Tool design is a specialized area of manufacturing engineering which comprises the analysis, planning, design, and construction and application of tools, methods and procedures necessary to increase manufacturing productivity.
RESPONSIBILITY OF A TOOL DESIGNER MUST BE FAMILIAR WITH THE FOLLOWING:
Cutting tools, tool holders, and cutting fluids.
Machine tools, particularly modified or special type
Jigs and fixtures
Gauges and measuring instruments
Dies for sheet metal cutting and forming Dies for forging, upsetting, cold finishing and extrusion
Fixtures and accessories for welding, riveting and other mechanical fastening.
OBJECTIVES OF TOOL DESIGNER
The tool designer must realize the following objectives or goals.
Reduce the overall cost of manufacturing a product by producing acceptable parts at the lowest cost.
Increase the production rate by designing tools that will produce parts as quickly as possible.
Maintain quality by designing tools which will consistently produce parts with the required precision.
Reduce the cost of special tooling by making every design as cost effective and efficient as possible.
Design tools which will be safe to operate.
THE DESIGN PROCESS
The design process consists of 5 basic steps:
Statement and analysis of the problem.
Analysis of the requirements.
Development of the initial ideas.
Development of design alternatives.
Finalization of design ideas.
a. PROCESS ENGINEERING
The process engineering takes place immediately after the product engineering has completed the design of a product. The output of product engineering namely part drawings, bill of material, engineering specifications, are the fundamental data on which process engineering function starts. In addition to above the data the information regarding the quantity of manufacture is of paramount importance in process engineering. An important decision has to be made at the outset as to what components are to be made in the plant, and what are to be bought from vendors. This goes by the name of “MAKE BY DECISION”
Make by decision must be based on various factors:
Quality, reliability and availability of supply.
Control of trade secrets and patents.
Research and development facility.
Flexibility and alternate source of supply.
Economic analysis.
b. PROCESS PLANNING
Process planning involves the determination of the method and the sequence of operations, tools and equipment required as well as the specification of the parameters of the process like speeds, feeds. The decision involved in this process has an important bearing on the cost of manufacturer and thus on the profitability of a manufacturing enterprises.
VARIOUS STEPS IN PROCESS PLANNING
Analysis of the part print.
Consultation with the product design, wherever clarification or modification regarding product design is required.
Preparation of rough method layout.
Detailed operation planning.
Final method of layout.
Fixing the in process dimensions and tolerances.
Writing out process shut and detailed operation plan.
2.0. ECONOMICS OF PROCESS AND MACHINE TOOL SELECTION
The managers and technicians are frequently encountered with the problem of process and machine tool selection. They are called upon to take decisions with regard to choice of our alternative from several proposals. There can be several alternative avenues for an investment to get more than a specified rate of return. In such situations break down concept play an important role in decision making.
BREAK EVEN ANALYSIS
In many situations encountered in engineering economy, the cost of 2 alternatives may be affected by a common variable. When such a condition occurs it may be desirable to find the value of the variable that will result in equal total cost for the alternatives considered. The value of the variable so designated is known as the break-even point. The total cost of any alternative is made up of fixed cost and variable cost.
FIXED COST
It is defined as that group of cost involved in a going activity who’s total will remain relatively constant throughout the range of operational activity.
VARIABLE COST
It is defined as that group of costs which vary in relationship to the level of optional activity. In general, all costs such as direct labor, direct material, direct power etc. which can readily be allocated to each unit of product, are considered to constitute the variable costs.
The above mentioned concept on breakeven point is also applicable for the evaluation of alternative machine tools for performing a particular operation on a particular component considering the costs of set up, tooling and operation time.
3.0. SEQUENCE OF OPERATION
While planning different operations to be performed on a part, it is important to decide which to follow which. There are several considerations to this effect. To analyze the relative importance of different considerations, operations can be classified in to:
Critical operations.
Secondary operations.
Qualifying operations.
Re qualifying operations.
CRITICAL OPERATION
The operation that must be given special consideration in order to accomplish some unique characteristics on or from some surface of the work places.
SECONDARY OPERATIONS
The operations which are not critical but have a function purpose on the work piece and are generally performed without much special effort. Drilling and tapping way serve as one example of two secondary operations performed sequentially.
QUALIFYING OPERATIONS
The operations performed on work pieces like cast or forged pieces to establish qualified locating surfaces prior to accomplishing process critical areas.
RE QUALIFYING OPERATIONS
The operations performed on the work pieces in order return it to its original machined geometry. Heat treatment operations are one of the major causes of re qualifying operations.
4.0. SELECTION OF PARAMETERS OF DIFFERENT PROCESSES
To select various parameters for different processes, it required to know how these affect the machinability of a given material. The term machinability is a complicated one to lead to any clear definition. In a simpler way, machinability means ‘the ease’ with which a given material may be machined with a specific cutting tool. Commonly used yardsticks to measure machinability are:
Cutting force and power.
Tool life.
Surface finish.
These three factors together represent the machinability. For a particular machine set up and for a particular process, the above factors are considerably influenced by various parameters of the process. These parameters of the processes can be divided broadly in to:
Machine variables.
Work material variables.
Cutting fluids, vibration and chatter.
The above variations can be listed as follows:
MACHINE VARIABLES/TOOL VARIABLES/WORK MATERIAL VARIABLES
Cutting speed.
Dimensions of cut (a) feed (b) depth of cut.
Rigidly and freedom from chatter of machine tool work and work holding devices.
Tool material.
Tool shape i.e., various angles of tool.
Tool temperature.
Hardness.
Tensile strength.
Chemical composition.
Microstructure.
Degree of cold work.
Strain harden ability.
Chemical, physical prosperities of cutting fluid.
There is no single theory available considering all these parameters of metal cutting. Only empirical relations are available, which were arrived at after conducting a series of experiments to study the relationship of above variables with cutting force and power, tool life and surface finish. Here the discussions would be about these relations taking turning process as example. The same principles can be applied to other process tool life, drilling, milling etc., with due modifications and considering special factors of the respective processes. Before starting this, it is required to know about cutting force and power, tool life and surface finish as applied to turning process.
CUTTING FORCE AND POWER
Below shows three components of forces in turning.
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| Fig.8.(a) |
The tangential force, Ft does most of the work in turning and responsible for most of the power consumed. Hence, when cutting force in turning is mentioned, for practical purpose, tangential force Ft need be taken.
The feed force Ff does very little work and the radial force Ft does not work. The tendency of this component of force is to deflect the work piece. The approximate ratios of…
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| Fig.9(b) |
Ft: Ff: Fr is 4: 2:1
The power consumed in turning operation is given by the formula:
Fe x v/4500 H.P.
Fc = Cutting force – (tangential force alone is considered here) in Kgf.
V = Cutting speed. In metre/mt.
Cutting force can be measured by the use of a dynamic meter. Cutting force can also be computed from the specific cutting pressure of the work material. Specific cutting pressure is the cutting force per unit area of under formed chip cross section. For any other chip thickness; the formula for specific cutting pressure corresponding to that thickness would be,
=Kc 1.1/(h)p
Where, kC 1.1 is the specific cutting pressure for unit area (1mm x 1mm)
H= undeformed chip
P= is an index
New cutting force = specific cutting pressure x feed per revolution x depth of cut.
TOOL LIFE
The life of the tool in between resharpening cab specified in several ways like:
Actual cutting time in between resharpening.
No. of pieces produced.
Volume of metal removed.
Equivalent Cutting speed. For example V 60, cutting speed at which 60 mts. Of actual cutting time of machine time is obtained, under a given set of cutting conditions.
Tool failures occur ordinarily, in any of the following ways:
By flank wear, wear on the flank below the cutting edge.
By crate wear; caused by the flow of chip on the rake face. This wear occurs slightly away from the cutting point and has cup shape, gradually spreads towards the cutting edge.
By chipping wear, breaking off of tool. This can occur due to built up edge formation or due to shock loads.
Due to temperature failure.
Some of the common criteria for judging the tool failure area;
Complete failure: Tool completely unable to cut.
Flank failure: By judging the width of wear land developed on the flank of the tool.
Finish failure: Loss of surface finish on the workpiece.
Size failure: Change in dimensions of the finished parts.
Cutting force or power failure: Increase in cutting force or increased in power consumed.
Thrust force failure: Difficulty in feeding indicative of tool failure.
SURFACE FINISH
A typical machined surface when enlarged would look as shown in below:
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| Fig.9(C) |
It shows peak and valleys super imposed on a wave form. The peaks and valleys themselves are produced due to feed marks and the part of built up edge left on the machined surface. The waviness of the surface is due to vibrations inherent in machining process.
How the various parameters of the processes affect cutting force and power, tool life and surface finish can be seen now.
CUTTING SPEED
a. Cutting Force and Power
Based on the results of a many investigations it can be said that the cutting force is independent of the cutting speed beyond 100 ft/mt it can be seen that cutting force increase as the cutting peed is reduced. This is because of built up edge formations at very low cutting speeds (please refer graph).
b. TOOL LIFE
Cutting speed is the parameter having maximum influence on tool life. There is a well known equation formulated by Taylor and which is
Vtn= constant (c)
Where, V =Cutting speed in meters/mt.
T = Tool life in mts.
h= Exponent whose value is primarily dependent on tool material and varies, to some extent, with other machine and work material variables.
C= Constant whose value is depends primarily on the material being cut and other machine and work other variable. Numerically equal to the cutting speed that gives a total life of /mt.
The approximate values of ‘n’ in the above equation are for
H.S.S. tool = 0.1 to 0.15
Carbides = 0.2 to 0.25
Ceramics = 0.6 to 1.0
These values can also be found by conducting machining at two different cutting speeds and noting the tool life in each case.
From the above relation, it can see that when cutting speed is increased, tool life is decreased and vice versa. But in the case of machining steel and cast iron with carbide tools it is seen that the tool life goes down when the cutting speed is reduced below a certain value, this is because of built up edge formation at very low cutting speeds.
c. SURFACE FINISH
As cutting speed is increased, there are fewer tendencies for built up edge formation because of lesser duration of contact and lesser strain hardening. Because of this, surface finish is improved at higher cutting speeds.
DIMENSIONS OF CUT OUT (Feed and depth of cut)
a. Cutting Force and Power:
Increase in both feed and depth of cut will result in increase of force and power. It can also be seen that increase in depth has more effect than increasing feed on cutting force and thereby on power. This is because when speed is increased, chip thickness is also increased and hence specific cutting pressure decreases, but the thickness does not change. Because of this reason for the same increase in area of chip, cross section depth will have more effect.
The relation can be expressed,
Fc = kdufw
K = Constant, when value depends mainly on the material being cut and true rake angle of tool,
v & w are exponents.
In average machining where the nose radius is small in relation to depth u = 1 and w = 0.8
Hence, fc = k x d1x f 0.8
b. TOOL LIFE
Tool life obtained at a given cutting speed is affected by dimensions of cut. Tool life is reduced when dimensions of cut are increased.
As there will be more contact area with chip and tool when depth of cut is increased, heat dissipation is better than in the case of increasing the speed. When feed is increased, contact area remains same. Hence increase in feed will reduce tool life faster than increase in depth of cut. The general empirical relation between cutting speed for chosen tool life and feed and depth of cut is,
Vt = CA/dxfy
VT = Equivalent lant cutting speed for a chosen tool life.
CA = A constant whose value is depends on work and other variables.
f = Feed per revolution (or feed per tooth)
d = Depth of cut
x&y = Exponents. In practice common machining of steel,
x = 0.14 and y = 0.43
For cast iron;
X = 0.10 and y = 0.30.
This formula states that a combination of larger depth of cut and a high rate of feed with a low cutting speed will allow a large amount of metal to be removed during a given life of the tool.
c. SURFACE FINISH
Depth of cut has no influence on surface finish. But indirectly more depth of cut may induce vibration of tool and work piece system, and in turn surface finish may be affected. To see the effect of increasing feed, first of all, how the feed ridges are formed must be seen. Figure shows two consecutive positions of a tool after a revolution of the work.
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| Fig.1 |
The dimension ‘h’ in the figure is the height of the feed ridge. It can be directly seen from the figure that if feed per revolution is reduced, height of the feed ridge will also be reduced thereby increasing the surface finish.
By calculation, it can be found out that,
H = fr2/8R
This means the height of the feed ridge varies as square of the feed revolution.
TOOL MATERIAL
a. Cutting Force and Power:
With a few exceptions like diamond tools, cutting force and power consumption are practically independent of tool materials.
b. TOOL LIFE
Tool materials have no effect on tool life.
c. Surface Finish: Diamond has a minimum tendency to weld on to metals and thus very little built up edge is formed and a high quality of surface finish can be achieved. Other than diamond all rest of the tool materials have no effect by themselves on the finish. But if the tool material is capable of machining at high cutting speeds than a better surface finish can be achieved because of high cutting speeds.
TOOL SHAPE AND ANGLES
a. Cutting Force and Power: When rake angle is increased in the positive direction, cutting force also reduces. It is proved by experiments that cutting force reduced by 1% if 1o rake angle is increased in the positive direction.
The clearance angles have no effect on cutting force and power. If the side cutting edge angle is increased, feed force reduces and radial component of force increases.
If the nose radius is increased, radial force increases. This indicates that, if a slender job is to be machined, too high a side cutting angle and nose radius will increased radial forces and bend the job.
b. TOOL LIFE
Cutting temperature is only slightly effected by the rake angle and so also tool life.
The side and clearance angles if provided to avoid rubbing, then they have negligible effect of temperature and so also the – tool life. If side cutting edge angle is increased, the length of the cutting edge is increased and heat is generated and distributed over a longer length. This renders better tool life. End cutting edge angle ordinarily has minor influence on temperature and hence no tool life.
c. SURFACE FINISH
Tools angles and shape have remarkable effect on surface finish. An increase in tool rake angle, reduces cutting forces and consequently reduces built up edge formation. This leads to better finish.
Coming to clearance angles, if equal size variation is allowed on work piece (because of tool wearing off), tool with larger clearance angle will give better surface finish than with a tool with lesser clearance. This is because of lesser amount of rubbing than in the latter case.
Figure -4 -, show how cutting and end cutting edge angles on surface finish side.
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| Fig.11. |
An increase in side cutting edge angle and a decrease in end cutting edge angle will improve surface finish because feed ridge height is reduced. By this reason, a broad nose tool should give a good surface finish.
An increase in nose radius invariably improves the surface finish, as it can be seen from figure 3, and from relation,
H = fr2/8R
HARDNESS AND TOUGHNESS OF THE WORK MATERIAL
a. Cutting Force and Power: As a general rule, cutting forces and power increases with increasing hardness and toughness.
b. TOOL LIFE
If the hardness of the work material is increased then the hard particles exist in the chip. As the chip impinging on tool force with high momentum, may pluck particles from tool surface, causing reduced tool life. But if work material is too soft like pure iron on low carbon steels, even then tool life will be less because of built up edge formation. In these cases, it is better to slightly increase the hardness of these materials and machine to get better tool life.
c. SURFACE FINISH
The softer materials produce poor surfaces because of the same reason, built up edge formation. Hardening these soft materials to certain degree will render better surface finish in subsequent machining. Thus a cold drawn bar of steel which is work hardened can be expected to produce surface finish than hot drawn bar.
MICRO STRUCTURE
Cutting Force and Power: Micro structure as such has little effect on force and power consumption excepts as it affects hardness or strength materials. As already mentioned, harder or stronger materials require more power to machine.
b. TOOL LIFE
Carbon steels consists of a materials with particles of hard iron carbide distributed out in geometrical patterns depending upon the conditions of heat treatment. While machining low carbons steel, whatever to be the micro structure, have poor machining characteristics because of built up edge formations. This affects the tool life. In medium and high carbon steel microstructure is very important. To avoid built up edge formation while machining: a harder structure is wished, that means a mean ferrite path large dispersion of carbide particles. But avoid hard particles impinging on tool face and increasing wear; it is require to have larger mean ferrite path with a relatively few, large, widely spaced carbide. This means an optimum structure is required for an optimum tool life. Empirically it can be said a particle structure in best suited for medium carbon steel and a spheroidised (larger mean ferrite path0 structure is most advantageous for high carbon steels. The dependence of optimum structure upon machining operation may be illustrated by comparison of turning and broaching. Broaching is a slow speed operation and hence is a tendency built up new formation. This calls for a harder structure in broaching than in turning. High carbon spheroidised steel is usually too soft and hence produces built up edge and less tool life in broaching.
c. SURFACE FINISH
For a good surface finish consideration also, an optimum structure in steels are required. In the case of cast iron, greater the size of graphite particles, rougher is the surface finish.
CUTTING FLUID
Cutting Force and Power:
Cutting fluid performs two functions in machining.
Lubrication.
Cooling.
Lubrication effect is more required in slow speed operations where chance of built up edge formation is high. Cooling effect is more required in high speed operations to cool the tool and work. In slow speed operations, especially cutting fluid reduce the cutting force because it reduces the effect of built up edge formation.
b. TOOL LIFE
Tool life is extended because of cutting fluids. Apart from reducing the effect of built up edge. It cools down tool and reduces the friction at tool chip interface, forming a thin film coating.
c. SURFACE FINISH
Surface finish is improved because of lesser built up edge formation. Nowadays the technology has been improved in tool design and tool process with a intention to achieve more productivity at lesser cost.
JIGS AND FIXTURES
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| Fig.12 |
Jigs and fixtures are special work holding devices used while machining a job in machine tool. Devices used to hold parts at a definite relation to each other during assembly, welding, brazing are also commonly called as “Fixtures.” Fixtures are also used for inspection. These help in increasing productivity and to maintain the dimension of the part within a specified tolerance limit.
The distinction between jigs and fixtures is of purely academic interest.
A jig is normally defined as a device for holding a component, for specific operation, in such a way that it will guide one or more cutting tools to the same position on any number of similar components that may be used upon it.
A fixture is a device to which a component is fastened for a specific operation, but the cutting tools are not guided. Some people distinguish between jigs and fixtures as follows:
A jig is a work holding device which is not fastened to the machine on which it is used.
A fixture is also a work holding device but one that is fastened to the machine.
ADVANTAGES OF JIGS AND FIXTURES
Eliminate laying out work piece before machining and consequently eliminates setting up the work piece on the machine tool to the layout lines.
Increases machining accuracy, because the work piece is automatically located without aligning on the machine tool. Tolerances are maintained and interchangeability is attained.
Increases productivity due to increasing the number of work piece simultaneously machined.
Due to higher clamping rigidity, higher speeds and feeds, depth of cut can be used and thereby increases in productivity.
Save operator’s labor.
Use of lower skilled operator made possible.
Decreases expenditure on quality control and reduces scrap.
Increases versatility of machine tool.
Either fully or partly automates the machine tool.
ELEMENTS OF JIGS AND FIXTURES
Locating elements properly positioning the work piece.
Clamping elements.
Power devices for operating the clamping element.
Tool guiding or cutter setting elements.
Indexing devices for accurately changing the position of the work piece in the jig and fixture.
Auxiliary elements (locating and holding assembly for machine tool)
Fastening parts which serve to hold the elements or components together.
Body, Base or Frame.
PRINCIPLES OF LOCATION
Locating elements help in positioning similar components, repeatedly in the same relative position with respect to the body of fixture. Two important principles relating to the design of locating elements are as follows:
Reduce all possible degrees of freedom of the component to zero.
Avoid redundancy of location. If unavoidable, the redundant location should be floating.
THE SIX DEGREES OF FREEDOM
A body in space is free to make in six definite ways. It consists of linear movement along the three axis OX, OY and OZ, and rotational movement about the three axis. Sum of these movements gives six degrees of freedom. In order to locate a component i.e. to define a component in space, it is necessary that all the six degrees of freedom are arrested.
CLAMPING DEVICES
For any machining operation, it is essential to make suitable clamping arrangements. The selection depends upon type of working. Different clamps are required depending upon whether the components are produced in batch quantities or in mass production.
The clamps are broadly classified in following varieties.
Simple hand operated clamps.
Quick acting hand operated clamps.
Power operated clamps – pneumatic or hydraulics.
The selection of suitable clamp is based on an effort to balance the cost of clamp against the cost of the operation, in order to obtain the lowest possible total cost of fixtures and operation.
Tool designer should use sound judgment for selection of specific clamping arrangements for the job in hand. In general, clamping arrangement should be as simple as possible. Complicated arrangements tend to lose their effectiveness as the parts become worn, necessitating excessive maintenance, which might readily off-set the saving of faster operation.
CLAMPING PRINCIPLES
The purpose of a clamp is to exert a force to press a work piece against the locating surfaces and hold it there in opposition to the action of cutting forces. Clamping forces should be directed within the locating area, preferable through heavy sections of the work piece directly upon locating spots or supports. Cutting forces should be taken by fixed locators in jig or fixtures as much as possible, but in case of some components, moments set up by the cutting forces must be counteracted by clamping forces. To be effective, a clamp should be designed to exert a force equal to the largest force imposed upon it in the operation.
TURNING FIXTURES
Main principle of turning is rotating the work against traversing tools. The work is ordinarily set upon a horizontal lathe, but heavy odd shaped jobs are mounted on a vertical lathe. In actual practice, the types of turning and boring fixtures are as many as the numbers of components exist. However an attempt is made to generalize the fixture design to some extent with regard to the type of components.
1) S.No.
Type of Fixture.
Type of Component.
Chucks: Standard and special.
L/D ratio up to 1.
2) Face drivers, centers and spring collects.
For components like shaft, L/D ratio up to 2.
3) Mandrels and Arbor type fixtures.
L/D about 1. Suitable for finish operation.
4) Face plate or angle plate type fixture.
Components for mostly unsymmetrical shapes e.g. bearing housing and components of hydraulic units.
5) Special boring fixture.
For semi finishing and finishing operation.
6) Special turning fixture. For components like engine cranks shaft.
BORING FIXTURE
Boring means internal work of enlarging a cored or drilled hole. The class of work allocated to boring machines consists mainly of boring and facing the bosses of components which are too large or difficult to be set on centre lathe.
Alignment of boring fixture is very important. Careful tool setting is required.
MILLING FIXTURES
Milling fixtures are to be designed for rigid work holding because of the very nature of operation for which they are used. Milling operations throws intermittent load of varying intensity during the course of operation, usually multipoint tools are used and the teeth which remove material intermittently unlike lathe tools.
Factors which dictate the use of fixtures is work piece accuracy, odd and complicated shapes, heavy metal removal, quick and fool proof loading.
FACTORS TO BE CONSIDERED WHILE DESIGNING A MILLING FIXTURE
Type of milling machine used horizontal, vertical or universal type.
Milling machine and accessories data like, dimensions of machine table, its travel and position in 3 directions with respect to column and spindle, centre – to- centre distance and dimensions of T slots etc.
Clamping elements and supporting body should be rigid and sufficient to withstand shock loads without deforming the job.
Work piece area to be milled, should always be located within the area termined by supporting points, or the supporting points should be under the area to be milled.
Supporting and locating elements should be rigid as the cutting forces are directed against them.
Centre setting block hardened and ground, should be provided in the fixture to set the cutter properly in relation to the locating surfaces of the job.
Provide lifting points for fixture handling and access for loading and locating the part on to the fixture.
The projecting parts of clamping studs and nuts should be kept to minimize height over the milled surface.
TOOL ROOM
It is well known to everybody that tool room is the heart of the factory as all development, inspection, tooling, maintenance, and production activities depend on it. Tool room functions just like a human heart, it gives fresh tooling, inspection aids and many other helps to productive body to function well.
HOW TOOL ROOM WORKS?
As soon as tool design completes design of any jig, fixture, tooling or inspection gauge, it is sent for manufacturing to tool room with tool room number. This number is then included a pre-planned program for the month.
After receiving the drawing, tool room people fill the requirement of the raw materials.
As soon as the raw materials come, it is inspected in stores and then it is sent to tool room for further operation.
Meanwhile for any tooling, jigs or fixtures, tool room decides the processing (or in simple words sequence of operations)
As soon as the processing is finalized, the job is sent on different machines for various operations.
Different companies have different working operations and various operating function depending upon the factory’s tooling system, but working principle is same to all organizations.
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