After reading this essay you will learn about tools, jigs and fixtures.
Cutting tools having distinct cutting edges are used to perform metal-cutting process by removal of chips. Finished surface of desired shape and dimensions is obtained by separating a layer from the parent work pieces in the form of chips e.g., turning, drilling, milling etc. The process of metal cutting in which chip is formed is affected by a relative motion between the work piece and the hard edge of a cutting tool held against the work piece.
Tools are of following types:
(i) Hand tools
(ii) Machine tools
(iii) Cutting tools
Various types of tool materials that have contributed to increased speeds, feeds and rigidity are high carbon steel, high speed steel, cast alloys, cemented carbides, diamonds, and ceramics etc.
The device by which materials are changed into their primary shapes and forms is called die. The parts manufactured by ordinary casting, forging, rolling, swaging, drawing etc. need further finishing for final product but with the use of dies the parts are manufactured with close tolerances that they do not need further finishing. If the parts are manufactured with further more close tolerances then the metal powders are placed in a metal mould or die and compacted under great pressure.
Die casting is practised now-a-days, refers to the forcing, by pressure of molten metal into a metal die. Therefore, the term die used in this process implies a metallic mould which is filled under pressure.
The two most widely used die casting methods are as under:
(a) Hot-chamber method.
(b) Cold-chamber method.
Dies for both the methods are similar in construction because there is little difference in the method of holding and operating them. They are made in two sections, to provide means or removing the castings. Metal enters the stationary side when the die is locked in closed position.
As the die opens, the ejector plate in the movable half of the die is advanced so that pins project through the die half and force the casting from the cavity of fixed cores. The life of the die depends on the metal to cast and may range from 10,000 to several million fillings.
The tools used in most presses come under the general heading of punches and dies. They have an opening to receive the punch, and the two must be in perfect alignment for proper operation. Punches and Dies are not interchangeable, but must work together as a single unit.
Dies may be classified according to either the type of press operation performed or their construction as given under:
(i) According to Construction or Method of Operation:
(ii) According to the Type of Press Operation:
(a) Shearing—punching, blanking, notching, perforating, trimming, shaving, slitting and lancing.
(b) Bending—folding, angle bending, curling, forming and seaming.
(c) Drawing—forming tubes, flanges, embossing, bulging, cupping and reducing.
(d) Sequeezing—coining, sizing, flattening, swaging, cold forging, riveting, upsetting, extruding and hot pressing.
Mass production requires that all parts be made according to rigid specifications and working drawing. The devices by which the operators and inspectors refer that the standards established by the engineering department are maintained in the shop are called gauges. According to the need several kinds of gauges are used, varying widely in shape and size. They are classified as under.
(i) Manufacturing Gauges:
Manufacturing or working gauges are those used by the machine operators in the actual production of parts. They are made with very small tolerances, the purpose being to keep the size near the centre of the limits of tolerance.
(ii) Inspection Gauges:
Inspection gauges are those used by inspector in the final acceptance of the product. They are to ensure that the product is made in accordance with the tolerance specifications on the working blue-prints.
(iii) Master Gauges:
In addition to the above two, a third type of gauge, known as a master gauge, is sometimes used. Such a gauge is merely a reference gauge with which inspection gauges are periodically compared. The equipment used consists of such tools as precision-gauge blocks, measuring machines, optical flats, microscopes and projecting equipment.
The classification of the gauges as given under is principally according to the shape or purpose for which it is used:
5. Form used for – (a) screw thread, (b) fillet, (c) centre, (d) drill point, (e) angle, (f) gear tooth, (g) special contour etc.
6. Thickness – (a) Precision gauge block, (b) feeler, (c) wire.
The superintendent of production control department is generally made in charge of tool room who has to supervise tool department personnel such as foremen, tool inspectors, clerks, group attendants, labourers and tool crib attendants.
The tool department is responsible for manufacturing, storing, issuing and maintenance of tools, jigs, fixtures, dies and gauges. Its organiser should have adequate knowledge and familiarity with tools and manufacturing process.
Tools are stored in tool cribs which are situated at the centre or nearer to the production department. Tool cribs are specially constructed and consist of racks so that tools may be kept in safe custody and can easily be issued whenever desired.
Location of Tool Cribs:
Methods for locating tool cribs are as given under:
(i) Central Tool Crib:
The crib of this system is made very large in size and is located at the central place. The central crib consists of all factory tools which are issued according to the requirements of the other cribs.
(ii) Sub-Tool Crib:
From this crib, tools are issued to the different department tool cribs and from departmental tool cribs, tools are issued for production purposes. The departmental tool crib consists of tool required by that department. The tools list may vary from department to department and requisition may be placed to the sub-tool crib whenever additional tools are needed.
(iii) Movable Tool Crib:
This crib contains tools needed by the entire plant. It consists shelves for storing the tools and supplies the tools at site against the requisitions of the department.
Factors Influencing Location of Tool Cribs:
Following are the factors which influence the location of tool cribs:
(i) Size and layout of the plant
(ii) Allocation of space to department
(iii) Nature of product
(iv) Methods of operation
(v) Grade of operators
Layout of Tool Crib:
A tool crib should have all the abilities that of a good store.
The factors and considerations on which crib space depends are given as under:
(i) The tool should be stored to an approachable height. In the beginning, the tools should be stored in bottom-most bin then in the upper side bins of the rack.
(ii) The size of the bins should be according to the size of the tools to be stored.
(iii) A sufficient aisle space should be left between bin to bin.
(iv) The space left between bin to bin should be limited to what is absolutely necessary but should not be so small as to over-crowd them.
Methods of Issuing Tools:
The method of issuing tools is described as under:
1. Single-Check System:
In this system each worker is provided with a brass check on which his clock number is stamped. When the worker desires to have a tool then he hands over his check and gets a tool or tool is exchanged. The crib attendant hangs the check on a hook by the place where the tool is kept.
It shows that which worker has taken a particular tool. The check is returned when the worker deposits the tool. It is very simple but disadvantage is that particulars of worker and tool are not recorded in this system.
2. Double-Check System:
In this system first check serves the purpose similar to that of the single check system. The second check showing the number of tools issued is hung against the hook number of the worker on a board. This system has also drawback that it does not show that when or for what job the tool has been issued to the worker.
3. Triplicate Tool-Slip System:
In this system, when a worker wants to draw a tool he fills out a tool-order or tool-slip in triplicate.
The tool-slip is printed with all the particulars such as:
(a) Tool order
(c) Department where the tool is to be used
(d) Bin number
(e) Clock number of the worker
(f) Quantity of tools issued
(g) Size of tool
(h) Description of tool
(i) Signature of the worker etc.
The three copies are of three distinctive colours. Generally the original is made white, the duplicate yellow and the triplicate of pink colour. One set of slips is used for one tool order and one item only.
The crib attendant issues the tool and returns the yellow copy of the slip to the worker. The white copy is retained by the crib attendant as a receipt and hung it in employee’s compartment under the clock number of that worker. The pink copy is filed in the tool compartment which is provided in lock system. All the white copies of the tool orders are fixed on a wall board from where it is visible at a glance that which worker is issued how many tools.
When a workman returns his tool, he also returns the yellow copy of the tool order. If the tool is in good condition, the crib attendant removes the white copy from the compartment and hands over it to the worker, then he also removes the pink copy from the tool records, it means that the tool has been returned.
When the tool returned by the worker is broken, then he has to submit an “explanatory report” approved by the foreman with that tool. The explanatory report specify the cause of the damage.
If the cause is due to carelessness of the worker then action has to be taken against the worker and tool is sent for repairs. After long uses when the tool becomes unserviceable then report is received from production department and tool is declared as scrap and is removed from the stock.
Record of Issue of Tools:
Following are the advantages of maintaining the record of tools issues:
(i) It prevent losses of tools.
(ii) It ensure maximum utilization of tools.
(iii) It ensure good condition of the tools.
(iv) It ensure proper handling and prevents the tools from loss.
(v) It maintains the history of the tools and guides for further purchasing.
(vi) Broken or damaged tools can be replaced immediately with the proper records.
Advantages of Triplicate Tool-Slip System:
(a) Number of tools received by a worker can be counted.
(b) Total number of tools of particular type can be counted.
(c) Time of issue of tool and physical position of the tool can be known.
(d) The inventory card for each tool is possible.
(e) As the explanatory report comes time to time, therefore, durability of the tools can be determined.
(f) Slow moving tools can be detected and removed accordingly.
(g) Responsibility of the damaged tool can be fixed easily.
(h) With due care and proper handling of the tools, the general efficiency of the plant may be raised.
(i) Tools are easily indicated when they should be repaired, replaced and declared as a scrap.
(J) The system indicates to the purchase department to purchase new tools.
(k) Information regarding tool transactions can be given to the accounts department with this system.
4. Electric Machine System:
This system consists of an electric machine by which tool order is printed in duplicate. When a work-man draws a tool, he signs the original tool order and submits both the orders to the tool crib attendant. The tool crib attendant then issues the tool to the worker and files the original order in the tool record file under the worker’s clock number. The duplicate is placed in the bin from which the tool is withdrawn.
When the tool is returned by the worker in good condition then the signed original copy of the tool order is returned to him. The duplicate copy is retained there and is used for further analysis of works.
If the tool is broken then the worker returns the tool with broken tool report approved by the foreman. Under this condition, the original copy is returned to the worker and the duplicate together with the broken tool report is filed until the case is decided.
Control of Tool Inventory:
This consists of control on procurement, quantities and disbursement of tools to different tool cribs.
The main records of the tool inventory system are given as under:
(i) Tool requisition.
(ii) Tool inventory ledger.
(iii) Bin cards.
Duties of Tool Keeper:
(i) To store the tools, jigs, fixture, dies and gauges.
(ii) To make efforts for purchasing the tools as per purchase requisitions.
(iii) To maintain tool ledgers and other records.
(iv) To issue the tool against the tool orders placed by the production department.
(v) To check the conditions of the tools returned by the production department.
(vi) To get repaired the damaged tools.
(vii) To undertake the physical inventory of tools.
(viii) To classify the tools and mark on them.
(ix) To assist the tool standardization committee in respect of standardizing the tools.
Standardization of Tools:
The type, fixed size, quality and measure of the tool are considered under this term. The relationships and values of the tool must be considered according to the scientific procedures.
A standardisation committee is constituted among the manufacturing engineers, tool designers, tool engineers, technical persons and suppliers of the tools, which also considers all the standards pertain to the tools.
The main advantages of the standardization of the cutting tools are given as under:
(i) It gives good performance of the tools and maintains good standards of quality of the work.
(ii) It reduces tool inventory, volume of tool and storage space, thereby lowering the cost.
(iii) A bulk purchase of such tools is possible which gives economy of quantity buying.
(iv) As it increases the sources of supply, therefore, tools can be purchased from large suppliers.
(v) The interchangeability of tools reduces down-time in changing tools on machines.
(vi) It helps in setting up new facilities in the manufacturing process.
(vii) It allows higher utilization of tools.
(viii) It promotes safety and increases operating efficiency.
(ix) It provides ease in inspection, grinding and maintenance of the tools.
(x) It reduces overall production cost of the work.
Jigs and Fixtures:
(i) They eliminate the marking out, measuring and other setting methods before machining.
(ii) They enable production of identical parts which are interchangeable.
(iii) They increase the machining accuracy of the work piece.
(iv) They increase the production capacity of the man and machine.
(v) They reduce the operator’s labour and consequent fatigue to him.
(vi) They enable semi-skilled operator to perform the operation, which saves labour cost.
(vii) They minimise the expenditure on the quality control of finished products.
(viii) They reduce the overall cost of machining by fully or partly automatising the processes.
Principles of Jigs and Fixtures Design:
Following are the essential factors which must be considered in designing a jig or a fixture:
(i) Study of the work piece.
(ii) Study of the machine tools.
(iii) Study of the locating elements.
(iv) Study of the locating and unloading system,
(v) Study of the clamping arrangement.
(vi) Study of the power devices for clamping the work piece.
(vii) Study of the clearance required between the jig and the work piece.
(viii) Study of indexing system and devices.
(ix) Study of the tool setting and guiding elements.
(x) Study of the fool-proofing arrangement.
(xi) Study of the ejecting systems.
(xii) Study of the dwarf removal system.
(xiii) Study of the rigidity and vibration proofing.
(xiv) Study of the table fixing arrangement.
(xv) Study of the safe factors and safety devices.
(xvi) Study of the methods by which the jigs are manufactured.
Types of Jigs and Fixtures:
The type of jig and fixture totally depends on the type of work to be machined and its quality required.
Some simple drill jigs are as below:
(i) Template jig
(ii) Plate jig
(iii) Diameter jig
(iv) Channel jig
(v) Ring jig
(vi) Box jig
(vii) Leaf jig.
They serve the purpose of holding work piece securely on the jigs or fixtures against the cutting forces.
For an efficient clamping, the following operational factors must be considered:
(i) The clamping pressure should be exerted on the solid supporting part of the work otherwise distortion will be developed.
(ii) The amount of clamping pressure should be just to hold the work against the cutting pressure.
(iii) There should be a limited movement of the clamp for loading and unloading the work.
(iv) The clamp should be so designed that it may be removed while unloading the work.
(v) The clamp should be very simple and fool-proof.
(vi) The clamp should be so strong that it may not allow bending.
(vii) The operating lever or nut of the clamp should be of same size and shape, and well knurled if operated by hand.
(viii) It should be case-hardened to avoid the wear of the clamping faces.
(ix) The setting of the clamp should be such that it may allow to perform most of the operations on the work,
(x) The clamping parts should be designed to make it non-detachable from the jig.
Types of Clamps:
The clamps which are commonly used with jigs and fixtures are:
(i) Flat clamp
(ii) Screw clamp
(iii) Pivoted clamp
(iv) Latch clamp
(v) Double acting clamp
(vi) Equalising clamp
(vii) Cam clamp
(viii) Swing, plate clamp
(ix) Wedge clamp.