Various tool bits, carbide inserts and holders

Typical hand ground cutting tool angles for the lathe.

Shearing action depicted.

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The term tool bit generally refers to a non-rotary cutting tool used in metal lathes, shapers, and planers. Such cutters are also often referred to by the set-phrase name of single-point cutting tool. The cutting edge is ground to suit a particular machining operation and may be resharpened or reshaped as needed. The ground tool bit is held rigidly by a tool holder while it is cutting.

Geometry[link]

Back Rake is to help control the direction of the chip, which naturally curves into the work due to the difference in length from the outer and inner parts of the cut. It also helps counteract the pressure against the tool from the work by pulling the tool into the work.

Side Rake along with back rake controls the chip flow and partly counteracts the resistance of the work to the movement of the cutter and can be optimized to suit the particular material being cut. Brass for example requires a back and side rake of 0 degrees while aluminum uses a back rake of 35 degrees and a side rake of 15 degrees.

Nose Radius makes the finish of the cut smoother as it can overlap the previous cut and eliminate the peaks and valleys that a pointed tool produces. Having a radius also strengthens the tip, a sharp point being quite fragile.

All the other angles are for clearance in order that no part of the tool besides the actual cutting edge can touch the work. The front clearance angle is usually 8 degrees while the side clearance angle is 10-15 degrees and partly depends on the rate of feed expected.

Minimum angles which do the job required are advisable because the tool gets weaker as the edge gets keener due to the lessening support behind the edge and the reduced ability to absorb heat generated by cutting.

The Rake angles on the top of the tool need not be precise in order to cut but to cut efficiently there will be an optimum angle for back and side rake.

Materials[link]

Steels[link]

Originally, all tool bits were made of high carbon tool steels with the appropriate hardening and tempering. Since the introductions of high-speed steel (HSS) (early years of the 20th century), sintered carbide (1930s), ceramic and diamond cutters, those materials have gradually replaced the earlier kinds of tool steel in almost all cutting applications. Most tool bits today are made of HSS, cobalt steel, or carbide.

Carbides and ceramics[link]

Carbide, ceramics (such as cubic boron nitride) and diamond, having higher hardness than HSS, all allow faster material removal than HSS in most cases. Because these materials are more expensive and brittler than steel, typically the body of the cutting tool is made of steel, and a small cutting edge made of the harder material is attached. The cutting edge is usually either screwed on (in this case it is called an insert), or brazed on to a steel shank (this is usually only done for carbide).

Inserts[link]

Almost all high-performance cutting tools use indexable inserts. There are several reasons for this. First of all, at the very high cutting speeds and feeds supported by these materials, the cutting tip can reach temperatures high enough to melt the brazing material holding it to the shank. Economics are also important; inserts are made symmetrically so that when the first cutting edge is dull they can be rotated, presenting a fresh cutting edge. Some inserts are even made so that they can be flipped over, giving as many as 16 cutting edges per insert. There are many types of inserts: some for roughing, some for finishing. Others are made for specialized jobs like cutting threads or grooves. The industry employs standardized nomenclature to describe inserts by shape, material, coating material, and size.

Form tools[link]

A form tool is precision-ground into a pattern that resembles the part to be formed. The form tool can be used as a single operation and therefore eliminate many other operations from the slides (front, rear and/or vertical) and the turret, such as box tools. A form tool turns one or more diameters while feeding into the work. Before the use of form tools, diameters were turned by multiple slide and turret operations, and thus took more work to make the part. For example, a form tool can turn many diameters and in addition can also cut off the part in a single operation and eliminate the need to index the turret. For single-spindle machines, bypassing the need to index the turret can dramatically increase hourly part production rates.

On long-running jobs it is common to use a roughing tool on a different slide or turret station to remove the bulk of the material to reduce wear on the form tool.

There are different types of form tools. Insert form tools are the most common for short- to medium-range jobs (50 to 20,000 pcs). Circular form tools are usually for longer jobs, since the tool wear can be ground off the tool tip many times as the tool is rotated in its holder. There is also a skiving tool that can be used for light finishing cuts. Form tools can be made of cobalt steel, carbide, or high-speed steel. Carbide requires additional care because it is very brittle and will chip if chatter occurs.

A drawback when using form tools is that the feed into the work is usually slow, 0.0005" to 0.0012" per revolution depending on the width of the tool. Wide form tools create more heat and usually are problematic for chatter. Heat and chatter reduces tool life. Also, form tools wider than 2.5 times the smaller diameter of the part being turned have a greater risk of the part breaking off.^[1] When turning longer lengths, a support from the turret can be used to increase turning length from 2.5 times to 5 times the smallest diameter of the part being turned, and this also can help reduce chatter. Despite the drawbacks, the elimination of extra operations often makes using form tools the most efficient option.

Toolholders[link]

By confining the expensive hard cutting tip to the part doing the actual cutting, the cost of tooling is reduced. The supporting tool holder can then be made from a tougher steel, which besides being cheaper is also usually better suited to the task, being less brittle than the cutting-edge materials.

The tool holders may also be designed to introduce additional properties to the cutting action, such as

• Angular approach - direction of tool travel.
• Spring loading - deflection of the tool bit away from the material when excessive load is applied.
• Variable overhang - the tool bit may be extended or retracted as the job requires.
• Rigidity - the tool holder can be sized according to the work to be performed.
• Direct cutting fluid or coolant to the work area.

Note that since stiffness (rather than strength) is usually the design driver of a tool holder, the steel used doesn't need to be particularly hard or strong as there is relatively little difference between the stiffnesses of most steel alloys.

Holders used on lathes[link]

Bit holder and toolpost[link]

The toolpost is the part of a metalworking lathe which either holds the tool bit directly or holds a toolholder which contains the tool bit. There are a great variety of designs for toolposts (including basic toolposts, rocker toolposts, quick-change toolposts, and toolpost turrets) and toolholders (with varying geometry and features).

Box tool[link]

A box tool is mounted on the turret of a turret lathe or screw machine. It is essentially a toolpost that brings its follower rest along with it. A tool bit (or several tool bits) and a compact follower rest (usually V-shaped or with two rollers^[2]) are mounted opposite each other in a body which surrounds the workpiece (forms a "box" around it). As the tool bit puts a lateral deflecting force on the workpiece, the follower rest opposes it, providing rigidity. A different and popular type of boxtool uses two rollers rather than a follower rest. One roller is called a "sizing roller" and the other roller is called a "burnishing roller". The rollers turn with the stock to reduce scarring on the finished turn. Opposing tool bits may be used (instead of a rest) to cancel each other's deflecting forces (called a "balanced turning tool"), in which case the box tool begins to overlap in form, function, and identity with a hollow mill.

Holders used on shapers, slotters, and planers[link]

Clapper box[link]

Shapers, slotters, and planers often employ a kind of toolholder called a clapper box that swings freely on the return stroke of the ram or bed. On the next cutting stroke, it "claps" back into cutting position. Its movement is analogous to that of a butterfly-style check valve.

Holders used on milling machines[link]

Fly cutters[link]

Fly cutters are a type of milling cutter in which one or two tool bits are mounted. The bits spin around with the rotation of the spindle, taking facing cuts. Fly cutters are an application of tool bits where the bits are part of a rotary unit (whereas most other tool bit use is linear).

History[link]

Tool bits have been used for centuries, yet their further technological development continues even today. Before about 1900, almost all tool bits were made by their users, and many machine shops had forges. In fact, good machinists were expected to have blacksmithing knowledge, and although the chemistry and physics of the heat treatment of steel were not well understood (as compared with today's sciences), the practical art of heat treatment was quite advanced, and something that most skilled metalworkers were comfortably acquainted with. Tool bits were made of carbon tool steels, which have high enough carbon content to take hardening well. Each bit was forged with a hammer, quenched, and then ground with a grindstone. The exact details of the heat treatment and tip geometry were a matter of individual experience and preference.

A substantial technological advance occurred in the 1890–1910 period, when Frederick Winslow Taylor applied scientific methods to the study of tool bits and their cutting performance (including their geometry, metallurgy, and heat treatment, and the resulting speeds and feeds, depths of cut, metal-removal rates, and tool life). Along with Maunsel White and various assistants, he developed high speed steels (whose properties come from both their alloying element mixtures and their heat treatment methods). His cutting experiments chewed through tons of workpiece material, consumed thousands of tool bits, and generated mountains of chips. They were sponsored in large part by William Sellers (a principal of Midvale Steel and Cramp's shipyard) and later by Bethlehem Steel.^[3] Not only did Taylor develop new materials to make single-point cutters from, but he also determined optimum geometry (rake angles, clearance angles, nose radiuses, etc.). After Taylor, it was no longer taken for granted that the black art of individual craftsmen represented the highest level of metalworking technology. This was part of a larger trend during the 19th and 20th centuries by which science was mixed with art in the material culture of everyday life (applied science).

Stellite soon joined high speed steels as a material for single-point cutters. Although diamond turning had been around for a long time, it was not until these new, expensive metals came about that the idea of cutting inserts became commonly applied in machining. Before this, most single-point cutters were forged entirely of tool steel (then ground at the tip). Now it became more common to attach a separate tip (of one material) to a holder (of another). With the development of commercially available cemented carbide (1920s) and ceramic inserts (post-WWII), this trend accelerated, because carbide and ceramic are even more expensive and even less suited to serving as a shank. The technological development, however, did not immediately displace the older ways. Between 1900 and 1950, it was still not uncommon for a machinist to forge a tool from carbon tool steel.

Today, among the single-point cutters used in mass production (such as of automotive parts), insert tools using carbide and ceramic far outnumber HSS or cobalt steel tools. In other machining contexts (e.g., job shops, toolrooms, and hobbyist practice), the latter are still well represented. An entire system of industry-standard notation has been developed to name each insert geometry type. The number of carbide and ceramic formulations continues to expand, and diamond is used more than ever before. Speeds, feeds, depths of cut, and temperatures at the cutting interface continue to rise (the latter counterbalanced by copious cooling via liquid, air, or aerosols), and cycle times continue to shrink. Competition among product manufacturers to lower the unit costs of production continually drives technological development by the tool manufacturers, as long as the costs of R&D and tooling purchase amortization are lower than the amount of money saved by productivity increases (e.g., wage expense reduction).

See also[link]

• Tool wear
• Diamond tools
• Balancing machine
• Tool management

References[link]

Bibliography[link]

• Hartness, James (1910), Hartness Flat Turret Lathe Manual, Springfield, Vermont, USA: Jones and Lamson Machine Company, http://books.google.com/books?id=LLY5AAAAMAAJ&pg=PA3 .
• Kanigel, Robert (1997), The One Best Way: Frederick Winslow Taylor and the Enigma of Efficiency, Viking Penguin, ISBN 0-670-86402-1 .

External links[link]

Wikimedia Commons has media related to: Tool bits

• Detecting and Correcting Unbalance in Toolholders for High-Speed Machining - http://www.hofmann-global.com/documents/ToolholderBalancing.pdf

A modern toolbox.

A tool is any item that can be used to achieve a goal, especially one that is not consumed in the process. Informally the word is also used to describe a procedure or process with a specific purpose. Tools that are used in particular fields or activities may have different designations such as Instrument, Utensil, Implement, Machine, or Apparatus. The set of tools needed to achieve a goal is equipment. The knowledge of constructing, obtaining and using tools is technology.

Tool use by humans dates back millions of years, and other animals are also known to employ simple tools.

History[link]

Prehistoric stone tools over 10,000 years old, found in Les Combarelles cave, France

Carpentry tools recovered from the wreck of a 16th century sailing ship, the Mary Rose.

Stone and metal knives

An upholstery regulator

Anthropologists believe that the use of tools was an important step in the evolution of mankind.^[1] Humans evolved an opposable thumb — useful in holding tools — and increased dramatically in intelligence, which aided in the use of tools.^[2] Because tools are used extensively by both humans and wild chimpanzees, it is widely assumed that the first routine use of tools took place prior to the divergence between the two species.^[3] These early tools, however, were likely made of perishable materials such as sticks, or consisted of unmodified stones that cannot be distinguished from other stones as tools. The beginning of the Stone Age marks the era when hominins first began manufacturing stone tools, and evidence of these tools dates back at least 2.6 million years in Ethiopia.^[4] One of the earliest distinguishable stone tool forms is the hand axe.

Tools are the most important items that the ancient humans used to climb to the top of the food chain; by inventing tools, they were able to accomplish tasks that human bodies could not, such as using a spear or bow and arrow to kill prey, since their teeth were not sharp enough to pierce many animals' skins.

The transition from stone to metal tools roughly coincided with the development of agriculture around the 4th millennium BC. Mechanical devices experienced a major expansion in their use in the Middle Ages with the systematic employment of new energy sources: water (waterwheels) and wind (windmills).

Machine tools occasioned a surge in producing new tools in the industrial revolution. Advocates of nanotechnology expect a similar surge as tools become microscopic in size.^[5][6]

Functions[link]

One can classify tools according to their basic functions:

• Cutting tools, such as the knife, scythe or sickle, are wedge-shaped implements that produce a shearing force along a narrow face. Ideally, the edge of the tool needs to be harder than the material being cut or else the blade will become dulled with repeated use. But even resilient tools will require periodic sharpening, which is the process of removing deformation wear from the edge. Other examples of cutting tools include gouges and drill bits.
• Moving tools move huge and tiny things. For example, concentrating-force tools: the hammer moves a nail, the maul moves a stake, or a whip moves flesh on a horse. These operate by applying physical compression to a surface. In the case of the screwdriver, the force is rotational and called torque. Writing implements deliver a fluid to a surface via compression to activate the ink cartridge. Also grabbing and twisting nuts and bolts with pliers, a glove, a wrench, etc. All these tools move items by some kind of force. Also trucks, rockets and airplanes move larger items and particle accelerators move very small items.
• Tools that enact chemical changes, including temperature and ignition, such as lighters and blowtorches.
• Guiding, measuring and perception tools include the ruler, glasses, set square, sensors, straightedge, theodolite, microscope, monitor, clock, phone, printer
• Shaping tools, such as molds, jigs, trowels.
• Fastening tools, such as welders, rivet guns, nail guns, or glue guns.
• Information and data manipulation tools, such as computers, middleware, IDE, spreadsheets

Some tools may be combinations of other tools. An alarm-clock is for example a combination of a measuring tool (the clock) and a perception tool (the alarm). This enables the alarm-clock to be a tool that falls outside of all the categories mentioned above.

There is some debate on whether to consider protective gear items as tools, because they do not directly help perform work, just protect the worker like ordinary clothing. They do meet the general definition of tools and in many cases are necessary for the completion of the work. Personal protective equipment includes such items as gloves, safety glasses, ear defenders and biohazard suits.

Tool substitution[link]

Often, by design or coincidence, a tool may share key functional attributes with one or more other tools. In this case, some tools can substitute for other tools, either as a makeshift solution or as a matter of practical efficiency. "One tool does it all" is a motto of some importance for workers who cannot practically carry every specialized tool to the location of every work task; such as a carpenter who does not necessarily work in a shop all day and needs to do jobs in a customer's house. Tool substitution may be divided broadly into two classes: substitution "by-design", or "multi-purpose" use, and substitution as makeshift. Substitution "by-design" would be tools that are designed specifically to accomplish multiple tasks using only that one tool. Substitution as makeshift is when human ingenuity comes into play and a tool is used for its unintended purpose such as a mechanic using a long screw driver to separate a cars control arm from a ball joint instead of using a tuning fork. In many cases, the designed secondary functions of tools are not widely known. As an example of the former, many wood-cutting hand saws integrate a carpenter's square by incorporating a specially shaped handle that allows 90° and 45° angles to be marked by aligning the appropriate part of the handle with an edge and scribing along the back edge of the saw. The latter is illustrated by the saying "All tools can be used as hammers." Nearly all tools can be used to function as a hammer, even though very few tools are intentionally designed for it and even fewer work as well as the original.

Tools are also often used to substitute for many mechanical apparatuses, especially in older mechanical devices. In many cases a cheap tool could be used to occupy the place of a missing mechanical part. A window roller in a car could easily be replaced with a pair of vice grips or regular pliers. A transmission shifter or ignition switch would be able to be replaced with a screw-driver. Again, these would be considered tools that are being used for their unintended purposes, substitution as makeshift. Tools such as a Dremel would be considered the substitution "by-design", or "multi-purpose". This class of tools allows the use of one tool that has at least two different capabilities. "Multi-purpose" tools are basically multiple tools in one device/tool. Tools such as this are often power tools that come with many different attachments like a Dremel does, so you could say that a power drill is a "multi-purpose" tool because you can do more than just one thing with a power drill.

Multi-use tools[link]

Bicycle multi-tool

A Multi-tool is a hand tool that incorporates several tools into a single, portable device; the Swiss army knife represents one of the earliest examples. Other tools have a primary purpose but also incorporate other functionality - for example, lineman's pliers incorporate a gripper and cutter, and are often used as a hammer; and some hand saws incorporate a carpenter's square in the right-angle between the blade's dull edge and the saw's handle. This would also be the category in which the "multi-purpose" tools since they are also multiple tools in one (multi-use and multi-purpose can be used interchangeably). These types of tools were specifically made to catch the eye of many different craftsman who traveled to do their work. To these workers these types of tools were revolutionary because they were one tool or one device that could do several different things. With this knew revolution of tools the traveling craftsman would not have to carry so many tools with them to job sites, being that their space would be limited to the vehicle they were driving. The problem of having to deal with so many different tools was solved with the overtaking of multi-use tools.

Tool metaphors[link]

A telephone is a communication tool that interfaces between two people engaged in conversation at one level. And between each user and the communication network at another. It is in the domain of media and communications technology that a counter-intuitive aspect of our relationships with our tools first began to gain popular recognition. Marshall McLuhan famously said "We shape our tools. And then our tools shape us." McLuhan was referring to the fact that our social practices co-evolve with our use of new tools and the refinements we make to existing tools.

In North American colloquialism, the word "tool" may be loosely used to describe an individual of low mental capacity (essentially, an "idiot") or someone who fails to realise that they are being taken advantage of by others.^[7] This metaphor is drawn from the understanding that tools (as understood by most of society) are objects that are used for specific tasks.

Use by animals[link]

Observation has confirmed that multiple species can use tools including monkeys, apes, several birds, and sea otters. Philosophers originally thought that only humans had the ability to make tools, until zoologists observed birds^[8] and monkeys^[9][10][11] making tools. Now the unique relationship of humans with tools is considered to be that we are the only species that uses tools to make other tools.^[12]

See also[link]

• Device paradigm
• List of tool-lending libraries
• Toolbank
• Toolmaker

References[link]

Look up tool in Wiktionary, the free dictionary.

Wikimedia Commons has media related to: Tools

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (August 2010)

A bit (a contraction of binary digit) is the basic capacity of information in computing and telecommunications; a bit represents either 1 or 0 (one or zero) only. The representation may be implemented, in a variety of systems, by means of a two state device.

In computing, a bit can be defined as a variable or computed quantity that can have only two possible values. These two values are often interpreted as binary digits and are usually denoted by the numerical digits 0 and 1. The two values can also be interpreted as logical values (true/false, yes/no), algebraic signs (+/−), activation states (on/off), or any other two-valued attribute. The correspondence between these values and the physical states of the underlying storage or device is a matter of convention, and different assignments may be used even within the same device or program. The length of a binary number may be referred to as its "bit-length."

In information theory, one bit is typically defined as the uncertainty of a binary random variable that is 0 or 1 with equal probability,^[1] or the information that is gained when the value of such a variable becomes known.^[2]

In quantum computing, a quantum bit or qubit is a quantum system that can exist in superposition of two bit values, "true" and "false".

The symbol for bit, as a unit of information, is either simply "bit" (recommended by the ISO/IEC standard 80000-13 (2008)) or lowercase "b" (recommended by the IEEE 1541 Standard (2002)).

History[link]

The encoding of data by discrete bits was used in the punched cards invented by Basile Bouchon and Jean-Baptiste Falcon (1732), developed by Joseph Marie Jacquard (1804), and later adopted by Semen Korsakov, Charles Babbage, Hermann Hollerith, and early computer manufacturers like IBM. Another variant of that idea was the perforated paper tape. In all those systems, the medium (card or tape) conceptually carried an array of hole positions; each position could be either punched through or not, thus carrying one bit of information. The encoding of text by bits was also used in Morse code (1844) and early digital communications machines such as teletypes and stock ticker machines (1870).

Ralph Hartley suggested the use of a logarithmic measure of information in 1928.^[3] Claude E. Shannon first used the word bit in his seminal 1948 paper A Mathematical Theory of Communication. He attributed its origin to John W. Tukey, who had written a Bell Labs memo on 9 January 1947 in which he contracted "binary digit" to simply "bit". Interestingly, Vannevar Bush had written in 1936 of "bits of information" that could be stored on the punched cards used in the mechanical computers of that time.^[4] The first programmable computer built by Konrad Zuse used binary notation for numbers.

Representation[link]

A bit can be stored by a digital device or other physical system that exists in either of two possible distinct states. These may be the two stable states of a flip-flop, two positions of an electrical switch, two distinct voltage or current levels allowed by a circuit, two distinct levels of light intensity, two directions of magnetization or polarization, the orientation of reversible double stranded DNA, etc.

Bits can be implemented in many forms. In most modern computing devices, a bit is usually represented by an electrical voltage or current pulse, or by the electrical state of a flip-flop circuit.

For devices using positive logic, a digit value of 1 (or a logical value of true) is represented by a more positive voltage relative to the representation of 0. The specific voltages are different for different logic families and variations are permitted to allow for component aging and noise immunity. For example, in transistor–transistor logic (TTL) and compatible circuits, digit values 0 and 1 at the output of a device are represented by no higher than 0.4 volts and no lower than 2.6 volts, respectively; while TTL inputs are specified to recognize 0.8 volts or below as 0 and 2.2 volts or above as 1.

Transmission and processing[link]

Bits are transmitted one at a time in serial transmission, and multple numbers of bits in parallel transmission. A bitwise operation optionally process bits one at a time, see below Bit-based_computing.

Storage[link]

In the earliest non-electronic information processing devices, such as Jacquard's loom or Babbage's Analytical Engine, a bit was often stored as the position of a mechanical lever or gear, or the presence or absence of a hole at a specific point of a paper card or tape. The first electrical devices for discrete logic (such as elevator and traffic light control circuits, telephone switches, and Konrad Zuse's computer) represented bits as the states of electrical relays which could be either "open" or "closed". When relays were replaced by vacuum tubes, starting in the 1940s, computer builders experimented with a variety of storage methods, such as pressure pulses traveling down a mercury delay line, charges stored on the inside surface of a cathode-ray tube, or opaque spots printed on glass discs by photolithographic techniques.

In the 1950s and 1960s, these methods were largely supplanted by magnetic storage devices such as magnetic core memory, magnetic tapes, drums, and disks, where a bit was represented by the polarity of magnetization of a certain area of a ferromagnetic film, or by a change in polarity from one direction to the other. The same principle was later used in the magnetic bubble memory developed in the 1980s, and is still found in various magnetic strip items such as metro tickets and some credit cards.

In modern semiconductor memory, such as dynamic random access memory or flash memory, the two values of a bit may be represented by two levels of electric charge stored in a capacitor. In programmable logic arrays and certain types of read-only memory, a bit may be represented by the presence or absence of a conducting path at a certain point of a circuit. In optical discs, a bit is encoded as the presence or absence of a microscopic pit on a reflective surface. In one-dimensional bar codes, bits are encoded as the thickness of alternating black and white lines.

Information capacity and information compression[link]

When the information capacity of a storage system or a communication channel is presented in bits or bits per second, this often refers to binary digits, which is a computer hardware capacity to store binary code (0 or 1, up or down, current or not, etc.). Information capacity of a storage system is only an upper bound to the actual quantity of information stored therein. If the two possible values of one bit of storage are not equally likely, that bit of storage will contain less than one bit of information. Indeed, if the value is completely predictable, then the reading of that value will provide no information at all (zero entropic bits, because no resolution of uncertainty and therefore no information). If a computer file that uses n bits of storage contains only m < n bits of information, then that information can in principle be encoded in about m bits, at least on the average. This principle is the basis of data compression technology. Using an analogy, the hardware binary digits refer to the amount of storage space available (like the number of buckets available to store things), and the information content the filling, which comes in different levels of granularity (fine or coarse, that is, compressed or uncompressed information). When the granularity is finer (when information is more compressed), the same bucket can hold more.

For example, it is estimated that the combined technological capacity of the world to store information provides 1,300 exabytes of hardware digits in 2007. However, when this storage space is filled and the corresponding content is optimally compressed, this only represents 295 exabytes of information.^[5] When optimally compressed, the resulting carrying capacity approaches Shannon information or information entropy.

Multiple bits[link]

There are several units of information which are defined as multiples of bits, such as byte (8 bits), kilobit (either 1000 or 2¹⁰ = 1024 bits), megabyte (either 8000000 or 8×2²⁰ = 8388608bits), etc.

Computers usually manipulate bits in groups of a fixed size, conventionally named "words". The number of bits in a word varies with the computer model; typically between 8 to 80 bits; or even more in some specialized machines.

The International Electrotechnical Commission's standard IEC 60027 specifies that the symbol for binary digit should be "bit", and this should be used in all multiples, such as "kbit" (for kilobit).^[6] However, the letter "b" (in lower case) is widely used too. The letter "B" (upper case) is both the standard and customary symbol for byte.

In telecommunications (including computer networks), data transfer rates are usually measured in bits per second (bit/s) or its multiples, such as kbit/s. (This unit is not to be confused with baud.)

A millibit is a (rare) unit of information equal to one thousandth of a bit.^[7]

Bit-based computing[link]

Certain bitwise computer processor instructions (such as bit set) operate at the level of manipulating bits rather than manipulating data interpreted as an aggregate of bits.

In the 1980s, when bitmapped computer displays became popular, some computers provided specialized bit block transfer ("bitblt" or "blit") instructions to set or copy the bits that corresponded to a given rectangular area on the screen.

In most computers and programming languages, when a bit within a group of bits such as a byte or word is to be referred to, it is usually specified by a number from 0 (not 1) upwards corresponding to its position within the byte or word. However, 0 can refer to either the most significant bit or to the least significant bit depending on the context, so the convention of use must be known.

Other information units[link]

Other units of information, sometimes used in information theory, include the natural digit also called a nat or nit and defined as log₂ e (≈ 1.443) bits, where e is the base of the natural logarithms; and the dit, ban, or hartley, defined as log₂ 10 (≈ 3.322) bits.^[3] Conversely, one bit of information corresponds to about ln 2 (≈ 0.693) nats, or log₁₀ 2 (≈ 0.301) hartleys. Some authors also define a binit as an arbitrary information unit equivalent to some fixed but unspecified number of bits.^[8]

See also[link]

• Byte
• Integer (computer science)
• Primitive data type
• Bitstream
• Entropy (information theory)
• Binary numeral system
• Ternary numeral system
• Bit (Tron character)

References[link]

External links[link]

• Bit Calculator - Convert between bit, byte, kilobit, kilobyte, megabit, megabyte, gigabit, gigabyte