Know and How of Compasses/Dividers and Calipers
Dividers and Calipers
Compasses or dividers and calipers are frequently shown among the tools of Roman carpenters, masons, wheelwrights and shipwrights. They are all "firm joint" compasses. In some cases one leg appears to have been slightly longer than the
other, so they may have been used as gauges, a tool which the Romans do not seem to have known. The earliest example of a pair of wing dividers occurs on Jost Ammann's picture of the Compass-maker's Shop of 1568. The modern cooper's pattern of spring compasses, made of ash bent in a "U" shape, held together and regulated by a crossbar with a screw of opposite hand at each end, is illustrated on the shop sign of John Jennion of London, dating to about 1730, but may have been in use both in England and France well before this. The earliest beam compass, said to have been six to twelve feet long, with a fixed point at one end and a single
moving trammel head, is shown in Roubo's book.
Outside Calipers
Size: Spring Joint Calipers- up to 300mm Firm Joint Calipers- up to 900mm
Material: Steel
Use: To transfer measurements from a rule to the work or to match two elements to fit.
Bowlegged outside calipers which clear the work are used to take outside measurements. Two kinds of calipers are available: firm jointed calipers, which are free to move but are held firmly in any position by friction between the two legs, or spring-jointed calipers which are controlled by a knurled nut on a threaded rod. Within limits, it is possible to take inside measurements with firm joint calipers.
Inside Calipers
Size: As outside calipers
Material: Steel
Use: As outside calipers
Inside calipers are available in the same size range as outsides calipers. They have straight legs, turned out at the top and are used to make inside measurements. They are available with firm or spring joints. As with outside calipers, it is possible within limits to measure external dimensions with firm joint inside calipers.
Transfer Calipers
Sometimes it is necessary either to open or close the legs of the calipers to remove them from the work, thus losing the measurements. In this case use transfer calipers which have a secondary leg locating on a lock nut situated on one of the primary legs. lock the secondary leg in position and obtain the require setting. Release the lock nut and remove the tool. Replace it in contact with the work with the secondary leg, secure the lock nut and read off of the measurement. A spring joint inside the caliper can be used for this job by closing the kegs by hand, taking care not to disturb the adjusting screw nut when removing the tool from the work.
(Sorry for the low res images)
Old Leg Calipers
Size: Up to 150mm
Material: Steel
Use: To scribe lines parallel to an edge
Old leg calipers can be used to find the center of round or square section metal stock. They have two hinged legs. one has either a hardened steel, or a replaceable point, and the other has a toe which runs against the edge of the workpiece. Be careful not to let the toe slip otherwise the line will wander. Some calipers have a toe which locates over the edge of the work. This particularly useful when marking sheet metal.
Spring Dividers
Size: Up to 300mm measured from center of roller to the point of the leg
Material: Steel
Use: To scribe arcs and circles on metal, or to "step off" divisions on a line
Spring dividers are similar to spring calipers, except they have a small knurled spigot to facilitate the scribing of circles. Adjustment is made by means of a knurled nut on a threaded rod. dividers normally have two identical flat legs with hardened points. They are sometimes fitted with removable points which can be adjusted for equal length and be replaced when worn.
Wing Compass
Size: Up to 450mm
Material: Steel
Use: To scribe arcs and circles on metal, or to "step off" divisions on a line.
Wing compasses have solid, square section legs for approximately half their length. Hinged at the joint they are adjusted by means of a curved wing attached to one leg and passing through a slot in the other. The legs are locked in position by means of a knurled screw bearing on this wing. The leg is moved by the operation of a flat spring riveted to it.
Beam Compass
Size: 120 to 180cm measuring capacity
Material: Heads- steel Beam- aluminium or wood
Use: To scribe large arcs and circles
A beam compass consists of a long metal or wooden bar which carries trammel heads. These heads slide along the bar and can be fixed in position by wedges or screws. They have hardened steel points with which to scribe large arcs and circles. One point can be replaced by a pencil.
Slide Caliper
Size: To measure up to 81mm
Material: Steel
Use: To measure internal and external dimensions
One jaw of the slide caliper is fixed to the main part of the tool. The other jaw is part of a slide, which moves in or out until both jaws come into contact with the work. The slide is fixed with a knurled clamp nut and had imperial or metric graduations.
Vernier Caliper
Size: Measuring capacity- 150mm to 1800mm
Material: Steel
Use: To obtain very fine measurements
The Vernier caliper, named after its inventor, is a development of the slide caliper, but is graduated to make finer readings. it is capable of measuring internal and external dimensions and can also be used as a depth gauge. Vernier calipers are available with imperial and metric graduations.
Micrometer Caliper
Size: up to 600mm
Material: Steel
Use: To obtain very fine measurements
Micrometers are designed to produce to design extremely fine measurements required in engineering. so that parts of a machine will meet with the minimum tolerance. There are micrometers for measuring depth, across screw threads, inside dimensions and, most commonly, outside dimensions. The size range varies, but the most popular micrometer measures outside dimensions of 0 to 1in. (25mm). The micrometer has a "U" shaped frame with an anvil on one side and an adjustable spindle extending from the other. The knurled thimble adjusts the spindle to required setting, which is then fixed by the lock nut. A ratchet stop is sometimes fitted to the end of the spindle. If the ratchet is used to adjust the spindle, it will click or slip when the anvil or spindle contact with the work. The latest development in micrometers is expensive, but extremely easy to use. When the spindle and anvil come in contact with the workplace, the measurements can be read directly from a digital display. It is very accurate and does not involve the computations needed by a standard micrometer.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2022 Temoor Dar
