Jigs and Fixtures: Elements and Design Steps | Machine Tools | Engineering

In this article we will discuss about:- 1. Meaning of Jig 2. Meaning of Fixture 3. Elements of Jigs and Fixtures 4. Design Principles of Jig and Fixture 5. Design Steps of Jig and Fixture 6. Use of Jig Body in Jig and Fixture 7. Basic Rules for Locating Jig and Fixture.

Meaning of Jig:

Jig may be described as a plate, or metal box, structure or a device usually made of metal on to or into which components can be clamped or fastened or located and held in positive manner in identical position one after the other 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 which may be used upon it. Jigs are, usually fitted with hardened steel bushings for guiding drills or other cutting tools.

Holes are bored in the structure, so that when tools are fed through them and into the component, holes are made in the component in the correct positions as required by the component drawing; the holes in the jig positively locating and guiding the cutting tools.

It is usually necessary for the work to be held in the jig by clamping. Jig is usually not fixed to the machine table by clamping. However, for drilling holes above 6 mm diameter, it is usually necessary to fasten the jig securely to the table. Jigs are used for mass drilling, reaming and tapping.

Meaning of Fixture:

Fixture may be described as a structure for locating, holding, and supporting a component or work piece securely in a definite position for a specific operation but it does not guide the cutting tool. The cutting tools are set in position by machine adjustment or by trial and error method.

Setting gauges or setting blocks and feeler gauges are often provided to enable the initial setting of work to the cutter to be quickly and easily accomplished before machining (Refer Fig. 28.1). The fixture is generally bolted or fixed securely to the machine table in such a position that the work is in the correct relationship to the cutter. Fixtures are used for mass milling, turning and grinding operations.

 

Elements of Jigs and Fixtures:

The employment of jigs and fixtures is an important aspect of workshop engineering for the production of articles in large quantities with a high degree of accuracy and inter-changeability at a competitive cost. The purpose of jigs and fixtures is to maintain low manufacturing costs and to increase industrial efficiency.

Thus the jigs and fixtures are mainly used to reduce costs and ensure inter-changeability which allows for rapid assembly. Its purpose is also to speed up machining times by eliminating time of handling and setting of the component parts. Further jigs and fixtures are taking the place of the skilled man in the production factory and making it possible to employ unskilled or semi-skilled operators.

Their use also ensures the uniformity of finished product which could not be expected otherwise, because of variation of skill between individuals. The primary object of the use of jigs and fixtures is to facilitate the holding and supporting of the component by using fixtures; to position it properly and guide the cutters so that every component will be uniform. It is also employed to accommodate several components at one setting and thus taking advantage of multiple machining.

It is particularly very suitable where correct positioning of various holes at various exact places is important and which otherwise would consume lot of time in marking etc. The distinction between jig and fixture is not important but it is generally understood that jig is that part which incorporates bushes, and guides the tools or cutters; the fixture holds and locates the work without necessarily providing definite guidance for the tools.

Design Principles of Jig and Fixture:

Jig and fixture design is based upon a number of fundamental principles but there are no hard and fast rules. The jig tool designer is likely to meet a new problem on almost every component he handles, but the underlying principles will be found to be similar.

There can be several ways of achieving the same results but the designer has to see the relative merits of various possible ways in respect of their cost, ease of making, facility of using, possibilities for accuracy etc. Each job is designed on its merits to suit individual components fulfilling all its requirements. However, with any design followed, the aim should be simplicity and economy.

For precision in machining operation, the work be properly positioned with respect to tool (referencing). For accuracy the designer has to ensure that the part is precisely located and rigidly supported. It should be ensured that the tool is easily loaded and unloaded and be fool-proof.

The work holder or clamp must be strong enough to resist the cutting forces and accurately hold the position of the part against the cutting forces. Clamps must also allow for rapid loading/unloading.

While designing the jigs and fixtures the following principles should be borne in mind:

1. The method of location and clamping should be such as to reduce idle time to a minimum. For this purpose the aim of the designer should be to arrange in such a way that the loading can take place on one batch of components whilst another is being machined. Location and clamping influence the accuracy and quality of a component.

Other principles to be followed for location purposes are:

(a) Locating surfaces should be as small as possible and the location must be done from the machined surfaces. For first operation upon rough un-machined surface, three point locations should be used where possible, and adjusting or expanding locators being used to allow for large variations in size.

(b) Sharp corners in the locating surfaces must be avoided. A good radius or chamfer on the end of locating pin permits the component to be placed on easily and quickly.

(c) As many degrees of freedom of movements as necessary to maintain the required accuracy should be curtailed. Redundant location features need not be provided.

(d) At least one datum surface should be established at the first opportunity. Location should be from the same machined surface (datum) for as many operations as possible to reduce the possibility of error.

(e) Adjustable locators should be provided for rough surface.

(f) Locating pins should be tapered and be easily accessible and visible to the operator.

(g) Quick acting clamps should be used wherever possible.

(h) The necessity of lifting the clamp by hand should be avoided by fitting a spring suitably to lift it.

(i) Location should be designed to be fool-proof so that a component can be loaded into only one correct position and it can’t enter into any other position.

(j) Enough clearance for clearing machining burrs should be provided. Location features should not be able to collect swarf.

(k) Operator’s safety and the manipulative difficulties in loading the component into the fixture should be given due consideration. Sometimes it may be desirable to use retractable location pins.

2. The basic principles for good clamping design are:

(a) Clamping should always be arranged directly above the points supporting the work, otherwise the work will be sprung and machined in a distorted position thus resulting in inaccuracies. The supporting points should be strong enough to resist bending. In other words, clamps should be applied to the component where it is rigid and well supported.

(b) Fibre pads should be riveted to clamp faces where metallic contact with the work would cause damage.

(c) The position of the clamps should be such that it provides best resistance to the cutting forces.

(d) Clamps should not cause deformation of the work piece, i.e. clamping forces should be controlled such that they do not distort the locators, work fixture, or the clamps themselves.

(e) Pressure of cutting force should act against the solid part of the jig and not against the clamps.

(f) All the clamps and adjustment should be on that side of the fixture from where the loading or unloading of the components will take place.

(g) Clamping arrangement should be designed in such a way that it can be easily and clearly removed clear of the work by avoiding the necessity of length unscrewing of nuts.

(h) Clamps should be quick acting and as simple as possible.

(i) Clamps should be well clear of cutting tools, and adequate clearance should be available with the clamps released to load and unload the work in safety.

(j) For ease of unloading use of ejectors may be considered.

(k) Sound ergonomic principles should be applied.

3. The process of loading and unloading the component should be as easy as possible. Ample space should be left for hand movements e.g. as between the walls of a jig and the position of the component. It should also take into account any possible size variation and some means should be provided for releasing the component if for some reason it sticks.

4. Work supports:

(a) The number of fixed supports on any surface should not be more than three and they should be as far apart as possible, because the component will sit on three points without rocking. If, however, more than three supports are necessary then spring loaded adjustable supports should be used for additional supports so that three fixed supports will locate the surface and spring loaded type will automatically rise to touch the component through the medium of the springs.

(b) The area of supporting surface should be least possible so that it can be kept clean. Its surface should not take dust or swarf.

(c) The supports should be visible and accessible to operator.

Loading and supporting surfaces should be renewable wherever possible. Generally such surfaces should be of hardened material. This is very important for the proper maintenance of the jigs and fixtures. They should be fitted into through holes and not blind holes.

5. Stability and Rigidity:

Jigs and fixtures should be rigid enough as this fault is generally not realised in normal working. For this purpose at least 4 legs should be provided on the jigs and suitable means should be provided for clamping the fixture on the machine bed. If possible, arrangements to damp out and absorb the vibrations should also be provided.

6. Clearance of chips. For cleanliness and accuracy, the importance of providing good swarf and chips clearance cannot be over-emphasised. Adequate space in the form of channel ways should be provided to enable the metal chips to be blown clear by using compressed air. The awkward little corners that cannot help but collect chips, should be avoided.

7. Coolant to the cutting edges. Adequate arrange­ments must be made for the supply of coolant to the cutting edges, so that at the same time as the cutters are cooled the swarf is kept clear.

8. Jig and fixture should be fool-proof. The design of the jig and fixture should be such that it is impossible for an operator to insert either the work piece or the cutting tools in any position other than the correct one.

This can be arranged by placing a pin or abutment in a position to clear the component when it is in its correct relative position, but fouling the component when the reverse is the case. Also the pilot bushes could be used of different sizes in order that the tools may not be applied through the wrong bushes in a jig.

9. Careful considerations should be given to the initial method of location with particular reference to the operations that follow.

10. Safety of the operation:

All the necessary precautions in this respect should be observed.

11. Design should be as simple as possible. As far as possible standard components of parts should be used and similar operations should be grouped.

12. Drilling jigs should be as light as possible consistent with strength to facilitate handling.

13. The job is normally not bolted to the machine table and should be provided with four feet so that it will rock if it is not standing square on the machine table and warn the operator.

14. Fixtures should be robust especially for inter­mittent cuts in order to prevent distortion and avoid vibrational effects.

15. Tennon strips should be provided for accurate and quick location of the fixture on the machine bed.

16.  Milling fixtures should be provided with a setting piece so that a feeler gauge may be used for setting the fixture relative to the cutters.

Design Steps of Jig and Fixture:

The following steps should be followed for designing jigs and fixtures:

(i) Draw the outline of the work piece in position of machine.

(ii) Inspect the drawing carefully and note all limited dimensions and features which are strictly related.

(iii) Consider the sequence of operations.

(iv) Draw the location system for locating the component in fixture.

(v) Draw the clamping system for clamping the component in the fixture.

(vi) Draw the tool guide (in case of drills) or tool setting dies (in case of fixtures), i.e. method of positioning the tool relative to the component.

(vii) Method of positioning the fixture relative to the machine: 

In the case of jig which is not clamped on machine, some form of stop must be provided against which the jig may be held to give a quick initial positioning.

(viii) Decide the most suitable jig and body for fixture.

(ix) Combine all the components are rigidly as possible.

(x) Safety aspects to protect the user whilst using the fixture should be considered.

Location:

In the design of jigs and fixtures, the location of the component is very important aspect as the correct location influences the accuracy of the finished product; and particularly in reference to the positional relationship with other surfaces on the component.

Further the location arrangements are closely related to other aspects of jig application as for example the faulty method of clamping may cause the component to lift away from the locating surface though the perfectly satisfactory method of location might have been employed.

Use of Jig Body in Jig and Fixture:

The tool body provides a rigid base for mounting locators, supports, clamps and other accessories needed to reference, locate and hold the part. The size and shape of the tool body is determined by the size of the part and the operation to the performed.

Body may be made either of cast iron by casting process or fabricated by welding together various slabs and bars of mild steel and heat treating it to relieve the stresses. The second method is most commonly used as it is cheap, light and easy in construction. Sometimes body is built up.

The choice of material and method of construction depends on economy, rigidity desired, accuracy and expected life.

Cast tool bodies may be made of cast iron, cast aluminium, or cast resins. Cast bodies are quite stable. Cast tool bodies are also beat for part nesting and offer vibration dampening. Initial cost is higher due to the required pattern and longer lead time. (Time spent between design and fabrication).

Welded tool bodies are usually made from steel, aluminium, or magnesium. These have high strength and rigidity. Other advantages are design versatility, ease of modification, and short lead time.

Built-up tool bodies are the most common form of tool body and can be made from almost any material such as steel, precast sections, aluminium, magnesium, and wood. The main advantages of using built-up tool bodies are adaptability, design versatility, ease of modification, use of standard parts, and short lead time.

Preformed materials like precision ground flat stocks, cast bracket materials, precision ground drill rods, structural steel sections, and precast tool bodies, readily available can greatly reduce the cost of tool body. Since preformed materials are available in a variety of sizes and shapes, the time required to machine a tool body is also greatly reduced.

The various types of bodies in common use are:

(1) Plain type

(2) Channel type

(3) Length type

(4) Box-Type

(5) Universal jigs

(6) Index jigs

(7) Built up jigs

(8) Welded jigs

The plain type is of simplest type and is used when plain holes have to be drilled. It is a simple plate jig having either drill bushes for guiding the tools or without bushes if holes are closely spaced. When the body of a jig is made up from a standard steel channel section, it may be described as a channel jig.

The built up jig can be either made by using dowels and screws for fabricating the members or of welded type, i.e. fabricating by welding. In built up jigs, standard steel sections are used for the limited number of details which are secured by means of screws and dowels.

The locating pins and blocks are positioned so that the greatest dimensional variations of the work piece may be accommodated. The leaf type of local jig is used in the case of medium and large components where it may be both unnecessary and undesirable to construct a jig to hold the complete component and where the machining operation is confined purely to a local section of the work piece. This type of jig is simple, made from a block of steel fitted with two adjustable locating screws and a spring loaded plunger.

The box type jig is used where a component requires drilling in more than one plane and the jig is to be provided with an equivalent number of drill bush plates. In this type of jig, generally feet would be needed opposite to each bush plate and one side of the box is to be fitted with a lid or latch to provide the necessary opening for inserting the component and for unloading it. This should be made as light as possible and special attention given for swarf disposal. This is probably the best choice where three-two- one type location is used.

Basic Rules for Locating Jig and Fixture:

Part locators, for restricting the movement of a part and its proper positioning, require skill and planning. They must be planed into the tool design and never be installed as an after-thought.

The basic rules for locating are:

(i) A tool designer must always ensure that locators are positioned to contact the work on a machined surface. This is essential for accurate placement of the part in the tool and to ensure the repeatability of jig or fixture.

(ii) The locators should be spaced as far apart as possible, thus using fewer locators and insuring complete contact over the locating surface.

(iii) Locators should be placed to avoid interference with chips or dust. Where it is not possible, the locators should be relieved. (Refer Fig. 28.2) Fixed-shop locators may be either machined into the tool body or be installed. Installed locators are normally more economical to use because of time taken to make the machined locators. Since the installed locators can be replaced when worn, the entire body need not be made again.

(iv) The tool tolerance should be between 20 to 50% of the part tolerance in order to maintain the required precision.

(v) Fool-proofing must be incorporated i.e. to insure that the part will fit into the tool only in its correct position. For this purpose fool-proofing pins should be incorporated at suitable location so that wrong insertion would not be permitted by this pin.

(vi) Use of duplicate locators, which not only is costly but causes inaccuracy, should be avoided. Once the reference surface for location is decided, there should not be other locator to cause interference in location with reference to the reference surface. For instance, locating a part both from its outside edge and the hole can create problem.