Machining on a grinding machine

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Machining on a grinding machine

Bykova T.A.

From the simplest grinding machine to the most complex, grinding machines can be classified as utility grinding machines, cylindrical grinding machines and surface grinding machines. grinding machine metal

Grinding machine is used to finish work pieces that must show high surface quality and high accuracy of shape and dimension. As the accuracy in dimensions in grinding is on the order of 0,000025 mm, in most applications it tends to be a finishing operation and removes comparatively little metal, a bout 0,25 to 0,50 mm depth. However, there are some roughing operations in which grinding removes high volumes of metal quite rapidly. Thus, grinding is a diverse field.

The grinding machine consists of a bed with a fixture, to guide and hold the work piece, and a power-driven grinding wheel spinning at the required speed. The speed is determined by the wheels diameter and manufacturers rating. The grinding head can travel across a fixed work piece, or the work piece can be moved while the grind head stays in a fixed position.

Fine control of the grinding head or table position is possible using a vernier calibrated hand wheel, or using the features of numerical controls.

Grinding machines remove material from the work piece by abrasion, which can generate substantial amounts of heat. To cool the work piece so that it does not overheat and go outside its tolerance, grinding machines incorporate a coolant. The coolant also benefits the machinist as the heat generated may cause burns. In highprecision grinding machines (most cylindrical and surface grinders), the final grinding stages are usually set up so that they remove about 200 nm (less than 1/10000 in) per pass - this generates so little heat that even with no coolant, the temperature rise is negligible.

These machines include the:

Belt grinder, which is usually used as a machining method to process metals and other materials, with the aid of coated abrasives. Sanding is the machining of wood; grinding is the common name for machining metals. Belt grinding is a versatile process suitable for all kind of applications like finishing, deburring, and stock removal.

Bench grinder, which usually has two wheels of different grain sizes for roughing and finishing operations and is secured to a workbench or floor stand. Its uses include shaping tool bits or various tools that need to be made or repaired. Bench grinders are manually operated.

Cylindrical grinder, which includes both the types that use centers and the centerless types. A cylindrical grinder may have multiple grinding wheels. The workpiece is rotated and fed past the wheel(s) to form a cylinder. It is used to make precision rods, tubes, bearing races, bushings, and many other parts.

- Surface grinder which includes the wash grinder. A surface grinder has a "head" which is lowered to a workpiece which is moved back and forth under the grinding wheel on a table that typically has a controllable permanent magnet for use with magnetic stock but can have a vacuum chuck or other fixturing means. The most common surface grinders have a grinding wheel rotating on a horizontal axis cutting around the circumference of the grinding wheel. Rotary surface grinders, commonly known as "Blanchard" style grinders, have a grinding head which rotates the grinding wheel on a vertical axis cutting on the end face of the grinding wheel, while a table rotates the work piece in the opposite direction underneath. This type of machine removes large amounts of material and grinds flat surfaces with noted spiral grind marks. It can also be used to make and sharpen metal stamping die sets, flat shear blades, fixture bases or any flat and parallel surfaces. Surface grinders can be manually operated or have CNC controls. - Tool and cutter grinder and the D-bit grinder. These usually can perform the minor function of the drill bit grinder, or other specialist toolroom grinding operations.

- Jig grinder, which as the name implies, has a variety of uses when finishing jigs, dies, and fixtures. Its primary function is in the realm of grinding holes and pins. It can also be used for complex surface grinding to finish work started on a mill.

- Gear grinder, which is usually employed as the final machining process when manufacturing a high-precision gear. The primary function of these machines is to remove the remaining few thousandths of an inch of material left by other manufacturing methods (such as gashing or hobbing).

Die grinder, which is a high-speed hand-held rotary tool with a small diameter grinding bit. They are typically air driven (using compressed air), but can be driven with a small electric motor directly or via a flexible shaft.

BONDING MATERIAL

Bond

The abrasive particles in a grinding wheel are held in place by the bonding agent. The percentage of bond in the wheel determines, to a great extent, the "hardness" or "grade" of the wheel. The greater the percentage and strength of the bond, the harder the grinding wheel will be. "Hard" wheels retain the cutting grains longer, while "soft" wheels release the grains quickly. Besides controlling hardness and holding the abrasive, the bond also provides the proper safety factor at running speed. It holds the wheel together while centrifugal force is trying to tear it apart. The most common bonds used in grinding wheels are vitrified, silicate, shellac, resinoid, and rubber.

Vitrified

A vast majority of grinding wheels have a vitrified bond. Vitrified bonded wheels are unaffected by heat or cold and are made in a greater range of hardness than any other bond. They adapt to practically all types of grinding with one notable exception: if the wheel is not thick enough, it does not withstand side pressure as in the case of thin cutoff wheels.

The grade of hardness should be selected as carefully as the grain size. A grinding abrasive wheel that is too soft will wear away too rapidly, the abrasive grain will be discarded from the wheel before its useful life is realized. On the other hand, if the wheel is too hard for the job, the abrasive particles will become dull because the bond will not release the abrasive grain, and the wheel's efficiency will be impaired.

Silicate

Silicate bond releases the abrasive grains more readily than vitrified bond. Silicate bonded wheels are well suited for grinding where heat must be kept to a minimum, such as grinding edged cutting tools. It is not suited for heavy-duty grinding. Thin cutoff wheels are sometimes made with a shellac bond because it provides fast cool cutting.

Resinoid

Resinoid bond is strong and flexible. It is widely used in snagging wheels (for grinding irregularities from rough castings), which operate at 9,500 SFPM. It is also used in cutoff wheels.

Rubber

In rubber-bonded wheels, pure rubber is mixed with sulfur. It is extremely flexible at operating speeds and permits the manufacture of grinding wheels as thin as 0.006 inch for slitting nibs. Most abrasive cutoff machine wheels have a rubber bond.

Factors Affecting the Grain Size

Grain size to be chosen when selecting a grinding wheel depends upon the factors described below.

The softer and more ductile the material, the coarser the grain size.

The larger the amount of stock to be removed, thecoarser the grain size.

The finer the finish desired, the finer the grain size.

Tensile Strength of Material

The tensile of material to be ground is the main factor in the selection of the abrasive to be used. Two types of abrasives are suited to different materials as shown below.

Silicon Carbide:

Gray and chilled iron, brass and soft bronze, aluminum and copper, marble and other stone, rubber and leather, very hard alloys, cemented carbides, unannealed malleable iron, aluminum oxide, carbon steels, alloy steels, high speed steels, annealed malleable iron, wrought iron, hard bronzes.

References

1. http://dic.academic.ru/dic.nsf/muller/913006

2. https://en.m.wikipedia.org/wiki/Grinding_machine

3. https://detroitmachine.tools/machining-handbook/chapter-8/page/3

4. https://www.youtube.com/watch?v=tnXGrmVoVEo

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