Measuring The Performance of a Cutting Wheel
The performance of a cutting wheel can be measured by Cutting Index that is a ratio which measures the number of cuts a wheel will produce a particular application.
The Cutting Index can be measured by the following method.
1.Measure the diameter of the wheel before cutting and calculate the surface area of the workpiece that needs to be cut.
2.Determine the number of cuts on the workpiece.
3.Now measure the diameter of the wheel after the cutting and calculate the surface area of the used wheel. Multiply the surface area of the workpiece into the number of cuts to calculate the total surface area of the cut work piece.
|Cutting index =||Surface area of the cut workpiece
Surface area of the cutting wheel before cutting
|– the surface area of the cutting wheel after cutting|
Measuring The Performance of a Grinding Wheel
The performance of the grinding wheel can be evaluated by measuring the Grinding Removal Ratio (GRR).
GRR can be measured by the following method
1.Weigh a new (unused) grinding wheel and a part of the work piece on weighing scale and note its weight.
2.Grind the work piece for a set time.
After grinding weigh the work piece and the grinding wheel.
Material lose/Wheel loss
Where material loss = Weight of work piece before grinding-weight of work piece after grinding
Wheel loss = Weight of grinding wheel before grinding-weight of grinding wheel after grinding.
The higher the numbers better the performance of the wheel.
Average range values for the 180mm grinding wheel is 10 to 14.
GRR should not be less than 6
Choice of Grinding Wheels
The Choice of the right grinding wheel depends on:
- Material to be ground and its hardness.
- The amount of stock removal and finish required.
- Wet or dry grinding.
- Wheel speed.
- Area of grinding contact.
- The severity of grinding operation.
- The condition of a machine.
The Three components of a grinding wheel are:
1. The ABRASIVE grains that do the actual cutting.
2. The BOND which holds the abrasive grains together.
3. The STRUCTURE which is the void or air space between adjacent abrasive grains coated with bond, so as to provide chip clearance.
The interrelation between the factors affecting the choice of wheel and the components of the wheel are elaborated below:
- Abrasive, Grit Size and Grade
- Grit size and Bond
- Grit Size, Grade, and Structure
- Abrasive and Grade
1. Material to be ground and its hardness:
This affects the choice of abrasive, grit size and grade.
The two main abrasive materials used are aloxite silicon carbide. Aloxite is best suited for alloy, carbon and high-speed steels, all having high tensile strength. Silicon carbide is ideal for metals of low tensile strength like cast iron, non-ferrous metals, and other non-metallic materials.
This means the grain size measured by the approximate number of openings per linear inch in the final screen used to size the grains. There are 23 standard grit sizes ranging from 8, used in coarse wheels to 320, used in very fine wheels. However, commonly used sizes are in the range of 16 to 120. A relatively fine grit works best on hard, brittle material; while a coarse grit can be used advantageously on soft and ductile materials.
This indicates the relative strength or holding power of the bond which supports the abrasive grains in a wheel. The grade of the wheel is represented by letters which move in the order of the alphabets. F is extremely soft, while Z is very hard. For most precision jobs, the grades required fall between F and N, while the grades for rough grinding and snagging range from M to Z.
A harder grade of the wheel should be used on soft materials and soft wheels on hard materials. This is because a hard wheel used on the hard material will tend to dull the wheel surface faster.
2. Amount of stock removal and finish required:
This affects grit size and bond
As a rule, a coarse grit is selected for fast cutting; and fine grit, for a high finish.
This is the adhesive substance which holds the abrasive grains. Commonly used bondings are vitreous and resin. The former is used for fast cutting and commercial finish, while the latter, for a high finish.
3. Wet or dry grinding:
This affects the choice of grade. Soft grade wheels must be used in dry grinding to minimize heat generation. In wet grinding, one grade harder wheel may be used, as the coolant reduces heat generated.
4. Wheel speed:
The wheel speed dictates the type of bond. Vitrified bond wheels should not be used where peripheral speeds exceed 33metres per second (6,500 feet per minute.)
In all cases, the safe operating speed shown on the label or blotter and measured in rpm should never be exceeded.
5. Area of Grinding contact
This affects choice of grit, grade, and structure.
A coarse grit is required for a relatively large surface area. However, as the area of contact becomes smaller a finer grid should be used.
Similarly, a soft grade of wheel is preferable for large surface areas; and harder grades, when area of contact becomes smaller.
Grain spacing in the wheel is indicated by a number and is determined by the proportion and arrangement the abrasive grains and bond. When the abrasive grains are close together, the wheel has a denser structure as indicated by a lower structure number such as 4 or 5 (refer structure chart).
6. Severity of Grinding operation:
This affects abrasive and grade.
A tough abrasive like A is required for severe operations like snagging while a mild abrasive like 32A or 38A is best suited for light grinding operations. Intermediate abrasives like 19A or 23A are used for grinding jobs of average severity.
Hard grades provide durable wheels for severe operations, while medium and soft grades are generally used for precision grinding.
The principal grinding operations can be classified as:
1.Grinding down and smoothing weld seams.
2.Cleaning the metal before welding.
3.Grinding out imperfections in casting and smoothing rough surfaces.
4.Smoothing surfaces on grey iron castings before painting.
5.Grinding off fins and painting on automobile castings.
6.Grinding punch press dies and patterns.
7.Grinding rail tracks.