Tool Design-3607 Locating Clamping Jigs Completed PDF

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Tool Design-3607 KP Imon | 122076 Reference book: Donaldson, Wilson + Chothaz  Locating Methods 1 TERMINOLOGY 1.1 LOCATING: The term locating refers to the dimensional and positional relationship between the workpiece and the cutting tool used on the machine. A work holding and positioning device ...

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Tool Design-3607 KP Imon | 122076 Reference book: Donaldson, Wilson + Chothaz

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Locating Methods 1 TERMINOLOGY 1.1 LOCATING: The term locating refers to the dimensional and positional relationship between the workpiece and the cutting tool used on the machine. A work holding and positioning device is usually employed to establish this relationship.

1.2 DEGREE OF FREEDOM: A work piece in space free to move in any direction around three mutually perpendicular axis and may be said to have twelve degree of freedom. From figure, it may move either three mutually perpendicular axes and may rotate in either of two opposite direction about each axis (clockwise or anti-clockwise)

1.3 COMPLETE LOCATION When 12 degree of freedom of workpiece is restricted then the workpiece is said to be completely located. Restriction of 12 degree of freedom is called complete location. Complete location has been accomplished by use of three points in (or parallel with) the reference plane, two points in a second plane, and one point in a third plane.

1.4 CONICAL LOCATION The method of locating from a circular surface by using cones is called conical location.

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1.5 RADIAL LOCATION When the workpiece contains accurate holes that one perpendicular to a finished locating surface, adequate locating may be achieved with two pins or plugs extending from a base surface. It is known as radial location.

1.6 PIN AND BUTTON LOCATOR A round pin or button is used to firmly support or hold the workpiece in position in this method of location. The main difference between pins and buttons is in legth. Buttons are generally shorter than pons and are used for vertical positions. Pins are usually used for horizontal locations. Larger sizes are sometimes referred to as plugs.

1.6.1 Advantages and disadvantages of Pin locator Used for horizontal locations. Different to chip disposal.

1.6.2 Differences between pin and button locator Pin locator 1. 2. 3. 4.

Size Chip removal Load carrying capacity Job holding direction

Larger Difficult Light Horizontal

Button locator Smaller Easier Heavy Vertical

1.7 NEST OR CAVITY LOCATION The term nest is usually used when referring to the area of the jig or fixture that receives the workpiece. The nest of a jig or fixture should be a pocket or cavity into which the workpiece is placed and located. This is an effecting method of locating.

1.7.1 Advantage of Nesting 1. The only freedom remaining is in an upward direction. 2. A smaller rest can be used to locate cylindrical workpiece 3. Different size of workpiece can be located

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4. No supplementary locating device such as pins, buttons are required.

1.7.2 Disadvantages of Nesting 1. Since the workpiece is completely surrounded, it is difficult to lift of the nest 2. Ejecting device is necessary 3. Higher costs for making the nest 4. Additional time is needed during operation

1.8 PARTIAL NESTING The location method in which two cavity is formed just like the two ends of nesting method. It eliminates most of the disadvantages of nesting method. As such1. Since they don’t have the entire contour of the workpiece, they require less time to make 2. Since they don’t completely confine the workpiece it is easily left out the work holder 3. No ejecting device is needed 4. Easy removal of chips

2 LOCATING 2.1 THE BASIC PRINCIPLES OF LOCATION Configuration is the major factor in determining how a workpiece will be located. The configuration is determined by a combination of flat, circular and irregular surface. A flat surface is one that lies in one plane. A circular surface is one that is made from the segment of a circle. Irregular surfaces are not flat nor circular. They may or may not be geometrically true.

2.1.1 Locating from plane surface The basic reference for location is a flat plane, generally a machine table. The machine table is usually at right angles or parallel with the machine’s feed movement. Most machines have three standard movements which move the workpiecei. ii. iii.

Up and down (vertical) Right or left (longitudinal) In or out (Cross)

All locating devices are made with regard to the basic reference plane (machine table). If a workpiece with a plane surface is placed upon the basic reference plane (machine table), it will remain in position because of the forces of gravity acting upon it. However, during a

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machine operation, the workpiece may move in any direction except towards the surface of machine table. The machine table acts as a stop and becomes a locating surface. It prevents movement of the workpiece when forces are imposed upon it by the vertical feed movement. If the workpiece doesn’t have a flat side to mate with the machine table, the flat plane of the machine table cannot be used as a locating surface. It becomes necessary to reduce the contact area of the locating surface. A series of sharp points would give the theoretical minimum amount of contact area, but in practical the points must have enough body to prevent rapid wear and breaking. A minimum of three points (or locators) must be used to locate the workpiece, although four or more may be used to provide adequate support.

 Minimum of three locators always establish the same location of the workpiece. Four or more locators do not establish this theoretical location because if the workpiece surface is not true, the position is still determined by three locators. This may be compared with two tables, one with three legs and one with four legs. The table with three legs always stand on an uneven floor using all three legs. But the table with four legs always have a leg that doesn’t touch the uneven floor. The table always tilt from one leg to another. The result is that the table top is in two different places. At this point the base plane is serving as a locator and prevents workpiece movement when forces are imposed upon it by the vertical feed movement.  If the workpiece is to be located in the same position each time, two stops or locators must be provided on side of the part.  Complete location has been accomplished by use of three points in (or parallel with) the reference plane, two points in a second plane, and one point in a third plane.  The locating points should be placed as far apart as possible to minimize the effect of in accuracies in the workpiece and locators. Why should they be placed as far apart as possible?

The locating points should be placed as far apart as possible to minimize the effect of inaccuracies in the workpiece and locators. This separation also provides greater support because the locators are nearer the outer extremities of the workpiece. Moreover if the locating points are placed as far apart as possible, the angle of error becomes smaller.

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2.1.2 Locating from circular surface: The basic reference for location is the flat plane of the machine tool table surface again. However, instead of locating the flat plane of the workpiece parallel to the reference plane, it is necessary to locate the axis of the circular workpiece. One of the common method of locating form a circular surface is by using cones, a method commonly referred to as conical location and usually employed when locating is done from a hole.

2.1.2.1 Conical location Closely related to conical location is the V method, used primarily to locate round workpieces or workpieces with convex circular surfaces. The best general V-angle is 90°. Shortcomings of V-locator 1. Smaller included angles hold a round workpiece more securely but are more susceptible to location errors caused by burrs, chips, dirt, and workpiece inaccuracies. 2. V-locator locates only the longitudinal axis of the cylindrical workpiece. When work is done perpendicular to the axis, the position of V-locator should be arranged to keep displacement of the workpiece to a minimum. 3. It should be noted that any variation in work size will cause the axis of the round workpiece to shift, the V should be directed in such a way that variations in workpiece size will not affect location on the workpiece. Proof: Best included angle of V-locator is 90° Let’s consider a V-locator with an included angle of 2x, which locates a circle of radius R1 with center A and another circle of radius R2 with center B. The distance between the centers is C. By similar triangles ∆�

� � = = �� � � � ⇒

and ∆�

,

� � = � � +

⇒ � +

� =� �

⇒� � + � =� �

⇒ � = � � −� � ⇒ � =� ⇒

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=

� −�

� −� �

� =

� −� �

× � � ���� = ⁄

−

� ����

The value of C is minimum when cosec(x) is minimum. Hence the value of x should be 90°, but then the value of included angle would be 2x = 180°, a straight line. However, as x approaches 90° there is less inclination for the circular section to seat positively in the V and more difficultly in retaining it. The best compromise ts achieved when x = 90° and the included angle of V-locator is 90°

2.1.2.2 Concentric location A method of locating from a circular surface so common that it is often overlooked is the use of standard chucks. The method incorporates both locating and clamping and is often referred to as concentric location. Because, the work is usually positioned to a common center. Chucks are usually thought of as clamping and locating devices to be used where both the chuck and workpiece revolve; however they may be quite useful for locating and clamping stationary workpiece to machine tables. Example: The use of standard chucks.

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The location of a round workpiece is not affected by variation in diameter, and location and clamping can be accomplished quickly and readily.

2.1.3 Locating from irregular surfaces Irregular surfaces are neither flat nor circular. They may or may not be geometrically true. For example, a parabolic or elliptical surface would be considered an irregular surface. A surface of a workpiece that may vary dimensionally from time to time would also be an irregular surface. Example: The raw edge or surface of a casting. The degree of roughness may determine whether a surface would be considered flat, circular or irregular. A rough flat surface may have to be considered as an irregular surface when determining locating methods, especially when workpiece dimensions vary from part to part. Locating methods used for flat and circular surfaces may be used for some irregular surfaces that are finished and geometrically true. For example, V locating methods may be used to locate certain parabolic surfaces and button locators maybe used to locate certain elliptical surfaces. It may be necessary to locate from an irregular surface only during the first machining operation, as it should produce holes or surfaces that can be used as reference or locating points from subsequent operations. The unevenness of the surface of a casting will allow a maximum of three contact points. More than three point will allow the casting to teeter. Moreover, if more than three points are used, the workpiece will deform when clamping pressure is applied. It is therefore necessary to use adjustable rest pins or equalizers to compensate for the unevenness of the workpiece surface. Examples, irregular surface: Casting, forging, weldments

2.1.3.1 Fixture Jack One of the simplest types of adjustable rest pins, commonly called a fixture jack, is used with non-adjustable locators. The workpiece is positioned on three non-adjustable locators, and the jack or jacks are adjusted until they touches the workpiece surface. The contact pressure between the jacks and the workpiece depends upon the judgment of operator and is a disadvantage when a number of jacks are used. Mechanical equalizing devices are sometimes used. They give nearly equal pressure between the two locators and are used to replace one of three supporting points on a rough workpiece surface. In other words, four contact points may be used, yet three-contact-point principal is still adhered to.

2.2 3-2-1 METHOD OF LOCATION A workpiece may be positively locating by means of six pins, so positioned that collectively they restrict the workpiece in nine of its degree of freedom. This is known as the 3-2-1 method of location.

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2.2.1 Three pins arrest five degrees of freedom The figure 2-7 shows the prism resting on three pins A, B and C. The faces of the three pins supporting the prism form a plane parallel to the plane that contains the X and Y axes. The prism cannot rotate about the X and Y axes and it cannot move downward in the direction of freedom 5. Therefore freedoms 1,2,3,4, and 5 have been restricted.

2.2.2 Five pins arrest eight degrees of freedom If fig 2-8, two additional pins D and E whose faces are in a plane parallel to the plane containing the X and Z axes prevent rotation of the prism about the Z axis. It is not free to move to the left in the direction of freedom 8. Therefore, freedoms 6,7, and 8 have been restricted and the prism cannot rotate.

2.2.3 Six pins arrest nine degrees of freedom Finally the addition of pin F as shown in fig 2-9, freedom 9 is restricted. Thus by means of six locating points; three in a base plane, two in a vertical plane, and one in a plane perpendicular tho the first two, nine degrees of freedom have been restricted.

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Clamping 3 TERMINOLOGY 3.1 CLAMP The mechanism used for clamping action are known as clamp.

3.2 CLAMPING The action of pressing the workpiece against the locating surface and hold it there agains forces acting upon it- is known as clamping. Clamping depends oni) ii)

Type of work piece Clamping action

4 THE BASIC PRINCIPLE OF CLAMPING 4.1 ESSENTIAL REQUIREMENTS OF CLAMPS (1) (2) (3) (4)

The workpiece must be held rigidly while the cutting tools are in operation; The time required for loading or unloading the tool must be as short as possible. Which means the clamping device must be quick acting; When subjected to vibration, chatter or heavy pressure, the clamping must be positive; and The clamp must not damage the workpiece.

4.2 RULES OF APPLYING CLAMPING FORCE (1) (2) (3) (4)

Clamping force should be directed towards the nest or locating surface. They should be arranged in such a manner that the thrust of cutting tool is away from the clamp. Clamping forces should be applied over a heavy part of the workpiece, whenever possible. Clamping mechanism should be designed in such a manner that they cannot be applied in any way except the correct one.

5 TYPES OF CLAMP While the types of clamps are numerous, they can be classified in seven basic groups. (1) (2)

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Strap Cam

(3) (4)

Screw Latch

(5) (6)

Wedge or key Toggle

(7)

Rack and pinion.

5.1 STRAP CLAMPS Simples and the most commonly used clamp. All strap clamps employ the principle of levers.

5.1.1 How it works The strap clamp can be slid onto and off of the workpiece for easy workpiece removal. A compression spring between the base and clamping bar holds the bar in position when the clamping knob is loosened. The pillar, or heel pin, rests in a groove on the underside of the bar to prevent it from turning when the clamping bar is loosened. A hardened washer is used next to the clamping knob to prevent it from rubbing the bar.

5.1.2 Advantages of Strap clamp (1) (2)

Simplest and commonly used It may be activated by hand knob

5.1.3 Disadvantages of Strap clamp (1) (2) (3)

Not suitable for heavy clamping Greater operator fatigue than other Comparatively slower than others

5.2 CAM CLAMPS The main types of cam clams are spiral and eccentric cam clamps.

5.2.1 Advantages (1) (2)

It provides an efficient and rapid means of clamping When it is desirable to push the workpiece against stops o into a nest

5.2.2 Disadvantages (1) (2)

It may be loosen under vibrating forces It should not be used where heavy clamping forces are required.

5.3 SCREW CLAMPS Screw clamps incorporate a screw thread to clamp a workpiece.

5.3.1 Advantages of Screw clamp (1) Exerting adequate forces (2) Resisting loosening tendencies set by vibration

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5.3.2 Disadvantages of Screw clamp (1) Relatively slow clamping (2) Not suitable for high production

5.4 LATCH CLAMPS: A latch clamp is consists of leaf, spring, drill bushing and jig foot

5.4.1 Advantages: (1) Ease of use (2) Speed of manipulation

5.4.2 Disadvantages (1) Cannot be used for heavy clamping (2) Difficult to secure rigidly (3) Used for relatively light work

5.5 WEDGE CLAMP A plain wedge clamp consists of a movable inclined plane which forces the workpiece against a fixed stop. The taper angle of a plain wedge may range from 6-18 degrees.

5.5.1

Advantages of wedge clamp

(1) Construction is simple (2) Quick acting

5.5.2 Disadvantages of wedge clamp (1) Have a tendency to loosen under vibration (2) A hammer or some sort is required to drive the wedge in and out (3) Loose tools can get lost or mislaid

5.5.3 How to improve a plain wedge clamp? A plain wedge clamp can be improved by adding levers and links. This arrangement is quick-acting and provides a partial lock when the center pivot goes past the centreline and the handle hits the stop pin.

5.6 TOGGLE CLAPS Clamps depending upon the movement of rigid links for its own movement are called toggle clamps.

5.6.1 Advantages of toggle clamps (1) Quick-acting (2) Suitable for heavy load (3) Gives ample clearance for loading and unloading

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5.6.2 Disadvantages of toggle clamps (1) (2)

Only used for sheet metal operation Adjustment is provided variation in stock thickness

5.7 RACK AND PINION CLAMPS Rack and pinion clamps are used extensively on universal jigs. They are irreversible and a lock is incorporated. This lock holds the shaft while it is being clamped.

6 HYDRAULIC AND PNEUMATIC CLAMPING Many of the previously mentioned clamps may be actuated by hydraulic or pneumatic methods when large production quantites justify them.

6.1 ADVANTAGES: (1) (2) (3)

Faster clamping Uniform and equalized clamping pressure Less operator fatigue

6.2 `DIFFERENCE BETWEEN PNEUMATIC AND HYDRAULIC Pneumatic 1. 2. 3. 4.

Working fluid Pressure of fluid Cylinder size Clamping pressure

Air Comparatively high Smaller Light

Hydraulic Liquid fluid Comparatively low Larger Heavy

6.3 PRINCIPLE OF THE AIR-TO-HYDRAULIC BOOSTER If the air-cylinder piston is subjected to 100 psi air pressure and the area of the piston is 10sq in., a force of 1000lb is placed upon the ram. If the ram area is 1 sq in. the pressure upon the hydraulic oil must be 1000psi. A hydraulic pressure of 100 psi has been produced from a 100 psi line air pressure, so the pressure is increased by a ratio of 1:10.

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Drill Jigs 7 INTRODUCTION TO DRILL JIGS A drill jig is a device for ensuring that a hole to be drilled, tapped, or reamed in a workpiece ঁ বি না েজ এেটা will be machined in the proper place. [বিবিন াপজোজে র যি া েজ রবিটা গিত খজে বেগ যি া ে জেই বিেজে িে অিথাজন গাইড ে জি পাজ ন অপাজ ট ] Basically it consists of a clamping device to hold the part in position under hardened-steel bushing through which the drill passes during the drilling operation. The drill is guided by the bushing. If the workpiece is is of simple construction, the jig may be cla...