It may seem strange to begin with a description of instrument parts, but some are used in more than one instrument, and this saves giving details each time. Sometimes there is a choice of which parts to use, depending on the tools and materials you have. For example, there are three types of bidirectional sights - slits, narrow V notches, and staggered tall sights. Fine slits are sawn with a junior hacksaw, or made by mounting metal strips side by side. But if you don’t have a junior hacksaw or straightedged strips, it is better to cut V-notch sights in thin sheet metal, with tin snips.


Eyebolts, T Nuts & Wing Nuts
Eyebolts are used in some instruments to clamp surfaces together or as screw levelling feet - the eyes can be easily gripped, and a plumb line can hang from the eye. Six mm.(1/4 inch) eyebolts are used to clamp instruments to tripods, because a 6 mm. thread will fit a camera tripod. A substitute for eyebolts is to cut the heads off 6 mm. metal screws and put the shafts into 1/4 inch operating knobs from electronics stores. Hex head bolts can be used as levelling feet; only a small force is needed to turn these, and the heads can easily be gripped.


When eyebolts are screwed in and out, there is the danger of losing them. Tie them to a length of string , and fasten the string to the instrument frame, like this:-


T nuts are used because they bite into timber and don’t turn; a good substitute is to bore a hole for an ordinary nut, push the nut in, and fasten it with adhesive, like this:-



Wing nuts are sometimes used to clamp surfaces tight. To stop wing nuts unscrewing, use a longer screw and put a locknut above the wing nut. Like this:-








The Plumb Line -is a length of string with a weight attached, which always hangs vertical. It is very old - it is mentioned often in the Old Testament, including Amos 7:7-8, 2 Kings 21:13, and Isaiah 28:17, and it was used before that.




The plumb line is almost never still. It is affected by body tremors if held in the hand, and a slight breeze will set it swaying. Steady it with a hand or foot, but stop before taking the measurement in case it affects the accuracy. For a plumb bob, any weight (e.g. a stone) will do, provided all tests are made with the line only, not the bob. To take sights from the bob, it must be symmetrical in shape and attached to the line on its axis, like the second one.


Iron bob weights from 150 to 500 grams are available, but a good cheap substitute is a lead fishing sinker of 200 to 500 grams. They are not as well made as a machined bob weight, and not all sinkers hang truly vertical, but with careful selection they are a good substitute. Two good patterns are shown on the right


Pendulums are used to show ‘plumb’ in several instruments, including the Tryall. They are not quite as precise as a plumb line, but tests show they are good enough for the purpose. They don’t tangle or need to be wound up when you have finished. The pendulums in these notes are made of wood, but they can be made of fencing wire, provided there is no other magnetic material near by for them to stick to. For example, if the Try-all pendulum is made of fencing wire, the stop screws must be brass.
Like plumb lines, they are affected by air currents, but are prevented from banging about by stop screws or screw eyes. If wind is moving the pendulum, stop it gently with one finger, then lift your finger away just before taking a reading.


Strapping Iron Strip, 16 mm.(5/8 in.) wide and 0.8 mm.(1/32 in.) thick, is supplied in coils for binding crates or bundles of timber; to open the crate or bundle, the strapping is cut with shears. The discarded strips are a good source of straight edged metal strip for making instrument parts - particularly sight vanes - and is well worth the trouble of getting some. The iron is springy enough to keep its shape in spite of rough treatment, but can be drilled, filed, ground, sawn with a hacksaw, cut with tin shears., or bent. It has a gloss black coating, but will rust, so it should be painted with flat black oil paint.


Wooden Parts. Most of the parts used in these instruments are made of wood. Unless the notes say something else, these should preferably be made from straight grained dressed hardwood, free of knots; it is less likely than softwood to twist or warp with age. Hardwood is also easier to drill accurately; a drill bit sometimes wanders off centre in softwood, and the instrument accuracy often depends on correct hole positions.


Open Sights, without lenses, are fitted to all sighting instruments. Both crosses have bidirectional vane sights, the dumpy levels have unidirectional slit and crosshair sights, and the dumpy plummet and crevel both have sights with horizontal and vertical slits, a crosshair, and a scale. Sights are used in pairs; the one furthest from the eye is the foresight, the one nearest the eye is the backsight. When looking the opposite way, bi-directional sights reverse their function - the foresight becomes the backsight and vice versa, and both sights are the same pattern.


Bidirectional Sights come in several different patterns, and it is up to the constructor which to fit; each instrument description includes a list of those which are suitable. They can be cut from light gauge sheet metal - galvanised iron, aluminium, brass, or tinplate from a can - even cardboard.

1. Narrow V-notch Sights. Like this:-


As the eye moves towards the tip of the notch, the width becomes less, and this effectively reduces the pupil diameter of the observer’s eye, giving it a greater depth of focus; the fore sight, only centimetres away, stands out sharply against the target, usually many metres away.


The degree to which this can be done depends on the light level; in daylight the eye can be close to the tip, giving a sharp image; in lower light levels the eye must move further away, but the image will not be as sharp.

. 2. Slit and V-notch Sights. A variation on the above is to cut a slit and shallow V-notch, partly covered by a pivoting shutter blade, like this:-



Each sight can be a V-notch fore sight, or a variable slit back sight, by opening or closing the shutter bar; which has a similar effect to moving your eye towards the tip of a notch (see remarks in 1, above, re. focus and light level). A slit back sight also gives a fixed point, even if your eye moves, and greatly reduces parallax error.
Cut the slit first with a fine tooth hacksaw blade, or better still with a junior hacksaw, keeping the slit central and vertical. To cut thin sheet metal, clamp it between two pieces of flat wood, and cut all three. Then cut the v-notch with tin snips or hacksaw, making sure the notch tip is in line with the slot. For heavier sheet metal, file the notch with a triangular file.

3. Hinge Sights. Another way to make sights is to cut slits or V-notches in one side of two 50 mm. butt hinges, which can be folded down when not in use. Existing holes in the side in which the notches are put must be blocked. On the mounting side, two existing holes may be used, but the front mounting hole may need to be drilled to allow the hinge to swivel around this screw when first lining it up, as described above.

Most hinges have slack in their pivots, and this must be removed to keep the sights firm in position, both when opened for use and when folded away. A good way to do this is to place countersink head screws under the moving parts of the hinge pivots.

This enables the moving surfaces to bear on the screw heads when the usual mounting screws are tightened, taking the play out of the pivot, and imparting a stiff action to the movement. Also, the height of each of these screws can be adjusted individually to set the sight exactly at right angles to its mounting. Like this:-

For steel hinges, use brass bearing screws if available. (Use metal thread screws or self tappers for a metal or plastic base (see ‘fitting sights to a commercial level’). For brass hinges, use steel screws - wood, metal thread, or self tapping, as appropriate. After the hinges are mounted, the bearing force can be varied by adjusting the tension on the mounting screws; if wear of the bearing screw heads or hinge pivots occurs with use, simply tighten the mounting screws. If side play in the hinges is excessive, either discard the hinges or try removing the play by striking the gap between the pivot joints with a centre punch.


4. Washer Sights. When making v-notch sights from thicker metal, such as found in hinges, it is hard to file a notch with a sharp point, or to cut a clean slot with a hacksaw. An alternative is to cut a gap in one side of a hinge, and mount two washers side by side to form a double notch, like this:-



The washers need to be large; a good size is 6 mm. (1/4 in.) panel washers, having an outside diameter of 25 - 30 mm.. They can be fastened with 5 mm. (3/16 in.) countersunk head machine screws; although the screws are a size too small, the heads pull into the washer holes and hold them firmly. Shutter bars may be pivoted on one of the washer holding screws, to make a backsight pinhole. They must be large enough to cover the gap between the washers, and have a 3 mm. (1/8 in.) hole, which is positioned over either the upper or lower notch to form the pinhole.

Turning Sights are used in some instruments; these are screwed to separate wood blocks, fastened to the instrument by a single machine screw and wing nut. The slit or notch of the sight must be exactly in line with the axis of the screw, so that when the sight is turned it doesn’t move sideways in any direction. Like this:-



Tall Sights are long slit sights, which give a large field of view at right angles to the sight line, making it a wide angle plane rather than a single line. Mounted vertically, they are useful for making horizontal layouts and measurements in hilly or rough country, where the height of the target may vary, and often avoids the need for range poles, which can be a source of error. Being large, they must be of the folding type.
Mounted horizontally, they can be used to make wide angle level or height measurements, but there is a risk of them sagging under these conditions, and short slit and crosshair sights are preferred. (See ‘Unidirectional Sights’).
In these notes tall sights are only fitted to the Crevel, but they could be fitted to a builders’ level, or the adjustable surveyors’ cross.
Tall sights with vane pattern No. 1 were used in the 18th and 19th centuries - two tall vanes mounted side by side with a slit between them, and with several cut-outs so the observer could see to line up on the target. Then the slit was used to make the actual sighting. Like this:-



The ones used here have two plates mounted side by side, but staggered. An ideal slit sight allows the slit width to be changed for different light levels when used as a backsight, and for a variable width of view of the target when used as a foresight, and staggering the plates lets this be done by simply turning the sights around their axis., like this:- .

When used as a foresight, the vanes are turned edge on to the sight line, and the target pattern is seen through a gap formed by two thin lines; the target point is then aligned central in the gap. When the foresight vanes are turned edge on, stray light reflection from their faces can cause sighting errors, so the vanes MUST be painted flat black.

A variation on this pattern is to increase the gap at the top of the vanes, like this:-

This form is the most versatile of the three patterns. It provides a backsight slit which can be varied in width at any height by simply turning the sight, and a narrow V notch foresight with a variable height tip, and very little obstruction of the field of view, again by turning the sight.
A possible problem with patterns 2 and 3 is that when taking a backsight, each sight must be turned about 90 º. This contradicts the idea that the instrument should not be touched between taking fore and back sights in case the instrument is knocked out of line. If you need to take many backsights, it might be better to stick to vane pattern no. 1, which doesn’t need turning.


Aligning open sights. As shown in the sketches above, the mounting foot of each sight has three screw holes. When aligning any of the sighting instruments, the sights are first mounted with only one screw in each foot. Each sight can then be tapped a little from side to side, to bring the whole instrument into line. Like this:-



Unidirectional Sights with slits, crosshairs or scales, are different for each instrument, and so are shown with the instrument descriptions in chapters 4, 5 and 6.



 Paint and Varnish. All wood surfaces should be protected to prevent water absorption, which can cause warping and throw the instrument out of alignment. This is particularly true of the floating dumpy levels, which use softwood floats. Also, any surface which can be seen when looking through a backsight should be flat black to stop stray light reflections as much as possible .
Where parts are made from tin cans, these should be protected from rust with a coat of - preferably - oil paint Varnish or oil paint are good for surfaces exposed to moisture, and oil paint is preferred for black surfaces. If these are not available, several coats of water based paint are better than nothing.



Bubble Level Indicators. To set up an instrument in a horizontal plane, level indication in two directions is needed, and a good way is to use bubble level indicators. These are available commercially in two types - a circular bubble housing, and one with two straight bubble phials set at right angles. Both have holes for screwing on a flat surface. . Like this:-


The double type is preferred; it uses ordinary bubble phials, which can sometimes be bought separately if you break one, it gives separate indication in two directions, and is sometimes more sensitive than the circular type. Plastic pocket levels are now so cheap that it may cost less to buy one, cut it up, and drill holes for it to be screwed flat on the instrument. The accuracy doesn’t matter - this can be corrected by setting up the instrument in each direction in turn, using end for end tests as in the making instructions, and packing under the mounting screws to bring the bubble to the middle. Test the sensitivity by standing the pocket level and a good level side by side on a board, and tilt it; the bubble in the cheap level should move just as far as the good one. The phials in some cheap levels, with small diameter bubbles, are sometimes sensitive in one direction only.

A modern builders’ level will work in any horizontal position; it will indicate level when stood on edge, and also when laid flat; how accurate it will be in this position depends on how the phial is set in the frame, but tests of some good levels show they are correct. This fact is used in the Crevel, where the same bubble is used to show level with the instrument in both the horizontal and vertical positions, and in the Adjustable Cross, where a builders’ level is placed on its side.

The bubbles in professional surveying instruments are more sensitive than those in builders’ levels - they move further for the same change in level. An easy way to improve an ordinary indicator is to wrap two small pieces of black plastic tape round it, spaced exactly the bubble length apart, and if there is a gap under the phial, to either paint the under surface white, or cover it with white tape; it is easier to see small changes in bubble movement. Tests showed an improvement of up to 7 times in the accuracy of an instrument when the tape is used.




Part 2 shows simple contact instruments,as would be used for everyday building tasks.