3 Fitting
MEASURING, MARKING-OUT AND TESTING
Accuracy is the word that comes to mind when we mention engineering. On bench work this accuracy is ensured by careful measuring, marking-out and testing.
The Rule. The most common measuring tool is the steel rule. These are available in lengths from 4 inches up to 6 feet. They are divided into 1/8 ", 1/16 ", 1/32 ", and 1/34 ", or if required, in 1/10 " and 1/100 ".
Scribers. The scriber (fig. 1) is the marking-out tool most used with the steel rule. It is made of hardened and tempered carbon steel. It should be ground to a point in such a way that the lines of the grindstone are in line with the axis as shown, otherwise the tip will break off more easily.
To ensure that scribed lines show clearly it is usual to clean the metal and then colour it with copper sulphate or some proprietary marking-out fluid. Castings can be rubbed with white chalk or given a coat of whitewash.
Dot Punch. Some engineers like to dot punch the lines after marking-out so that the scribed lines are not "lost" when further work is being done on the workpiece. Dot punching is done with a sharp centre punch (fig. 2). The dot punch mark should be heavy enough to be seen, but not so large that it disfigures the work.
Centre Punch. Before a hole is drilled the exact centre must be centre punched (fig. 2).
Dividers. Spring dividers, trammels, odd-leg calipers (also known as jenny calipers or hermaphrodite calipers) are used for marking-out (fig. 3).
Calipers. Calipers are used to facilitate measuring, both for outside work such as checking the diameter of a rod, or for internal work, e.g. checking the diameter of a bored hole. These are shown in figure 4. Both spring type and firm joint calipers are available for internal and external use. Spring calipers are used in schools more than the firm joint kind because they are easier for a beginner.
Firm joint calipers are sometimes known as "tap" calipers because many engineers tap them to open and close them (fig. 4). By tapping a very fine degree of accuracy is obtainable.
Try Squares or Engineers' Square. Testing or "trying" a right angle is done with a try square. These are precision tools and should never be dropped or otherwise misused. They are available in many sizes from those with a 2" blade up to very large ones with 36" blades. Various qualities are obtainable: a good one for workshop use has a 5" hardened and tempered blade and a case-hardened stock. Figure 5 shows one method of using a try square. It is important that the stock be kept firmly pressed against the work.
Combination Set. A combination set is shown in figure 6. It comprises a rule which is usually 12" long, a centre head or centre square which can be used to find the centre of circular work, a square head with which angles of 900 and 450 can be set out, and a protractor head for obtaining angles which can be read direct from the scale.
Surface Plate (fig. 7). Surface plates provide an accurate flat surface from which to mark out work. They are made of fine grained cast iron with a thick top and heavy ribbing underneath to resist distortion. They stand on three legs for stability. The surface is planed and on the best plates it is hand scraped. A wooden cover into which a piece of thin felt is stuck should be used for protection when the plate is not in use. The felt inside the cover should be soaked in oil to prevent rust on the plate.
Surface Gauge or Scribing Block. The surface gauge is used on the surface plate for marking-out lines parallel to the surface of the plate (fig. 8). The height of the point is set against a rule which is stood vertically on the plate or against the rule of the combination set held upright in the square head. The fine adjusting screw allows the point to be raised or lowered fractional amounts. The base of the scribing block is vee-shaped so that it can rest on round sections. Often there are two small pins which can be pushed to protrude below the bottom surface. These are used in special circumstances such as in figure 8a.
Vee-Blocks. Vee-blocks are useful for holding round work for marking-out on the surface plate, as shown in figure 9.
Vee-blocks are made in pairs and each block of the pair is stamped with the same number. They should always be used as a pair unless only one block is required. The larger blocks are usually made of cast iron, but the smaller, more accurate, vee-blocks are made of mild steel, case hardened and accurately ground. These are made with grooves along the side for clamps which secure the work as shown in figure 9.
Angle Plates. Angle plates are made from fine grained cast iron. They are available in a large variety of sizes and qualities. Small ones are about 3" long and the largest ones take two men to lift them. They are made either machined all over or with webbed ends (fig. 10). Large irregular shaped work pieces can be clamped against them for marking-out. The ends of the angle plates are machined so they can be stood on end if required.
Micrometer Caliper. This is an almost indispensable precision measuring instrument in the Mctalwork Room. They are available in sizes ranging from 0 " to 1", 1" to 2", 2" to 3" and so on. Figure 11 shows a 0" to 1" micrometer.
In principle they are simple. One end of the spindle is threaded and this screws into the sleeve. The screw has 40 threads to the inch which means that each revolution of the spindle opens or closes the gap between the anvil and the spindle face 1/25", or 0-025". The spindle and the barrel move together. The barrel, which moves over the sleeve, is divided into 25 parts so that each division represents ^ of a turn or o-oo 1". The sleeve is marked off and numbered in tenths of an inch: 1, 2, 3 and so on up to 10. Each one of these tenths is divided into 4, i.e. 1/40" or 0.025". When the micrometer is closed, that is when the spindle and anvil are in contact, the zero on the thimble will be coincident with the zero on the datum line. The micrometer in figure 11 is open to 0.354".
Largest number on sleeve 3 = 0.3
Subdivisions visible 2 = 0.050
Lines on thimble counted
from 0-4 = 0.004
_____
Reading 0.354
Figure 11A shows a reading of 0.539.
Largest number on sleeve 5 = 0.5
Subdivisions visible 1 = 0.025
Lines on thimble 14 = 0.014
_____
Reading 0.539
The ratchet at the top is provided so that the same pressure will be applied whoever uses the instrument. However, most engineers know when they are exerting the right pressure by "feel" so that they seldom use the ratchet. The locknut sets the instrument at a given reading, but this can cause damage if used by beginners because so often they try to force it over the work being measured. The locknut is useful when large numbers of components of similar size are being checked. Figure 12 shows the best way of holding the micrometer. It takes a little practice to learn the knack of holding the micrometer in one hand, but it is invaluable because it leaves the other hand free to hold the component to be measured.
Vernier Caliper Gauges. These are precision instruments used for taking internal and external measurements to within o-oo 1". Figure 13 shows a vernier caliper. For normal workshop use they are 12 inches long, although much larger ones are available.
For less precise work smaller verniers are made which read to 1/100". Reading the measurement on these is a little easier than those which measure to o-oo1", but the principle is the same.
Nine divisions on the fixed scale equals 0-9". On the vernier or sliding scale the 0.9" is divided into 10 spaces. 0.9 ÷ 10 = 0.09". Therefore each division on the vernier or sliding scale is 0.01" shorter than each division on the fixed scale. Figure 14A shows this. At B a reading of 1.17" is shown.
The verniers which read to 1/1000" have a main scale which is either divided into 1/20 " or 1/40 ". Figure 15 shows a vernier with a main scale divided into 1/20- On this 49 divisions on the main scale are matched by 50 divisions on the sliding scale. Forty-nine divisions on the fixed scale is 49 x 0.-050 = 2.450". On the vernier or sliding scale the 2.450" is divided into 50 spaces. 2.450 ÷ 50 = 0.049", therefore each division on the sliding scale is 0.001" shorter than each division on the main scale. A reading of 1.332" is shown.
On the vernier which has a main scale divided into 1/40" 25 divisions on the sliding scale are matched by 49 divisions on the fixed scale. Forty-nine divisions on the fixed scale is 49 x 0.025 = 1.225"- On the vernier or sliding scale the 1.225 's divided into 25 spaces: 1.225 ÷ 25 = 0.049". Two divisions on the fixed scale = 0.050" so each division on the vernier is 0.001" shorter than each pair of divisions on the fixed scale.
To read the vernier to 1/1000" it is helpful to use a magnifying glass to check the coincidence of the line on the fixed scale with the one on the sliding scale.
Screw Pitch Gauge. This is used for checking the pitch of a thread. It is also useful for checking the radius at the crest and the root of the threads when screwcutting on the lathe. The pitch of the thread and its depth is clearly marked on each blade and they are available in the standard thread angles, i.e. B.A. 47 1/2 °, BSF and WHIT 55°, Metric 6o°. Figure 16 shows a typical screw pitch gauge. It has 14 blades.
Radius or Fillet Gauge. Radius gauges are used to check radii on internal and external work. These are bought in sets and are available in decimal, fractional or metric radii. Figure 17 shows a radius gauge which has 20 blades.
Feeler Gauge. These are in sets as shown in figure 18. The thickness of the blades ranges from 0.0015" to 0.025". They are used for checking narrow gaps.
Thread Angle Gauge. These are used for checking the angle when grinding a tool for screw cutting and for setting the tool in correct relationship to the axis of the work on the lathe (fig.19).
Bench Work
The bench vice is the important holding device at which most of the bench work is done. Usually each pupil in the metalwork room has a vice place.
Engineers parallel jaw vices (fig. 20) are usually made of cast iron with inset steel jaws. These may or may not have a quick release mechanism. This is for quick opening and closing of the vice and it is operated by a lever which lifts and lowers a half nut which engages on a buttress thread. Heavy hammering or bending should be done on the leg vice (Chapter 6, fig. 3) not on the bench vice. The roughened surface of the jaws is sometimes covered with vice clamps to protect the work.
These are made of some soft metal such as copper, aluminium or lead.
Hand Vices (fig. 21). These are useful for holding thin pieces of metal when using the drilling machine. Hand vices are made of drop forged steel. The jaws are roughened to improve the grip, but for this reason it is advisable to use two pieces of thin cardboard as temporary vice clamps to prevent damage when holding non-ferrous sheet metal.
Tool Makers Clamps. As the name suggest, these are much used by toolmakers who often make their own. Many tool-makers prefer the kind shown in figure 21A to those at B. because they have one jaw uninterrupted by the head of a screw. This plain jaw can be rested on a flat surface such as a drilling machine table. When gripping work the jaws should be adjusted so that they are parallel.
Toolmakers clamps are made of case hardened mild steel. It is a mistake to make these from small square section stock because the strength is required in one direction which is better obtained using oblong section stock.
Pin Vices (fig. 22). They are used for holding small work. The handle is hollow to allow long lengths of metal to be held. Pin vices will hold work up to 1/16 " diameter.
Pliers (fig. 23). Some of these can be used as holding devices, although many pliers, such as side cutting and round-nosed, have other uses.
Hack Saws (fig. 24). The illustration shows a tubular frame hack saw, but there are other types available. In principle they are all similar. There is always provision for adjusting the frame to suit the length of blade, and for turning it axially through 900 as shown. The blade is always held in tension.
The adjustment for length on the saw shown is provided by the tubular frame which slides through the top of the handle and is locked in place by a knurled set screw.
The blades are usually 10" or 12" long, 1/2 " deep and 0.025" thick. The number of teeth varies between 32 per inch to 18 per inch. Note the length of a hacksaw blade is measured from the outside edges of the holes. These are made of either high speed steel, which is more expensive but will last longer and cut harder metals, or of low tungsten steel. Low tungsten steel blades are made either as "flexible", which have teeth hardened but the back soft and are very tough, or "all hard" type blades, which are hard throughout. These are preferred by skilled men because they are more rigid.
When choosing a blade it is important to select one which will have a minimum of 3 teeth always in contact with the work (fig. 24A) otherwise "chatter" will result and teeth will start to break off. When inserting a blade in the hacksaw frame, the teeth must always point forward, i.e. away from the operator. Remember to use the full length of the blade when hacksawing and to take about one second for each cut, i.e. don't try to cut too quickly because this often causes the blade to twist or brake and results in bad work.
Junior Hacksaws. These are useful for smaller work (fig. 24B). The blade, which is 6" long, is held in tension by the spring of the frame itself. The blades have a small pin inserted at each end by which they are held in the frame.
Piercing Saws (fig. 24C). These are for very fine work such as that done by jewellers and silversmiths, but they are sometimes useful in the school workshop for piercing out motifs in sheet metal.
For piercing the saw should be used in the upright position and the work held by hand on a bench pin which is made of wood as shown. The teeth of the blade should point downwards, i.e. towards the handle.
"Abrafile" Tension File. This is really a file but it is used as a saw. The blade which is like a thin round file is held taut in the frame (fig. 24D).
Tension files are available in lengths from 6" to 11 "'approximately fa" diameter. Special links are made for using these blades in a hacksaw frame.
Power Hacksaws (fig. 25). These are used in workshops where a lot of sawing is necessary. The machine might be a small bench model with a jaw capacity of 2" by 2", or a heavy duty type with a capacity of 10" by 10". Special large blades are made for these machines.
Chisels. These are usually called cold chisels because they are used for cutting cold work. The flat chisel is the most used.
Cold chisels can be made from hardened and tempered carbon steel or bought made from "non-temper chrome alloy steel". These are extremely tough and yet can be sharpened with a file. Flat carbon steel chisels should be ground as in figure 26. The lines left by the grinding wheel should be as shown, i.e. they should be in line with the axis thus helping to prevent the extreme edge from breaking off. The cutting edge is ground in a slight arc so that when cutting the main thrust is taken in the centre. The top of the chisel which is soft will become "mushroomed" with continued use unless ground off from time to time. If this mushroom is not ground off it can be dangerous because a glancing blow from the hammer might cause a piece to fly off.
Flat chisels arc useful for shearing thin stock as shown in figure 26A, or for cutting out sheet metal on a chipping block as shown at B.
Half round chisels are often used for cutting oil grooves and "cleaning up" corners as shown at C.
Cross cut or cape chisels are used where a narrow groove is required such as a key-way (D).
The diamond chisel can be used for cutting into sharp internal corners. A typical example is shown at E.
Hammers. The engineer's ball pein hammer is the type most used in the metalwork room. The heads are made from hardened and tempered carbon steel. Popular weights in the workshop are J, § and 1 pound. The handle is of hickory or ash. The hammer head is made fast on the handle by a wood wedge which is often firmly held in place by a metal wedge. The section in figure 26F shows how the hole is tapered both ways.
Files. The file is the most used hand cutting tool in the engineering workshop. It is made of hardened and tempered high carbon steel but the tang is left soft. The length of a file is measured from the tip to the shoulder and does not include the tang. Files are available from 4" to 16" long in a variety of sections and are available either single or double cut. The grades of cut range through: bastard, second cut, smooth and dead smooths
To prevent clogging of the teeth (pinning) it is best to cut soft metals with the coarser grade of files. Special files known as "Dreadnought" and "Millenicut" are used when a lot of filing on soft metal is done* Non-ferrous metals require new files for best results. The files can afterwards be used on steel. File handles are bought separately and various sizes are available. To prevent splitting, a steel ferrule is fitted. It is usual to fit these by heating the tip of the file tang to red heat and then forcing it into the handle. Care must be taken to keep the file handle in line with the file. Figure 27 shows some of the more common kinds of file.
Flat files can be used on a wide variety of work. These are available in lengths from 4" to 16" in grades of cut: bastard, second cut, smooth and dead smooth.
Hand files are available from 4" to 16" long in all grades of cut. This file is parallel in width but it tapers in thickness towards the end. One edge has no teeth (safe) so that it can be used against shoulders or in corners where another type of file might cause damage.
Round files are 4" to 16" long, available in all cuts. They are often used to enlarge holes and to file radii. The smaller sizes are sometimes known as rat tail files.
Half round files are 4" to 16" long, available in all cuts. Used for filing concave surfaces and inside large holes.
Three square files are 4" to 16" long, are available in all cuts, and are used for filing sharp corners and vees.
Square files are 4" to 16" long, available in all cuts. Often used for fifing drilled holes square and for filing slots.
Warding files are 4" to 8" long, available in all cuts. They are parallel in thickness but taper in width. They are useful for filing narrow slots such as are found on keys and inside locks.
Knife files are 4" to 12" long, available in all cuts, and are used for filing into sharp corners etc.
Needle or Swiss files are for fine work. These vary in length from 4!" to 7". They do not need a handle: the shaft is knurled to provide a grip. Finer cuts than smooth and dead smooth are made. A large number of shapes are available, one of which is shown (fig. 28).
Rifflers are used by engravers, die makers and silversmiths. They are usually double ended and scores of different shapes are available. One of these is shown (fig. 29).
Filing. Before commencing to use a file, make sure it is clean. This is best done by pushing a file pricker (fig. 30) a number of times across the teeth until it takes up the shape of the teeth and removes all the dirt and metal particles. The file pricker can be made from a thin piece of mild steel filed to a chisel end. Some people use a file card for this, but continued rubbing of the hard steel bristles on a file tends to spoil the edge of a file particularly when this is done almost as a habit in the school workshop.
The workpiece should be held securely and as low as possible in the vice and the edge to be filed held horizontally. The work should be at a comfortable height, but of course it.is not possible to make the height of the vice suitable for all people. A right handed person should stand at the vice with feet apart—left foot foremost—more or less in the stance of a boxer. The file handle is held in the right hand with the right elbow close to the body. The tip of the file is held with the left hand. The weight of the body should supply the fo'ree and weight should be transferred from the right foot to the left on the forward stroke. It must be remembered that a file cuts only on the forward stroke.
On the return stroke the pressure is taken off the file, but it is not lifted from the work. Use the full length of the file and don't try to rush. The aim usually is to prevent rocking and to obtain a flat surface. For heavy filing more weight is put on the end of the file and the file is pushed diagonally across the work, first from one side and then from the other, thus removing the crests of the ridges made by the previous stroke. For light filing the tip is held lightly between the thumb and forefinger of the left hand.
Drawfiling (Fig. 31) is a finishing process and if done properly gives a smooth flat surface. Chalk rubbed on the file helps to prevent pinning (the clogging of the teeth by small particles of metal) and gives a better finish to the work. Any oil or grease on the surface being filed makes the file cut less efficiently. Even perspiration from the fingers, when taking the last fine cuts, tends to make the file skid. For this reason avoid, as far as possible, touching the surface being filed. As the work nears completion it should be taken out of the vice and checked frequently with a try square or a rule, or both.
If a lot of metal has to be removed by filing, it is an aid to make criss-cross cuts in the metal first (fig. 32) either with a hacksaw or the edge of a file. Another method is to rough chamfer the work first as shown in figure 32A and then to file flat as at B.
Care of files.
1. Don't knock or rub files together. Remember they are hard and will damage each other.
2. Keep them in a rack slightly apart from each other.
3.Don't attempt to file hardened steel.
4.Avoid using a new file on the hard sandy skin of a casting— old files will do for this.
5.Don't throw away old files, they make good scrapers and hand turning and wood turning tools, but must be properly tempered otherwise they might break and cause injury to the user.
Scraping. Scrapers are made from hardened and tempered carbon steel and the edges are sharpened, after grinding, on a fine oil stone. Three scrapers are shown in figure 33. These are used for removing small amounts of metal usually when working on something which has to be accurate, e.g. when making a surface plate or scraping a bearing.
When scraping a flat surface it is usual to lightly coat a master flat surface, such as a good surface plate, with a thin coat of engineers blue. The work is then rubbed on the master surface or the master surface rubbed on the work, and any high spots which show blue scraped off. This is repeated as often as required until all the high spots are removed. Scraping, however, is a slow and therefore expensive process which has been largely superseded by surface grinding.
Taps and Tapping. Taps are used for making internal threads They are available in sets of three (fig. 34) made from high speed steel or carbon steel.
Before tapping a suitable hole must be drilled which is usually a little larger than the core diameter of the tap. (See chart)
Start tapping with the taper tap which is held in either a bar or a chuck type tap wrench (figs. 34A and 34B). It is important to keep the tap in alignment with the hole and once the tap has started to cut it is advisable to reverse it a fraction of a turn periodically to break the chips. This tap is then followed by the second tap and then by the plug. If a shallow open ended hole is being tapped the taper tap alone will do the work. When tapping a blind hole the plug tap must be used finally and the chips knocked out of the hole from time to time. Taps are brittle and will break off in the hole if too much pressure is applied. To reduce the friction when tapping it is advisable to use a lubricant on all metals except brass and cast iron.
LUBRICANTS FOR THREADING
Material to be threaded Lubricant Aluminium Paraffin Brass None Bronze and copper Paraffin or lard oil Cast iron None Mild steel Sulphur base oil
Broken Taps. A lot of time can be wasted in trying to remove a broken tap from a piece of work. Often it is quicker to discard the part and start again. However, there are several methods used for the removal of broken taps.
1. Use a tap extractor. These work better on the larger sizes. They have three or four prongs, according to the kind of tap being removed, which fit into the flutes of the broken tap.
2.Break out the tap using a small chisel and a hammer.
3.On an open ended hole it is sometimes possible to drive the tap out with a punch from the back.
4.If a carbon steel tap is broken in the hole it is possible to heat the tap and the surrounding metal with a blow pipe flame to cherry red and allow to cool slowly then centre punch and drill out. This is not possible with high speed steel taps because the temperature required is too high to be obtained with the blowpipe.
5.If the tap is broken in a blind hole, build a small wall of plasticine about half an inch high all round the broken tap and carefully pour in concentrated nitric acid. This will etch away a little of the workpiece as well as part of the tap. Wash away every trace of the acid and unscrew the broken tap with a hammer and a small punch.
After a tap has been removed by any of these methods it is often necessary to drill and tap a size larger.
Dieing. Stocks and dies arc used for cutting external threads. These are shown in figure 35. The dies are made from hardened and tempered steel and may be of the circular split die pattern or loose dies.
The circular split die is the one popularly used in school. There are three adjusting screws but the adjustment is only slight. The middle screw, which is pointed, forces the split open and the two side screws are for closing the die.
When starting to cut an external thread the die should be "open", i.e. with the centre screw tight; also the side screws must be screwed down to prevent any tendency lor the die to twist. The end of the rod to be threaded should be slightly tapered to allow the die to start. The taper can'be filed. Use a lubricant as for tapping and keep the die "square" to the rod. Reverse the die a little from time to time to break the chips. After the first cut try the thread in the tapped hole. If it is too tight take another cut with the dies after slackening the centre screw and tightening the outside ones.
Riveting. Rivets are mentioned in the chapter onjoining metals and the more common kinds are shown there.
Riveting in school requires a ball pcin hammer, a dolly and a set and snap (fig. 36). Before riveting the parts must be drilled with the right size drill. It is important that the rivet be a good fit in the hole otherwise it might bend when being hammered.
Countersunk-head rivets are often used in school work because they can be filed flush and make a clean job. When using a countersunk rivet, support the head on a flat surface such as the face of the anvil (fig. 36A). The length of the rivet depends on the depth of the countersink, but usually if the amount left for riveting is equal in length to the diameter of the rivet an adequate head can be formed leaving enough to clean off with a file (fig. 36B).
Snaphead rivets are supported underneath with a rivet snap held in a vice, which acts as a dolly (fig. 36 (1)). First'the two pieces are set down to make sure the p lates are properly together. This operation is important on large sheets which might be distorted. The head is first formed with a ball pein hammer (fig. 36 (2)) and finished off with the snap (fig. 36 (3)). The amount of rivet above the hole required for forming a snap head is usually equal to slightly more than the diameter of the rivet.
For special riveting jobs such as we meet when making model steam engines it is often necessary to make up special dollies.
Spanners and Screwdrivers (fig. 37). Nuts and bolts are loosened or tightened with a spanner. These are made to suit the various sizes of nuts and bolts available. It is important to use the right size spanner otherwise the nut or bolt head soon becomes badly worn at the corners. When using an adjustable spanner be sure that it is properly tight. Many kinds of spanners are available but the most popular ones are those shown
Screwdrivers should fit the slot of the screw reasonably well. The sides should be ground so that they are almost parallel otherwise they tend to lift out of the slot when twisted.
Screw Threads. Screw threads are standardised but there are many standards made by different countries. Figure 38 shows those most used in our workshops.
British Standard Whitworth (B.S. W.) The BSW thread form was devised by Sir Joseph Whitworth and first used in 1841. This thread has been widely used since it was first introduced, but as engineering work has become more and more varied other standard threads have been introduced such as British Standard Fine and British Standard Pipe.
British Standard Fine {B.S.F.) and British Standard Pipe {B.S.P.). These both use the Whitworth thread form but have a finer pitch. In the B.S.F. thread this fine pitch gives a larger core and thus more strength, except that the thread is a little weaker. But the nut is less likely to be loosened by vibration. For pipe work the thread needs to be fine otherwise it would break through the wall of the pipe.
British Association (B.A.). This is used often on threads below a quarter of an inch diameter. These threads have a fine pitch
(metric) and the sizes are closely graded starting from O.B.A. which is the largest (see table at back). B.A. threads are used for delicate work.
Square Thread. This is used for transmitting motion in cither direction such as on the cross slide of a lathe. There is less friction on this kind of thread than on the vee threads.
Acme Thread. This is often used on the lead screws of lathes because it is easier to engage the split nuts on this slightly vee'd thread than on a square thread. It transmits motion in both directions and is easier to cut than the square thread.
Buttress Thread. Wherever the thrust is required in one direction, as on some screw jacks, this thread is used. It combines the easy transmission qualities of the square thread with the strength of the vee thread.
Terms Used for Threads
Pitch. The pitch of a thread is the distance between two threads measured axially; for convenience, it is the distance between the crests of adjacent threads.
Lead. The lead of a screw is the distance it advances in the nut in one revolution. This is only the same as the pitch on single start threads, but on a double start thread it moves twice the pitch and on a treble start thread it moves three times and so on. It is possible to see how many starts a thread has by looking at the end of the bolt. A double start thread has two threads of the same depth running round the cylinder and a three start thread, three. Multi-start threads, as these arc called, are used on machinery where quick axial movement of the thread is required.
Core Diameter. The core diameter of a thread is the outside diameter minus twice the depth of the thread. It must be measured at right angles to the axis.
Outside Diameter. The outside diameter is the diameter measured over the crests of the threads. Screws, bolts etc. are known by this dimension, e.g. a 1/2" Whitworth bolt has an outside diameter
of 1/2 ".
