Unit 1-3 Fixtures and Tools

Jigs and Fixtures

Jigs and fixtures are the devices which help in increasing the rate of identical parts and reducing the human efforts required for producing these parts. Objects can be machined involving the use of drilling, milling, planning, and grinding machines, etc. If such objects are to be produced in identical shapes and sizes on a mass scale, suitable devices have to be used for holding and locating purposes so that the repetition work can be done. These devices are jigs and fixtures.

The construction of jigs

A jig may be defined as a device which holds and locates a workpiece, and guides and controls one or more cutting tools. The holding of the work piece and guiding of the tool will make sure that they are located in the correct positions relative to each other.

In construction, a jig comprises a plate, a structure, or a box made of metal or in some cases non-metal. Its functions are handling the components in identical positions one after the other, and correcting the tools' incorrect positions relative to the workpiece in accordance with the drawing, specification, or operation layout.

The construction of fixtures

A fixture is a device which holds and locates a workpiece during an inspection or a manufacturing operation. The fixture does not guide the tool.

In construction, the fixture comprises different standard or specially designed components and is clamped on the machine to hold the workpiece in position. These tools are set at the required positions relative to the workpiece and adjusted by gauges or manually.

The main elements of jigs and fixtures

1) Body. It is a plate, box or frame-type structure in which the components to be machined are located. It should be quite sturdy and rigid.

2) Locating elements. These elements locate the workpiece in a proper position relative to the cutting tool.

3) Clamping elements. These elements firmly secure the workpiece in the located position.

4) Grinding and setting elements. These elements guide the cutting tool in case of jigs and help in proper tool acting in case of fixtures.

5) Positioning elements. These elements include different types of fastening devices, which are used in securing the jig or fixture to the machine at the proper position.

6) Indexing elements. They are not always provided. But many workpieces may have to be indexed to different positions in order to perform machining operations on different surfaces or different locations. In such cases, these elements will have to be incorporated in the jig or fixture.

Fasteners and Tools

Fasteners

Fasteners are devices which permit one part to be joined to a second part and, hence, they are involved in almost all designs.

The importance of fasteners can be realized when referring to any complex product. In the case of the automobile, there are literally thousands of parts which are fastened together to produce the total product. The failure or loosening of a single fastener could result in a simple nuisance such as a door rattle or in a serious situation such as a wheel coming off. Such possibilities must be taken into account in the selection of the type of fastener for the specific application.

The advantages of fasteners are:

1) Disassembly for inspection and repair.

2) Modular design where a product consists of a number of subassemblies. Modular design aids manufacturing as well as transportation.

There are three main classifications of fasteners, which are described as follows:

1) Removable. This type permits the parts to be readily disconnected without damaging the fastener. An example is the ordinary nut-and-bolt fastener.

2) Semipermanent. For this type, the parts can be disconnected, but some damage usually occurs to the fastener. One such example is a cotter pin.

3) Permanent. When this type of fastener is used, it is intended that the parts will never be disassembled. Examples are riveted and welded joints.

(1) Screws

A screw is a type of fastener characterized by a helical ridge, known as a male thread (external thread), as Figure 1-4 shows. Screws are used to fasten materials by digging in and wedging into a material when turned, while the thread cuts grooves in the fastened material that may help pull fastened materials together and prevent pull-out. There are many screws for a variety of materials; those commonly fastened by screws include wood, sheet metal, and plastic.

(2) Bolts

As Figure 1-5 shows, a bolt is a form of threaded fastener with an external male thread. Bolts are very closely related to screws. The distinction between a bolt and a screw is commonly misunderstood. There are several practical differences, but most have some degrees of overlap between bolts and screws. Bolts are for the assembly of two unthreaded components, with the aid of a nut. Screws in contrast are used in components which contain their own thread, and the screw may even cut its own internal thread into them. Many threaded fasteners can be described as either screws or bolts, depending on how they are used.

(3) Heads

Screws are manufactured with a variety of different heads to suit different needs. Figure 1-6 shows the heads. Some common cruciform or internal polygon heads are as follows:

1)Pan—a raised disc with a large surface area and rounded edges.

2) Button or dome head—hemispherical, has a rounded top.

3) Flat (countersunk) head—conical, with a flat outer face and tapering inner face allowing it to sink into the material. The angle of the screw is measured as the full angle of the cone. Countersunk heads will sit flush with material when used in conjunction with a countersink bit.

4) Bugle head—similar to countersunk, but there is a smooth progression from the shank to the angle of the head, similar to the bell of a bugle. Many drywall screws are bugle head screws.

Bolts are also manufactured in a variety of different heads, but instead are typically external polygon heads. Common bolt heads include:

1) Hexagonal.

2) Square.

3) 12 Points.

(4) Nuts

A small block of metal with a central, threaded hole that is designed to fit around and secure a bolt or screw and Figure 1-7 shows the nuts.

Fastening tools

Fastening tools are simply tools that help you apply fasteners, such as nails, screws, bolts, and other specialty hardware.

(1) Hammers

As Figure 1-8 shows, a hammer is a tool consisting of a weighted“head”fixed to a long handle that is swung to deliver an impact to a small area of an object. This can be, for example, to drive nails into wood, to shape metal (as with a forge), or to crush rock. Hammers are used for a wide range of driving, shaping, and breaking applications.

(2) Mallets

A mallet is a kind of hammer with a relatively large head. Mallet heads may be of softer material, like rubber, for use in striking applications that require a softer and more blunt impact. Figure 1-9 shows the mallets.

(3) Screw drivers

As Figure 1-10 shows, a screwdriver is a tool, manual or powered, used for screwing (installing) and unscrewing (removing) screws. A typical simple screwdriver has a handle and a shaft, ending in a tip the user puts into the screw head before turning the handle. Typical screw driver tips include:

1) Phillips (+).

2) Flat head (-).

3) Torx (star).

(4) Wrenches

As Figure 1-11 shows, a wrench or spanner is a tool used to provide grip and mechanical advantage in applying torque to turn objects—usually rotary fasteners, such as nuts and bolts—or keep them from turning. The most common shapes are called open-end wrench and box-end wrench.

(5) Ratchets or Socket Sets

A ratchet is a type of wrench that inserts into a socket to turn a fastener, typically in the form of a nut or bolt, just as Figure 1-12 shows. A ratchet incorporates a reversible ratcheting mechanism which allows the user to pivot the tool back and forth to turn its socket instead of removing and repositioning a wrench to do so.

(6) Hex keys

A hex key is a simple tool used to drive bolts and screws with hexagonal sockets in their heads, as shown in Figure 1-13. The tool is usually formed of a single piece of hexagonal rod of hard steel, with blunt ends that are meant to fit snugly into the screw's socket, bent in an“L”shape with unequal arms. The tool is usually held and twisted by the long arm, creating a large torque at the tip of the short arm. Reversing the tool lets the long arm reach screws in hard-to-reach places.