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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.
1.1 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.
1.2 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.
1.3 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.
1.4 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.
1.5 Control
1.5.1 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.
1.6 Cutting and Feed Movements for Conventional Machines
Machine Cutting Movement Feed Movement Types of Operations
Lathe Workpiece rotates Tool and carriage Cylindrical surfaces, drilling, boring, reaming and facing
Boring machine Tool rotates Table Drilling, boring, reaming and facing
Planer Table traverses Tool Flat surfaces (planing)
Shaper Tool traverses Table Flat surfaces (shaping)
Horizontal milling
machine Tool rotates Table Flat surfaces, gears cams, drilling, boring, reaming and facing
Horizontal boring Tool rotates Tool traverses Flat surfaces
Cylindrical grinder Tool (grinding wheel) rotates Table and/or tool Cylindrical surfaces (grinding)
Drill press Tool rotates Tool Drilling, boring, facing and threading
Saw Tool Tool
and/or workpiece Cut off
Broaching Tool Tool External and internal surfaces
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