The use of special nozzles (bits) in assembly work was at one time due to the rapid failure of the tips of conventional screwdrivers during their professional use. In this regard, replaceable bits, invented in the first half of the 20th century, turned out to be more profitable and convenient.
When tightening several hundred self-tapping screws with a screwdriver with a tip, they began to change not the screwdriver, but only its nozzle, which was much cheaper. In addition, when working with several types of fasteners at once, many different tools were not required. Instead, in a single screwdriver, it was enough to change the nozzle, which took only a few seconds.
However, the main motivation behind the use of bits was the invention of centered fastener heads. The most common of them were cruciform – PH and PZ. With a careful study of their designs, it can be established that the tip of the nozzle, pressed into the center of the screw head, does not experience significant lateral forces that throw it out of the head.
According to the scheme of a self-centering system, other types of fastening heads used today are also built. They allow you to twist the elements not only at low speeds, but also at significant speeds with a large axial load.
The only exceptions are the S-type straight bits. They were historically designed for the very first hand-drilled screws. Bit alignment in slots does not occur, therefore, with an increase in rotation speed or a decrease in axial pressure, the nozzle slips out of the mounting head.
This is fraught with damage to the front surface of the element to be fixed. Therefore, in the mechanized assembly of critical products, the connection with elements with a straight slot is not used.
Its use is limited to less critical fasteners with low twisting speeds. When assembling products with a mechanical tool, only those types of fasteners are used in which a reliable fit of the nozzle to the fastener is ensured.
Fastening bits can be classified according to several criteria:
- type of fastening system;
- head size;
- bit rod length;
- rod material;
- metal coating;
- design (single, double);
- the possibility of bending (normal and torsion).
The most important is the division of bits into types of fastening systems. There are many of them, the most common will be discussed in a few paragraphs.
Almost every species system has several standard sizes, differing in the size of the tool head and the fastener slot corresponding to it. They are designated by numbers. The smallest ones start from 0 or 1. The recommendations for the type indicate the thread diameters of the fasteners for which the bit under a specific number is intended. So, the PH2 bit can be used with fasteners with a threaded diameter of 3,1 to 5,0 mm, PH1 is used for self-tapping screws with a diameter of 2,1–3,0, etc.
For ease of use, bits are available with different shaft lengths – from 25 mm to 150 mm. The sting of a long bit reaches the slots in those places where its more voluminous holder cannot penetrate.
Materials and coating
The alloy material from which the bit is made is a guarantee of its durability or, conversely, the softness of the structure, in which, when the specified forces are exceeded, it is not the fastener that breaks, but the bit. In some critical joints, just such a ratio of strengths is required.
However, in the vast majority of applications, the user is interested in the maximum possible number of fastener twists with one bit. To obtain strong bits that do not break due to the brittleness of the alloy, do not deform at the most loaded touch points, various alloys and steels are used. These include:
- high-speed carbon steels from R7 to R12;
- tool steel S2;
- chrome vanadium alloys;
- alloy of tungsten with molybdenum;
- alloy of chromium with molybdenum and others.
An important role in ensuring the strength properties of bits is played by special coatings. Thus, a layer of chromium-vanadium alloy protects the tool from corrosion, and the deposition of a layer of titanium nitride significantly increases its hardness and wear resistance. Diamond coating (tungsten-diamond-carbon), tungsten-nickel and others have similar properties.
The titanium nitride layer on the bit is easily recognizable by its golden color, the diamond one by the characteristic luster of the tip of the sting. It is more difficult to find out the brand of metal or alloy of bits, the manufacturer usually does not give or even hides this information in commercial interests. Only in some cases, the steel grade (S2, for example) can be applied to one of the faces.
By design, the bit can be single (sting on one side, hexagonal shank on the other) or double (two stings at the ends). The latter type has a double service life (both stings are the same) or ease of use (stings differ in size or type). The only disadvantage of this type of bit is the impossibility of installing it in a manual screwdriver.
Bits can be produced in regular and torsion versions. In the latter design, the tip itself and the shank are connected by a strong spring insert. It, working on twisting, transmits torque and allows you to bend the bit, which increases the possibility of access to inconvenient places. The spring also absorbs some of the impact energy, preventing the bit from breaking the splines.
Torsion bits are used with impact drivers in which the impact force is applied tangentially to the screwing circle. Bits of this type are more expensive than conventional bits, last longer, allow you to twist long fasteners into dense materials that conventional bits cannot cope with.
For ease of use, bits are produced in different lengths. Each one following the main standard size (25 mm) is 20-30 mm longer than the previous one – and so on up to 150 mm.
The most important characteristic of the bit is the duration of operation. Usually it is expressed in the number of fasteners screwed before the tool fails. The deformation of the sting manifests itself in the gradual “licking off” of the ribs in the process of the bit slipping out of the slot. In this regard, the most resistant bits are those that are not subjected to efforts that throw them out of the slot.
Of the most used, they include the H, Torx systems and their modifications. In terms of strong contact between bits and fasteners, there are many other systems, including anti-vandal ones, but their distribution is limited for a number of technical reasons.
The main types of bits used
The number of types of bits, including those that have become obsolete due to low technical suitability, is estimated at several dozen. Today, the following types of screwdriver bits have the greatest scope of application in fastener technology:
- PH (Phillips) – cruciform;
- PZ (Pozidriv) – cruciform;
- Hex (denoted by the letter H) – hexagonal;
- Torx (indicated by the letters T or TX) – in the form of a six-pointed star.
The PH Phillips Blade, introduced after 1937, was the first self-centering tool for driving screw-threaded fasteners. The qualitative difference from a flat sting was that the PH cross did not slip out of the slot even with a quick rotation of the tool. True, this required some axial force (pressing the bit against the fastener), but ease of use has increased dramatically compared to flat slots.
Clamping was also required in flat-slotted screws, but when tightening the PH bit, it was not necessary to apply attention and efforts to limit the possibility of the tip slipping out of the slot. The twisting speed (productivity) has increased dramatically even when working with a manual screwdriver. The use of a ratchet mechanism, and then pneumatic and electric screwdrivers, generally reduced the labor intensity of assembly operations by several times, which gave significant cost savings in any type of production.
The PH sting has four blades, tapering in thickness towards the end of the bit. They also capture the mating parts of the fastener and tighten it. The system is named after the engineer who implemented it in fastener technology (Phillips).
PH bits are available in five sizes – PH 0, 1, 2, 3 and 4. Shaft length – from 25 (basic) to 150 mm.
Approximately 30 years later (in 1966) the PZ fastening system (Pozidriv) was invented. It was developed by the Philips Screw Company. The shape of the PZ sting is cruciform, like that of the PH, however, both types have such serious differences that they do not allow a bat of one system to qualitatively tighten the fasteners of another. The angle of sharpening the end of the bit is different – in PZ it is sharper (50 º versus 55 º). The blades of the PZ do not taper like those of the PH, but remain equal in thickness throughout their entire length. It was this design feature that reduced the force of pushing the tip out of the slot at high loads (high twisting speeds or significant rotational resistance). The change in the design of the bit improved its contact with the head of the fastener, which increased the service life of the tool.
The PZ nozzle differs from the PH in appearance – grooves on both sides of each blade, forming pointed elements that are absent on the PH bit. In turn, to distinguish from PH, manufacturers apply characteristic notches on PZ fasteners, shifted by 45º away from power ones. This allows the user to quickly navigate when choosing a tool.
PZ bits are available in three sizes PZ 1, 2 and 3. Shaft length is from 25 to 150 mm.
The greatest popularity of the PH and PZ systems is explained by the good possibilities of automatic tool centering in in-line assembly operations and the relative cheapness of tools and fasteners. In other systems, these benefits have less significant economic incentives, so they have not been widely adopted.
The shape of the tip, denoted by the letter H in the marking, is a hexagonal prism. The system was invented back in 1910, and enjoys unflagging success today. So, confirmatory screws used in the furniture industry are twisted with H 4 mm bits. This tool is capable of transmitting significant torque. Due to the tight connection with the fastener slot, it has a long service life. There is no effort to push the bit out of the slot. Nozzles H are available in sizes from 1,5 mm to 10 mm.
Torx bits have been used in technology since 1967. They were first mastered by the American company Textron. The sting is a prism with a base in the form of a six-pointed star. The system is characterized by close contact of the tool with fasteners, the ability to transmit high torque. Widely distributed in the countries of America and Europe, in terms of popularity, the volume of use is close to the PH and PZ systems. The modernization of the Torx system is an “asterisk” of the same shape, supplemented by a hole in the axial center. Fasteners for it has a corresponding cylindrical protrusion. In addition to even tighter contact between the bit and the screw head, this design also has an anti-vandal property, excluding unauthorized unscrewing of the connection.
Other types of nozzles
In addition to the popular nozzle systems described, there are less well-known and less commonly used types of bits for a screwdriver. Bits fall into their classification:
- under a straight slot type S (slotted – slotted);
- hexagon type Hex with a hole in the center;
- square prism type Robertson;
- fork type SP (“fork”, “snake eye”);
- three-blade type Tri-Wing;
- four-bladed type Torg Set;
- and others.
Companies develop their unique bit-fastener systems both to prevent non-experts from accessing instrument compartments and to protect against vandals looting contents.
A good bat can perform many more fastener tightening operations than its simplified counterpart. To choose the desired tool, you need to contact a trading company whose employees you trust and get the necessary recommendations. If this is not possible, choose bits from well-known manufacturers – Bosch, Makita, DeWALT, Milwaukee.
Pay attention to the presence of a hardening coating of titanium nitride, and, if possible, to the material of the product. The best way to choose is to try one or two pieces of equipment in your own business. So you not only establish the quality of the product yourself, but also be able to give recommendations to your friends. Perhaps you will stop at an inexpensive option that has clear economic or technical advantages over the originals of eminent companies.