A workpiece undergoes different machining operations during a manufacturing process, which often renders its surface rough.
The grinding process helps refine the surface of the workpiece by removing unwanted material and producing a smooth surface with the desired finish.
But what exactly is grinding? What are its different types? And what are the tools used for grinding?
Grinding is a machining operation that removes material from a workpiece’s surface via friction by bringing the workpiece in contact with a rotary wheel embedded with abrasives. The abrasive materials on the grinding wheel surface aid in material removal by increasing frictional contact.
This article comprehensively covers all these aspects, providing a thorough understanding of grinding and its relation to machining.
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What is Grinding (Machining)?
Grinding is a finishing process used to refine the surface of the workpiece by removing a fine layer of material to achieve the desired tolerance.
Unlike ball grinding where the workpiece is entirely rushed into fine powdered form, grinding is performed on the surface of the workpiece to remove the material and enhance its surface finish.
There are generally three main mechanisms that drive the grinding process: cutting, plowing, and sliding.
The cutting mechanism occurs when the abrasive grinding wheel can effectively penetrate the workpiece and remove material.
There is sufficient clearance for the chip to flow away via the coolant flow and wheel movement.
Plowing occurs when the grinding wheel cannot effectively remove material, pushing the material inward relative to its edge.
It involves plastic deformation of the workpiece without removing the material from its surface.
Sliding occurs due to an inadequate cutting depth, low clearance between the wheel and workpiece surface, or when the abrasives on the grinding wheel wear out.
This results in sliding of the grinding wheel over the workpiece's surface without effectively removing the material, resulting in excessive rubbing of the two surfaces and scratches.
To attain the best results for grinding, it is important to clamp the workpiece firmly by using clamping devices such as magnetic chucks, jaw chucks, etc., to minimize vibrations.
Parameters Affecting the Grinding Process
The grinding process is governed by various parameters, such as machine tool geometry, tool composition, work material, etc.
Diameter and Speed of the Grinding Wheel
The speed of rotation of the grinding wheel depends upon its size.
A large wheel requires greater torque to rotate and therefore requires a powerful grinding machine, whereas a small wheel can be rotated at high speeds with relative ease.
Generally, for grinding operations, it is advised to maintain a high rotational speed to achieve a smooth surface finish.
As a result, it is preferable to use small-sized grinding wheels that can be easily rotated at high speeds.
However, using a very small-sized wheel for machining of large workpiece can generate high frictional heat and damage the grinding wheel.
Therefore, it is advised to use a moderate-sized wheel that can be rotated at high speed and provides enough surface area to effectively dissipate the generated heat.
Apart from that, it is also important to regulate the RPM because a low speed will lead to insufficient grinding, while an extremely high speed can damage the grinding wheel.
For instance, a 5" (127 mm) grinding wheel should operate in the range of 7000 to 12000 rpm for optimal grinding.
Crossfeed of the Workpiece
The crossfeed is the movement of the workpiece surface relative to the grinding wheel.
When performing a grinding operation, the workpiece is moved in a uniform to-and-fro motion to grind the entire length of the workpiece.
Generally, it is advised to perform short strokes and utilize around 25% of the wheel width to attain a high surface finish.
Contrarily, increasing the crossfeed reduces surface quality while enhancing productivity.
Therefore, you can also utilize around 90% of the width when performing roughing operations.
Grinding Ratio
The grinding ratio is the ratio of the amount of work material removed per unit wear of the grinding wheel.
A higher grinding ratio is preferable for long tool life. Hence, if the grinding ratio is too low, it is advised to modify the machining parameters to improve tool life.
Grinding Wheel Composition
Grinding wheels are usually composed of aluminum oxide, silicon carbide, cubic boron nitride, or diamond abrasives in addition to a steel or aluminum disc.
This composite material composition is strongly bonded together, which aids in the grinding process.
Harder abrasives like diamond are preferable for grinding harder workpieces like stainless steel, whereas silicon carbide abrasives are used for grinding softer materials like aluminum.
Types of Grinding Operations
Cylindrical Grinding
In this type of grinding, the workpiece and the grinding wheel rotate while they are in contact.
The workpiece can either be mounted onto a separate rotating chuck or the same rotating shaft as the grinding wheel.
For the grinding operation to occur, the workpiece can either be fed into the grinding wheel or moved across the wheel face.
Cylindrical grinding utilizes straight wheels, usually 0.5" (12.7 mm) to 3" (76.2 mm) in diameter, with Cubic Boron Nitride or Diamond abrasives.
Surface Grinding
Surface grinding involves the movement of the workpiece under the grinding wheel by either mounting the workpiece on a horizontal spindle that moves it to and fro or on a rotary table that rotates and moves the workpiece in a circular pattern during grinding.
This type of grinding is best suited for obtaining smooth, flat surfaces and machining grooves that would otherwise require finishing milling operations.
The wheel specifications and geometry are similar to that of cylindrical grinding.
Centerless Grinding
Centerless grinding is ideal for obtaining cylindrical shapes with extremely tight tolerances.
Instead of mounting the workpiece on a central support, it is supported by the grinding wheel, feed wheel, and work support blade at three different locations, with the grinding wheel and feed wheel rotating in the same direction.
The workpiece between them rotates in the opposite direction, where the support blade raises the workpiece position above the centreline for efficient grinding.
Through-feed Centerless Grinding
This category is better suited for workpieces having an overall cylindrical shape. The feed wheel moves laterally, feeding the workpiece against the grinding wheel.
Infeed Centerless Grinding
Infeed grinding is ideal for stock that has varying diameters or protrusions. The feed wheel, in this case, moves the workpiece in a downward direction.
Endfeed Centerless Grinding
The grinding wheel, feed wheel, and support blade position are such that a tapered or conical surface is achievable by passing the workpiece through this configuration.
Creep Feed Grinding
This grinding process is a one-pass operation that machines a deep cut of about 1" (25.4 mm) at low RPMs.
A second pass is also achievable but with a smaller depth of cut of about 0.002" (0.0508 mm).
This process requires continuous and large amounts of coolant flow as its power requirements and heat generation levels are high.
Typically, steel and iron workpieces undergo this grinding.
Snagging Grinding
Snagging grinding usually employs straight or straight-cup grinding wheels mounted vertically or horizontally, with the workpiece mechanically fed against the wheel for grinding.
This type of grinding is generally used to remove unwanted material like castings, weld protrusions, risers, etc., from the surface of the workpiece, and therefore does not produce a high surface finish.
Cut-off Operations
This grinding performs cutting operations as an alternative to conventional cutting operations, like milling, laser cutting, etc.
The grinding wheel possesses multiple cutting points due to the abrasives embedded on its surface. The surface speed during this operation is generally as high as 2 to 3 miles per minute.
This process requires a powerful grinding machine, and a rule of thumb dictates that for every additional inch of diameter of the grinding wheel, the required power increases by one horsepower.
Tools Used for Grinding
The grinding wheel is the backbone of the complete grinding operation. It is an aluminum or steel disc with abrasive particles pressed and bonded to its surface.
Grinding tools are classified into two main categories: handheld grinders and stationary grinders.
Handheld Grinders
Angle Grinder
An angle grinder is a popular and common grinder comprising a grinding wheel positioned at an angle to the main power shaft that runs it.
This type of grinder is not only beneficial for polishing and smoothening surfaces but also for cutting and material removal operations.
Fabrication and construction industries commonly utilize this type of grinding tool.
Die Grinder
Die grinders are similar to angle grinders. However, it is more prevalent in hobbyist applications due to its low cost and small size.
Cabinet makers often employ this grinder for woodworking and related operations.
Linear Grinder
Linear grinders comprise grinding wheels positioned perpendicularly to the grinder holding bar. It is used for surface polishing and smoothening.
The grinding wheel, in this case, has a thick width, making it more durable.
Automobile body professionals commonly use this category of grinders for surface finishing.
Power Drill
Although the grinding operation using a drill is time-consuming and not as effective, it is cheaper, making it a popular alternative for hobbyists.
Stationary Grinders
Bench Grinder
A bench grinder is usually fixed onto a workbench and can be used for different types of applications.
Using a suitable grinding wheel allows you to use this type of grinder for roughening and finishing operations.
Belt Grinder
Belt grinders comprise a grinding belt coated with abrasives instead of a grinding wheel. The belt can be positioned at any angle, making grinding more convenient.
The belt has a longer life and can be used for various applications, such as deburring and polishing. Knife-making often utilizes this grinding.
Types of Abrasives Used in Grinding Wheels
The abrasives are mainly categorized into two types: natural and manufactured.
Abrasives, like aluminum oxide, silicon carbide, cubic boron nitride, etc., are manufactured for specific applications, whereas diamond abrasives use very fine particles of natural diamonds.
However, with the advancement in technology, even diamond abrasives are slowly being replaced with synthetic diamonds.
| Abrasive Material | Applications |
|---|---|
| Aluminum Oxide | Grinding Steel and ferrous alloys |
| Silicon carbide | Grinding cast irons, non-ferrous metals like copper, non-metals like ceramics, and cemented carbides |
| Cubic Boron Nitride (CBN) | Grinding high-speed super hard steels, stainless steels, cast irons, etc. |
| Diamond | Grinding cemented carbides, marble, granite, and stone. |
Aluminum oxide
Aluminum oxide is prepared from refining bauxite ore in a furnace.
Different composition levels of aluminum oxide constituent elements lead to varying degrees of toughness.
Aluminum-coated wheels are usually employed for grinding steel and other ferrous alloys.
Silicon carbide
Silicon carbide wheels consist of pure white quartz heated and mixed with petroleum coke, sawdust, and salt in a furnace. It is harder and more brittle compared to aluminum oxide.
There are two categories of silicon carbide wheels: black and green.
Black wheels are ideal for grinding soft materials like cast iron, copper, and non-metals like ceramics, while green wheels are less durable and ideal for grinding cemented carbides.
It must be noted that separate grinding wheels must be used for ferrous and non-ferrous metals to avoid contamination.
Cubic Boron Nitride (CBN)
CBN is extremely hard and can withstand temperatures up to 2500 °C. It is manufactured from a series of high-temperature and high-pressure processes, similar to those involved in manufacturing synthetic diamonds.
It is used for grinding applications involving high-speed super hard steels, stainless steels, cast irons, etc.
Diamond
Diamonds are a crystalline form of carbon, with natural diamonds being more expensive than synthetic diamonds.
Both categories are the hardest and strongest abrasives rendering them ideal for use in grinding cemented carbides, marble, granite, and stone.
Nomenclature of a Grinding Wheel
All grinding wheels are marked and labeled with their type, size, specification, maximum operating speed, safety precautions, and miscellaneous information.
Wheel Type
The wheel type is labeled with an ISO number that signifies its shape. For instance, an ISO Type 52 label signifies that the wheel is spindle-mounted.
Wheel Size
The grinding wheel size is also clearly marked on the wheel surface.
For example, a 200 x 5 x 20 mm grinding wheel labeling means that the wheel diameter, thickness, and hole size are 7.87 in (200 mm), 0.2 in (5 mm), and 0.79 in (20 mm), respectively.
Wheel Specification
The grinding wheel specification is marked as a combination of numbers and letters.
For instance, the specification C 30 F 3 S 12 represents the type of abrasive material, the grit size, the grade, the structure, and the bond type.
C represents silicon carbide, 30 represents medium grain size, F represents soft abrasive, 3 represents dense structure, and S stands for silicate bond that binds the abrasives to the wheel's disc.
Wheel Speeds
For grinding wheels having a diameter of 3.15 in (80 mm) or more, the maximum operating speed is always marked on the wheel surface, whereas for smaller wheels, this speed is mentioned in a separate manual.
For high-speed wheels, color codes or colored stripes may also be present on the grinding wheel apparatus.
Each color represents a different speed range, identifying the minimum speed, optimal speed range, and maximum speed.
Miscellaneous Information
Miscellaneous information comprises the manufacturer’s trademark/name, safety standard test record, expiry date if the wheel contains organic bonds, a code number that traces back to the manufacturer details, and a mounting arrow that identifies the heaviest point of the wheel.
Difference Between Machining and Grinding
| Machining | Grinding |
|---|---|
| Generally involves high MRR | Generally a surface finishing process with a low MRR |
| Comparatively less accuracy and tolerance | Better dimensional accuracy and tighter tolerances |
| Uses a single-point or multi-point cutting tool | Uses a grinding wheel coated with abrasives |
| Cutters have a specific geometry with well-defined features | Grinding wheels have certain specifications but abrasives have random features |
| Rake angle can be positive or negative ranging from -15° to 15° | Different rake angles that vary from +60° to –60°, or even more |
| Clearance angle cannot be zero or negative, usually varies from +3° to +15° | Clearance angles can be zero or negative |
| Every cutting edge performs cutting operation | A small percentage of total abrasives contribute in the grinding action |
| Shearing is the main mechanism | Rubbing, scratching, and ploughing along with shearing occurs |
| Power required is relatively lower | Power required is relatively higher due to a higher loss of energy |
| Majority of the generated heat is lost through chip removal | A significant amount of heat transfers in the workpiece |
| Extremely hard, brittle, and tough materials are difficult to machine | A material with any degree of hardness, brittleness, and toughness can be ground |
Machining removes a relatively higher amount of material in the form of chips, compared to grinding which primarily smoothens the workpiece surface.
A machining operation can semi-finish a workpiece with tolerances varying from 1 to 50 micrometers, whereas grinding can achieve tolerances of 0.5 to 2 micrometers leading to higher accuracy.
In machining, a cutting tool along with carbide or diamond inserts can machine hard materials, but smoothening and polishing are impossible.
Cutters have well-defined shapes and angles, which is also the case in grinding wheels, except abrasives can form varying angles and shapes on the grinding wheel surface.
The rake angle in grinders varies significantly, and the clearance angle can be zero or negative, which is not the case in cutters used for machining.
In grinding, a small portion of the abrasives is actively engaged in the ground at a particular instant. In contrast, in machining, every edge comes in contact with the workpiece surface and removes material.
Due to the additional mechanisms of plowing, scratching, and rubbing, in addition to shearing, grinding generates more heat and requires greater power.
Extremely hard, brittle, or tough materials can damage the cutter, but in the case of grinding, the abrasives enable the grinding wheels to grind almost any material with ease.
Frequently Asked Questions (FAQ)
What is the material removal rate (MRR)?
Material removal rate (MRR) is the rate of volume of material removed from a workpiece in any machining operation.
Can grinding be automated?
Yes, grinding can be automated by implementing CNC programming and robotic arms.
What safety precautions should be exercised during the grinding operation?
The safety precautions that should be taken during a grinding operation include wearing safety shoes, gloves, a face shield, and headgear and using a machine guard around the grinding wheel for enhanced protection from chips removed from the workpiece.
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