Hand tools

Guidelines for tool design In any area with so broad a background and so long a history as that of the design of hand tools it is difficult to make any dramatic changes simply by the application of the expertise of human engineering. Designs that have stood the test of millennia tend to be good. However, while the broad principles of tool design have been handed down from generation to generation, not all designs have survived without occasional loss of the subtleties that distinguish the good from the bad. Nor is it necessarily true that no improvements can be made in even the simplest of tools; and furthermore, with the development of power tool technology much can yet be done to ensure that the interface between the tool and the user is optimal. General requirements Essentially a tool comprises a head and a handle, with sometimes a shaft, or in the case of a power tool, a body. It may be difficult to determine where the junctions of head, shaft, and handle actually occur. In a double-ended wrench or spanner, one head and a portion of the shaft act as the handle for the other. In the case of a hammer the handle and shaft are continuous, while in the case of the screwdriver the same applies to the head and shaft.A tool requires some motive power which, in the simple hand tool, is supplied by the musculature of the user, while in the caes of the power tool it is provided by a motor mounted in the body or handle, or from some external source. In every case, however, there is some form of handle and it is at the handle that the greater portion of the human interface of any tool is found. Thus the handle is of prime significance as the means whereby human input is applied to the system as a motive force, or guiding and stabilizing force, or some combination. In a power tool the

controls and the means of mounting the head are also part of the interface.In other more complex person-machine systems the information is also the site for the provision of information on the state of the system. That information is usually provided in the form of visual displays. In a person-tool system, however, most of the feedback, other than tactile and kinaesthetic (which are indeed important sources), occurs by direct visual monitoring of the result. Formal visual displays are almost non-existent except in such tools as the torque-wrench, devices for measurement, and a few special purpose displays. Some of these requirements will be discussed later. In the meantime consideration will be given to the requirements for handles. G rasp

To understand handles it is first necessary to understand grasp. Several approaches have been made towards defining the characteristics of grasp. Some of these have been purpose oriented, and some have been function oriented. A simple and yet embracing viewpoint has, however, been outlined by Napier (1956) who defines the prehensile movements of the human hand in terms of a power grip and a precision grip. Each grip has different functional characteristics but virtuallly all manual activities, excluding the hook grip which will be discussed later, can be classified in terms of their requirement for a precision or power grip, either separately or in combination.In a power grip the object is held in a clamp formed by the partly flexed fingers and the palm, with counter pressure being applied by the thumb lying more or less in the plane of the palm. Such a grip is found for example in holding a heavy hammer. In a precision grip the object is pinched between the flexor aspects of the fingers and the opposing thumb, as for example in tapping with a light hammer. In fact the position of the thumb and forefinger in relation to the handle determines the relative amount of power versus precision. For precise tapping movements with a light hammer the thumb will be aligned along the handle and the forefinger separated from the others such that the hammer is held in a triangle comprising thumb, forefinger, and middle finger. For a heavy force with a large hammer the fingers are curled around the handle with the thumb giving additional support in a firm power grip. The posture of the thumb and fingers in a precision grip ensures that the sensory surfaces of these digits are used to the best advantage in providing the greatest opportunity for delicate adjustments of grip in response to sensory feedback.Napier outlines other factors which influence the posture of the hand during function. O f these, the shape influences the grip only in so far as the eventual use of the object will determine how it is going to be held. Thus, other things being equal, it may be more convenient to hold

a cylinder in a power grip and a ball shape in a precision grip, but if the cylinder-shaped object is going to be used for a precision purpose, for example a chipping hammer, it will be held in a precision grip, while if the ball-shaped object is going to be used for forceful activity, for example the front handle of a power sander, it will be held in a power grip.From the point of view of design and operation it is difficult to combine a grasp function with a control function. This indeed can be done, as witness the trigger or lever of many power tools. Greenberg and Chaffin (1976) however, cite one study of a sander with a near-dome shaped handle on its top. The handle of course is used for directing the sander. Downward pressure of the dome, however, also activates the sander. The fingers, therefore, not only are required for grasping but also have to exert counter pressure upwards to allow the palm to compress the dome downwards. This action, which requires prolonged and intensive activity of the small muscles of the hand, is of course intrinsically more fatiguing than a simple grasping action. Such complex requirements should be avoided.It must also be recognized that while two varieties of grip may be defined for the sake of analysis, the two types of prehensile activity, namely precision and power, are not of course mutually exclusive. One or other may dominate in a given action, but one may yield to the other during the course of an activity, and consequently the design of a tool may have to meet the needs of both. Thus in driving a screw the initial activity is one of precision but as the screw becomes set the requirement for power becomes dominant and the grip changes.The relationship of the hand to the forearm shows differences between the two grips. In the precision grip the wrist is dorsiflexed and stabilized while the tool lies in the axis of what would be the extended forefinger. In the power grip the hand is deviated towards the ulnar side and the wrist is held in the neutral position between full extension and full flexion.As noted earlier, another form of grip, namely the hook grip, is found where there is no requirement for precision but where something heavy, such as a power tool, needs to be carried. In the hook grip the object is suspended from the flexed fingers, with or without support from the thumb. Since this grip can be maintained for more prolonged periods than a power grip, heavy tools should be designed in such a manner that they can be so carried. H an dedness

Consideration of the requirements of grasp leads to consideration of the problem of handedness. For a single-handed activity the vast majority of people have a hand preference, some 92 per cent favouring the right hand. A few are completely ambidextrous and all can learn to function adequately with either hand, although few will have the strength and

dexterity of their favoured hand in the less favoured hand, even with training.While the number of persons who are clearly left-handed is relatively small, their requirements should be borne in mind wherever possible. The fitting of handles to tools should make the tool applicable to both left and right-handed persons, for example, in the position of controls in a power tool, unless it is clearly inefficient to do so. It might be noted that the driving of screws and fasteners utilizes the powerful supinating movement in a right-handed person and the less powerful pronating movement in a left-handed person. This limitation has to be accepted since the provision of left-handed threads is not a feasible solution for the purpose. H a n d strength

Hand strength has been the subject of several surveys, although largely confined to somewhat selected populations. The studies quoted by Damon, et a l (1966) refer to United States military and civilian workers and have a mean value for hand strength ranging from 41.9 to 59.8 kg (94 to 134 lb) for males and, in another population group including British and United States workers, 24.5 to 33.0 kg (55 to 74 lb) for females. Other studies, quoted by Greenberg and Chaffin (1976), suggest an average grip strength of 43.3 kg (110 lbs) for men. S h a p e o f handle

The shape should conform to the natural holding position of the hand. In the resting stage, the right hand of a right-handed person holding a tool in such a manner as to meet the requirements of both precision and power will be held more than half-supinated, with the wrist abducted about 15 degrees and slightly dorsiflexed, the little finger in almost full flexion, the others less so, the first finger less than half flexed, and the thumb adducted and slightly flexed. The combination of adduction and dorsiflexion at the wrist with varying flexion of the fingers and thumb generates an angle of about 78 degrees between the long axis of the arm and a line passing through the centre point of the loop created by the thumb and first finger, that is, the transverse axis of the fist (Figure 10.1). While the wrist is being used, of course, that angle will not be continuously maintained. For example, in hammering, the wrist will move from full adduction to full abduction; on the other hand during the operation of a power drill the angle will very largely be maintained continuously.In general, the shape of a handle corresponds basically to that of a cylinder, or a truncated cone, or occasionally a sector of a sphere, although the basic shape may undergo flattening, or other curves may be

superimposed upon it. Because of its attachment to the body of a tool, a handle may also take the form of a stirrup, a T-shape, or an L-shape, but the portion that is held by the hand will commonly be in the form of a cylinder or cone.While a cylindrical form is the basic shape for most handles, a true cylinder is indeed not the desirable shape, except where a handle is intended virtually solely as a hook grip for carrying. Instead, the cylinder should be modified into the form of a curved and truncated cone, such as is found in hammers, screwdrivers, chisels, files, and so on, or in more complex modifications as in the handles of saws and power tools. The truncated curved cone derives from the varying degree of flexion in the fingers during the resting grasp. The space enclosed by the grasp is of course not in fact cylindrical but is complex and multicurved.It would be a relatively simple matter, of course, to make a casting of that shape and build a handle to match it. This, however, would be a highly undesirable approach since the resulting handle would be appropriate only to the hand on which it was modelled, and only under the circumstances of use under which the model was made. The shape also varies in its dimensions from hand to hand and during use of the hand. Instead of using a contour-matching shape, it is necessary to develop a shape which, with minimal obstruction, will meet the requirements of both hand and function. In fact, any form of specific shaping to a hand is undesirable, such as ridges and valleys for fingers, fluting, indentations, and so on, since with varying shape and size of hands, and varying mode of function the resulting shapes do not in fact fit the hands of a significant number of users. Shapes then should be generalized and basic, sectors of spheres, flattened cylinders, long contoured curves, flat planes, put together in such a manner as to conform in general to the contours of the space of the grasping hand, but not specifically. Particular examples

of this shaping will be discussed later in connection with examination of individual tools.Consideration must also be given to other shapes. The sphere, or portion of a sphere, is in fact not commonly found except in some forms of stabilizing handle. It has the advantage that it can be readily grasped from many angles. It is desirable, however, to use a sector of a sphere as the dome at the head of tools where some of the drive comes via the palm of the hand, such as screwdrivers, chisels, and planes. It provides a good contour fit to the palm with no sharp projections. In this regard it should be noted that no edges should be permitted in the handles of tools. All potential edges should be smoothly curved off. Many contemporary tools, particularly some screwdrivers, still have relatively sharp edges to the fluting that has been designed into the plastic handles with the object of improving the grip. Continued pressure from the edges of tool handles give rise to discomfort, inefficiency, and eventually damage to the hand of the user.Still another shape, which can be regarded as a modification of the cylinder, is the hexagonal section. It is of particular value for small calibre implements which would otherwise be cylindrical in section. It is easier to maintain a stable grip on a hexagonal section of small calibre than on a cylindrical section. Square or cuboidal sections can be rotated less freely, and have sharper edges, but are also of value. A hexagonal section for the handle of a small screwdriver can be very effective.Where the grasp has to undergo dynamic change, such as in the use of pliers and shears, rather than remain static, such as with a hammer, there are still other considerations with respect to shape, but the same principles apply. One is still concerned with a cylindrical or conical form, but in this case one is concerned with sections of the cone or cylinder which continuously change in diameter along the handles. The specific requirements for the handles of such tools will be examined later. T hickness (w idth) o f handles

With respect to thickness, again it is desirable for the handle to conform to anthropometric requirements. Surprisingly, however, very little work has been done to determine the specifications of human grasp, although fairly adequate information exists on the recommended dimensions of knobs, selector switches, and so on. One study on human grasp involving construction workers (Bobbert, 1960), indicates an average maximum inside grasp between thumb and index finger, encompassing at least 70 degrees of a cylinder, to be 7.4 cm (2.9 in).Design requirements for knob controls, which have some of the requirements of handles, include the following (note that all dimensions are approximate):

From a review of several studies (particularly Ayoub and LoPresti, 1971; Konz, 1974) it is apparent that a grip diameter of 40 mm (IV 2 in) is most appropriate for a power grip. The grip force at 50 mm (2 in) is 95 per cent of that at 40 mm, and 70 per cent of that at 65 mm (2V2 in). For precision grip, diameters of less than 6 mm (V4 in) tend to cut into the hand and do not give sufficient control.Although 75 mm (3 in) is given as the recommended maximum, a handle of that size would be unsatisfactory. In practice, most handles should range between 25 and 40 mm (approximately 1V2 in). Indeed the capacity to apply torque becomes reduced when the diameter of the handle exceeds approximately 50 mm (2 in), (Pheasant and O’Neill, 1975). This latter figure should be considered the operational maximum. For a hook grip a diameter of 20 mm (% in) is recommended (Woodson and Conover, 1966). For females all the foregoing recommended limits should be reduced by 10 per cent, and indeed additional as yet unspecified limitations might be required by reason of ethnic heritage.The actual width for the individual tool will of course vary with its function and size. Thus, where the tool is large and a power grip is required, for example in a heavy hammer or the handle of a power drill, the width will be found at the upper limit of the range. Where the tool is small and demands a precision grip, for example a jeweller’s screwdriver or a dental drill, the width of the handle will lie at the lower end, or even below, for special purposes. Where a power grip is required, however, even where the tool head is small, as in a small driver for wood

screws, it is necessary to provide a grip at the higher end of the calibre range. Where rotation of the tool handle is unwanted, as in the case of a hammer, it is desirable to have a bilateral flattening, rather than retain a circular or near-circular section.In general, the thicker the handle the less is the load on the hand muscles. However, because of the shape of the space enclosed by the grip, the calibre of a handle should not normally be the same throughout its length. It will, of course, normally be wider at the thumb end and narrower at the little finger. Representative dimensions for the classic ‘pistol grip’, which is applicable to many forms of tool handle, are shown in Figure 10.2 . L en g th o f handle

While in some cases the handle of a tool merges indistinguishably into the shaft, as for example in a hammer, in others the length is determined by the working position of the hand. It is thus fixed by the critical anthropometric dimensions and the nature of the grip used. Particulars will be considered later when examining specific tools, but ideally the length must meet the maximum expected dimension at the level of the 97.5th percentile or higher. Thus, for example, a power tool handle must accommodate the maximum closed grasp at the 97.5th percentile, that is approximately 100 mm (4 in), bearing in mind the possible need for gloves, while the heavy screwdriver, used partly in a precision grip and partly in a power grip, must accommodate the length from palm to flexed knuckle of the forefinger, again approximately 100 mm (4 in). Short handles are unsuitable for tools requiring a power grip. A handle with a length so short that it cannot be grasped between thumb and

forefinger, that is approximately 19 mm ( 3/4 in), is unsuitable for any tool. Drillis (1963) notes that the length of handle for a tool such as a file, or for that matter a screwdriver, should be one ‘thumb-ell’ in folk terminology, or the distance between the ulnar edge of the hand and the tip of the outstretched thumb. W eight o f tool

The weight of the handle should be considered in relationship to the weight and balance of the tool. In the case of percussion tools it is desirable to reduce the weight of the handle to the minimum, and have as much weight as possible in the head. In other tools the balance should be evenly distributed where possible. In tools with small heads, and bulky handles, such as small screwdrivers, this may not be feasible, but the handle ideally should then be made progressively lighter as its bulk increases relative to the size of the head and shaft.In their study on hand tools and small presses Greenberg and Chaffin (1976) note that the weight of a tool determines its ability to be moved and hence that a heavy weight reduces the proficiency of the worker. While advocating an 11 kg (25 lb) limit on weight they also recommend that requirements for lifting the tool be limited to five times or less per minute. While 11 kg can be considered an upper limit, it would be well to add that for most efficient purposes the weight of a tool should not exceed 4.5 to 5.5 kg (10 to 12 lb). Lifting handles or designated grasp points should be provided in heavy tools so that two persons can cooperate in moving the device. A lifting bolt or hole should be provided for the better use of a hoist or tool balancer to assist in lifting and lowering the tool. The grasp points or handles should allow the fingers to wrap around the surface for at least 270 degrees. A n gu la tio n o f handles

The line of transmitted force in using a hand tool passes along the fingers, then through a centrally located carpal bone in the wrist to the radius bone, and up the arm. Although the middle finger is the central finger, and also normally the longest finger, the axis around which the hand operates is not in fact that of the middle finger but that of the index finger. Thus the axis of function of a tool grasped by the hand, whether it is a hammer, a screwdriver, or a power drill, is along the line of the pointing index finger, a fact that has been sometimes overlooked in the design of tools with angled handles. Therefore any angulation of handles that is necessary, for example in power tools or single-handed shears, should be undertaken with this anatominal relationship in mind. Thus the handles should not only reflect the axis of the grasp (that is, about 78 degrees from the horizontal), but the handle or handles should be so oriented

It is not by accident that for millennia wood was the material of choice for tool handles. In addition to being readily available and easily worked, it has qualities that make it desirable as the link between hand and metal. Its inherent elasticity provides the degree of resistance to pressure and shock absorbency that permits comfortable application offeree; its thermal conductivity permits a rate of heat exchange between the tool and the skin such that subjectively it feels neutral in warmth; its frictional resistance allows the application of torque with minimal discomfort at the skin, and also even when the skin is wet with sweat or other liquid; it is light in relation to its bulk, and it is visually and tactually pleasing. On the other hand, although moderately hard wearing, it can be damaged fairly readily and it will easily become stained and impregnated with grease and oil. Even more significantly, it will break and become detached from its mounting. Inevitably the wooden handles of many tools have given way to handles of polystyrene or some other form of impact resistant and stain resistant plastic, which has something of the same qualities as wood but is more durable, more colourful, and more economical. In one quality, however, namely the aesthetic of texture, plastic cannot replace wood. Texture of course is not merely an aesthetic quality; it is also functional. A tool handle requires a readily identifiable texture to provide an input to the sensory nervous system to assist in maintaining the grip. It is desirable in fact to ensure that some distinctive surface texture is incorporated into the otherwise smooth plastic handles for this purpose. As already noted, flutings, ridges, and indentations, which were intended to provide texture and increase frictional resistance, may in fact cause pressure injury on the fingers. Some dull roughening, palpable to the skin of the hand, but neither sharp nor injurious, can serve the purpose better.Deep recesses of greater than 3 mm (Vs in) are not recommended because of the variation in morphology of the finger throughout the population. In particular, a person with large fingers may create compression forces on the lateral surfaces of the fingers, which are abundant in superficial nerves, arteries and veins; or a person with small fingers may be forced to attempt compression of two fingers into one recess with similar results. In general finger recesses should only be

provided when the primary force is pulling across the palm, as in a tool used to insert or pull apart objects, and then only when small forces of no greater that about 6.5 kg (15 lb) are expected. If larger forces are required a pistol grip tool should be provided. The use of a flange and thumb stop on the handle is also recommended for tools used to insert or press parts together.If recesses are placed on both handles of, for example, a pair of pliers, there is a possibility of damage to the palm of the hand as well as to the fingers; it is therefore recommended that the force bearing area, where high or repetitive forces are expected, should be designed to span the breadth of the palm, which for 95 per cent of male workers would require a length of no less than 10 cm (4 in), and it should have a curvature of no greater than about 1.3 cm (V2 in) over its entire length.Several other recommendations in this regard can be quoted from the work of Greenberg and Chaffin (1976). Bright highly polished surfaces should be avoided. Smooth surfaces should only be provided when small forces are needed frequently to actuate the tool. Non-reflective ripple coatings should be used in most cases. Cast or machine surfaces should, if possible, be coated with matt paint or other similar material. Should this not be practicable, such surfaces should be sandblasted or otherwise surface treated so that the sharp surface peaks are rounded, thereby reducing the abrasive characteristics of the surface.All exterior edges of a tool which are not part of the functional operation and which meet at an angle of 135 degrees or less should be rounded with a radius of at least 0.8 mm (V32 in) approximately. Similarly, all corners formed by the intersection of three or more surfaces of which two form an angle of 135 degrees or less should be rounded to at least 1.6 mm (Vi6 in). If rounding is not feasible a layer of plastic or rubber may be overlaid and securely attached. The same principle applies to distinct surface protrusions which should be removed, relocated or countersunk; failing that, the part should be covered with a rounded pliable material. In some situations, for example, a projecting bolt, it might be necessary to cover it with a guard.A related problem may occur with openings. Exterior openings may be found in large power tools. These openings can catch clothing, tear skin, or even injure joints if, for example, a finger should be caught. Although this is perhaps unlikely, such openings should be eliminated wherever possible, or covered where otherwise necessary. Where the openings are inherent in the action of the tool, such as in the jaws of pliers, or clamps, consideration should be given, where it is compatible with the intended action of the tool, to so design the jaws that even when fully closed they could not compress a finger. Even if this is not feasible, at least it should be ensured that the handles of other moving parts of the tool do not come sufficiently close together that they could trap the skin of the operating hand, as may happen with improperly designed

pliers or clamps. Furthermore, operational procedures should be designed to minimize the likelihood of entrapment of body members.Metal handles may be used in some tools in place of wood or plastic, but to meet the requirements of shock absorbency, thermal conductivity, frictional resistance and texture, they have to be covered with a rubber, leather, or synthetic sheath of a thickness appropriate to the material used. S iz e o f tools

Various studies have shown that the size of an object, taken in conjunction with its weight, have a multiplicative effect over and above that of either size or weight alone. The size of a hand tool, however, is only of significance when it is unusually large, a situation that is relatively uncommon. A significantly stressful factor in this regard is the horizontally measured displacement of the centre of gravity of the load from the torso (Greenberg and Chaffin, 1976). The size of the load may be a major factor in determining this effect. Where the centre of gravity is displaced from the front of the torso by greater than 25.4 cm (10 in) the ability to lift and lower an object is greatly decreased for both men and women. Consequently, objects that have to be handled, including heavy tools, should be designed to locate the centre of gravity as close to the person’s torso as possible. Intructions or markings should indicate either where to grasp the tool to achieve this result, or where the centre of gravity is located, if it is not close to the perceived geometric centre. S e x

Ducharme (1975) surveyed the opinions of female United State’s military personnel on the suitability of a variety of tools and equipment in a variety of craft skills, such as electrical maintenance, vehicle maintenance, aircraft maintenance, metal working, structural work, electronic maintenance, and so on. Each craft had at least one tool or piece of equipment considered to be unsuitable by more than 10 per cent of the female work force. The average age of the workers was 21V2 years, the average height 165 cm (65 in), the average weight 50.2 kg (127 lb) and the average hand length 17.5 cm (6.9 in). A selection of some of the offending tools and pieces of equipment from different trades is shown below, with the percentage of workers complaining, and the cause for complaint (Table 10.3): The foregoing represent only one small selection from a small study, undoubtedly there are many more. For example, soldering guns found unsuitable for the above noted population have a somewhat similar operational action to that of power drills but are lighter. Presumably power drills would also be found to be unsatisfactory. Similarly shears and rivet cutters which were found unsatisfactory by up to 22 per cent of the female work force examined are similar in action to a variety of

methods. Drillis and his colleagues (1963) showed that the efficiency of the system is a function of the distance from the mass centre to the line of action and the radius of gyration with respect to the centre of action.Thus for the efficiency of the system to be maximum the centre of mass must lie in the head of the tool on the line of action, an impossibility of course with a shafted tool. The stone hand-held hammer may well have been mechanically efficient, even if the delivered blow was weak.The hammer, although occurring in many varieties, is one of the simplest tools ever developed, comprising a shaped head and a shaft or handle. Normally it is used in a power grip, but the ordinary carpenter’s hammer is not uncommonly held in a precision grip which merges into a power grip as the character of the work changes. Light-weight carpentry and panel-beating hammers, or chipping hammers, and so on, are also commonly held in a precision grip. Thus a hammer handle must meet the need of a wide variety of activities. In fact a straight cylindrical bilaterally flattened wooden handle of calibre within the range of 25-50 mm (1-1 Vi in), appropriate to the weight of the head, is indeed

very effective. The length of the shaft is also a function of the activity. It has been shown that the mean weight for a chopping action should be no greater than 2 per cent of the operator’s weight, 6.5-7.5 kg (3-3 V2 lb), and the mean length of the handle, 35 per cent of the operator’s height (Drillis, 1963). Appropriate dimensions for several types of hammer are shown in Figure 10.4.There are many varieties of hammers. The contemporary model originated in the Age of Metals and has changed little. A sledge-hammer, weighing for example up to 3.3 kg (7¥2 lb) with a handle approximately 60 cm (24 in), may be used double-handed for heavy driving or working of wrought iron; a fitter’s hammer with a 0.9 kg (2 lb) head and a handle of 25-30 cm (10 to 12 in) is a single-handed tool which combines strength with speed; the ballpeen or hemispherical back of the head is used for rivetting. Geologist’s and boilermaker’s hammers have longer narrower heads for use in restricted space and for concentrating the blow on a smaller area. The back of a carpenter’s hammer may have either a narrow straight edge for driving nails with small heads or a claw for extracting

nails. The claw hammer, in fact is known from Roman times. Heads made of soft metal, rubber, rawhide, or synthetic materials may be used to avoid damage to the material being struck. Some indeed may be hollow and weighted with lead, while lead itself is used as the head of a plumber’s dresser, and wood as the head of a carpenter’s mallet or a wooden maul.Axes and adzes are also striking tools, but with a cutting edge. The essential difference is in the relationship of the head to the handle. In the adze the plane of the head is at right-angles to the handle. The weight and shape of the axe head is adjusted to the operation it has to perform, varying from 0.5 to 2.2 kg (1 to 5 lb) or more. 2. S cra p in g tools Saw s: Primitive stone tools were of course scraping tools as well as percussive tools, but the saw as an implement did not become specialized until the seventh century B.C., with the beginnings of the Metal Age. It was originally used with a pull cut only. The push cut utilized by most saws today originated with the Romans. Pruning saws, fret saws, and coping saws, with thin narrow blades, may have pull cuts as also do powered reciprocating and sabre saws. The concept of the M-shaped teeth with variable set used in contemporary hand-saws was developed in the Middle Ages, but the modern saw blade originated from rolling mill stock in the eighteenth century.The action of heavy sawing essentially involves a fixed grasp in the power position, with repetitive flexion and extension at the elbow, while the action of light sawing, such as with a fret saw, requires a precision grip with some manipulation at the wrist. Very heavy crosscut sawing with a two-man saw may indeed require the use of two hands, one superimposed upon the other, but the grasp is the same as for one hand. Thus basically there are two types of handles required for saws, one for a power grip and one for a precision grip.For the heavier work the broad compound curve of the pistol grip provides a comfortable efficient handle, where the major limiting feature is the width of the gloved or ungloved hand. To conform to the flat planes of the saw the sides of the handle can be flattened without loss of effectiveness, provided that the bilateral width of the handle does not become less than 25 mm (1 in). Narrower widths increase the pressure loading on the thenar eminence and palm and give rise to limiting discomfort. While the compound curve of the pistol grip is desirable it is not mandatory, providing the angle of the handle between vertical and horizontal conforms to the approximate 78 degrees which represents the angle of the resting grasp. All edges however must then be rounded. A high quality design which uses an almost rectangular section with curved edges has been marketed.Representative dimensions for saw handles are shown in figure 10.5.