Jigs & Fixtures

Jigs and Fixtures:

Some  machining operation are so simple’ which are done quite easily, such as turning, the job is held in position in the chuck and turning operation is done easily. No other device is required to hold the job or to guide the tool on the machine in such an operation. But some operations are such type in which the tool is required to be guided by means of another device and also some jobs are of such forms which are required to be held in position on the machine by means of another device.

The device which guides the tool is called jig and the device which holds the job in position is called fixture.

Jigs and fixtures are special purpose tool which are used to facilitate production (machining, asslembling and inspection operations), when work piece is based on the concept of interchangeability according to which every part will be produced within an established tolerance. Jigs and fixtures provide on means of manufacturing interchangeable parts since they establish a relation with predetermined to tolerance between the work and cutting tool. They eliminate the necessity of a special set up for each individual park. So’ A jig is  may be de-fined as a device which hold and position the work; locate or guides the outing tool relative to the work piece and usually not fixed to the m/c table. It is usually lightly in construction.                                                                                                                  

A fixture is a work holding device and position the work; but doesn’t guide ‘locate or position the cutting tool’ the setting of the tool is done by machine adjustment and a setting blocker using slip gauges. A fixture is hold or clamp-ed to the machine table. It is usually heavy in construction. Jigs are used on drilling , reaming , tapping and couter boring operations , while fixtures are used in connection with turning , milling , grinding , shaping , planning and boring operations.

The use of jig and fixture makes possible more rapid and more accurate manufacturing at a reduction of cost.

Uses of Jigs and Fixtures:

1. Jigs and fixtures are used to reduce the cost of production as there use elimination being out work and setting up of tools.
2. To increase the production.
3. To assure the high accuracy of the parts.
4. To provide for interchangeability.
5. To enables heavy and complex shaped parts to be machined by holding rigidly to a machine.
6. To control quality control expenses.
7. Less skilled labor.
8. Saving labor.
9. There use partially automates the machine tool.
10. Improve the safety at work, thereby lowering the rate of accidents.

Jigs

The most-common jigs are drill and boring jigs. These tools are fundamentally the same. The difference lies in the size, type, and placement of the drill bushings. Boring jigs usually have larger bushings. These bushings may also have internal oil grooves to keep the boring bar lubricated. Often, boring jigs use more than one bushing to support the boring bar throughout the machining cycle.

In the shop, drill jigs are the most-widely used form of jig. Drill jigs are used for drilling, tapping, reaming, chamfering, counter boring, countersinking, and similar operations. Occasionally, drill jigs are used to perform assembly work also. In these situations, the bushings guide pins, dowels, or other assembly elements.

Jigs are further identified by their basic construction. The two common forms of jigs are open and closed. Open jigs carry out operations on only one, or sometimes two, sides of a work piece. Closed jigs, on the other hand, operate on two or more sides. The most-common open jigs are template jigs, plate jigs, table jigs, sandwich jigs, and angle plate jigs. Typical examples of closed jigs include box jigs, channel jigs, and leaf jigs. Other forms of jigs rely more on the application of the tool than on their construction for their identity. These include indexing jigs, trunnion jigs, and multi-station jigs.

Specialized industry applications have led to the development of specialized drill jigs. For example, the need to drill precisely located rivet holes in aircraft fuselages and wings led to the design of large jigs, with bushings and liners installed, contoured to the surface of the aircraft. A portable air-feed drill with a bushing attached to its nose is inserted through the liner in the jig and drilling is accomplished in each location.

Fixtures

Fixtures have a much-wider scope of application than jigs. These workholders are designed for applications where the cutting tools cannot be guided as easily as a drill. With fixtures, an edge finder, center finder, or gage blocks position the cutter. Examples of the more-common fixtures include milling fixtures, lathe fixtures, sawing fixtures, and grinding fixtures. Moreover, a fixture can be used in almost any operation that requires a precise relationship in the position of a tool to a workpiece.

Fixtures are most often identified by the machine tool where they are used. Examples include mill fixtures or lathe fixtures. But the function of the fixture can also identify a fixture type. So can the basic construction of the tool. Thus, although a tool can be called simply a mill fixture, it could also be further defined as a straddle-milling, plate-type mill fixture. Moreover, a lathe fixture could also be defined as a radius-turning, angle-plate lathe fixture. The tool designer usually decides the specific identification of these tools.

Elements of Jigs and Fixtures.

Various elements of jigs and fixtures and their details are follows.

1: Body                   2: Locating devices

3: Clamping devices   

4: Tool guide(jigs bushing)

1: Body:

The jig body is generally made of cost iron by casting process or fabricated by welding together various slabs and bars of mild steel. It may be heat treated to relief the stresses. Body is the most prominent feature of the jig. Its main purpose is to support and house the job.

The various jig body are follows:

(A):Plane Type Jig:

Plane type jig is the simplest type, it is used when plane holes are to be drilled. It has either drill bushes for guiding the tools or the holes without bushes.

(B): Channel Type Jig:

Channel type is made up from standard steel channel section.

(C): Box type Jig:

Box type jig is used where a component requires drilling in more than one plane and the jig is to be provided with on equilant number of drill bush plates. One side of the box is fitted with a lid which can be opened for inserting the component and for unloading it. It should be made as light as possible.

(D): The Built Up Jig:

The built up jig used dowels and screws for fabricating member welded type. Standard steel sections are used in it for the limited numbers of details, which are secured by means of screws and dowels, the locating pins and the blocks are positioned so that the greatest dimensional variation of the work piece may be accommodated.

(E): Leaf Type Jig:

Leaf type jig is simple made from a block of steel fitted with two adjustable locating screws and a spring loaded plunger. It is used in case of measured large components where it may be both unnecessary and construct a jig to hold the complete component , where madding is purely confined to a local section of the work piece.

2:Locating Devices

The pins of various design and made of hardened steel are the most common locating devices used to locate a work piece in a jig or fixture. The shank of the pin is press fitted or driven into the body of jig or fixture. The locating diameter of the pin is made larger than the shank to prevent it from being forced into the jig or fixture body due to the weight of the work piece or cutting forces. Depending upon the mutual relation between the work piece and the pin.

The pin may be classify as follows:

(A): Locating Pins:

When reamed or finally finished holes are available in work piece, these can be used for locating purpose of the manner as shown , these are two types of locating pins:

·        Conical locating pins

·        Cylindrical locating pins

(B): Support Locating Pins:

With these pins (also known as rest pins) buttons or pads the work piece with flat surfaces supported at convenient. In the fixed support pins the locating face is either ground flat or curved. Support pins with flat head are usually employed and provided location and support to machine surface, because more contact area is available during location. It would insure accurate and stable location. The spherical head or round head rest buttons are used for supporting rough surfaces (un machined and cast surfaces) because they provide a point support which may be stable under these circumstances. Adjustable type support pins are used for work piece whose dimension can vary. For example sand casting, forging or unmachined faces.

(C): Jack Pins:

Jack pins or spring pins are also used to locate the work piece whose dimension are subjected to variation. The pin is allow to come up under spring pressure or conversely is pressed down by the work piece. When the location of the work piece is secured the pin is locked in this position by means of locking screw.

Elements of jigs and fixtures:

3: Clamping devices:

If the work piece can not be restrained by the locating devices or elements, it become necessary to clamp the work piece in jig or fixture body. The most common example of clamping devices is bench vice. The purpose of the clamping is to exert a pressure to press a work piece against the locating surfaces and hold it there in a position to the cutting forces. In bench vice the movable jaw of the vice exert force on the work piece , their by holding it in correct position of location in the fixed jaw of the vice.

 The commonly used clamping devices are follows:

(A): Clamping Screws:

Clamping screws are used for light clamping. Clamping screws are shown in fig.

(B): Hook Bolt Clamp:

This is very simple clamping device and is only suitable for light work and where usual tip of the clamp is inconvenient. The typical hook bolt clamp is shown.

(C): Bridge Clamp:

It is very simple and reliable clamping device. The clamping force is applied by spring loaded nut.

(D): Heel Clamp:

These consists of a rusted plate, center stud and heel. This trap should be strengthen at the point where the hole for the stud is cut out, by increasing the thickness around the hole. The design differ from simple bridge clamp in that a heel is provided at the outer end of the clamp to guide its sliding motion for loading and unloading the work piece.

(E): Swinging Strap(Latch Clamp):

This is a special type of clamp which provides a means of intry for loading and unloading the work piece. For this the strap(latch or lid) can be swing out from the work piece. The typical swing strap or latch clamp is shown in figure.

(F): C-Clamp:

To unload the work piece, the locking nut is unscrewed by giving it about one turn and this releases the c- clamp. When the clamp is removed or swing away the work piece can freely pass over the nut. To reverse procedure is adopted for loading the work piece.

>: Element of jigs and fixtures:

4: Tool Guide or Jig Bushing:

Sometimes the stiffness of the cutting tool may be in sufficient to perform certain machining operations. Then to locate the tool relative to the work, use is made of guiding parts such as jigs bushing and templates. These must be precise, were resistance and changeable.

Jig bushes are used in drilling and boring, a bush fits into the hole of the jig, through which the drill passes. The diameter of the bush depends on the diameter of the drill. Different type of bushes are spot welded or screwed with the jig. Headless type bushes are press fit into the hole of the job. Bushes are general made of a good grade of tool steel to insure hardening at a fairly no temperature and to lesson the danger of fire cracking. Sometime the bushes for guiding tools may be of cast iron. Hardened steel bushes are always preferable for guiding drills, reamers and taps etc.

American standard bushes are classified in three categories.

1: press- fit wearing bushes

2: renewable wearing bushes

3: linear wearing bushes

Types of bushes (tool guide/jig bushes):

1: Press fit wearing bushes:

These bushes are used when little importance in put on accuracy or finish and tool used is a twist drill. These bushing are installed directly in the jig body and are used mainly for short protection. There are two design of press fit bushing:

A>   Plain or headless bush

B>   Headed or flanged bush

2: Renewable bushes:

When the guide bushes requires periodic replacement (due to wear of the inside diameter of the bush). Its replacement is simply by using a renewable bush. These are of the flanged types and sliding fit into the linear bush, which is installed press fitted into the jig plate. The linear bush provides hardened wear resistance, mating surface to the renewable bush. The renewable bushes must be prevented from rotating or lifting with the drill. One common method is to use a retaining screw.

3: Linear bushes:

These bushes are also known as master bushing, are permanently fixed to the jig body. These acts as guides for renewable type bushing. These bushes are be with or without head.

Types Of Fixtures:

Milling Fixtures:

A Milling fixture is a work holding device which is firmly clamped to the table of the milling machine. It holds the work piece in correct position as the table movement carries it past the cutter or cutters.

Essentials of Milling Fixtures:

1: Base:

A heavy base is the most important element of a milling fixture. It is a plate with a flat and smooth under face. The complete fixture is built up from this plate. Keys are provided on the under face of the plate which are used for easy and accurate aligning of the fixture on the milling machine table. By inserting them into one the T slot in the table. These keys are usually set in keyways on the under face of the plate and are held in place by a socket head cap screw for end key. The fixture is fastened to the machine table with the help of two T bolts engaging in T slots of the work table.

2: Setting Blocks:

After the fixture has been securely clamped to the machine table , the work piece which is correctly located in the fixture , has to be set in correct relationship to the cutters. This is achieved by the use of setting blocks and feeler gauges. The setting blocks is fixed to the fixture. Feeler gauges are placed between the cutter and refrence planes on the setting block so that the correct depth of the cut and correct lateral setting is obtained. The block is made of hardened steel and with the refrence planes(feeler surfaces) grooved. In it correct setting , the cutter should clear the feeler surfaces by at least 0.08cm to avoid any damage to the block when the machine table is moved back to unload the fixture. The thickness of the feeler gauge to be used should be stamped on the fixture base near the setting block.

3: Locating and Clamping Elements:

The same design principles of location and clamping apply for milling fixtures have been discussed above.

Some Design Principles for Milling Fixtures:

1: Pressure of cut should always be against the solid part of the fixture(fig a).

2: Clamps should always operates from the front of the fixture (fig B)

3: The work piece should be supported as near the tool thrust as possible(fig c)

Types Of Drilling Jigs:

Drilling jigs may be classified as follows:

1.     Template jig

2.     plate type jig

3.     Open type jig

4.     Channel jig

5.     Leaf Jig

6.     Box type jig

1: Template Jig:

This is the simplest type of jig; It is simply a plate made to the shape and size of the work piece; with the require number of holes made it. It is placed on the work piece and the hole will be made by the drill; which will be guided through the holes in the template plate should be hardened to avoid its frequent replacement This type of jig is suitable if only a few part are to be made.

2: Plate Type Jig:

This is an improvement of the template type of jig. In place of simple holes, drill bushes are provided in the plate to guide the drill. The work piece can be clamped to the plate and holes can be drilled. The plate jig are employed to drill holes in large parts, maintaining accurate spacing with each other. 

3:Open Type Jig:

In this jig the top of the jig is open; the work piece is placed on the top.

4 Channel jig;

The channel jig is a simple type of jig having channel like cross section. The component is fitted within the channel is located and clamped by locating the knob. The tool is guided through the drill bush.

5: Leaf Jig:

It is also a sort of open type jig , in which the top plate is arrange to swing about a fulcrum point , so that it is completely clears the jig for easy loading and unloading of the work piece. The drill bushes are fitted into the plates , which is also known as leaf , latch or lid.

6: Box Type Jig:

When the holes are to drill more than one plane of the work piece , the jig has to be provided with equalant number of bush plates. For positioning jig on the machine table feet have to be provided opposite each drilling bush plate. One side of the jig will be provided with a swinging leaf for loading and unloading the work piece, such a jig would take the form of a box. Such a jig should be as light as possible. Since it will have lifted again and again. Typical figure of box type jig is shown:

Design principles Common to jigs and fixtures:

There are some principles which are useful to design jigs and fixtures.

1: Rigidity:

Jigs and fixtures should be sufficiently stiff to secure the preset accuracy of machining.

2: Fool proofing:

It can be defined as “ the incorporation of design feature in the jig or fixture that will make it possible to lead the work into jig and fixture, in an improper position , but will not interfere with loading and unloading the work piece.” There are many fool proofing devices , such as fooling pegs, blocks or pins which clears correctly position parts but prevent incorrectly loaded parts from entering the jig and fixture body.

3: Clearance:

Clearance is provided in the jig or fixture body

(A)   to allow for any variation in component sizes specially casting and forging.

(B)   To allow for hand movements so that the work piece can easily placed in the jig or fixture and removal after machining .

4: Burr Grooves:

A burr raised on the work piece at the start of the cut is termed a minor burr and at the end of a cut is called a major burr. Jigs should be designed so that the removal of the work piece is not obstructed by these burr for this suitable clearance grooves or slots should be provided.

5: Ejectors:

The use of ejection devices to force the work piece out from the jig or fixture is important in two positions.

(A)   the work piece is heavy

(B)   machining pressure forces the work piece to the slides or based on the jig or fixture and the pressure and oil or coolant fill will cause the work to strick and difficult to remove on small jigs and fixtures , a pin located under the work will remove the part radially.

6: Inserts:

To avoid any damage to fragile and soft work piece and also to the finished surfaces of the work piece while clamping. Inserts of some soft material such as copper, lead , fiber , leather , hard rubber and  plastic should be fitted to the faces of the clamps.

7: Design for Safety:

Jigs and fixtures must be safe and convenient in use, following are the some

Factors for the safety of worker working on jigs and fixtures.

(A): Sharp corner on the body of jig and fixture should be avoided.

(B): Sighting surfaces should be cleared.

(C): Bolt and nut should be inside the body of jig or fixture and not protrude on the surface.

8: Sighting Surface:

Machining on the work piece must be clearly visible to the worker. He should not be required to bend is neck for seeing the work piece or work surfaces.

9: Simplicity in design:

Design of the jig and fixture should be a simple one. A completed design require a large maintenance. They should be easily to set , cheap in manufacture.

10: Economical:

Jig and fixtures should be simple in construction, give high accuracy , be sufficiently rigid and lightly weight. To satisfy these conditions an economical balance has to be made.

APPLICATIONS FOR JIGS AND FIXTURES

Typically, the jigs and fixtures found in a machine shop are for machining operations. Other operations, however, such as assembly, inspection, testing, and layout, are also areas where work holding devices are well suited. Figure 1-7 shows a list of the more-common classifications and applications of jigs and fixtures used for manufacturing. There are many distinct variations within each general classification, and many work holders are actually combinations of two or more of the classifications shown. 

EXTERNAL-MACHINING APPLICATIONS:

Flat-Surface Machining

   • Milling fixtures

   • Surface-grinding fixtures

   • Planing fixtures

   • Shaping fixtures

Cylindrical-Surface Machining

   • Lathe fixtures

   • Cylindrical-grinding fixtures

Irregular-Surface Machining

   • Band-sawing fixtures

   • External-broaching fixtures

INTERNAL-MACHINING APPLICATIONS:

Cylindrical- and Irregular-Hole Machining

   • Drill jigs

   • Boring jigs

   • Electrical-discharge-machining fixtures

   • Punching fixtures

   • Internal-broaching fixtures

NON-MACHINING APPLICATIONS:

Assembly

   • Welding fixtures

   • Mechanical-assembly fixtures

     (Riveting, stapling, stitching, pinning, etc.)

   • Soldering fixtures

Inspection

   • Mechanical-inspection fixtures

   • Optical-inspection fixtures

   • Electronic-inspection fixtures

Finishing

   • Painting fixtures

   • Plating fixtures

   • Polishing fixtures

   • Lapping fixtures

   • Honing fixtures

Miscellaneous

   • Layout templates

   • Testing fixtures

   • Heat-treating fixtures

Modular Fixtures

Modular fixtures achieve many of the advantages of a permanent tool using only a temporary setup. Depicted in Figure 1-4, these workholders combine ideas and elements of permanent and general-purpose workholding.

Figure 1-4. Modular workholders combine ideas and elements of both permanent and temporary workholding to make inexpensive-yet-durable workholders.

The primary advantage of modular fixtures is that a tool with the benefits of permanent tooling (setup reduction, durability, productivity improvements, and reduced operator decision-making) can be built from a set of standard components. The fixture can be disassembled when the run is complete, to allow the reuse of the components in a different fixture. At a later time the original can be readily reconstructed from drawings, instructions, and photographic records. This reuse enables the construction of a complex, high-precision tool without requiring the corresponding dedication of the fixture components.