Design Of Fixture

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Tool Design

DESIGN OF FIXTURES Introduction A fixture is a device for holding a work piece during machining operation. The name is derived from the fact that a fixture is always fastened to a machine or bench in a fixed position. It does not contain special arrangements for guiding the cutting tool, as drill jigs do. In a setup using a fixture the responsibility for accuracy depends upon the operator and the construction of the machine tool. Other types of tooling used for positioning parts relative to each other for fabricating purpose are also commonly referred to as fixture. Assembly fixtures and weld fixtures are example of this type. Many machining operations can be performed by clamping the work piece to the machine table without using a fixture, especially when a few parts are to be machined. However, when the number of parts is large enough to justify its cost, a fixture is generally used for holding and locating the work.

Types of Fixtures Examples of common types of fixtures are vise, milling, boring, broaching, lathe and grinding. Each will be described in the following section.

1. Vise Fixtures: Standard machine vises adopted with special jaws provides an easy way on holding parts for machining. They may be used with various types of machine tools and therefore may be classed as a type of fixture by themselves. All machinists know that is very easy to clamp a work piece with parallel sides in a vise but that work pieces with round or irregular contours are very difficult to clamp properly. It is also well known that unless precautions are taken, the clamping action of vise jaws is likely to mar finished surfaces. Special jaws are designed to hold work pieces with irregular contours properly and at the same time avoid damaging important surfaces. When it is necessary to hold the work piece firm against pressure in all directions the wedge type jaws may b useful. All the jaws considered up to now here held the work piece more or less in vertical position. However, many work pieces requires surfaces machines at a specified angle other than 90o for these workspaces. One of the jaws is recessed to take the work piece at the proper angle. Small casting or forgings with irregular contours can be held in standard machine vises by using epoxy resin to from a nest within the vise jaws to fit the irregular contour.

2. Milling Fixture A mill fixture holds the parts in the correct relation to the milling cutter as the table movement carries the parts through the cutters. It consists of five main parts, the base, clamps, rest blocks or rest, locating points and gauging surfaces. i) The Base: The base of a mill fixture consists of a base plate which has a flat and accurate undersurface and forms the main body on which various components are mounted. A heavy base is the most important element of a milling fixture. The complete fixture is built up from this plate, Keys are provided on the under surface of the plate which are used for easy and accurate Roll # 08MR-20

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CH # 04

Tool Design

aligning on the fixture on the milling machine table by inserting them into one of the T-slots in the table.

ii) Locating Point / Clamping Elements: The design of clamping and locating elements are same as milling fixtures so apply the all principles locating and clamping devices on milling fixtures. iii) Setting Block: After the fixture has been squarely clamp to the machine table, the work piece which is correctly located in the fixture, has to be set in correct relation ship to the cutters. This is achieved by the use of a setting block and feeler gauges. The setting block is fixed to the fixture and feeler gauges are placed between the cutter and reference plane on the setting block. So, that correct depth of cut and correct lateral setting is obtained. The block is made of steel among hardened and ground with the reference plane.

Milling Machine vise: For many jobs, the standard machine vise can be used to clamp the work piece under the cutter. The moveable jaw can be operated by a cam or a screw for various jobs. Special jaw may be attached to the plane jaws of the standard vise and vees also be fitted instead of the standard plane jaws, for short runs the special jaws may be made of aluminum alloy for soft mild steel for long runs, the varying surfaces of the special jaws should be hardened and should be of renewable type. Cast iron jaws are also frequently used. Design Principle for milling Fixtures a) Pressure of cut should always be against the solid part of the fixture. b) Clamps should always operate from the front of the fixture. c) The work piece should be support as near the tool thrust as possible. Classification of Milling Fixtures: There is no standard method of classification of milling fixtures, because each fixture is different and must be designed to meet the certain requirements. A. According to type of Milling Operation: According to this criteria, the milling fixtures can be face milling fixtures, shop milling fixtures, slotting fixtures, straddle milling fixture, gang milling fixture, string or line milling fixture, profile milling fixture and so on. B. According to the way of work piece is clamped: Under this category, we have hand clamping fixture, power clamping fixture, toggle clamping fixtures and automatic fixture. C. According to the way of the work piece is located: Such as center fixture, V-block fixture and pin or stud fixture. D. According to the method of pressing the work piece to the center: a) Cradles fixtures: Work piece is rocked or rated with in a given angle during milling. b) Rotary fixture: Work piece is rotated under the cutter. c) Drum fixture: Work piece is mounted on the rotating drum. Roll # 08MR-20

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Tool Design d) Indexing fixture: Work piece is indexed into the next position during the machining cycle of the mill. e) Rise and fall fixtures: These fixtures allow rising and lowing of the work piece in conjunction with the mill feed.

3. Lathe Fixtures A large majority of lathe operations can be accomplished by using standard chucks and holding methods. However, many parts such as casting and forging cannot readily be mounted by any of the standard methods. It is therefore necessary to manufacture special work-holding fixtures for machining these parts. The basic principles of fixture design apply to lathe fixtures. However, these are additional considerations that apply specially to lathe fixtures, since the fixture and the work piece revolve. They are as follow: 1- Clamps and other holding devices should be designed in such a way that they will not be loosened by centrifugal force. 2- The work piece should be gripped on its largest diameter, and whenever possible the gripping diameter should be larger than the diameter being machined. 3- The fixture should be well balanced. Irregular-shaped work pieces may have to be counterweighted to prevent vibration. It is not necessary to achieve prefect balance unless the work piece is rotating at high speeds. 4- The fixture should be as light as possible, since it is rotating. 5- Projections and sharp corners should be avoided as the operator may not be able to see them while the tool is rotating. 6- Thin section of the work pieces may require support to resist the pressure of the lathe tool. Lathe holding methods are usually some adaptation of chucks, faceplates and mandrels or a combination of the three. Each of the foregoing methods has a standard counterpart, such as three-jaw universal chucks, spring collets, rubber flex collets, standard mandrels etc. Such standard work-holding methods are universal, i.e. they will hold a variety of work sizes as long as they are of the same shape or geometry. Chucks: - Widely used lathe holding devices are universal self-centering scroll chucks with two, three, four and six jaws and independent four-jaw chucks used for odd-shaped work pieces. There are several variations, such as combination chucks with self-centering and independent jaw action and jaw powered by air or hydraulic cylinders on the rear of the spindle. Special top jaws may be designed for better fit to the gripping diameter of the work piece. Standard jaws usually will not be in full contact with the work piece. Soft top-jaw blanks available from chuck companies can be machined to the desired shape directly on the chuck. The jaw height should be kept short inn order to grip parts as close to the chuck face as possible. The finished jaw may be hardened or left soft as desired. Faceplate Fixture: - A lathe-faceplate fixture is fastened to a standard lathe faceplate and is usually built up from standard clamping and locating devices. It may be similar in appearance to mill fixture, but it is quite different in that it is rotating around the lathe axis. The base of the fixture may be a flat plate bolted on the faceplate or it may be an angle plate. It is usually located on the faceplate by means of two dowel pins and secured by cap screws inserted through the faceplate into tapped holes in the fixture. Sometimes faceplate fixtures with Roll # 08MR-20

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Tool Design

circular bases indicating grooves or accurately machined holes for the purpose of aligning the fixture by a dial indicator. Mandrels: - Mandrels are shafts especially made to hold work to be machined concentrically around a previously bored or drilled hole. There are two general types in common use, a) Plane mandrels and b) Expanding mandrels. a) Plane mandrels have about a 0.006” taper per foot. Work pieces are pressed on the plain mandrel by an arbor press. There must be a mandrel for each hole size, and the tolerance must be quite close on hole size to prevent the mandrel from completely passing through the work piece. Mandrels are generally not suited for production purpose. b) Expanding mandrels are more satisfactory for production work since the work piece in most cases is easier to remove and there is less wear on the mandrel as well as less damage to the bore of the work piece.

4. Grinding Fixtures The design of grinding fixtures is basically the same as the design of fixtures for use on other machine tools. For instance, there is little difference in a mill fixture and a fixture to be used on surface grinder. Likewise, a lathe fixture is similar to a fixture designed for use on a cylindrical grinder. There are, however, conditions unique to the grinding process that must be taken into consideration. The grinding process is usually a finishing operation where the work piece dimensions are held to close tolerance; therefore great accuracy is required in the design and construction of grinding fixture. Work pieces cannot deflect under clamping pressure and locating methods must be precise and exacting. Location of work pieces is made easier, however, since by the time they are ready for grinding, most work pieces have sufficient machined surface by which they can be positioned and held. For the time of grinding at the point of grinding heat generated at the point of grinding-wheel contact is controlled by a heavy application of coolant. Coolant nozzles must be designed properly and located where they will do the most good. The entire contact zone between the wheel and work should be flooded with an abundant supply of clean coolant. It should be applied in large quantities under very little pressure.

Roll # 08MR-20

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