金工实习英文讲义-磨工

Mechanical Engineering Training

Grinding

Name: Student NO.:

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1. Introduction to Grinding

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

A wide variety of machines are used for grinding:

(1) Hand-cranked knife-sharpening stones (grindstones)

(2) Handheld power tools such as angle grinders and die grinders

(3) Various kinds of expensive industrial machine tools called grinding machines (4) 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 \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 \machining, it is usually better suited to taking very shallow cuts, such as reducing a shaft’s diameter by half a thousandth of an inch or 12.7 μm.

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, and shears a tiny chip that is analogous to what would conventionally be called a \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 \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.

In this training course, considering the availability of required equipment in the training center, we will focus on the training of metal casting methods.

2. Types of Grinding Process

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

Surface grinding uses a rotating abrasive wheel to remove material, creating a flat surface. The tolerances that are normally achieved with grinding are ± 2 × 10?4 inches for 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 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 photo of a surface grinding machine is shown in Figure 1. The machine you are going to use in this training course is the surface grinding machine. You will learn about the working principles of the machine and manipulate the machine to grind a workpiece according to a technical drawing.

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Figure 1 Surface grinding machine

Cylindrical Grinding

Cylindrical grinding (also called center-type grinding) is used to grind the cylindrical surfaces and shoulders of the workpiece. The workpiece is mounted on centers and rotated by a devise known as a drive dog or center driver. The abrasive wheel and the workpiece are rotated by separate motors and at different speeds. The table can be adjusted to produce tapers. The wheel head can be swiveled.

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 work. 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 micro-inches (metric: 50 nm to 3 um), with typical finishes ranging from 8-32 micro-inches. (metric: 0.2 um to 0.8 um) Figure 2 shows a cylindrical grinding machine.

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Figure 2 Cylindrical grinding machine

3. Working Principle of the Surface Grinding Machine

Figure 3 Structure of a surface grinding machine

As can be seen in Figure 3, the surface grinding machine consists of a table 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 wheel’s diameter and manufacturer’s rating. The grinding wheel can travel across a fixed work piece, or the work piece can be moved while the grind wheel stays in a fixed position. The work piece is usually firmly

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fixed on the table through electromagnetic power to make sure it won’t move under the rotatory force of the grinding wheel. So when we say the work piece moves, we actually mean the table that fixes the work piece moves. Fine control of the grinding head or table position is possible using a vernier calibrated hand wheel. From Figure 3, we can see there are three hand wheels, in which the Longitudinal Feed Hand Wheel controls the longitudinal movement of the table, the Cross Feed Hand Wheel controls the horizontal movement of the table while the Vertical Feed Hand Wheel controls the vertical movement of the grinding head. With the hand wheels, we can precisely control the amount of material to be removed and finally meet the technical requirement.

Figure 4 The grinding process

As can be seen form Figure 4, the Grinding machine removes material from the surface of the workpiece 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. During the grinding process, the coolant is continuously supplied to the grinding wheel where it contacts the workpiece to remove the heat.

4. Grinding Wheel

A grinding wheel is an expendable wheel that is composed of an abrasive compound used for various grinding (abrasive cutting) and abrasive machining operations. The wheels are generally made from a matrix of coarse 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. They may also be made from a solid steel or aluminum disc with particles bonded to the surface. Figure 5 shows the photo of a grinding wheel that is used in the surface grinding machine.

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Figure 5 Grinding wheel

The manufacture of these wheels is a precise and tightly controlled process, due not only to the inherent safety risks of a spinning disc, but also the composition and uniformity required to prevent that disc from exploding due to the high stresses produced on rotation. Common materials for manufacturing grinding wheels include: Aluminum Oxide, Silicon Carbide, Ceramic, Diamond and Cubic Boron Nitride. Grinding wheels with diamond or Cubic Boron Nitride (CBN) grains are called super-abrasives. Grinding wheels with Aluminum Oxide (corundum), Silicon Carbide or Ceramic grains are called conventional abrasives.

5. Use of the Micrometer

In the training practice, you are supposed to grind the workpiece according to a technical drawing where size and tolerance of the finished workpiece are specified. Your finished workpiece must conform to all the specifications in the technical drawing. Therefore, in order to check if the workpiece is qualified, you have to learn about the use of the micrometer. A micrometer, sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw widely used for precise measurement of components in mechanical engineering and machining as well as most mechanical trades. Micrometers are usually, but not always, in the form of calipers (opposing ends joined by a frame), which is why micrometer caliper is another common name. The spindle is a very accurately machined screw and the object to be measured is placed between the spindle and the anvil. The spindle is moved by turning the ratchet knob or thimble until the object to be measured is lightly touched by both the spindle and the anvil. Figure 6 shows a micrometer.

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Figure 6 The micrometer

But how to read the micrometer? Let us see an example in Figure 7.

Figure 7 Micrometer thimble reading 5.78mm

The spindle of an ordinary metric micrometer has 2 threads per millimeter, and thus one complete revolution moves the spindle through a distance of 0.5 millimeter. The longitudinal line on the frame is graduated with 1 millimeter divisions and 0.5 millimeter subdivisions. The thimble has 50 graduations, each being 0.01 millimeter (one-hundredth of a millimeter). Thus, the reading is given by the number of millimeter divisions visible on the scale of the sleeve plus the particular division on the thimble which coincides with the axial line on the sleeve. Suppose that the thimble were screwed out so that graduation 5, and one additional 0.5 subdivision were visible (as shown in Figure 7), and that graduation 28 on the thimble coincided with the axial line on the sleeve. The reading then would be 5.00 + 0.5 + 0.28 = 5.78 mm.

6. Training Practice

In this training course, you are supposed to grind the workpiece according to a technical drawing. The drawing will be given to you in class, so before you start working, first read the drawing carefully and make sure you have understood all the specifications on the drawing. Then following the guidance of the teacher, you can manipulate the grinding machine. When you have finished, use the micrometer to check if the workpiece meets the specifications, if not, you have to repeat the process until the specifications are all met.

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7. Safety Rules

(1) The grinding wheel rotates in a very high speed, so do not try to use your hands to

touch the wheel or workpiece when the machine is running.

(2) Sparks may occur when the grinding machine is working, so you shall stay away

from the end of the machine to avoid being burnt.

(3) After the workpiece is finished, do not try to pick it up with bare hand. Gloves are

needed in case you get your fingers injured by the heat from the workpiece.

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