Classnotesforyou 5f1m6b

ME09 L14: Design of Jigs & Fixtures Nikhil N.S. APME JECC

Location • To ensure precision in any machining operation, the W/P must be properly positioned with respect to the cutting tool. The position of the locator is analyzed with respect to degree of freedom , space and surface. • Locating- refers to the dimensional and positional relationship b/w the w/p and the cutting tool. • Locator- it is a device to establish and maintain the position of a part in a jig or fixture to ensure the repeatability of the workholder.

Principles of location It is one of the factor to be considered for Design of Jigs and Fixtures: • Locating surface should be small as possible and the location must be done from the machined surface. • Sharp corners in the locating surfaces must be avoided. Locating pin should be easily accessible and visible to the operator. • Adjustable locators should be provided for rough surfaces.

Continued…. • The locating device should be designed such that each successive w/p when loaded and clamped will occupy the same position in the jig or fixture. • The selection of work holding device mainly depend on nature of w/p and requirement of the machining operation. • DOF-To locate a w/p accurately it is necessary to restrict it against movements in any of the six DOF with the help of suitable locating pins.

Basic rules for locating. • • • • •

Positioning the locators Accuracy and tolerance Fool proofing Duplicate location Motion economy

• • • • •

Six point location principle or 3-2-1 principle of location. Principle of least points. Principle of Extreme positions. Principle of mutually perpendicular planes. Principle of fool proofing.

3-2-1 principle • According to this principle a w/p can be completely restrained by providing three points in one plane, two location points in the second plane and one location point in the third plane.

Types of location • There are different methods used for location of a work. The locating arrangement should be decided after studying the type of work, type of operation, degree of accuracy required. • Volume of mass production to be done also mattes a lot.

Plane surface

• Flat Locator

Flat locators are used for location of flat machined surfaces of the component. • In first case an undercut is provided at the bottom where two perpendicular surfaces intersect each other. This is made for swarf clearance. • The middle figure shows flat headed button type locator. There is no need to made undercut for swarf clearance. The button can be adjusted to decide very fine location of the workpiece. • There can be a vertical button as shown in third figure, which is a better arrangement due to its capacity to bear end load and there is a provision for swarf clearance automatically.

Six point

Cylindrical location • It is used for locating components having drilled holes. • The cylindrical component to be located is gripped by a cylindrical locator fitted to the jig’s body and inserted in the drilled hole of the component. • The face of the jig’s body around the locator is undercut to provide space for swarf clearance.

Conical location • This is used for locating the workpieces having cylindrical hole in the workpiece. • The workpiece is found located by ing it over the conical locator inserted into the drilled hole of the workpiece. • A conical locator is considered as superior as it has a capacity to accommodate a slight variation in the hole diameter of the component without affecting the accuracy of location. • Degree of freedom along z-axis can also be restrained by putting a template over the workpiece with the help of screws.

Vee location • This is quick and effective method of locating the workpiece with desired level of accuracy. • This is used for locating the circular and semi-circular type of workpieces. • The main part of locating device is Vee shaped block which is normally fixed to the jig. • This locator can be of two types fixed Vee locator and adjustable Vee locator. • The fixed type locator is normally fixed on the jig and adjustable locator can be moved axially to provide proper grip of Vee band to the workpiece.

Radial location • Radial locators restrict the movement of a workpiece around a concentric locator.

Profile location • The work piece can be positioned on the sighting plate in such a way that, there is equal margin on all the sides. • The profile of the workpiece can be located by confining the profile with cylindrical locating pins. • When there is considerable variation in work piece dimensions from batch to batch, an eccentric locator can be used. • The eccentricity of the locator can be varied by rotating it to suit the workpieces in the batch.

Continued…………..

Nest location • The inside profile of the nest matches with the outside of the work piece. • The height of the work piece should be lesser than the work piece to permit grip over the workpiece for unloading. • For thin sheet metal work pieces figure slots or ejection arrangements should be provided for unloading work piece.

Redundant location

Extreme locations • The locating points should be placed as far away from one another as possible, to achieve the greatest accuracy in location. • This is essential to keep the workpiece under stable equilibrium. • This principle would ensure min. misalignment for a certain displacement of any point with respect to another if the distance between the points is increased.

Clamping • Once a work is located, it is necessary to press it against the locating surfaces and hold it there against the force acting upon it. The tool designer refers to this action as clamping and the mechanism used for this action are known as clamps. • Clamps should be positioned above the points ing the component in order to minimize distortion and they should be strong enough to resist bending. Quick acting clamps by hand if the job can be done by fitting a spring to lift it.

Clamping principles 1.The clamp should press against a strong portion of the workpiece without distorting the workpiece. 2.It should hold the work firmly and transmit the clamping force through the work, directly over a fixed beam. 3.It should be capable of overcoming the maximum possible force excerted on it by the work piece. If possible ,the minimum clamping force applied by the clamp should be able to accomplish this.

4. A fixed locator or should oppose the force of cutting. The clamp should not be designed or required to resist the force of cutting. 5. Simple clamps are always preferred than complicated clamps. 6. Clamp should be easy to operate, to be positioned or loosened. They should never interfere with the positioning or removal of the workpiece. 7.Clamp should be designed so that they retract enough to clear the workpiece. 8.Whenever possible clamp should be designed so that they can be operated from the front or the operator’s side of the fixture. 9.When side clamps are used, they should be designed so that they press down and in and tend to seat the work.

10.When cams or wedges are used,they should be designed so that louds or vibrations will tend to tighten rather than loosen them. 11. The clamp should not damage the work piece 12.When estimating the cutting force, that a clamp is to overcome, it should be ed that these forces can be increase because of uneven depth of cut, the dulling of cutters, vibrations in cutting rates or stock removal, or changes in the material hardness. It is best to over design the clamp if there is some doubt has to be accumulated forces that will be present.

Types of clamps 1. Screw Clamp • Screw Clamps are threaded devices with knurled collar, Hand knob or spanner flats for rotating and tightening the screw. • The clamping area of a screw clamp can be increased by a provision for a pad.

2.Strap or Plate Clamps • These are made of rectangular plates and act like levers. • In its simplest form clamp is tightened by rotating a hexagonal nut on a clamping screw.one end of the cam presses against the work piece and the other on the heel pin, thus loading the clamp like a simply ed beam. • The clamping face of the clamp is curved and the pressure face of the heel pin is made spherical to take care of any deviation in work piece. • Spherical washer-helps to operate the clamp satisfactorily even if there is considerable variation in work piece height.

3.Swing Clamp • This is a special type of clamp which provides a means of entry for loading and unloading the workpiece without removing the strap fully. • For this purpose the strap can be swung in or out.

4.Wedge Clamp

5.Multiple Clamp

6.Latch Clamp

7.Toggle Clamp

Hinged Two Way Clamp

8.Self Locking Clamp

10.Hydraulic and Power Clamping

Fluid Power Clamping

Magnetic Clamp

Vacuum clamping

Electrostatic Clamping