JIG AND FIXTURE DESIGN EDWARD G HOFFMAN PDF
Edward Hoffman Jig and Fixture Design, 5E Delmar Learning Drafting (Copyright Cengage Learning. All Rights Reserved. May no). This outstanding Jig And Fixture Design Edward G Hoffman Author is released to give the Get them for file format pdf, word, txt, rar, ppt, zip, and also kindle. Jig And Fixture Design Edward G Hoffman Author. Introduction To Jigs And Fixtures - National Institute Of introduction to jigs and fixtures. introduction a jig.
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Jig and fixture design Edward G Hoffman. This newly revised text presents the basic concepts of the design of workholding devices. Topics covered include the . Download as PDF Print Jig and fixture design / Edward G. Hoffman Send to Email Jig and fixture design / Edward G. Hoffman. keep coming resourceone.info you need a jig and fixture design edward g hoffman author, you can download them in pdf format from our resourceone.info file format that can.
Every part produced must be held while it is machined, joined, or inspected or has any number of other operations performed on it.
So, whether the operation requires a simple drill press, a multiple-axes computer, or numerically controlled machining center, the workpiece must be accurately located and securely held throughout the operation. The part, not the process, is the primary consideration in workholding. Just as no single machine tool will perform every required operation, no individual jig or fixture can possibly hold every part. However, each workholder variation has basic similarities to other types and styles of jigs and fixtures.
The subject of this text is these similarities rather than the differences. This text helps the reader develop a thorough understanding and working knowledge of how and why jigs and fixtures are designed and built as they are.
To do this, the discussion starts with the fundamentals of jigs and fixtures and works through the various elements and considerations of design. Throughout the text, two fundamental tool design principles are constantly stressed: simplicity and economy.
Jig Fixture Design by Edward Hoffman
To be effective, a workholder must save money in production. To this end, the construction of the tool must be as cost-effective as possible while ensuring that the tool has the capacity to perform all of the intended functions.
The reader becomes familiar with working with part drawings and production plans showing the sequencing of operations in the shop. The 20 units of this text are divided into four major sections. The first section Units 1—5 gives the reader an overview of the basic types and functions of jigs and fixtures, as well as a detailed description of the way these workholders are designed and built.
The reader learns the basic elements of supporting, locating, and clamping the part and then is introduced to the basic principles of workholder construction. This section provides the background information for the more advanced study later in the text.
The second section Units 6—8 introduces the reader to the primary considerations of design economics and the basic methods used to initiate and prepare the design drawings.
In keeping with the worldwide standard of measurement and modern drafting practices, the SI system International System of Units is introduced and explained. The process of geometric dimensioning and tolerancing is also presented; many manufacturing drawings in industry use this system of dimensioning, and the reader should be comfortable with its basic principles.
New material covering the applications of Computer Aided Design CAD and how it is applied to jig and fixture design has also been added to this section to inform the reader of the developments in this important area of tool manufacturing. The third section Units 9—14 introduces and explains the processes involved in designing and constructing the basic types and forms of jigs and fixtures.
From simple template and plate-type workholders to more detailed and complex channel and box-type tooling, each basic style is thoroughly explained and illustrated. The final section Units 15—21 covers the specialized workholding topics in manufacturing as they relate to jig and fixture design.
Covering power workholding methods and equipment, modular workholding systems and low-cost tooling practices, and designing jigs and fixtures for inspection and welding and for numerically controlled machine tools, this section has been expanded to include those areas of tooling technology that will service the needs and requirements of industry for years to come.
The product, a method for manufacturing, made available. A prototype goes one more steptooling concepts, and a quality plan are developed that beyond the solid computer model. The prototype, asuits the selected manufacturing facility.
In this way, single physical part provided prior to formal produc-problems are not discovered on the production floor, tion, is a valuable tool for understanding more com-but are corrected early in the concurrent process.
This plex part geometries.
Prototypes are manufacturedultimately saves time and money while speeding up using conventional Computer Numerical Controlthe process of getting product to market earlier. Con- CNC machine tools or some of the newer technolo-current engineering allows a company to have a dis- gies such as stereolithography or a layered object man-tinct economic advantage in a global market.
Both the stereolithography and LOM develop the part The tool designer develops a plan for maintain- geometry using a system of layering the medium anding the concepts developed by the team with respect solidifying or cutting out that layer with a laser. Theto economic guidelines.
Expert computer systems result is a solid object made one layer at a time whereare now part of the design environment, and they the layers may be no more than. Whethersupport an integrated approach for tracking time and analyzing the prototype and the part drawing or justmoney allocated for the project and provide imme- the part drawing, the designer must consider the fol-diate information at any point in the concurrent lowing factors that directly influence the design choic-process.
This istime wisely spent and results in an efficient and cost- Production Planeffective tool design. The design process is not as lin-ear as it used to be. Communication models between The production plan Figure 1—2 is an itemized list ofteam members include e-mail and electronic transfer the manufacturing operations and the sequence of theof materials and may make use of sophisticated tech- operations chosen by the process planning engineer.
Team members The production plan can take many forms, dependingmay consist of customers, designers, and builders in on the needs of each company. At the least, it shoulddifferent locations that may take them halfway include a brief description of each machining operationaround the world.
Part Drawings The tool designer also uses this plan to assist in the design. The production plan can include the following:The tool designer receives a duplicate of the part geom-etry that will be used to make the part Figure 1—1.
Using check each operation? Answering these questions and others related to the specific task, the tool designer develops alternative solu-Alternatives tions. From these alternative solutions, the most effi- cient, dependable, and cost-effective design is chosen. One of the first steps in problem solving is determiningthe alternative solutions. Duringthis phase of the design, the tool designer must analyze The tool designer has many manufacturing responsi-all important information in order to answer the follow- bilities.
In addition to technical design duties, the tooling questions: designer may be responsible for obtaining materials, toolroom supervision, and tool inspection. UNIT I Pupose of Tool Design 5Design designer possesses, these skilled toolmakers can often see solutions that may not be obvious to theIn this phase, the tool designer is responsible for designer.
For this reason, it is always a good idea todeveloping the drawings and sketches of the tool build a good working relationship with your toolmak-design ideas. Design drawings are usually subject to ers. In tool design, a cooperative relationship betweenapproval by a chief designer. However, in smaller the designer and the toolmakers is essential.
Not onlycompanies, the tool designer often makes the tooling does working together make the task at hand easier,decisions. Often a tool designer is responsible for obtaining theSupervision for a single section, such as design or materials to make the tool. In either supply materials and parts that meet the design speci-case, the ability to lead others is helpful.
When selecting a vendor, a good practice is to choose the company that offers the most service to One resource a tool designer may often use to its customers. Services such as design assistance andhelp resolve design problems is the group of skilled problem solving, where their product is involved, arepeople in the toolroom. The toolroom is the area in a important factors to consider before making a finalshop where the machine tools and the skilled work- selection.
Another point to consider is whether theforce are found. These skilled trades employees are vendor can supply special parts or components whencapable of taking the prints for the individual compo- necessary. Generally, the specialty vendors can fur-nents of a tool and manufacturing them, assembling nish special items for much less than those items costthe parts, and verifying their accuracy. A variety of to make in-house. Regardless of the level of skill aImage not available due to copyright restrictions Copyright Cengage Learning.
First, the tool itself is inspectedfor compliance with the tool drawing. Second, several The following important concepts were presented intest parts are produced with the tool and are carefully this unit:checked to ensure that they conform to the specifica-tions shown on the part print. List the seven objectives of tool design. Determine the source of the following data by indicating 1 for the part drawing, 2 for the pro- duction plan, and 3 for additional instructions.
Time allocation b. Overall size and shape of the part c. Required accuracy d. Sequence of operations e. Type and size of machines used f. Money available Copyright Cengage Learning. Number of pieces 3. What does the term concurrent mean and how ish.
Previous machining it applied to the design of tooling? Locating surfacesj. Material specifications 4. Describe a toolroom. Type of cutters needed 5. List the skills of a tool designer. Type of machining required Copyright Cengage Learning. Jigs are usually fitted with hard- ened steel bushings for guiding drills or other cuttingAfter completing this unit, the student should be tools Figure 2—1A. If, however, holes above.
Set blocks and feeler or thickness gauges are used with fixturesJigs and fixtures are production-workholding devices to reference the cutter to the workpiece Figure 2—1B. The A fixture should be securely fastened to the table ofcorrect relationship and alignment between the cutter, the machine upon which the work is done.
Jigs and fixtures
Thoughor other tool, and the workpiece must be maintained. Fixtures vary in design from relatively simple Jigs and fixtures are so closely related that the tools to expensive, complicated devices. Fixtures alsoterms are sometimes confused or used interchangeably.
It is a production Jigs may be divided into two general classes: boringtool made so that it not only locates and holds the jigs and drill jigs. Boring jigs are used to bore holesworkpiece but also guides the cutting tool as the oper- that either are too large to drill or must be made an odd size Figure 2—2. Drill jigs are used to drill, 8 Copyright Cengage Learning. The more than one side.
The names used to identify thesebasic jig is almost the same for either machining oper- jigs refer to how the tool is built. The only difference is in the size of the bushingsused. Template jigs are normally used for accuracy rather than speed. Templates are the least expensive and simplest type of jig to use. TheyDrill jigs may be divided into two general types, open may or may not have bushings. When bushings are notand closed.
Open jigs are for simple operations where used, the whole jig plate is normally hardened. Closed, or Figure 2—2 Boring jig. Figure 2—4 Template jigs. Plate jigs are similar to templates Figure 2—5. Figure 2—6 Table jig. The only difference is that plate jigs have built-inclamps to hold the work.
These jigs can also be madewith or without bushings, depending on the numberof parts to be made. Figure 2— Mill a slot. Box fixture 2. Duplex fixture 3. Drill four. Drill four holes,. Plate jig 1. Box jig 2. Angle-plate jig 2. Angle-plate jig 3.
Channel jig 3. Template jigFigure 2—26C. Drill four holes two Figure 2— Channel jig 2. Plate jig 3. Box jig E. Mill a shoulder. Plate fixture 2.
Angle-plate fixture 3. A tool de- for jigs and fixtures. This is called referencing. To on a machined surface. This permits accurate place-ensure the desired accuracy, the tool designer must ment of the part in the tool and ensures the repeatabil-make sure the part is precisely located and rigidly sup- ity of the jig or fixture.
Repeatability is the feature ofported. Locators, in addition to properly positioning the tool that allows different parts to be machinedthe part, make sure that the tool is easily loaded and consistently within their required tolerances.
They must also make the tool foolproof. Lit- rate location is an important element in the repeata-tle is gained if the parts take too long to load or unload, bility of any tool. Locators should be spaced as far apart as possible.
The tool designer must also provide rigid support This permits the use of fewer locators and ensuresfor the part. If the part locators are designed properly, complete contact over the locating surface.
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Wherethey can serve as supports as well as locators. If 3—2. This is necessary to maintain the required preci-this is not possible, the locators should be relieved sion.
Specifying tool tolerances closer than 20 percent Figure 3—1.
Generally, tolerancesTolerance greater than 50 percent do not guarantee the desired precision. The single factor that should determine thisWhen designing a tool, the designer must keep the part decision is the specified accuracy of the part beingtolerance in mind. As a general rule, the tool tolerance machined.
If it were made to its Figure 3—1 Methods of relieving locators. Figure 3—2 Tolerance relationship. Any Foolproofingparts made within these sizes would be correct.
If thetool is made to fit the part at its design size of 1. To prevent correct position. The part in Figure 3—4A must bethis, the tool must be made to fit the parts at their machined on the tapered end, so the tool designerlargest or smallest limits of size, depending on how the includes a pin to prevent the part from being loadedpart is located.
This pin foolproofs the tool. The part in Figure 3—4B shows a hole that must be drilled with Figure 3—4 Foolproofing. A simple pin erence surface is the flange, as in Figure 3—5B, theplaced in one of these holes makes it impossible to hub locator is not necessary. If the hub is the refer-load the tool incorrectly. To correct this, the tool designer must Other foolproofing devices are just as simple.
If first determine which surface is to be referenced. Duplicate Locators Locational inaccuracies develop because of the difference in position and location tolerances betweenThe use of duplicate locators should always be avoid- the tool and the work Figure 3—6.
Locating the parted. Figure 3—5 shows examples of duplicate locators. First, the location of the pins in the tool iscould cause inaccuracies. Second, the location of the holes in the part is variable within For example, the flange in Figure 3—5A is located limits. When a part is placed in the tool that is at eitheron both the underside of the flange and the bottom of extreme of the part tolerance, it may not fit.
To elimi-the hub. Since these are parallel surfaces, only one is nate this possibility, the hole locator can be madeneeded and the other should be eliminated. If the ref-Figure 3—5 Duplicate locators. Figure 3—6 Position and locational differences. UNIT 3 Supporting and Locating Principles 25smaller to accommodate the variation, but if this isdone, the effectiveness of the hold locator is mini-mized and the locator becomes useless.
To avoid thisproblem, the tool designer must specify whether thepart is to be located from its holes or its edges, neverboth. An unrestricted object is free to move in any oftwelve possible directions.
Figure 3—7 shows an ment 2, 5, 1, 4, and 12 are restricted Figure 3—9.
An object is free to revolve around chance of error by limiting the area of contact andor move parallel to any axis in either direction. To raising the part above the chips. Flat bases may alsoillustrate this, the planes have been marked X-X, be used, but these should be installed rather thanY-Y, and Z-Z. The directions of movement are num- machined into the base. Installed locators are lessbered from one to twelve.
If button or flat locatorsTo accurately locate a part in a jig or fixture, move- are used, the most important consideration is keep-ments must be restricted.
This is done with locators ing the part above the chips and in constant contactand clamps. The fixture for the part in Figure 3—8 illustrates To restrict the movement of the part around the Z-the principle of restricting movement. By placing Z-axis and in direction eight, two more pin-type loca-the part on a three-pin base, five directions of move- tors are positioned Figure 3— To restrict direction seven, a single-pin locator is used Figure 3— TheFigure 3—7 Planes of movement.
This three-two-one, or six- point, locating method is the most common external locator for square or rectangular parts. When a workpiece having holes is located, the holes provide an excellent method of locating the complete part. As shown in Figure 3—12, the center hole is used as a primary locator, and one of the other holes is used as a secondary locator.
Here the primary locator is a round pin, and the secondary locator is a diamond pin. As shown, the base plate with the round pin positioned in the center hole will restrict nine degrees of movement 1, 2, 4, 5, 7, 8, 10, 11, and The diamond pin, located as shown, further restricts another two degrees of movement 6 and 3. Togeth- er, these locators restrict eleven degrees of move- ment.
The only direction the workpiece can move in Copyright Cengage Learning. Figure 3—10 Five-pin base restricts eight directions of movement. The tool designer must be able to accurately from a flat surface: To do and equalizing supports. These locators set the verticalthis, the tool designer must know the various types of position of the part, support the part, and prevent dis- tortion during the machining operation.
Figure 3—12 Primary and secondary locators. They can be The terms locator and support are usedeither machined into the tool base or installed Figure interchangeably when the devices used under a work-3— This type of support is normally used when a piece are discussed. The locating devices used to ref-machined surface acts as a locating point. Adjustable supports are used when the surface is Before choosing a support, the tool designer mustrough or uneven, such as in cast parts.
There are many consider the shape and surface of the part and thestyles of adjustable supports.
A few of the more com- type of clamping device to be used. The supportmon are the threaded Figure 3—14A , spring Figure selected must be strong enough to resist both the3—14B , and push types Figure 3—14C.
The threaded clamping pressure and the cutting forces. The clampsstyle is the easiest and most economical, and it has a should be positioned directly over the supports tolarger adjustment range than the others. Adjustable avoid distorting or bending the part. Locating from an Internal Diameter Equalizing supports are also a form of adjustable Locating a part from a hole or pattern is the most effec-support Figure 3— They provide equal support tive way to accurately position work.
Nine of thethrough two connected contact points. As one point is twelve directions of movement are restricted by usingdepressed, the other raises and maintains contact with a single pin, and eleven directions of movement arethe part. This feature is especially necessary on uneven restricted with two pins. When possible, it is logical tocast surfaces.
Jig and fixture design / Edward G. Hoffman
Figure 3—13 Solid supports. Figure 3—14A Adjustable supports, threaded type. Figure 3—14C Adjustable support, push type. Several types of locators are used for locating where the construction of the tool will not permit thework from holes. Figure 3—16 shows a few locators other type to be pressed out. Another type has theused for large holes. When large holes locate the advantages of the press-fit and the locking properties ofwork, fasten the internal locator with both screws and a thread.
Under normal conditions, two dowels andtwo screws are needed to hold the locator. With more Pin-type locators are used for smaller holes and forforce, it is better to use larger dowels and screws aligning members of the tool Figure 3— When therather than to increase their number. Pins used for part location are made with eitheruse the press-fit locator rather than the threaded locator tapered ends or rounded ends, allowing the parts to befor accuracy.
Threaded locators are useful in areas installed and removed easily Figure 3— Figure 3—16 Internal locators. Figure 3—17 Pin locators and bushing. Figure 3—18 Round and tapered locators. UNIT 3 Supporting and Locating Principles 31 The main difference between the pins used for To be effective, the diamond pin must always belocation and the pins used for alignment is the amount placed to resist this movement.
Figure 3—20 showsof bearing surface. Alignment pins usually have a how two diamond pins could be used to locate a part. Locating pins usually have a Notice how each restricts the direction of movement ofcontact area of one-eighth to one-half of the part thick- the other. Two diamond pins should be used to locate aness. More than this makes placement and removal part when the part has adequate locational tolerance.
In addition to the diamond pin relieved locator, Another style of pin common to jigs and fixtures other types are used for some workholders. A fewis the diamond or relieved pin, which is normally examples of relieved locators are shown in Figureused along with the round pin to reduce the time it 3— The specific design of any relieved locator istakes to load and unload the tool.
It is easier to locate determined by the workpiece and the type of locationa part on one round pin and one diamond pin than to required. Relieved locators reduce the area of contactlocate it on two round pins. In use, the round pin between the workpiece and the locator. Decreasinglocates the part and the diamond pin prevents the the contact area has little or no effect on the overallmovement around the pin Figure 3— Notice the locational accuracy; however, reducing the contactdirection of movement the part has around the round area helps make the jig or fixture easier to load andpin.
By installing the diamond pin as shown, this unload and lessens the problems caused by dirt, chips,movement is restricted. Figure 3—19 Locating with one relieved locator. Figure 3—20 Locating with two relieved locators. Figure 3—21 Relieved locators. Here, rather than using the complete thickness of the elliptical form of the pin shown at D. This differencepart for location, the locator is relieved in the middle, in diameter is what causes the cylindrical pin to bind.
This design provides full location However, as shown in Figure 3—24B, a sphericaland makes the locator less likely to bind in the locator always has the same diameter regardless ofworkpiece.
This can be seen by the three positions of the diameter The raised contact locator, shown in Figure 3—23, lines. Regardless of where the diameter, d, is mea-is an example of relieving a locator for better function. Here the top and bottom contact areas of the locatorhave been removed.
This design reduces the contactarea and raises the point where the locator and work-piece touch. Moving this contact point off the baseplate, to the middle of the workpiece, helps reduce theeffects of dirt, chips, or burrs. The raised contact designsupplies a complete locating surface and reduces thechance of the locator binding in the hole.The locator shown at the right has asimilar design, but it uses a relief groove in place ofthe lower relief angle.
From simple template and plate-type jigs to complex channel and box-type tooling, this newly revised edition features more than illust By emphasizing similarities among types and styles, "Jig and Fixture Design, Fifth Edition speeds readers to a complete understanding of the why's and how's of designing and building a variety of different workholders for manufacturing.
Pulleys, collars, and gears are some ofthe parts that use this type of jig. Con- CNC machine tools or some of the newer technolo-current engineering allows a company to have a dis- gies such as stereolithography or a layered object man-tinct economic advantage in a global market.
May not be copied, scanned, or duplicated, in whole or in part.
JIG AND FIXTURE DESIGN
Drill four holes,. In some cases, where jigin the locator could also be added if the relieved feet are used, the work can be machined on three sides. Regardless of the level of skill aImage not available due to copyright restrictions Copyright Cengage Learning. To be effective, a workholder must save Despite the many advancements and changes in money in production.