In the field of Automobile Engineering, several instruments are employed for precise measurement and accurate drawing. Among them, the Trammel, Surface Plate, Surface Gauge, and Scribing Block play particularly vital roles. Whether it is drawing large circles, checking the flatness of a surface, or marking fine lines on mechanical components, the correct use of these instruments is indispensable.
In today’s discussion, we shall explore in detail the structure and application of the Trammel, as well as how the Surface Plate, Surface Gauge, and Scribing Block contribute to achieving precision in engineering operations. This lesson is essential not only for understanding mechanical design but also for practical applications in workshops and industries.
Table of Contents
Trammel
When a divider exceeds 250 mm (10 inches) in size, it becomes difficult to handle. Therefore, for larger work, it is more convenient to use a Trammel. The legs of a trammel are not connected like those of a divider; rather, they are mounted independently on a straight round bar. By rotating the set screw, the legs can be fixed at any desired position. The functions of a trammel are similar to those of a divider.
A trammel is extremely useful for drawing large circles. Like a divider, its legs are straight and narrow; however, if necessary, curved legs similar to those of callipers can be fitted, allowing the trammel to be used as a calliper. The method of using it is often demonstrated with the help of diagrams. It can be employed in both inside and outside calliper operations.
Surface Plate
A Surface Plate is a flat, rectangular metal block, usually made of grey cast iron. Its top surface is smooth and flat, with all corners meeting at right angles (90°). To make it portable, handles are provided on both sides. These plates typically range in size from 300 mm to 1500 mm (12 to 54 inches) in length and 250 mm to 1200 mm in width.
The surface plate is used with instruments such as the indicator gauge, run-out gauge, surface gauge, scribing block, and V-block to test or mark the flatness, accuracy, and alignment of components such as crankshafts and camshafts. After scraping an object, its flatness is also checked by applying colour and rubbing it on the plate.
The surface plate must always be kept clean and free from rust. Care should be taken to prevent dust and metal particles from accumulating on its surface. No chiselling or filing should be carried out on objects placed upon it. Oil and a wooden cover should be used to preserve its surface.
Surface Gauge
The Surface Gauge is an instrument used to support and guide a scriber in various marking and measuring operations. The different parts of a surface gauge and their functions are illustrated in standard engineering diagrams.
Base:
The base of the gauge is designed to rest firmly on the surface plate. It is carefully finished to ensure that it is perfectly flat, with no convex or concave irregularities.
Clamp Nut:
By tightening this nut, scribers of different heights can be attached and secured to the spindle (upright rod).
Scriber:
The scriber has two ends—one straight and the other bent at an angle of 90°. The bent end is used for measuring heights, while the straight end is used for drawing straight lines.
V-Slot:
This slot enables the surface gauge to be conveniently positioned on cylindrical or round objects.
Gauge Pin:
When pressed, this pin protrudes below the base, allowing the surface gauge to be moved parallel to the edge of the surface plate or aligned with a vertical surface.
Adjusting Screw:
By turning this screw, the spindle can be adjusted to a fine angular position. The scriber is clamped to the spindle by the clamp nut, enabling fine control of the scriber’s tip during use.
Swivel Nut:
This nut allows the spindle to be set at a greater angle and locked in that position when required.
Uses of the Surface Gauge:
The surface gauge can be employed in several ways, commonly demonstrated in eight principal operations for measurement and marking work.
Scribing Block
The Scribing Block has a simple and straightforward construction. It is primarily used for marking (scribing) work in workshops and is widely known as a Marking Gauge. Its construction and parts are illustrated in engineering diagrams.