Tool bit
Encyclopedia
The term tool bit generally refers to a non-rotary cutting tool used in metal lathes
, shaper
s, 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.
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.
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.
, 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).
. 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.
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. 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.
The tool holders may also be designed to introduce additional properties to the cutting action, such as
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.
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).
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) 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.
s, 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
.
s. In fact, good machinists were expected to have blacksmith
ing 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 steel
s, 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 steel
s (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
. 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).
Lathe (metal)
A metal lathe or metalworking lathe is a large class of lathes designed for precisely machining relatively hard materials. They were originally designed to machine metals; however, with the advent of plastics and other materials, and with their inherent versatility, they are used in a wide range of...
, shaper
Shaper
A shaper is a type of machine tool that uses linear relative motion between the workpiece and a single-point cutting tool to machine a linear toolpath. Its cut is analogous to that of a lathe, except that it is linear instead of helical...
s, and planers
Planer (metalworking)
A planer is a type of metalworking machine tool that uses linear relative motion between the workpiece and a single-point cutting tool to machine a linear toolpath. Its cut is analogous to that of a lathe, except that it is linear instead of helical...
. 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
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.
Steels
Originally, all tool bits were made of high carbon tool steels with the appropriate hardeningHardening (metallurgy)
Hardening is a metallurgical and metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain...
and tempering
Tempering
Tempering is a heat treatment technique for metals, alloys and glass. In steels, tempering is done to "toughen" the metal by transforming brittle martensite or bainite into a combination of ferrite and cementite or sometimes Tempered martensite...
. 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
CarbideTungsten carbide
Tungsten carbide is an inorganic chemical compound containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery,...
, 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
Brazing
Brazing is a metal-joining process whereby a filler metal is heated above and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere, usually a flux...
on to a steel shank (this is usually only done for carbide).
Inserts
Almost all high-performance cutting tools use indexable insertsTipped tool
A tipped tool generally refers to any cutting tool where the cutting edge consists of a separate piece of material, either brazed, welded or clamped on to a separate body. Common materials for tips include tungsten carbide, polycrystalline diamond, and cubic boron nitride...
. 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
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. 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
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 fluidCutting fluidCutting fluid is a type of coolant and lubricant designed specifically for metalworking and machining processes. There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, aerosols , and air or other gases. They may be made from petroleum distillates, animal...
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.
Bit holder and toolpost
The toolpost is the part of a metalworking latheLathe (metal)
A metal lathe or metalworking lathe is a large class of lathes designed for precisely machining relatively hard materials. They were originally designed to machine metals; however, with the advent of plastics and other materials, and with their inherent versatility, they are used in a wide range of...
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
A box tool is mounted on the turret of a turret latheTurret lathe
The turret lathe is a form of metalworking lathe that is used for repetitive production of duplicate parts, which by the nature of their cutting process are usually interchangeable...
or screw machine
Screw machine
A screw machine may refer to a:* Screw machine , a small- to medium-sized automatic lathe that is mechanically automated via cams...
. 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) 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.
Clapper box
ShaperShaper
A shaper is a type of machine tool that uses linear relative motion between the workpiece and a single-point cutting tool to machine a linear toolpath. Its cut is analogous to that of a lathe, except that it is linear instead of helical...
s, slotters, and planers
Planer (metalworking)
A planer is a type of metalworking machine tool that uses linear relative motion between the workpiece and a single-point cutting tool to machine a linear toolpath. Its cut is analogous to that of a lathe, except that it is linear instead of helical...
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
Butterfly valve
A butterfly valve is a valve which can be used for isolating or regulating flow. The closing mechanism takes the form of a disk. Operation is similar to that of a ball valve, which allows for quick shut off. Butterfly valves are generally favored because they are lower in cost to other valve...
check valve
Check valve
A check valve, clack valve, non-return valve or one-way valve is a mechanical device, a valve, which normally allows fluid to flow through it in only one direction....
.
Fly cutters
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
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 forgeForge
A forge is a hearth used for forging. The term "forge" can also refer to the workplace of a smith or a blacksmith, although the term smithy is then more commonly used.The basic smithy contains a forge, also known as a hearth, for heating metals...
s. In fact, good machinists were expected to have blacksmith
Blacksmith
A blacksmith is a person who creates objects from wrought iron or steel by forging the metal; that is, by using tools to hammer, bend, and cut...
ing knowledge, and although the chemistry and physics of the heat treatment
Heat treatment
Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass...
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 steel
Tool steel
Tool steel refers to a variety of carbon and alloy steels that are particularly well-suited to be made into tools. Their suitability comes from their distinctive hardness, resistance to abrasion, their ability to hold a cutting edge, and/or their resistance to deformation at elevated temperatures...
s, which have high enough carbon content to take hardening well. Each bit was forged with a hammer, quenched, and then ground with a grindstone
Grindstone (tool)
A grindstone is a round sharpening stone used for grinding or sharpening ferrous tools. They are usually made from sandstone.Grindstone machines usually have pedals in which to speed and slow the stone to sharpen metal to the point of perfection....
. 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
Frederick Winslow Taylor
Frederick Winslow Taylor was an American mechanical engineer who sought to improve industrial efficiency. He is regarded as the father of scientific management and was one of the first management consultants...
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 steel
High speed steel
High speed steelMost copyeditors today would tend to choose to style the unit adjective high-speed with a hyphen, rendering the full term as high-speed steel, and this styling is not uncommon . However, it is true that in the metalworking industries the styling high speed steel is long-established...
s (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
William Sellers
William Sellers was a mechanical engineer, manufacturer, businessperson, and inventor who filed more than 90 patents, most notably the design for the United States standard screw thread...
(a principal of Midvale Steel
Midvale Steel
Midvale Steel was a succession of steel-making corporations whose flagship plant was the Midvale Steel Works at Nicetown, Philadelphia, Pennsylvania, which operated from 1867 until 1976...
and Cramp's shipyard) and later by Bethlehem Steel
Bethlehem Steel
The Bethlehem Steel Corporation , based in Bethlehem, Pennsylvania, was once the second-largest steel producer in the United States, after Pittsburgh, Pennsylvania-based U.S. Steel. After a decline in the U.S...
. 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
Applied science
Applied science is the application of scientific knowledge transferred into a physical environment. Examples include testing a theoretical model through the use of formal science or solving a practical problem through the use of natural science....
).
Stellite
Stellite
Stellite alloy is a range of cobalt-chromium alloys designed for wear resistance. It may also contain tungsten or molybdenum and a small but important amount of carbon...
soon joined high speed steels as a material for single-point cutters. Although diamond turning
Diamond turning
Diamond turning is a process of mechanical machining of precision elements using lathes or derivative machine tools equipped with natural or synthetic diamond-tipped tool bits...
had been around for a long time, it was not until these new, expensive metals came about that the idea of cutting inserts
Tipped tool
A tipped tool generally refers to any cutting tool where the cutting edge consists of a separate piece of material, either brazed, welded or clamped on to a separate body. Common materials for tips include tungsten carbide, polycrystalline diamond, and cubic boron nitride...
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
Cutting fluid
Cutting fluid is a type of coolant and lubricant designed specifically for metalworking and machining processes. There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, aerosols , and air or other gases. They may be made from petroleum distillates, animal...
), 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).