Grinding (abrasive cutting)
Encyclopedia
Grinding is an abrasive machining
process that uses a grinding wheel
as the cutting tool.
A wide variety of machines are used for grinding:
Grinding practice is a large and diverse area of manufacturing
and toolmaking
. It can produce very fine finishes and very accurate dimensions; yet in mass production contexts it can also rough out large volumes of metal quite rapidly. It is usually better suited to the machining of very hard materials than is "regular" machining (that is, cutting larger chips with cutting tools such as tool bit
s or milling cutter
s), and until recent decades it was the only practical way to machine such materials as hardened steels. Compared to "regular" machining, it is usually better suited to taking very shallow cuts, such as reducing a shaft's diameter by half a thousand of an inch (thou) or 12.7 um.
Grinding is a subset of cutting, as grinding is a true metal-cutting process. Each grain of abrasive functions as a microscopic single-point cutting edge (although of high negative rake angle
), and shears a tiny chip that is analogous to what would conventionally be called a "cut" chip (turning, milling, drilling, tapping, etc.). However, among people who work in the machining fields, the term cutting is often understood to refer to the macroscopic cutting operations, and grinding is often mentally categorized as a "separate" process. This is why the terms are usually used in contradistinction in shop-floor practice, even though, strictly speaking, grinding is a subset of cutting.
Similar abrasive cutting processes are lapping
and sanding
.
Surface grinding uses a rotating abrasive wheel to smooth the flat surface of metallic or nonmetallic materials to give them a more refined look or to attain a desired surface for a functional purpose. The tolerances that are normally achieved with grinding are ± 2 × 10−4 inches for a grinding a flat material, and ± 3 × 10−4 inches for a parallel surface (in metric units: 5 μm for flat material and 8 μm for parallel surface).
The surface grinder is composed of an abrasive wheel, a workholding device known as a chuck
, either electromagnetic or vacuum, and a reciprocating table.
Typical workpiece materials include cast iron and minor steel. These two materials do not tend to clog the grinding wheel while being processed. Other materials are aluminum, stainless steel, brass and some plastics.
The five types of cylindrical grinding are: outside diameter (OD) grinding, inside diameter (ID) grinding, plunge grinding, creep feed grinding, and centerless grinding.
A cylindrical grinder
has a grinding (abrasive) wheel, two centers that hold the workpiece, and a chuck, grinding dog, or other mechanism to drive the machine. Most cylindrical grinding machines include a swivel to allow for the forming of tapered pieces. The wheel and workpiece move parallel to one another in both the radial and longitudinal directions. The abrasive wheel can have many shapes. Standard disk shaped wheels can be used to create a tapered or straight workpiece geometry while formed wheels are used to create a shaped workpiece. The process using a formed wheel creates less vibration than using a regular disk shaped wheel.
Tolerances for cylindrical grinding are held within five ten-thousandths of an inch (+/- 0.0005) (metric: +/- 13 um) for diameter and one ten-thousandth of an inch(+/- 0.0001) (metric: 2.5 um) for roundness. Precision work can reach tolerances as high as fifty millionths of an inch (+/- 0.00005) (metric: 1.3 um) for diameter and ten millionths (+/- 0.00001) (metric: 0.25 um) for roundness. Surface finishes can range from 2 to 125 microinches (metric: 50 nm to 3 um), with typical finishes ranging from 8-32 microinches. (metric: 0.2 um to 0.8 um)
With CFG it takes 117 sec to remove 1 in.3 of material, whereas precision grinding would take more than 200 sec to do the same. CFG has the disadvantage of a wheel that is constantly degrading, and requires high spindle power, 51 hp (38 kW), and is limited in the length of part it can machine.
To address the problem of wheel sharpness, continuous-dress creep-feed grinding (CDCF) was developed in the 1970s. It dresses the wheel constantly during machining, keeping it in a state of specified sharpness. It takes only 17 sec. to remove 1 in3 of material, a huge gain in productivity. 38 hp (28 kW) spindle power is required, and runs at low to conventional spindle speeds. The limit on part length was erased.
High-efficiency deep grinding (HEDG) uses plated superabrasive wheels, which never need dressing and last longer than other wheels. This reduces capital equipment investment costs. HEDG can be used on long part lengths, and removes material at a rate of 1 in3 in 83 sec. It requires high spindle power and high spindle speeds.
Peel grinding, patented under the name of Quickpoint in 1985 by Erwin Junker Maschinenfabrik, GmbH in Nordrach, Germany, uses a tool with a with superabrasive nose and can machine cylindrical parts.
VIPER (Very Impressive Performance Extreme Removal), 1999, is a process patented by Rolls-Royce and is used in aerospace manufacturing to produce turbine blades. It uses a continuously dressed aluminum oxide grinding wheel running at high speed. CNC-controlled nozzles apply refrigerated grinding fluid during the cut. VIPER is performed on equipment similar to a CNC machining center, and uses special wheels.
Ultra-high speed grinding (UHSG) can run at speeds higher than 40,000 fpm (200 m/s), taking 41 sec to remove 1 in.3 of material, but is still in the R&D stage. It also requires high spindle power and high spindle speeds.
Internal grinding is used to grind the internal diameter of the workpiece. Tapered holes can be ground with the use of internal grinders that can swivel on the horizontal.
Centerless grinding
is when the workpiece is supported by a blade instead of by centers or chucks. Two wheels are used. The larger one is used to grind the surface of the workpiece and the smaller wheel is used to regulate the axial movement of the workpiece. Types of centerless grinding include through-feed grinding, in-feed/plunge grinding, and internal centerless grinding.
Pre-grinding When a new tool has been built and has been heat-treated, it is pre-ground before welding or hardfacing commences. This usually involves grinding the OD slightly higher than the finish grind OD to ensure the correct finish size.
Electrochemical grinding
is a type of grinding in which a positively charged workpiece in a conductive fluid is eroded by a negatively charged grinding wheel. The pieces from the workpiece are dissolved into the conductive fluid.
Electrolytic in-process dressing (ELID) grinding is one of the most accurate grinding methods. In this ultra precision grinding technology the grinding wheel is dressed electrochemically and in-process to maintain the accuracy of the grinding. An ELID cell consists of a metal bonded grinding wheel, a cathode electrode, a pulsed DC power supply and electrolyte. The wheel is connected to the positive terminal of the DC power supply through a carbon brush whereas the electrode is connected to the negative pole of the power supply. Usually alkaline liquids are used as both electrolytes and coolant for grinding. A nozzle is used to inject the electrolyte into the gap between wheel and electrode. The gap is usually maintained to be approximately 0.1mm to 0.3 mm. During the grinding operation one side of the wheel takes part in the grinding operation whereas the other side of the wheel is being dressed by electrochemical reaction. The dissolution of the metallic bond material is caused by the dressing which in turns results continuous protrusion of new sharp grits.
Physical property changes include the possible loss of magnetic properties on ferromagnetic materials.
Chemical property changes include an increased susceptibility to corrosion because of high surface stress.
Abrasive machining
Abrasive machining is a machining process where material is removed from a workpiece using a multitude of small abrasive particles. Common examples include grinding, honing, and polishing...
process that uses a grinding wheel
Grinding wheel
A grinding wheel is an expendable wheel that is composed of an abrasive compound used for various grinding and abrasive machining operations...
as the cutting tool.
A wide variety of machines are used for grinding:
- Hand-cranked knife-sharpening stones (grindstonesGrindstone (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....
) - Handheld power toolPower toolA power tool is a tool that is actuated by an additional power source and mechanism other than the solely manual labour used with hand tools. The most common types of power tools use electric motors. Internal combustion engines and compressed air are also commonly used...
s such as angle grinderAngle grinderAn angle grinder, also known as a side or disc grinder, is a handheld power tool used for cutting, grinding and polishing.Angle grinders can be powered by an electric motor, petrol engine or compressed air. The motor drives a geared head at a right-angle on which is mounted an abrasive disc or a...
s and die grinderDie grinderA die grinder is a handheld power tool used to grind, sand, hone, polish, or machine material, typically metal but also plastic or wood. They are usually pneumatically driven, although versions with electric and flexible shaft drive also exist...
s - Various kinds of expensive industrial machine toolMachine toolA machine tool is a machine, typically powered other than by human muscle , used to make manufactured parts in various ways that include cutting or certain other kinds of deformation...
s called grinding machineGrinding machineA grinding machine, often shortened to grinder, is a machine tool used for grinding, which is a type of machining using an abrasive wheel as the cutting tool...
s - Bench grinderBench grinderA bench grinder is a type of benchtop grinding machine used to drive abrasive wheels. A pedestal grinder is a larger version of a bench grinder that is mounted on a pedestal, which is bolted to the floor...
s often found in residential garages and basements
Grinding practice is a large and diverse area of manufacturing
Manufacturing
Manufacturing is the use of machines, tools and labor to produce goods for use or sale. The term may refer to a range of human activity, from handicraft to high tech, but is most commonly applied to industrial production, in which raw materials are transformed into finished goods on a large scale...
and toolmaking
Tool and die maker
Tool and die makers are workers in the manufacturing industry who make jigs, fixtures, dies, molds, machine tools, cutting tools , gauges, and other tools used in manufacturing processes...
. It can produce very fine finishes and very accurate dimensions; yet in mass production contexts it can also rough out large volumes of metal quite rapidly. It is usually better suited to the machining of very hard materials than is "regular" machining (that is, cutting larger chips with cutting tools such as tool bit
Tool bit
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...
s or milling cutter
Milling cutter
Milling cutters are cutting tools typically used in milling machines or machining centres . They remove material by their movement within the machine or directly from the cutter's shape .-Features of a milling cutter:Milling cutters come in several shapes and many sizes...
s), and until recent decades it was the only practical way to machine such materials as hardened steels. Compared to "regular" machining, it is usually better suited to taking very shallow cuts, such as reducing a shaft's diameter by half a thousand of an inch (thou) or 12.7 um.
Grinding is a subset of cutting, as grinding is a true metal-cutting process. Each grain of abrasive functions as a microscopic single-point cutting edge (although of high negative rake angle
Rake angle
Rake angle is a parameter used in various cutting and machining processes, describing the angle of the cutting face relative to the work. There are three types of rake angles: positive, negative, and zero...
), and shears a tiny chip that is analogous to what would conventionally be called a "cut" chip (turning, milling, drilling, tapping, etc.). However, among people who work in the machining fields, the term cutting is often understood to refer to the macroscopic cutting operations, and grinding is often mentally categorized as a "separate" process. This is why the terms are usually used in contradistinction in shop-floor practice, even though, strictly speaking, grinding is a subset of cutting.
Similar abrasive cutting processes are lapping
Lapping
Lapping is a machining operation, in which two surfaces are rubbed together with an abrasive between them, by hand movement or by way of a machine.This can take two forms...
and sanding
Sandpaper
Sandpaper, also known as glasspaper, is a heavy paper with abrasive material attached to its surface.Sandpaper is part of the "coated abrasives" family of abrasive products. It is used to remove small amounts of material from surfaces, either to make them smoother , to remove a layer of material...
.
Processes
Selecting abc which of the following grinding operations to be used is determined by the size, shape, features and the desired production rate.Surface grinding
The surface grinder is composed of an abrasive wheel, a workholding device known as a chuck
Chuck (engineering)
A chuck is a specialized type of clamp used to hold an object, usually an object with radial symmetry, especially a cylindrical object. It is most commonly used to hold a rotating tool or a rotating workpiece...
, either electromagnetic or vacuum, and a reciprocating table.
Typical workpiece materials include cast iron and minor steel. These two materials do not tend to clog the grinding wheel while being processed. Other materials are aluminum, stainless steel, brass and some plastics.
Cylindrical grinding
Cylindrical grinding (also called center-type grinding) is used in the removing the cylindrical surfaces and shoulders of the workpiece. The workpiece is mounted and rotated by a workpiece holder, also known as a grinding dog or center driver. Both the tool and the workpiece are rotated by separate motors and at different speeds. The axes of rotation tool can be adjusted to produce a variety of shapes.The five types of cylindrical grinding are: outside diameter (OD) grinding, inside diameter (ID) grinding, plunge grinding, creep feed grinding, and centerless grinding.
A cylindrical grinder
Cylindrical grinder
The cylindrical grinder is a type of grinding machine used to shape the outside of an object. The cylindrical grinder can work on a variety of shapes, however the object must have a central axis of rotation...
has a grinding (abrasive) wheel, two centers that hold the workpiece, and a chuck, grinding dog, or other mechanism to drive the machine. Most cylindrical grinding machines include a swivel to allow for the forming of tapered pieces. The wheel and workpiece move parallel to one another in both the radial and longitudinal directions. The abrasive wheel can have many shapes. Standard disk shaped wheels can be used to create a tapered or straight workpiece geometry while formed wheels are used to create a shaped workpiece. The process using a formed wheel creates less vibration than using a regular disk shaped wheel.
Tolerances for cylindrical grinding are held within five ten-thousandths of an inch (+/- 0.0005) (metric: +/- 13 um) for diameter and one ten-thousandth of an inch(+/- 0.0001) (metric: 2.5 um) for roundness. Precision work can reach tolerances as high as fifty millionths of an inch (+/- 0.00005) (metric: 1.3 um) for diameter and ten millionths (+/- 0.00001) (metric: 0.25 um) for roundness. Surface finishes can range from 2 to 125 microinches (metric: 50 nm to 3 um), with typical finishes ranging from 8-32 microinches. (metric: 0.2 um to 0.8 um)
Creep-feed grinding
Creep-feed grinding (CFG) was invented in Germany in the late 1950s by Edmund and Gerhard Lang. Unlike normal grinding, which is used primarily to finish surfaces, CFG is used for high rates of material removal, competing with milling and turning as a manufacturing process choice. Depths of cut of up to 6 mm (0.25 inches) are used along with low workpiece speed. Surfaces with a softer-grade resin bond are used to keep workpiece temperature low and an improved surface finish up to 1.6 micrometres RmaxWith CFG it takes 117 sec to remove 1 in.3 of material, whereas precision grinding would take more than 200 sec to do the same. CFG has the disadvantage of a wheel that is constantly degrading, and requires high spindle power, 51 hp (38 kW), and is limited in the length of part it can machine.
To address the problem of wheel sharpness, continuous-dress creep-feed grinding (CDCF) was developed in the 1970s. It dresses the wheel constantly during machining, keeping it in a state of specified sharpness. It takes only 17 sec. to remove 1 in3 of material, a huge gain in productivity. 38 hp (28 kW) spindle power is required, and runs at low to conventional spindle speeds. The limit on part length was erased.
High-efficiency deep grinding (HEDG) uses plated superabrasive wheels, which never need dressing and last longer than other wheels. This reduces capital equipment investment costs. HEDG can be used on long part lengths, and removes material at a rate of 1 in3 in 83 sec. It requires high spindle power and high spindle speeds.
Peel grinding, patented under the name of Quickpoint in 1985 by Erwin Junker Maschinenfabrik, GmbH in Nordrach, Germany, uses a tool with a with superabrasive nose and can machine cylindrical parts.
VIPER (Very Impressive Performance Extreme Removal), 1999, is a process patented by Rolls-Royce and is used in aerospace manufacturing to produce turbine blades. It uses a continuously dressed aluminum oxide grinding wheel running at high speed. CNC-controlled nozzles apply refrigerated grinding fluid during the cut. VIPER is performed on equipment similar to a CNC machining center, and uses special wheels.
Ultra-high speed grinding (UHSG) can run at speeds higher than 40,000 fpm (200 m/s), taking 41 sec to remove 1 in.3 of material, but is still in the R&D stage. It also requires high spindle power and high spindle speeds.
Others
is a specialized type of cylindrical grinding where the grinding wheel has the exact shape of the final product. The grinding wheel does not traverse the workpiece.Internal grinding is used to grind the internal diameter of the workpiece. Tapered holes can be ground with the use of internal grinders that can swivel on the horizontal.
Centerless grinding
Centerless grinding
Centerless grinding is a method of material removal through grinding, similar to centered grinding except for the absence of the spindle. It has high throughput, i.e. a large number of parts can be manufactured in a short time....
is when the workpiece is supported by a blade instead of by centers or chucks. Two wheels are used. The larger one is used to grind the surface of the workpiece and the smaller wheel is used to regulate the axial movement of the workpiece. Types of centerless grinding include through-feed grinding, in-feed/plunge grinding, and internal centerless grinding.
Pre-grinding When a new tool has been built and has been heat-treated, it is pre-ground before welding or hardfacing commences. This usually involves grinding the OD slightly higher than the finish grind OD to ensure the correct finish size.
Electrochemical grinding
Electrochemical grinding
Electrochemical grinding is a process that removes electrically conductive material by grinding with a negatively charged abrasive grinding wheel, an electrolyte fluid, and a positively charged workpiece. Materials removed from the workpiece stay in the electrolyte fluid...
is a type of grinding in which a positively charged workpiece in a conductive fluid is eroded by a negatively charged grinding wheel. The pieces from the workpiece are dissolved into the conductive fluid.
Electrolytic in-process dressing (ELID) grinding is one of the most accurate grinding methods. In this ultra precision grinding technology the grinding wheel is dressed electrochemically and in-process to maintain the accuracy of the grinding. An ELID cell consists of a metal bonded grinding wheel, a cathode electrode, a pulsed DC power supply and electrolyte. The wheel is connected to the positive terminal of the DC power supply through a carbon brush whereas the electrode is connected to the negative pole of the power supply. Usually alkaline liquids are used as both electrolytes and coolant for grinding. A nozzle is used to inject the electrolyte into the gap between wheel and electrode. The gap is usually maintained to be approximately 0.1mm to 0.3 mm. During the grinding operation one side of the wheel takes part in the grinding operation whereas the other side of the wheel is being dressed by electrochemical reaction. The dissolution of the metallic bond material is caused by the dressing which in turns results continuous protrusion of new sharp grits.
Grinding wheel
A grinding wheel is an expendable wheel used for various grinding and abrasive machining operations. It is generally made from a matrix of coarse abrasive particles pressed and bonded together to form a solid, circular shape, various profiles and cross sections are available depending on the intended usage for the wheel. Grinding wheels may also be made from a solid steel or aluminium disc with particles bonded to the surface.Lubrication
The use of fluids in a grinding process is necessary to cool and lubricate the wheel and workpiece as well as remove the chips produced in the grinding process. The most common grinding fluids are water-soluble chemical fluids, water-soluble oils, synthetic oils, and petroleum-based oils. It is imperative that the fluid be applied directly to the cutting area to prevent the fluid being blown away from the piece due to rapid rotation of the wheel.| Work Material | Cutting Fluid | Application |
|---|---|---|
| Aluminum | Light duty oil | Flood |
| Brass | Light duty oil | Flood |
| Cast Iron | Heavy duty emulsifiable oil, light duty chemical oil, synthetic oil | Flood |
| Mild Steel | Heavy duty water soluble oil | Flood |
| Stainless Steel | Heavy duty emulsifiable oil, heavy duty chemical oil, synthetic oil | Flood |
| Plastics | Water soluble oil, dry, heavy duty emulsifiable oil, dry, light duty chemical oil, synthetic oil | Flood |
Workholding methods
The workpiece is manually clamped to a lathe dog, powered by the faceplate, that holds the piece in between two centers and rotates the piece. The piece and the grinding wheel rotate in opposite directions and small bits of the piece are removed as it passes along the grinding wheel. In some instances special drive centers may be used to allow the edges to be ground. The workholding method affects the production time as it changes set up times.Workpiece materials
Typical workpiece materials include aluminum, brass, plastics, cast iron, mild steel, and stainless steel. Aluminum, brass and plastics can have poor to fair machinability characteristics for cylindrical grinding. Cast Iron and mild steel have very good characteristics for cylindrical grinding. Stainless steel is very difficult to grind due to its toughness and ability to work harden, but can be worked with the right grade of grinding wheels.Workpiece geometry
The final shape of a workpiece is the mirror image of the grinding wheel, with cylindrical wheels creating cylindrical pieces and formed wheels creating formed pieces. Typical sizes on workpieces range from .75 in. to 20 in. (metric: 18mm to 1 m) and .80 in. to 75 in. in length (metric: 2 cm to 4 m), although pieces between .25 in. and 60 in. in diameter (metric: 6 mm to 1.5 m) and .30 in. and 100 in. in length (metric: 8 mm to 2.5 m) can be ground. Resulting shapes can range from straight cylinders, straight edged conical shapes, or even crankshafts for engines that experience relatively low torque.Effects on Workpiece Materials
Mechanical properties will change due to stresses put on the part during finishing. High grinding temperatures may cause a thin martensitic layer to form on the part, which will lead to reduced material strength from microcracks.Physical property changes include the possible loss of magnetic properties on ferromagnetic materials.
Chemical property changes include an increased susceptibility to corrosion because of high surface stress.
See also
- Cryogenic grindingCryogenic grindingCryogenic grinding, also known as freezer milling, freezer grinding, and cryomilling, is the act of cooling or chilling a material and then reducing it into a small particle size. For example, thermoplastics are difficult to grind to small particle sizes at ambient temperatures because they soften,...
- Diamond grindingDiamond grindingDiamond grinding is a pavement preservation technique that corrects a variety of surface imperfections on both concrete and asphalt pavements. Most often utilized on concrete pavement, diamond grinding is typically performed in conjunction with other concrete pavement preservation techniques such...
- Diamond grinding of pavement
- Flat honingFlat honingFlat honing is a metalworking grinding process used to provide high quality flat surfaces. It combines the speed of grinding or honing with the precision of lapping. It has also been known under the terms high speed lapping and high precision grinding.- Origin of term :The term is derived from...
- Grinding dresserGrinding dresserA grinding dresser or wheel dresser is a tool to dress the surface of a grinding wheel. Grinding dressers are used for making different profile on grinding wheel. The dressing of a wheel in order to return the wheel to its original shape...
- High stock removal
- Honing (metalworking)Honing (metalworking)Honing is an abrasive machining process that produces a precision surface on a metal workpiece by scrubbing an abrasive stone against it along a controlled path...
- Hydro-erosive grindingHydro-erosive grindingHydro-erosive grinding is a process for radiusing orifice edges and hole intersections by flowing an abrasive fluid through. This process can calibrate the holes to flow a certain static flow rate.-External links:...
- SwarfSwarfSwarf, also known as turnings, chips, or filings, are shavings and chippings of metal — the debris or waste resulting from metalworking operations including milling and grinding. It can usually be recycled, and this is the preferred method of disposal due to the environmental concerns regarding...