规定 to 检测与验证. Caliper characteristics: ISO 13385-1; verification of calipers in China follows JJG 30.

Planned for the lab

Prismatic workpieces with flatness and parallelism errors are planned for KELIBRON (intake batch W1), together with the caliper’s contact model for prisms and a surface-plate indicator scene. Until then, the cylindrical housings already on the bench stand in: their measurable features are diameters, and the housing scene rehearses the same discipline — clean the part, seat the jaws square, read, judge — on a drawing that mixes several characteristics.