The main part of the steel tape measure box has an extra long steel connecting plate at the end of the ruler than the conventional rule box. The connecting plate is fixed with a secondary ruler and a bead, and the front end of the lower part is fixed with a positioning iron; there is an inner concave arc on the connecting plate of the rule box The arc-shaped groove and the arc surface coincide with the back arc of the steel ruler. The steel ruler can only slide longitudinally back and forth after being restricted by the auxiliary ruler and the bead in the concave arc-shaped groove.

The secondary ruler is a steel strip with 50 scale graduations. The distance of each division is 0.98mm. When fixed on the connecting plate of the ruler box, the zero position should be vertically coincident with the inner side of the rear positioning iron. The shape of the steel ruler is the same as that of a conventional steel measuring tape, but there is no scale at the end, which is used to fix the ruler end positioning iron. The zero position of the steel ruler and the inner side of the ruler end positioning iron are vertically overlapped, and the end of the steel ruler and the retracting spring The wheels are connected to each other and are wound on the reeling spring wheel, and are all installed in the ruler box. The inner end of the spring bar of the retracting spring wheel is fixed on the rule box. The rule end positioning iron is fixed longitudinally on the steel rule by a positioning thin iron sheet, which is different from the conventional horizontal fixation, and has an inner positioning surface and an outer positioning surface, which can be used to measure the outer circle and inner hole of the workpiece respectively. There is a hardened tip inside the positioning iron of the ruler end. The rear positioning iron is the same as the rule end positioning iron.

Among the above components, the ruler box is composed of a box body and a connecting plate. The steel connecting plate is first processed during manufacturing, and then the box body is made by injection molding. The steel connecting plate can be placed in the injection mold in advance. That is, it is cast as a whole with the box body; or two separate parts can be processed first and fixed with adhesive or screws. The steel ruler requires the scale to be as accurate as possible, and the finer the line, the better. The rule-end positioning iron and the rear positioning iron should be made of steel with a carbon content of more than 0.45%, so that the tip can be hardened after heat treatment. All components of the steel tape measure can be manufactured by conventional methods.

When the steel tape is used for measurement, you can hold the positioning iron at the end of the ruler with your hand, and pull out the steel ruler, so that the end positioning iron and the rear positioning iron are closely attached to the measured part of the workpiece, and you can start from the scale of the steel ruler. Read the value above the millimeter, and read the value below the millimeter from the sub-ruler. For example, if the twelfth scale of the secondary ruler coincides with the scale of the main ruler, the value is 0.24 mm, and the addition of the value of the main ruler and the value of the secondary ruler is the actual measured size. When measuring the inner circle of the workpiece, the outer side of the positioning iron is used to close the workpiece, and the measurement result should be noted to add two positioning iron width values. The ruler can also use the positioning iron tip to directly draw measurement marks on wooden parts, copper, aluminum parts and steel parts that have been coated with white powder after the measurement is completed.