English preparation

Production technique

Topic:

BASIC MACHINE-TOOL ELEMENTS

OF CUTTING MACHINES

including

LATHE, BORING MACHINE, SHAPER, PLANER, MILLING MACHINE

BASIC STRUCTURES, FRAMES, ELEMENTS, DRIVES,

HOLDING AND HANDLING WORKPIECE, CONTROL

Basic Machine Tool Elements (Figure 6 [S. 429])

Most machine tools are built by using two or more components. These components, although they may have different functions in such machines as a lathe, miller or drill press, have some common characteristics.

Structures and Frames

Stability of the machine structure is required to prevent chatter.

We differ three large elements of machine tools:

The structure they have,

the material they are of

and the methods by which they are controlled with.

Castings, forgings, and hot or cold-formed shapes usually require machining. The variety of size, shapes and materials calls for diverstiy in machining.

Machine tools differ not only in the number of cutting edges they employ, but also in the way the tool and workpiece are moved in relation to each other. In some machines (shapers, drill presses, milling machines and grinders) the workpiece remains virtually motionless and the tool moves. In others (planers, lathes and boring mills) the tool is virtually fixed and the workpiece moves. But it should be pointed out that sheldon are these simple principles applied without modification.

The single-point tool-shaping machines are the easiest to visualize. The lathe and the boring machine are kinematic inversions employing the single-point tool. In the case of the lathe the cutting tool is stationary. In the boring machine the tool rotates while the work is stationary. Though the lathe tool and the boring machine worktable are not truly stationary, this is overlooked for the moment. To feed a tool carriage past rotating work is usually more acceptable than to feed rotating work with headstock and supports past a stationary tool post.

Elements

Metal cutting machines are made up of self-contained units, each having a special function. The units or elements consist of headstock, column, table, saddle, bed, base or runways and cross or slide rails. Machines receive their names from the combination of units used to assemble the machine. For example there are four distinct types of horizontal boring, drilling and milling machines; table, floor, planer and multiple head.

The table type machine has a table and a saddle and the workpiece is placed on the table. The floor-type machine combines floor plate sections and runways. The planer-type machine derives its name from a reciprocating (hin- und herbewegen) table; and the multiple head machine incorporates additional headstocks, a cross rail and column supports. These basic untis can be employed in a number of ways to machine a part.

Methods of holding workpiece

The method used depends upon the workpiece, the machine, and the extent to which rapid production is desired. In the case of quantity production machines, such as lathes, the holding devices are usually actuated hydraulically, by air, electricity, or cam action in order that the clamping time can effort and be minimized. In the case of automated or numerically controlled machines, the holding devices may be progammed to release the workpiece at the conclusion of machining and to automatically clamp the next part to feed it.

Methods of handling workpieces

The customary method of handling a workpiece is manually if the mass is less than 10 to 25 kg or by crane or conveyor if heavier. A variety of mechanisms is available to load, position control the cycle, and unload the workpiece. If the volume warrants, systems are available that can completely process and assemble the item. Mechanical loading reduces operator fatigue. If a workpiece has a mass of 16kg, at a production rate of 60 pieces per hour the operator will load and unload about 7600kg in a working day. Therefore semi-automatic loading-unloading machines are used.

Control

Numerical Control

Numerical control refers to the operation of machine tools from numerical data stored on paper or magnetic tape, tabulating cards, computer storage, or direct information.

The design of NC machine tools now incorporates the many advantages that NC offers such as programed optimization of cutting speeds and feeds, work positioning, tool selecting and even chip removal. Because of optimum metal removal, chips must be controlled and removed by conveyors.

Numerical control starts with the part programmer who, after studying the engineering drawing, visualizes the machine operations required to machine the workpiece. He prepares a program by listing codes that define a sequence. A reference point between the workpiece and the machine tool is required. Cutting tools, holding devices and their location and when and where they get in action are specified. Also cooling fluid pressure and different lubrication fluids are added where they are needed.

After that the progam is getting stored, and the data can read back if necessary.

Cutting and Feed Movements for Conventional Machines

The cutting machines

Devices which are shaping workpieces with the help of chip cutting are called Chip controlled Cutting Machines. Shaping with the help of chip control is a very expensive and cost intensive work-procedure, because of the lost of material in form of chips. Besides, the workpiece and the machine itself are subject to wear (tear).

To guarantee the most economic way of working, workpieces are often given to a pre-treatment through cast, roll or forge machines as well as then to bring the workpiece with the help of cutting machines into the right size (measurement).

Cooling of cutting machines.

Cooling is very important in every machinery process. Without cooling the tool, regardless of what type the tool it is about, it would overheat and get glow, especially loose cutting efficience and the cutting edge will be torn very quickly. In fact, the conversion of cooling is caused by a special cooling fluid. This fluid is a special mixture of water (for the cooling itself) and oil (for lubrication). So cooling is not only for a smooth finish, but also to take care of the tool.

Lathe / Turning Machine

In the area of the cutting machines, turning ist the most frequently used and most important process for producing any cylindrical workpieces. This is because of the three following causes:

T  Most components of the manufacturing industry are axels, cylinders, disks, bolts, tins, wheels and screws, so the raw molds are rotating bodies like cylinders, and only processed on the shell surface.

T  Turning tools are relatively simple and therefore also cheap.

T  Because of the uninterrupted, continous chip cutting during the turning process outstanding cut performances and speeds can be reached.

Lathe figure (in work) Elements of a lathe (Figure 1 [S. 492])

Controls, Feed reverse, Speed change, levers, Headstock, Tool holder, Workpiece, Compound rest, Tailstock, Leadscrew, Feed rod, Traverse rod, Spindle control, Apron, Chip chain, Rapid traverse Feed-thread change handle.

The turning process

During the turning process the wedge-shaped edge of the turning chisel enters the surface of the material and takes the cutted piece in chipform away. The chisel must be kept in the centre plain of the workpiece at all times to fulfill its task.

We distinguish longitudional- and plan turning.

During the longitudional turning process the physical motion of the turning chisel is parallel to the cylinder axle, during the plan turning process the physical motion of the turning chisel is vertical to the cylinder axle. The favour can be from the center inwards or outwards to the center of the cylinder.

If the favour is diagonal to the axle, cone-types come into being.

Boring / Drilling Machine

Types of Drilling Machines

Drilling machines are classified according to their general construction:

A. portable drill

B. non portable drill

Upright drilling machine, a.o.

There are a few kinds of non-portable drilling machines, but the most popular, the Upright drilling machine is worth to explain.

Upright drills have power-feeding machanisms for the rotating drills and are designed for heavier work. Under Point <drilling machine figure> a one meter machine with a box-type column is shown. It is more rigid, than a round column machine and, consequently, is adapted to heavier work.

If several operations must be performed, such as drilling two different-sized holes and reaming them, four spindles are set up. This type of production is made on a so-called Drilling Machine. It can be attended by only one operator. The work itself can be done mechanically or automatically with the help of NC.

Drilling Machine figure (in work). Elements of a Drilling Machine (Figure 2 [S.538])

The boring process

Boring is the enlarging of a hole that has already been drilled or cored. Principially, it is an operation of truing a hole that has been drilled previously with a single-point lathe-type tool. For this operation on a drill press, a special holder for the boring tool is necessary.

Drilling produces a hole in an object by forcing a rotation drill against it. The same can be accomplished by holding a drill stationary and rotating by a chuck (Spannfutter).

Reaming is the enlarging of a machined hole to proper size with a smooth finish. Although this operation can be done on a drill press, other machine tools are equally well adapted to perform them.

Other methods of producing a hole are by punching, flame cutting, coring, circular saw cutting, fly cutting and ultrasonic and electric discharge machining.

The punching process, which is very rapid and specially adapted to thin materials, produces accurate holes, but the punches and holes are very expensive, if the amount of holes are large.

Flame cutting by the use of oxyacetylene or oxygen lances will cut holes through any thickness of commerial material, though the holes are accurate neither in size nor in shape.

Coring is used principially on large holes in castings to save metal and ruduce machining costs.

Circular saw cutters and fly cutters are both used for cutting large diameters in thin metal. Ultrasonic and electric discharge machining produce holes in hard materials.

Point Angle

To obtain good service from a drill it must be properly ground. The point angle should be correct for the materials that is to be drilled. The usual point angle on most commercial drills is 118 degrees for steel which is satisfactory for soft steel, brass, and most metals. For harder metals, larger point angels give better performance.

Milling Machine / Rotary Grinder

A milling machine removes metal when the work is fed against a rotating cutter. The milling cutter has a series of cutting edges on its circumference (Umfang), each of which acts as an individual cutter in the cycle of rotation. The work is held on a table which controls the feed against the cutter. In most machines, there are three possible movements, longitudional, crosswise, and vertical -but in some the table may also possess a swivel or rotaional movement. Since all table movements have micrometer adjustments, holes and other cuts can be spaced accurately.

The milling machine is the most versatile (wendig) of all machine tools. Flat or formed surfaces may be machined with excellent finish and accurancy.

Types of Milling Machines

The milling machine is versatile because of the large variety of cutters available. These cutters are usually classified according to their general shape, although in some cases they are classified by the way the are mounted, the material used in the teeth, or the method used in grinding the teeth.

There are three general designs of cutters:

Arbour (Walzenfräser) cutters: These cutters have a hole in the center for mounting on an arbour.

Shank (Schaftfräser) cutters: Cutters of this type have either a straight or tapered shank integral with the body of the cutter. When in use these cutters are mounted in the spindle.

Face (Stirnfräser) cutters: These cutters are bolted or held on the end of short arbours and are generally used for milling surfaces.

Milling Machine figure (in work). Elements of a Milling Machine (Figure 3 [S. 559])

Shaper

A Shaper is a machine with a reciprocating cutting tool, of the lathe type, which takes a straight-line cut. Perfection is not dependent on the accurancy of the tool as it is when a milling cutter is used for the same type of work (by usually taking smaller chips). By special tools, attachments and devices for holding the work, a shaper can also cut external and internal keyways, spiral grooves, gear racks, dovetails, T-slots an other miscellaneous shapes.

The Plain Horizontal Shaper

Commonly used for production and general-purpsoe work, a horizontal shaper, consisting of a base and a frame that supports a horizontal ram, is quite simple in construction. The ram, which carries the tool, is given a reciprocating motion equal to the length of the stroke desired. The quick return mechanism driving the ram is designed so that the return stroke of the shaper is faster than the cutting strike which reduces the idle time of the machine to minimum. The tool head at the end of the ram, which can be swiveld through an angle, is provided which means for feeding the tool into work.

Shaper figure (in work). Elements of a Shaper (Figure 4 [S. 579])

Shaping process

Cutting speed on horizontal shapers is defined as the average speed of the tool during the cutting stroke and depends primarily on the number of ram strokes per minute and the length of the stroke. If the stroke length is changed and the number of strokes per minute remains constant, the average cutting speed is changed. The ratio of cutting speed to return speed enters into the calculation, as it is necessary to determine what proportion of time the cutting tool is working. The ratio is about 3/5 of the time working and 2/5 of the time for the return stroke.

Shaper tools

Shaper tools are similar to lathe tools and are frequently held in the same type of holders.

Planer

A Planer is a machine tool designed to remove metal by moving the work in a straight line against a single-edge tool. Similar to the work done on a shaper, a planer is adapted to much larger work. The cuts, which are mainly plane surfaces, can be horizontal, vertical, or at an angle. Planers are not longer important for production work, as most plane surfaces are now machined by milling, broaching (Räumen), or obrasive machining.

The shaper and the planer both use single-point cutting tools. But the smaller workpiece on the shaper is more efficient machined than on the planer.

Differences between Planer and Shaper

Although both the planer and shaper are adapted to the machining of flat surfaces, there is not much overlapping in their fields of usefulness; the differ widely in construction and in the method of operation. When the two machines are compared in construction, operation and use the following differences may be seen:

A) Most planers differ from shapers in that they approach more constant velocity cutting speeds.

B) On the planer the tool is fed into the work; on the shaper the work is usually fed across the tool.

C) On the planer the work is moved against a stationary tool; on the shaper the tool moves accross                       the work which is stationary.

D) The planer is especially adapted to large work; the shaper can do only small work.

Planer figure (in work). Elements of a Planer (Figure 5 [S. 586])

Vocabulary