Jigs and Fixtures PDF

WEEK 1 **Jigs** - It is a work holding device that holds, supports and locates the workpiece and guides the cutting tool for a specific operation. \- is a type of tool used to control the location and/or motion of another tool **Fixture** - It is a work holding device that holds, supports and loc...

WEEK 1 **Jigs** - It is a work holding device that holds, supports and locates the workpiece and guides the cutting tool for a specific operation. \- is a type of tool used to control the location and/or motion of another tool **Fixture** - It is a work holding device that holds, supports and locates the workpiece for a specific operation but does not guide the cutting tool. It provides only a reference surface or a device **TYPES OF JIGS AND FIXTURES Types of Jigs:**  1. Template jig  6. Box jig 2. Plate jig  7. Leaf jig 3. Diameter jig   8. Angle plate jig 4. Channel jig  9. Indexing jig 5. Ring jig   10.Trunion jig **Types of Fixtures: ** 1. Plate fixture  4. Indexing fixture 2. Angle plate fixture  5. Multistation fixture 3. Vise-jaw fixture    6. Profile **ADVANTAGES OF JIGS AND FIXTURES**  1. Requires less skilled operation.  2. They reduce the production cost.  3. Increases machine and labor utilization.  4. They simplify the work handling. **DISADVANTAGES OF JIGS AND FIXTURES**  1. Can wear away over time.  2. Another one is the high initial set up costs (and time).  3. And can use a lot of material and be bulky. **WEEK 2** **Mass production** - aim at high productivity to reduce unit cost and interchangeability to facilitate easy assembly **Jig borer** - is a machine tool used to locate accurately and to make the numerous holes so necessary for dies, jigs, fixtures and many other precision parts **Bushing** - is a device used to guide the cutting tool or the twist drill in production runs. **Parts of a Jig:** 1. **Jig body** -- a part that houses or mainly supports the subject. 2. **Jig cover** - a part that supports the subject together with the clamping device. 3. **Clamping device** -- a part that prevents or secures the jig from moving on the drill table. 4. **Bushing** -- a part that guides the twist drill to eliminate replacements of parts where bushings are installed. 5\. **Locating points** -- a part that locates the positions of holes or of the subject on the device. **Drill Jig terms:**  1. **Open jig** -- also called plate jig or drill template The simplest type of drill jig 2. **Drill bushing** -- precision tools that guide cutting tools such as drills and reamers. 3\. **Locking pins** -- inserted to lock or hold the workpice. Types of Jigs: 1. **Template Jigs** - They are the simplest type used more for accuracy than speed. This type of jig fits over, on, or into the work and is not usually clamped. 2. **Plate Jigs** - They are similar to templates excepting that these have built-in clamps to hold the work. 3. **Table Jig** - This is similar to plate jig except that legs are provided to raise the jig off the table for large work. 4. **Sandwich Jig** - This type of jig is used for drilling thin or soft parts to avoid bending, warping etc. 5. **Angle-Plate Jigs** - These are used to hold parts which are to be drilled at right angles or some other angles to their mounting locators. 6. **Box Jigs** - These jigs usually totally surround the part. Since the part is completely located and held within the box, it is possible to drill holes from all the sides 7. **Channel Jig** - It is the simplest form of box jig. The jig is placed over the work and is located by one side of work and clamped from other side 8. **Leaf Jigs** - These are small box type jigs with a hinged leaf to permit easy loading and unloading of part. 9. **Indexing Jigs -** These are used to accurately space holes or other machined areas around a pant 10. **Trunnion Jigs** - These are used for drilling holes on very large or odd-shaped parts from different sides 11. **Multi-Station Jig** - On this jig, several operations can go on simultaneously **WEEK 3** **Clamping Elements** - hold the workpiece firmly engaged with the locating elements during operation **Clamping Device** - purpose is to hold work in the correct relative position in the fixture and to ensure that the job is not displaced under cutting forces **Strength** - The clamping system should be capable of holding the workpiece against the forces developed during the operation. **Productivity** - Clamping time should be minimized by using hand knobs, tommy bars, knurled screws, hand wheels and handles, so that the clamp can be tightened **Operator fatigue** - should be taken into account. If a considerable number of clamps are to be tightened and loosened repeatedly **Workpiece Variation** - The clamping points should be provided with ample radius to make the clamp operable even if there is variation in the workpiece. **Types of Clamping Device:** 1. **Lever of Strap Clamps** - This is the most popular clamping device used in workshops, and tool rooms of jigs and fixtures 2. **Screw Clamps** - These are widely used for jigs and fixtures. These have lower costs. However, their operating speed is quite slow 3. **Cam-action clamp** - when properly selected and used, provide a fast, efficient, and simple way to hold work 4. **Hinged Clamps** - These utilise hinged lids for loading and unloading the components. Generally the clamp is made integral with the hinged lid \- is often required when it is necessary to move both the clamp and the bolt completely out of the way for the loading of component 5. **Two-Way Clamps** - rapid clamping in two directions from one screw. Clamping force is applied to the top and one side of the work piece. The clamp has a quick release action. 6. **Wedges Operated Clamps** - having angle of 1 -- 4° are self-holding type and normally hold the work without additional attachments 7. **Toggle Action Clamps** - These are fast acting clamps. These have the natural ability to move completely free of the work, allowing for faster insertion/taking out of parts 8. **Power-activated Clamps** - may operate under hydraulic power, pneumatic power, or with an air-to-hydraulic booster 9. **Chucks and Vises** - Commercially available chucks and vises with slight modifications of jaws to suit the job can be used for a variety of applications 10. **Non-mechanical Clamps** - like magnetic and vacuum chucks are used to hold parts which are difficult to be held by other devices because of their size **WEEK 4** **Locating points** - Good facilities should be provided for locating the work. \- The position of workpiece should be accurate with respect to tool guiding in the jig or setting elements in fixture. **Locating guidelines** - No single form of location or type of locator will work for every workholder **Positioning locators** - The primary function of any locator is to reference the workpiece and to ensure repeatability. **Controlling chips** - The final consideration in the placement of locators involves the problem of chip control. **Avoiding redundant location** - Another condition to avoid in workholder design is redundant, or duplicate, location. **Preventing improper loading** - Fool proofing prevents improper loading of a workpiece. The problem is most prevalent with parts that are symmetrical or located concentrically **Using spring-loaded locators** - One method to help ensure accurate location is the installation of spring-loaded buttons or pins in the workholder **Determining locator size and tolerances** - The workpiece itself determines the overall size of a locating element. **Dimensions and tolerances** - Parts of the machine designer's task is to specify the parts and components necessary for a machine to perform its desired function **Common Dimensioning Terminology:** **Nominal size** -- the size we use in speaking of an element. **Limits** -- the stated maximum and minimum dimensions. **Tolerance** -- the difference between two limits. **Bilateral tolerance** -- the variation in both directions from the basic dimensions **Unilateral tolerance** -- the basic dimension is taken as one of the limits, and variation is permitted in only one direction. **Clearance** -- a general term that refers to the mating of cylindrical parts such as a bolt and a hole. **Interference** -- the opposite of clearance, for mating cylindrical parts in which the internal member is larger than the external member **Allowance** -- the minimum stated clearance or the maximum stated interference for mating parts. **Fit** -- the amount of clearance or interference between mating parts.