---
title: "Surface plate: materials, grades and use"
subject: "Metrology"
chapter: 6
key: "semester_3/metrology:surface-plate-materials-and-uses"
source: https://www.bctengineer.com/q/semester_3/metrology/surface-plate-materials-and-uses
site: BCT Engineering
---

# Surface plate: materials, grades and use

Metrology · Chapter 6: Surface Measurement

- Asked in: year not given, 4 marks
- Video: [Surface Plate in Metrology (Measurements) | Types, Uses, Material, Care & Calibration |](https://www.youtube.com/watch?v=LofO0P04cPs) — MECHANICAL ENGINEERING SKILLS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/metrology/surface-plate-materials-and-uses
- Whole chapter: https://www.bctengineer.com/md/semester_3/metrology/chapter-6

## Question

Write short notes on surface plates covering the materials, grades, manufacture and uses.

## Short answer

Surface plates are cast iron (scraped) or granite (burr-free, non-magnetic). Grades 00,0,1,2. Made by casting, aging, grinding, and three-plate scraping. Used as reference for marking out, flatness checks, height measurement, and base for sine bars and gauges.

## Answer

A **surface plate** is a heavy, rigid plate with a flat upper face that gives a reference plane for marking out, measurement and inspection of flat surfaces and of heights (with a height gauge or dial indicator).

#### Materials
- **Cast iron:** fine-grained, stress-relieved and ribbed underneath for stiffness; it can be scraped and is self-lubricating. It is the usual choice and is widely used in machine tool workshops.
- **Granite:** hard, and a scratch or chip does not raise a burr; it is non-magnetic, rust-free, has little thermal expansion and good wear resistance, so it is preferred in inspection rooms.
- **Glass or ceramic:** for small plates in light use.

#### Grades
Plates are made in grades 00, 0, 1 and 2 (IS 2285): 00 and 0 for laboratory and inspection, 1 for the inspection room, and 2 for workshop use. The permissible flatness error increases with size and with grade number.

#### Manufacture
The plate is cast, aged, rough machined and finished by grinding and then by hand **scraping**. The flat surface is produced by the **three plate method**: three plates are scraped in turn against each other (A against B, B against C, C against A) until all three mate perfectly; only true planes can fit each other in every combination.

#### Uses
Marking out, checking flatness with a straight edge, checking squareness and parallelism with a dial indicator and angle plate, as a base for sine bars and slip gauges, and for measuring heights with a vernier height gauge. The plate is kept clean and covered, supported at three points and checked regularly.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is granite preferred for surface plates in inspection rooms?
   A. It can be scraped by hand
   B. Dust does not raise a burr when scratched, and it is non-magnetic, rust free and has low thermal expansion
   C. It is the cheapest
   D. It is self-lubricating
   Answer: B. Granite is hard, non-magnetic, rust free, wear resistant and does not raise burrs when scratched. (Key point: Granite advantage)

2. Why are three plates scraped against each other in turn to make a flat surface?
   A. It is faster than using two
   B. Only a true plane can fit both other plates in every position
   C. The plates become lighter
   D. Each plate becomes convex
   Answer: B. Two plates could mate as convex and concave, but three scraped in turn all mate only if every one is a true plane. (Key point: Three plate method)

3. Which grades of surface plate are for laboratory and inspection use?
   A. Grade 2 only
   B. Grades 00 and 0
   C. Grade 1 only
   D. All grades equally
   Answer: B. Grades 00 and 0 are for laboratory/inspection, 1 for inspection room and 2 for workshop use. (Key point: Grades of plates)

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