In this article we will discuss about:- 1. Meaning of Machine Tools 2. Classification of Machine Tools 3. Characteristics 4. Cutting Motion 5. Requirements 6. Process Capability 7. Maintenance.
Meaning of Machine Tools:
A machine tool is a power driven device in which energy is utilised in deformation of material for shaping, sizing or processing a product to a desired accuracy by removing the excess material in the form of chips.
The machine tools are generally used for two purposes:
(i) To produce certain form.
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(ii) To produce finished surfaces.
The form of surfaces produced depends upon the shape of the cutting tool and the relative path of motion between the cutter and work-piece. The relative motion, also known as working motion, between cutter and workpiece can be obtained either by the motion of workpiece or cutting tool, or by a combination of both.
The working motions are of two types, i.e., primary cutting motion (drive motion), and feed motion. Working motions are powered by external source of energy. In addition to working motion, auxiliary motions such as for clamping/unclamping of workpiece, tool holder, idle travel, changing speed, engaging/disengaging of working motions, etc. are also provided on machine tools.
Classification of Machine Tools:
According to the type of the surface generated, the machine tools can be classified as follows:
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(a) Machine Tools for Cylindrical Work:
In this case, the workpiece is rotated about an axis, or alternatively the work may be at rest and the tool is rotated as well as traversed. Various machines used for this type of work are lathe, turret and capstan lathes, boring machine, cylindrical grinder and surface grinder. Refer Figs. 11.1 (i) and (ii).
(b) Machine Tools for Flat Surface Work:
For generating a straight-cut plane surface, the work-piece is moved past the cutting tool in a straight path and the tool is traversed in perpendicular direction or alternatively the tool is moved in straight path and work is traversed in perpendicular direction. In both these cases, the tool or the work is traversed in perpendicular direction only after one cut has been completed. The machines doing this type of operation are planning, shaping, slotting and broaching. Refer Fig. 11.1 (iii).
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The flat surface could also be produced by having rotary motion of multi-cutting edged tool and translatory motion of job. The examples of machines employing such motions are milling, surface grinding machines. Refer Fig. 11.1 (iv).
Holes are produced in workpieces by drilling operation. Refer Fig. 11.1 (v).
(i) For Lathes and Boring Machines:
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Drive motion—rotary motion of workpiece
Feed motion—translatory motion of cutting tool in the axial or radial direction
(ii) For Drilling Machines:
Drive motion—rotary motion of drill
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Feed motion—translatory motion of drill
(iii) For Milling Machine:
Drive motion—rotary motion of the cutter
Feed motion—translatory motion of the workpiece
(iv) For Shaping, Planing and Slotting Machines:
Drive motion—reciprocating motion of cutting tool
Feed motion—intermittent translatory motion of workpiece
(v) For Grinding Machines:
Drive motion—rotary motion of the grinding wheel
Feed motion—rotary as well as translatory motion of the workpiece.
Another way of classifying the machine tools is as follows:
(a) General Purpose or ‘Basic’ Machine Tools:
These are used for performing all metal cutting operations within their range. These include lathes, shapers, drilling machines, milling machines, grinding machines, planning machines etc.
(b) Production Machine Tools:
These are used to reduce the manufacturing cost and to increase the rate of production. These are generally multi-station tooling machines and designed to do one type of job at a time. These include Capstan and turret lathes, semi- automatic lathes, production milling machines, multiple head drilling machines etc.
(c) Special Purpose or Single Purpose Machine Tools:
These are used for mass production and generally one machine is capable of producing only one type of job. These include gear generators, camshaft grinders, piston turning lathes, thread rolling machines etc.
The surface finish depends upon the lay and the feed of the tool or work. Various machines are capable of producing different types of finish.
In the increasing order of good surface finish, various operations used for machining are arranged as:
i. Cutting,
ii. Sawing,
iii. Hand grinding,
iv. Filing,
v. Turning,
vi. Shaping,
vii. Milling,
viii. Boring,
ix. Drilling,
x. Surface grinding,
xi. Cylindrical grinding,
xii. Boring,
xiii. Lapping,
xiv. Polishing super finishing,
xv. Buffing etc.
The most important requirement to be fulfilled by any machine tool is that it should be able to produce jobs within high degree of accuracy consistently over a long period of time. For this, it is essential that they possess static as well as dynamic rigidity. Rigidity of a machine tool is its capability to resist deformation produced due to the introduction of cutting forces generated while machining.
(d) Flexible Manufacturing System:
These are the latest machines which can be used to machine any jobs and these are designed to achieve following objectives :
i. Less machining time per part: reduced loading, tool changing and other non-cutting time ;
ii. More flexibility ;
iii. Greater compatibility with systems ;
iv. Reduced operator involvement: improved safety and less noise.
These mainly employ machining centres, very versatile machines and robots, etc. for material handling.
Machine tools can be classified into several other ways, viz. as per weight, as light duty (upto 1 ton), medium duty (upto 10 t), or heavy duty (more than 10 t); as per degree of automation as manual, or semi-automatic or automatic machine tools.
Characteristics of Machine Tools:
i. Repeatability:
Machine tool should be designed to produce same dimensions with one setting, repeatedly. This is also known as process capability of machine tool.
ii. Reliability:
The mean time between failure (MTBF) should be high. All non-wearing parts should be designed to be fail proof. All parts subject to wear and tear should be designed for easy replacement, i.e., mean time to repair (MTTR) should be least possible.
iii. Easy Maintenance:
The parts requiring attention should be easily accessible. The oiling and greasing nipples should be easily visible and accessible. The parts needing adjustment should also be easily approachable. The complete machine tool should be easy service.
iv. Ease of Operation:
The controls for operation of machine tool should be located so as to cause least strain on operation. The relative location should follow standard conventional procedures to avoid possibility of maloperation.
Loading and unloading of parts should be as simple and fast as possible. Quick acting clamping devices should be used to avoid strain on operator. In the nutshell the operator should not feel fatigue in operation of machine tool in one full shift of operation.
v. Accuracy:
The machine should be able to produce accuracy in regard to dimensional geometry and surface finish without calling for much expertise of operator. For this purpose the machine tool must have adequate static stiffness and dynamic rigidity to withstand the cutting forces and other dead loads.
The alignments of various parts should be as per requirement and means provided to take up wear and lost motion. The machine tool should be properly erected and thoroughly tested for all alignment tests.
vi. Compactness:
The overall size of machine tool should be least possible. It depends on the sophistication of design.
vii. Safety:
Safety is of prime importance. No loose ends can be tolerated in this regard. All possibilities of accident must be foreseen and suitable guards provided. Adequate interlocks should be incorporated so that wrong operation will not be allowed.
viii. Production Capacity:
The machine tool should be designed for maximum metal removal rate and maximum production time, and least setting up time. The operator should exploit fully the capabilities of machine tool to achieve optimum results.
ix. Standardisation:
As far as possible only standard parts should be used so that their replacement is easy. The design should be simple, using minimum parts which can be easily assembled.
x. Aesthetic Appeal:
Machine tool should possess aesthetic appeal, i.e., it should not have any awkward or protruding parts. The colour should be soothing one. The layout of different parts in relation to one another should be elegant. There should be no sharp edges. The visible surfaces and covers should give pleasing appearance. The controls and indications should be located at convenient places and give nice appeal.
xi. Initial Cost and Operating Cost:
For same technical performance, the first cost and the operation and maintenance cost over its life time should be least possible to find market.
xii. Features of Advanced Machines:
All modern machine tools tend to have as much automation as possible. These are robust in construction and the rate of utilisation is highest. The design ensures for least friction, elimination of backlash, complete avoidance of stick-slip motion. It incorporates tool magazines and automatic tool changers to enable quick change of tool.
Cutting Motion in Machine Tools:
The process of chip removal is effected by the relative motion between tool and work piece. The motions of machine tool may be transmitted either to the cutting tool, or to the work or to both simultaneously.
Working motions of a machine tool are:
(a) Primary cutting motion,
(b) Feed motion, and these are specified by speed or feed rate.
(a) Primary Cutting Motion:
It provides for cutting the chip from the blank.
The most commonly used types of primary cutting motions are:
(i) Rotary primary cutting motion,
(ii) Reciprocating primary cutting motion.
(i) Rotary Primary Cutting Motion (Refer Fig. 11.2):
Rotary motion may be transmitted either to the work as in lathe group of machine tools or to the tool as in grinding machine, drilling machine or milling machine.
(ii) Reciprocating Primary Cutting Motion (Refer Fig. 11.3):
This motion can be transmitted to the tool as in shaper and slotter or to the work as in planer. In machine tools using reciprocating primary cutting motion the cutting cycle consists of a working stroke during which the tool cuts the chips and the idle or return stroke during which the tool or work returns to its initial position.
In some machine tools, the primary cutting motion may be a combination of rotary and reciprocating motions.
(b) Feed Motion:
The feed motion enables the cutting process to be extended to the whole surface to be machined on the work. The rate of feed is substantially less than the cutting speed. Other conditions being equal, the rate of feed determines the cross sectional area of the chip. A machine tool is subjected in addition to its static load, to the dynamic loads due to rotation/reciprocation, cutting forces and friction forces.
Feed motion can be expressed as feed per revolution or feed per stroke, or feed per revolution, or feed per tooth etc.
Requirements of Machine Tools:
Any machine tool should meet the following requirements:
(i) Ability of produce desired shape and size.
(ii) Design for required accuracy and surface finish.
(iii) High productivity.
(iv) Simple design—ease in operation and maintenance.
(v) Good ergonomics, i.e., convenience of controls and safe design.
(vi) Aesthetics, i.e., good appearance and finish.
(vii) Low cost of manufacturing and operation.
For any machining process, the layout of machine tool must be such that necessary combination of forming and setting motions are provided. Adequate rotary and translatory motions, either powered or manual, are provided to realise the required relative motion between the cutting tool and workpiece.
Layout usually consists of one stationary block and a number of moving blocks capable of being translated or rotated on respective guide ways. Different designs and layouts are possible by arranging the portion of stationary and movable blocks to obtain a particular order.
For machining heavy parts, these must be placed on a bed having level near the ground, the workpiece should remain stationary or may have only horizontal displacement ; the horizontal moving block must be close to the stationary block. Factors like static and dynamic stiffness of structure, ease of control, cost of manufacture, aesthetics, etc. would be deciding factors to narrow down the choice among various functionally acceptable alternatives.
The accuracy of a machine tool is dependent upon its geometrical and kinematic accuracy. It is important that the accuracy be maintained during operation and over the life span of the machine tool. The guiding elements, guide ways; power screws should have desired accuracy.
The traversing members should have uniform and jerk-free movement. The kinematic accuracy of a machine tool can be high by using short kinematic trains, manufacturing and assembling the components with a high degree of accuracy.
The static stiffness of machine tool structure should be high so that deformation due to cutting forces is within tolerable limits. The dynamic stiffness should also be high to reduce vibrations during machining. The measuring instruments, sensors, transducers, dials, and read-out devices used should have desired accuracy.
The relative position between the cutting tool and workpiece should remain unaffected due to thermal deformations during the machining operation. Surface finish is dependent upon speed, dynamic stiffness and feed etc.
The productivity of a machine tool can be increased by cutting down machining time (by providing high cutting speeds in adequate steps, high feed rates, providing multiple cuts etc.); cutting down non-productive time (using suitable jigs and fixtures to reduce clamping/unclamping time, using soft automation, etc.); improving reliability by proper operation and maintenance.
Simple design of machine tools using standardised components results in ease of manufacture, operation and maintenance. Good appearance of machine tool produces soothing effect, increasing productivity. Simplicity of design, incorporating safety features, good in appearance, external shape, finish and colour contribute to overall aesthetic quality.
Process Capability of a Machine Tool:
Process capability of a machine tool is defined as its ability to produce jobs within specified degree of accuracy consistently over a long period of time. The process capability of a machine tool depends upon its rigidity which is defined as its capability to resist deformation produced due to the introduction of cutting forces generated during machining. A machine tool must possess both static as well as dynamic rigidity.
Process capability can be improved by reducing number of links and changing their rigidity and reducing range of cutting forces.
Compliance of a Machine Tool:
Compliance of a machine tool is the reciprocal of rigidity. The compliance of a centre lathe can be calculated by considering the rigidity and deflection of various components.
Maintenance of Machine Tools:
A machine tool can continue to produce accurate work pieces within specified limits throughout its working life, if the wear of the machine tool does not exceed certain limits and parts which become faulty due to wear or other damages are replaced/repaired timely. The improved maintenance would reduce machine tool down time and lead to higher productivity.
Various maintenance techniques are:
(i) Preventive Maintenance:
Preventive maintenance is the planned maintenance of machine tools and equipment. The aim of preventive maintenance is to reduce wear and tear and take timely action before failure condition. Lubrication is most important part of preventive maintenance. All moving parts need timely lubrication. Lubrication schedules should be strictly followed.
If certain points are not checked periodically, they may lead to major break-down. All wearing parts in the machine tool or the parts subjected to fatigue should be replaced before failing as per the instructions of the manufacturer.
Preventive maintenance is essential to keep the machine in order for production and safety, and this increases the reliability and availability of machine tool.
A proper preventive maintenance schedule should be followed making daily/weekly/ monthly checks.
Daily Checks include cleaning the machine, checking lubricating oil levels and oil flow in sight glasses, checking coolant level, and to keep the maintenance department informed of even the minor defects noted in the performance of the machine.
Weekly Checks include checking of all lubrication levels, checking coolant, filters, hydraulic and pneumatic lines.
Monthly Checks include checking of spindle drive belts for wear, hydraulic pumps and hydraulic oil system, and movement of all axes under manual control.
Six Monthly Checks include checking of machine alignment and replacement of oils and filters.
(ii) Corrective Maintenance:
It includes replacement of worn parts and to carry out repairs. The maintenance engineer should regularly inspect the specific elements of the machine in order to determine whether wear has reached the stage when corrective maintenance should be carried out before failure occurs.
It is essential to maintain a record of the nature and cost of all repairs carried out. This will enable prediction of those elements of machines which need improved servicing, more frequent inspection.
(iii) Reconditioning:
It is the rebuilding of the machine to give the required production. The need to recondition the machine can be determined by the frequency of the corrective maintenance, because after certain life span it becomes unserviceable inspite of the best possible planned and preventive maintenance carried out. This is the stage when major overhaul or reconditioning is called for.
Thorough inspection of machine tool should be made. The inspection report should indicate the details of the components required to be replaced during reconditioning and the skilled labour required to accomplish the task. Generally it is undesirable to recondition the machine tool if the cost of parts to be replaced and repairs exceed 50% of the prevailing cost of the new equipment.
