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
OTHER NAMES: Bow calipers,egg calipers
SIZE: Spring joint calipers: up to12in.; Firm joint calipers to 36in.
MATERIAL: Steel
USE: To transfermeasurements from a ruleto the work or tomatch two elementsto 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
OTHER NAME: Straight
calipers
SIZE: As outside calipers
MATERIAL: Steel
USE: As outside calipers
SPRING-KNURLED NUT
THREADED ROD
Inside calipers are available in the same size range as outside calipers. They have straight legs, turned out at the top and are used to take 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.
Adjustment
Adjustment is different depending on the type of caliper. For firm joint calipers close or open the legs to approximate the required measurement.
Make fine adjustments by either tapping one leg on a hard surface to close the gap or by tapping the jointed end of the tool to open the legs slightly.
Spring joint calipers are adjusted by the knurled nut. Close the legs against the spring to take the load off the nut, which can easily be adjusted to the approximate position required. Fine adjustment is made by a slight turn on the nut.
Practice fine adjustment of calipers with one hand; this will leave you a free hand to steady the work.
Care and maintenance
Unless they are misused, calipers require little attention other than an occasional oiling of the moving parts. Take care not to over-oil firm joint calipers. Avoid using them to measure moving parts of machinery, as this can wear down the points of the tool.
Setting to the work
Hold one toe of the caliper in contact with the work and adjust the other leg until a slight "drag" can be felt on the other toe as you rock it back and forth. This "feel" can only be achieved by practice.
Setting calipers to a rule
For accuracy when setting any calipers to a rule, it is important to read off the measurement at eye level.
The same method is used to measure both inside and outside dimensions, but in a hole, centre the calipers by rocking them sideways as well as from front to back.
To set outside calipers, one leg over the end of the rule and adjust the other leg to the required measurement
To set inside calipers, rest both the toe of one leg and the end of the rule on aflat surface and adjust the other leg as before.
Transferring caliper measurements
Place the toes of the tools in contact, steadying them with a finger of one hand and adjust to match.
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 measurement. 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 required 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 the measurement.
A spring joint inside caliper can be used for this job by closing the legs by hand, taking care not to disturb the adjusting screw nut when removing the tool from the work.
Odd Leg Calipers
OTHER NAMES: Jenny
calipers; hermaphrodite calipers SIZE. Up to 6in. MATERIAL: Steel USE: To scribe lines parallel to an edge
Odd leg calipers can be used to find the centre 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 is particularly useful when marking sheet metal.
Odd leg calipers have a prehensile toe for positive location on the work edge.
Spring Dividers
OTHER NAMES: Spring compass, bow compass
SIZE: Up to 12in. measured from centre 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
OTHER NAME: Wing dividers
SIZE: Up to 18in.
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. On some patterns, fine adjustment is provided by
means of a knurled nut which operates on a screw thread attached to the
end of the wing. The leg is moved by the operation of a flat spring
riveted to it.
Setting or reading off a rule
Place the points of dividers in the incised divisions of a metal rule when reading off a measurement.
Stepping off measurements
Measure increments along a line by twisting the knurled spigot between thumb and forefinger, "walking" the dividers from one point to another. Take care to keep the leg in contact with the work upright to avoid slipping.
Scribing arcs and circles
To scribe metal, mark the centre point with a punch to hold the point. If the finish is important, first patch the centre with masking tape. Lean the tool in the direction of movement and scribe the circle or arc by revolving the spigot between thumb and forefinger.
Beam Compass
OTHER NAME: Trammel
SIZE: Measuring capacity: 4 to 6ft.
MATERIAL: Heads: steel; Beam: aluminum or wood
USE: To scribe large arcs and circles
FINE
ADJUSTMENT SLIDE
BEAM
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
OTHER NAME: Caliper rule
SIZE: To measure up to 3iin.
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 has imperial or metric graduations.
Outside dimensions
Locate the inside edges of both jaws on the work and read off the scale against the edge of the fixed jaw.
Inside dimensions
Locate the tips of the jaws on both sides of the work and read off against the special line marked on the body of the tool against the scale.
Vernier Caliper
SIZE: Measuring capacity: 6 to 72in.; 150 to 1,800mm
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.
Reading a Vernier caliper
Imperial:
The main scale is graduated in inches which are subdivided into 0.1 in,, which in turn are subdivided into 0.025in. The Vernier scale runs beneath it. Each Vernier graduation represents 0.001 in. Read off the measurement in the main scale indicated by the zero mark on the Vernier scale.
When the zero mark does not coincide with a line on the main scale, note which line on the Vernier scale does. Add whatever line is indicated on the Vernier scale to the total for an accurate reading.
Metric:
The metric main scale is graduated in lmm and subdivided into 0.25mm. Each metric Vernier section represents 0.01mm. Read off as for imperial. When the Vernier zero line does not coincide with a main scale line, find a line that does and add the Vernier measure to the total.
Micrometer Caliper
SIZE: Range: 0 to to 6 to 12in.; 0 to 13 to 150 to 300mm; specials up to 24in., or 600mm
MATERIAL: Steel
USE: To obtain very fine measurements
Micrometers are designed to produce the 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 lin. (25mm).
The micrometer has a U-shaped frame with an anvii on one side and an adjustable spindle extending from the other. The knurled thimble adjusts the spindle to the 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 and 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 workpiece, the measurement can be read directly from a digital display. It is very accurate and does not involve the computations needed by a standard micrometer.
Reading a micrometer
Micrometers are marked in imperial or metric graduations,
Imperial
The sleeve scale on an imperial micrometer has major divisions representing O.lin., subdivided into four equal parts, each representing 0.025in. When the anvil and spindle are in contact with the work, read off the measurement on the sleeve against the edge of the thimble.
If the thimble edge falls between graduations on the sleeve scale, use the thimble scale. This is graduated in 0.001in. and is numbered every 0.005in. Read off these measurements against the centre line of the sleeve scale and add them to the sleeve scale reading.
Some micrometers have a Vernier scale marked along the sleeve. This is graduated in 0.0001in. If the centre line on the sleeve falls between two graduations on the thimble scale, read off the Vernier scale to see which graduation coincides with the line on the thimble scale and add the Vernier reading to the total.
Metric
The sleeve scale on the metric micrometer is graduated in 1mm above the line and subdivided into 0.5mm below the line. The thimble scale is graduated in 0.01mm. Read off and calculate the total measurement as for imperial.
Handling the micrometer
Depth Gauge
SIZE: To measure up to 15in. or 375mm
MATERIAL: Steel
USE: To measure the depth of holes and mortises
A depth gauge is a graduated rule with a sliding head which has a machined flat base.
The flat base is placed on the surface of the workpiece, the rule adjusted to touch the bottom of the recess, and locked by a clamping screw. Remove the gauge and read off the measurement from the rule.
Hold a small micrometer conveniently in one hand by resting the frame in the palm of the hand and supporting it with the ring finger. This leaves the other fingers and thumb free to adjust the tool.
Alternatively hold the frame between the fingers of one hand and adjust the micrometer with the other.
Keep the micrometer free from dust and grease and store it carefully when not in use.
Measuring at an angle
You can use the depth gauge to measure the sloping side of a tapered hole.
Wire Gauge
SIZE: To measure imperial standard: 1 to 36in.; Metric: 0.2 to 10mm
MATERIAL: Steel
USE: To measure the gauge of wire and thickness of sheet metal
The wire gauge is a template which is used to measure the gauge or thickness of sheet metal or wire. Around the edge are graduated, numbered slots which fit wire or sheet metal. The metal or wire is tried in the various slots until it just fits without being forced.
Feeler Gauge
OTHER NAME: Thickness gauge
SIZE: To measure lj to 25in. in 0.001 in.; 3 to 100mm in 0.001mm
MATERIAL: Steel
USE: To measure very fine gaps
A feeler gauge has a number of thin metal blades of various thicknesses that fan out from a steel case, which protects them from damage when not in use. Each blade is marked with its thickness. The blades are used to measure small gaps, such as between a shaft and its bearing or between electrical contacts.
Screw Pitch Gauge
SIZE: To fit a range of threads
MATERIAL: Steel
USE: To measure the pitch of a machined uiread
Like the feeler gauge, the screw pitch gauge consists of a series of metal blades in a metal case. The edge of each blade is notched to match the shape and spacing of a range of threads cut in bolts or holes.
Measuring a screw thread
Hold the edge of the notched blade against the screw thread to see if it fits snugly.
Marking Gauge
SIZE: 64 to 9iin.
MATERIAL: Hardwood, steel
USE: To mark a line parallel to an edge
A marking gauge is a beam of hardwood or steel upon which slides a stock, that acts as a fence to guide a pointed pin a set distance from the edge of a workpiece. The steel pin is permanently fixed near one end of the beam and projects approximately j^in. from the underside. The stock is fixed in the required position by a thumb screw on one side. The guide face of the stock can be reinforced with brass facing strips let in flush with the surface. Sometimes the beam is graduated for setting the stock the required distance from the pin. If it is not, simply use a steel rule.
Using a marking gauge
iJress the stock against the edge of the workpiece with the beam resting on the surface. Rock the tool toward you until the pin touches the work at an angle, then push it away from you.
Finding centre of rail
When setting out a dowel joint you need to mark the centre of the rail. Set the pin as near as possible to the centre line and prick a hole in the work. Turn the work around and make a second mark alongside the other. If they do not correspond exactly make slight adjustments by tapping the end of the beam on the bench. Tapping the end nearest the pin will shorten the gap between the stock and pin; tapping the other end increases the gap. Adjust the pin until the two marks meet from both sides of the work.
If the marks fall short of the line, lengthen the gap between the stock and the pin.
If the marks overshoot the line, shorten the gap between the stock and the pin.
When the two marks correspond exactly on the centre line, the gauge is set correctly.
Cutting Gauge
MATERIAL: Hardwood
USE: To mark a line parallel to an edge across the grain of lumber
The cutting gauge is constructed and operated in exactly the same way as the marking gauge, but instead of the pointed pin it has a fiat blade secured with a small wedge. This is designed to cut across the grain of lumber to mark a line where a standard marking gauge would tend to tear the grain. The blade must be periodically removed and honed to a sharp edge.
Marking across the grain
Use the cutting gauge as you would a marking gauge to mark across wood grain.
Cutting wood veneer
To cut parallel strips of veneer shoot the edges square on a shooting board and align the edges with a straight edged
board. Hold the veneers down firmly with a batten and run the cutting gauge against the edge of the board.
Mortise Gauge
SIZE: 9-iin.
MATERIAL: Hardwood
USE: To mark two parallel sides of a mortise and tenon
The mortise gauge, like the marking gauge, has a beam and an adjustable stock. However, it has two pins to mark both sides of a mortise simultaneously. One pin is fixed and the other is attached to an adjustable slide. On some tools this is a simple pull slide, on others it may be adjusted by a thumb screw mounted at the end of the beam. Most mortise gauges have another pin mounted on the underside of the beam so that they can be used as a standard marking gauge.
Marking out a mortise and tenon
1. Mark one shoulder of the mortise with a try square. Position the side of the rail against this line and mark the mortise width against the other.
2. Square across with a try square. Set the pins and the gauge to correspond with the width of the chisel blade.
3. Adjust the stock to position the pins to make the mortise in the centre of the rail. Fix the stock with the knurled screw. Mark the mortise between the two shoulder lines.
4. Mark the shoulder line for the tenon then use the gauge, set as for the mortise, to mark out the tenon on both sides and across the end grain.
Leather Creaser
OTHER NAME: Leather crease
SIZE: 8in.
MATERIAL: Handle: hardwood; Blade: steel
USE: To mark a line on leathe
A leather crease has a blade with a blunt edge which is used to mark a dark line on leather as a decorative motif. The creaser is often used cold, but if heated produces a darker line.
A tool with a simple blade is known as a "single creaser" while, a "double creaser" has two parallel edges.
Using a screw creaser
To mark a parallel line use a screw creaser which has two blades linked by a thumb screw. One blade runs against the edge of the workpiece to guide the progress of the other.
