One of the most important steps in the manufacturing of any product is accurate measurement. The progress of mankind through the ages has been directly connected with the development of better ways to measure. A wide range of marking and measuring tools have been designed, which makes it possible for the skilled craftsman to mark and measure the work pieces to extremely high accuracy. Some of the marking and measuring tools are described below:
The steel rule is one of the most useful tools for taking linear measurement of the articles. It consists of a strip having line graduations etched or engraved at intervals of equal fraction of a standard unit of length.
These are usually graduated in both English and Metric System. In English System, the inches are sub-divided into 1/8, 1/16, 1/32 and 1/64 inches, whereas in Metric System, the centimeters are sub-divided into millimeters. For general purpose, steel rule of 300 mm length is most common. They are made rigid or flexible, narrow or wide according to the work for which they are intended. For small jobs, a pocket rule of 150 mm length is very useful. Accuracy of steel rule is 0.5 mm.
Classification – Steel rules are classified by length of the graduated portion.
Material – Steel rules are made up of high carbon steel, hardened and tempered
(a) Never allow a rule to get rusty. Removal of rust will affect its accuracy.
(b) One end of the rule is rounded for safety.
(c) A Small hole is provided to facilitate the rule to be hung in the store room.
Method of Use – Put the rule flat on the object and read the measurement from the rule. If the thumbnail is pressed against the rule, measurements can be read more easily. Always keep the rule at a right angle to the object and read the measurement directly above the rule. If the end of the rule is inaccurate, start measuring at the 1 cm mark but do not forget to subtract 1 cm from the total reading.
Steel Tape Rule – It is made of spring steel and has elasticity. It is used for measuring distances and has the advantage that it can be rolled & packed into a tiny round box. The tape is available in various sizes. This tape can be pulled in and out of its housing by means of a spring inside the housing. There is a hook at the starting end of the tape to hook the tape up against the object to be measured. This hook can only be used for measuring the length of an object. It cannot be used for any internal measurements unless it is differently designed. This tape is specially handy for measuring curved objects.
Flexible Rule – These rules are used for measuring greater lengths of curved or irregular shapes. It can also be rolled & packed into a tiny round box. The tape is available in various sizes. These are generally made of canvas. One side of the tape is graduated in English system and other side is graduated in Metric System. A ring is attached to its one end. By pulling the ring, we can pull out the tape according to our need. A small handle is provided for folding the tape.
There are some other types of rules available such as folding rule and zigzag rule, which are normally used by carpenters.
These are used to set out distances, scribe arcs and circles.
Classification – Dividers are classified by the length of the legs.
Material – Legs are made of high carbon steel and points are hardened & tempered. Adjusting arm is made of mild steel and spring is made of spring steel.
Method of Use – Open the divider correctly and set out or scribe arcs by holding on the top. When transferring dimensions from a scale to the job, one point should be placed in the second division or marking of the scale or rule to avoid any possibility of error. The divider may be kept at an angle while measuring. For marking, it is to be held perpendicular to the working surface with the points of the legs pressed.
Precautions – The accuracy of the divider will be lost if the legs are opened beyond 90º. The points should be kept clean and sharp. To sharpen the points, close the dividers and sharpen them on the outside of the legs only. The points may be protected by sticking them into a cork.
It is used for setting outlines at right angles to an edge or surface and for checking right angular work for trueness. It has a blade and stock which are fixed together exactly at right angle.
Classification – It is classified by the length of the blade.
Material – It is made of high carbon steel, hardened and tempered.
Method of Use – For checking the outside and inside corners, stock of the fitter’s square should be held firmly against one side of the object. Then it should be moved until the blade touches the adjacent perpendicular side of the object. No light should be visible between the blade and the object to ensure the object is absolutely square.
Precautions – To preserve the accuracy of the square, it should be handled carefully at all times. It should be kept away from cutting tools and stored in the box, when not in use.
Testing of Accuracy – This may be done by checking the square for trueness against an accurately machined right angular work, e.g. against vee block, or a master square, if available.
If vee block or master square is not available, a test may be made by placing the stock against the true edge of a flat surface and scribing a line on the surface, using the outside edge of the blade as guide. Turn the square over and check the outside edge of the blade against the line. If the square is accurate the blade edge will coincide. In a similar manner, test the inner edge of the blade.