Grinding (abrasive cutting)

Sketch of how abrasive particles in a grinding wheel remove material from a workpiece.

Grinding is a machining process that uses an abrasive wheel as the cutting tool.

A wide variety of machines are used for grinding. They include:

• hand-cranked knife-sharpening stones;
• handheld power tools such as angle grinders and die grinders;
• various kinds of expensive industrial machine tools called grinding machines; and
• the bench grinders often found in residential garages and basements.

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 bits or milling cutters), 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 thou.

Technically, grinding is a subset of cutting, as grinding is a true metalcutting 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 technically grinding is a subset of cutting.

Similar abrasive cutting processes are lapping and sanding.

Types of grinding processes

Selecting which of the following grinding operations to be used is determined by the size, shape, features and desired production rate.

• Surface grinding is the most common of the grinding operations. A rotating wheel is used in the grinding of flat surfaces. However, the grinding wheel is not limited to just a cylindrical shape, but can have a myriad of options that are useful in transferring different designs to the object being worked on. When surface grinding an object, one must keep in mind that the shape of the wheel will be transferred to the material of the object like a mirror image. It is common practice to what is called "dress" the stone. So as to make sure the shape of the wheel is what you want. This is done by using a diamond to remove the abrasive material not wanted and to give the desired geometry. Surface grinding is a finishing process that smooths the surface of metallic or nonmetallic materials and gives them a more refined look. The surface grinder is comprised of an abrasive wheel, a work-holding device known as a chuck either electromagnetic or vacuum, and a reciprocating table. Depending on the material being worked on and the desired surface finish, the wheel's abrasive material can vary from aluminum oxide, silicon carbide, diamond, and cubic boron nitride. The chuck is a device that is used to hold the material in place while it is being worked on. It can do this one of two ways; metallic pieces are held in place by a magnetic chuck, while nonmetallic pieces are vacuumed in place. During the grinding process, the workpiece must be flooded with a coolant to keep it from getting too warm, to act as a lubricant, and also to keep the excess shavings from building up. There are different types of coolants that are used depending on the material that is being worked on; the most common are: water-soluble chemical fluids, water-soluble oils, synthetic oils, and petroleum-based oils. In applying the coolant, care should be taken to apply the liquid directly to the location of the grind. This ensures the coolant reaches the material and is not deflected by the grind wheels high speeds. Typical workpiece materials include cast iron and minor steel. These two materials don't tend to clog the grinding wheel while being processed. Other materials to are aluminum, stainless steel, brass and some plastics. The three most important factors to consider when surface grinding are: the material of the grinding wheel, the material of the piece being worked on, and the grinding fluid. Not using the proper grinding fluid can result in the surface finish not being completely smooth. Of course, when grinding at high temperatures, the material tends to become weakened and is more inclined to corrode. This can also result in a loss of magnetism in materials where this is applicable. The tolerances that are normally achieved with grinding are ± 2 × 10⁻⁴inches for a grinding a flat material, and ± 3 × 10⁻⁴inches for a parallel surface. When working with any type of grinder, one should always be cognizant of safety procedures in order to avoid injury.

Cost elements of surface grinding:

• Time it takes to setup
• Time it takes to unload and load
• Time that the grinder is not in use
• Time it takes to grind the material
• Time it takes to change the tools
• The cost of tools/machinery
• Worker pay
• Sale price of finished product
• Paying for use of tools and equipment

Calculations

• Time to grind = Number of Grind Wheel Passes × the Length of the item to be ground ÷ the Feed rate of the material
• Number of Grind Wheel Passes = the Width of ground item ÷ (width of grind wheel - the distance the grind wheel Overlaps previous runs) × thickness of the grind ÷ depth grind wheel removes per pass

Cylindrical grinding

• Cylindrical grinding is also called center-type grinding and 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.

Process/Equipment

Process Characteristics

• Creates a piece that is formed, straight or tapered.
• Can only be cylindrical in shape
• Is a final machine process used to create highly accurate and smooth surfaces

Setup and Equipment

The setup of a cylindrical grinding machine 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.

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. and .80 in. to 75 in. in length, although pieces between .25 in. and 60 in. in diameter and .30 in. and 100 in. in length can be ground. Resulting shapes can range from straight cylinders, straight egded conical shapes, or even crankshafts for engines that experience relatively low torque.

Abrasive Materials

The most commonly used abrasive materials are Aluminum oxide, silicon carbide, diamond, and cubic boron nitride (CBN), with aluminum oxide being the most common of the four. Diamond and CBN wheels are often made of a cheaper core with outer layer of abrasive material to make the wheel less expensive. Diamond and CBN wheels are very hard and can grind down materials such as ceramic and carbides economically. There are five characteristics that describe the abrasive material on a cutting wheel.

• The material itself, designated by a letter.
  • A for Aluminum Oxide
  • C for Silicon Carbide
  • D, MD, SD for diamond
  • B for Cubic Boron Nitride
• Grain size ranging from 8 (coarsest) 600 (finest).
• The wheel grade is marked be a letter from A (soft) to Z (hard).
• The grain spacing is labeled by a number from 1 (densest) to 16 (least dense).
• The grinding wheel bonds in which the six most common are
  • Vitrified (V)
  • Resinoid (R)
  • Silicate (S)
  • Shellac (E)
  • Rubber (R)
  • Oxychloride (O)

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.

Lubrication and Cooling

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

Tolerances and Surface Finishes

Tolerances for cylindrical grinding are held within five ten-thousandths of an inch (+/- 0.0005) for diameter and one ten-thousandth of an inch(+/- 0.0001) for roundness. Precision work can reach tolerances as high as five hundred-thousandths of an inch (+/- 0.00005) for diameter and one hundred-thousandth of an inch (+/- 0.00001) for roundness. Surface finishes can range from 2 to 125 microinches, with typical finishes ranging from 8-32 microinches.

Workpiece Materials

Typical 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.

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.

Material Property	Effects of Grinding
Mechanical	Residual surface stresses Possible forming of a thin martensitic layer on surface Reduced fatigue strength
Physical	Possible loss of magnetic properties on ferromagnetic materials
Chemical	May increase susceptibility to corrosion because of high surface stress

Form grinding

Form grinding is a specialized type of cylindrical grinding where the grinding wheel has the exact shape of the final product. The grinding wheel does not transverse the workpiece.

• Internal grinding is used to grind the inside diameter of the workpiece. Tapered holes can be ground with the use of internal grinders that can swivel on the horizontal.internal grinding

• 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.

• Creep-feed grinding is used for high rates of material removal. 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 Rmax

• 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. electrochemical grinding

See also

• Hydro-erosive grinding

References

Bibliography

• Adithan, M.; Gupta, A. B. (2002), Manufacturing Technology, New Age International Publishers, ISBN 9788122408171.
• Jones, Franklin; Ryffel, Henry; Oberg, Erik; Mcauley, Christopher; Heald, Ricardo (2004), Machinery's Handbook (27 ed.), New York: Industrial Press, ISBN 0831127007.
• Todd, Robert; Allen, Dell; Alting, Leo (1994), Manufacturing Processes Reference Guide, New York, New York: Industrial Press, ISBN 0-8311-3049-0 {{citation}}: Text "author-link Robert Todd" ignored (help).