machining technology - jig & fixtures
Thursday, July 07, 2011
By
st_fanuc
In metalworking, a jig is a type of tool used to control the location and/or motion of another tool. A jig's primary purpose is to provide repeatability, accuracy, and interchangeability in the manufacturing of products. A jig's main function is to guide the tool during machining process. A jig is often confused with a fixture; a fixture holds the work in a fixed location. A device that does both functions (holding the work and guiding a tool) is called a jig.
As the CNC technology grows rapidly, with the ability to move according to exact path generated from NC code, the use of jigs is beginning to decrease. Nevertheless, the fixtures still greatly needed to set-up the workpiece faster.
Successful fixture designs begin with a logical and systematic plan. With a complete analysis of the fixture's functional requirements, very few design problems occur. When they do, chances are some design requirements were forgotten or underestimated.
The workpiece, processing, tooling, and available machine tools may affect the extent of planning needed. Preliminary analysis may take from a few hours up to several days for more complicated fixture designs.
Fixture design is a five-step problem-solving process. The following is a detailed analysis of each step.
1. DEFINE REQUIREMENTS
To initiate the fixture-design process, clearly state the problem to be solved or needs to be met. State these requirements as broadly as possible, but specifically enough to define the scope of the design project.
The designer should ask some basic questions: Is the new tooling required for first-time production or to improve existing production? If improving an existing job, is the goal greater accuracy, faster cycle times, or both? Is the tooling intended for one part or an entire family of parts?
The tooling designer must determine how much freedom and input there is on each project. If many choices regarding machine tools, operations, and cutting tools have already been made, the designer's role will have a relatively narrow focus.
2. GATHER INFORMATIONS
Collect all relevant data and assemble it for evaluation. The main sources of information are the part print, process sheets, and machine specifications. Make sure that part documents and records are current. For example, verify that the shop print is the current revision, and the processing information is up-to-date. Check with the design department for pending part revisions.
An important part of the evaluation process is note taking. Complete, accurate notes allow designers to record important information. With these notes, they should be able to fill in all items on the "Checklist for Design Considerations." All ideas, thoughts, observations, and any other data about the part or fixture are then available for later reference. It is always better to have too many ideas about a particular design than too few.
Four categories of design considerations need to be taken into account at this time: workpiece specifications, operation variables, availability of equipment, and personnel. These categories, while separately covered here, are actually interdependent. Each is an integral part of the evaluation phase and must be thoroughly thought out before beginning the fixture design.
Workpiece specifications usually are the most important factors and have the largest influence on the fixture's final design. Typically, these considerations include the size and shape of the part, the accuracy required, the properties of the part material, the locating and clamping surfaces, and the size of the run.
Operation variables include the type of operations required to make the part, number of operations performed, sequence of operations, inspection requirements, and time restrictions.
Availability of equipment required to machine, assemble, and inspect a part often determines whether the fixture is designed for a single part or multiple parts. A process engineer sometimes selects the equipment to machine parts before the tooling designer begins the design. The tooling designer should verify what equipment will be used for each operation.
A vertical milling machine, for example, is well suited for some drilling operations. But for operations that require a drill jig, a drill press is the most cost-effective machine tool. Typically, equipment criteria include the following factors: types and sizes of machines, inspection equipment, scheduling, cutting tools, and plant facilities.
Personnel considerations deal with the end user, or operator, of the equipment. Fixture designers should put themselves in the machine operator's shoes and consider all the operational scenarios they can. Designers should consider not only correct usage of the fixture, but also possible incorrect usage. They must ask, "Is there any way for me to hurt myself while operating this equipment?"
Additional factors usually considered in this category are operator fatigue, efficiency, economy of motion, and the speed of the operation. The designer also must know and understand the general aspects of design safety and all appropriate government and company safety rules and codes.
3. DEVELOP SEVERAL OPTIONS
This phase of the fixture-design process requires the most creativity. A typical workpiece can be located and clamped several different ways. The natural tendency is to think of one solution, then develop and refine it while blocking out other, perhaps better solutions. A designer should brainstorm for several good tooling alternatives, not just choose one path right away.
During this phase, the designer's goal should be adding options, not discarding them. In the interest of economy, alternative designs should be developed only far enough to make sure they are feasible and to do a cost estimate.
The designer usually starts with at least three options: permanent, modular, and general-purpose workholding. Each of these options has many clamping and locating options of its own. The more standard locating and clamping devices that a designer is familiar with, the more creative he can be.
Areas for locating a part include flat exterior surfaces (machined and unmachined), cylindrical and curved exterior surfaces, and internal features (such as holes and slots). The choice of standard locating devices is quite extensive.
Similarly, there are countless ways to clamp a part, using a wide array of standard clamping devices. For example, a workpiece can be clamped from the top, or by gripping its outside edge or an internal surface.
For preliminary drawings of the fixture, use several colored pencils. Often black is used to sketch the fixture, red for the part, and blue for the machine tool. Use isometric graph paper to keep the sketch proportional.
The exact procedure used to construct the preliminary design sketches is not as important as the items sketched. Generally, the preliminary sketch should start should start with the part to be fixtured. The required locating and supporting elements, including a base, should be the next items added. Then sketch the clamping devices. Finally, add the machine tool and cutting tools. Sketching these items together helps identify any problem areas in the design of the complete fixture.
4. CHOOSE THE BEST OPTION
The fourth phase of the tool-design process is a cost/benefit analysis of different tooling options. Some benefits, such as greater operator comfort and safety, are difficult to express in dollars but are still important. Other factors, such as tooling durability, are difficult to estimate.
In analyzing fixture costs, the emphasis is on comparing one method to another, rather than finding exact costs. Estimates are acceptable. Sometimes these methods compare both proposed and existing fixtures, so that, where possible, actual production data can be used instead of estimates.
To evaluate the cost of any workholding alternative, first estimate the initial cost of the fixture. To make this estimate, draw an accurate sketch of the fixture. Number and list each part and component of the fixture individually. Here it is important to have an orderly method for outlining this information.
For modular fixtures, total component cost should be amortized over the system's typical lifetime. Although somewhat arbitrary, dividing total component cost by 100 (10 uses per year, for 10 years) gives a fair estimate.
The next step is calculating the cost of material and labor for each tooling element. Once again it is important to have an orderly system for listing the data. First list the cost of each component, then itemize the operations needed to mount, machine, and assemble that component. Once those steps are listed, estimate the time required for each operation for each component, then multiply by the labor rate. This amount should then be added to the cost of the components and of the design to find the estimated cost of the fixture.
The total cost to manufacture a part is the sum of per-piece run cost, setup cost, and tooling cost. Expressed as a formula:
These variables are described below with sample values from three tooling options: a modular fixture, a permanent fixture, and a hydraulically powered permanent fixture.
Run Cost. This is the variable cost per piece to produce a part, at shop labor rate (material cost does not need to be included as long as it is the same for all fixturing options).
In our example, run costs for the permanent and modular fixtures are the same, while power workholding lowers costs by improving cycle time and reducing scrap.
Tooling Cost. This is the total cost of labor plus material to design and build a fixture. The modular fixture is least expensive because components can be re-used.
5. IMPLEMENT THE DESIGN
The final phase of the fixture-design process consists of turning the chosen design approach into reality. Final details are decided, final drawings are made, and the tooling is built and tested.
The following guidelines should be considered during the final-design process to make the fixture less costly while improving its efficiency. These rules are a mix of practical considerations, sound design practices, and common sense.
Use standard components. The economies of standardized parts apply to tooling components as well as to manufactured products. Standard, readily available components include clamps, locators, supports, studs, nuts, pins, and a host of other elements.
Most designers would never think of having the shop make cap screws, bolts, or nuts for a fixture. Likewise, no standard tooling components should be made in-house. The first rule of economic design is: Never build any component you can buy. Commercially available tooling components are manufactured in large quantities for much greater economy. In most cases, the cost of buying a component is less than 20% of the cost of making it.
Labor is usually the greatest cost element in the building of any fixture. Standard tooling components are one way to cut labor costs. Browse through catalogs and magazines to find new products and application ideas to make designs simpler and less expensive.
Use prefinished materials. Prefinished and preformed materials should be used where possible to lower costs and simplify construction. These materials include precision-ground flat stock, drill rod, structural sections, cast tooling sections, precast tooling bodies, tooling plates, and other standard preformed materials. Including these materials in a design both reduces the design time and lowers the labor cost.
Eliminate finishing operations. Finishing operations should never be performed for cosmetic purposes. Making a fixture look better often can double its cost. Here are a few suggestions to keep in mind with regard to finishing operations.
Machine only the areas important to the function and operation of the component. For example, do not machine the edges of a baseplate. Just remove the burrs.
Harden only those areas of the fixture subject to wear.
Grind only the areas of the fixture where necessary for operation.
Keep tolerances as liberal as possible. The most cost-effective tooling tolerance for a locator is approximately 30% to 50% of the workpiece's tolerance. Tighter tolerances normally add extra cost to the tooling with little benefit to the process. Where necessary, tighter tolerances can be used, but tighter tolerances do not necessarily result in a better fixture, only a more expensive one.
Simplify tooling details. Elaborate designs often add little or nothing to the function of the fixture. More often, a power clamp can do the same job at a fraction of the cost.
Keep the function and operation of a fixture as simple as possible. The likelihood of breakdowns and other problems increases with complex designs. These problems multiply when moving parts are added to the design. Misalignment, inaccuracy, wear, and malfunctions caused by chips and debris can cause many problems in the best fixture designs.
Reducing design complexity also reduces misunderstandings between the designer and the machine operator. Whenever possible, a fixture's function and operation should be obvious to the operator without instructions.
Once sketches and the basic fixture design have been completed, final engineering drawings, also called shop prints, are used in the toolroom to build the fixture.
The easiest way to reduce manual drawing time is by simplifying the drawing. Words or symbols should be used in place of drawn details where practical. All extra or unnecessary views, projections, and details should be eliminated from the drawing.
Drawing a complete clamp assembly, for example, adds very little to the total design. Simply showing the nose of the clamp, drawn in its proper relation to the workpiece and labeled with its part number, conveys the same information in a fraction of the time.
For drawings that require more detail, use tracing templates to reduce drawing time. These templates show most standard components in several views. If necessary, they may be enlarged or reduced on a copier to any scale needed for a drawing.
Once the proper tracing template is selected, simply slip it under the drawing sheet and align it with the drawing. When the template is properly positioned, tape it down and trace the component on the drawing sheet. Tracing templates save drawing time and improve the quality of the drawing.
Computers are rapidly replacing drawing boards as the preferred tool for preparing engineering drawings. Almost every area of design is affected by the computer. Computers, from large mainframes to micros, are becoming standard equipment in many design departments.
A standard tooling library often is used to add the fixturing components and elements to the drawing. Using a standard library in designing the fixture dramatically reduces drawing time. All components are drawn to full scale in a variety of views. Scaling down is best done in the final drawing, not when storing standard-component drawings. Storing a large fixture base at 1/4 scale does little good, because all components will have to be 1/4 scale to fit on it. For ease of use, all components should be stored at full scale. Each component can be called up from the library and placed on the drawing where it is required.
A CAD system also can be useful during the initial phase of the workholder design as numerous tooling options are developed. CAD is sometimes faster than sketching by hand, especially when detailed cost estimates are required.
Once drawings have been thoroughly checked, the next step is actually building the actual fixture. During the building stage, the designer should make sure the toolroom personnel know exactly what must be done when making the fixture. By periodically checking with the fixture builder, the designer can help eliminate any possible misunderstandings and speed the building process. If there are any difficulties with the design, the designer and builder, working together, can solve the problems with a minimum of lost time.
After the fixture is completed and inspected, it should be tested. The fixture is set up on the machine tool and several parts are run. The designer should be on hand to help solve any problems. When the fixture proves itself in this phase, it is ready for production.
As the CNC technology grows rapidly, with the ability to move according to exact path generated from NC code, the use of jigs is beginning to decrease. Nevertheless, the fixtures still greatly needed to set-up the workpiece faster.
Successful fixture designs begin with a logical and systematic plan. With a complete analysis of the fixture's functional requirements, very few design problems occur. When they do, chances are some design requirements were forgotten or underestimated.
The workpiece, processing, tooling, and available machine tools may affect the extent of planning needed. Preliminary analysis may take from a few hours up to several days for more complicated fixture designs.
Fixture design is a five-step problem-solving process. The following is a detailed analysis of each step.
1. DEFINE REQUIREMENTS
To initiate the fixture-design process, clearly state the problem to be solved or needs to be met. State these requirements as broadly as possible, but specifically enough to define the scope of the design project.
The designer should ask some basic questions: Is the new tooling required for first-time production or to improve existing production? If improving an existing job, is the goal greater accuracy, faster cycle times, or both? Is the tooling intended for one part or an entire family of parts?
The tooling designer must determine how much freedom and input there is on each project. If many choices regarding machine tools, operations, and cutting tools have already been made, the designer's role will have a relatively narrow focus.
2. GATHER INFORMATIONS
Collect all relevant data and assemble it for evaluation. The main sources of information are the part print, process sheets, and machine specifications. Make sure that part documents and records are current. For example, verify that the shop print is the current revision, and the processing information is up-to-date. Check with the design department for pending part revisions.
An important part of the evaluation process is note taking. Complete, accurate notes allow designers to record important information. With these notes, they should be able to fill in all items on the "Checklist for Design Considerations." All ideas, thoughts, observations, and any other data about the part or fixture are then available for later reference. It is always better to have too many ideas about a particular design than too few.
Four categories of design considerations need to be taken into account at this time: workpiece specifications, operation variables, availability of equipment, and personnel. These categories, while separately covered here, are actually interdependent. Each is an integral part of the evaluation phase and must be thoroughly thought out before beginning the fixture design.
Workpiece specifications usually are the most important factors and have the largest influence on the fixture's final design. Typically, these considerations include the size and shape of the part, the accuracy required, the properties of the part material, the locating and clamping surfaces, and the size of the run.
Operation variables include the type of operations required to make the part, number of operations performed, sequence of operations, inspection requirements, and time restrictions.
Availability of equipment required to machine, assemble, and inspect a part often determines whether the fixture is designed for a single part or multiple parts. A process engineer sometimes selects the equipment to machine parts before the tooling designer begins the design. The tooling designer should verify what equipment will be used for each operation.
A vertical milling machine, for example, is well suited for some drilling operations. But for operations that require a drill jig, a drill press is the most cost-effective machine tool. Typically, equipment criteria include the following factors: types and sizes of machines, inspection equipment, scheduling, cutting tools, and plant facilities.
Personnel considerations deal with the end user, or operator, of the equipment. Fixture designers should put themselves in the machine operator's shoes and consider all the operational scenarios they can. Designers should consider not only correct usage of the fixture, but also possible incorrect usage. They must ask, "Is there any way for me to hurt myself while operating this equipment?"
Additional factors usually considered in this category are operator fatigue, efficiency, economy of motion, and the speed of the operation. The designer also must know and understand the general aspects of design safety and all appropriate government and company safety rules and codes.
3. DEVELOP SEVERAL OPTIONS
This phase of the fixture-design process requires the most creativity. A typical workpiece can be located and clamped several different ways. The natural tendency is to think of one solution, then develop and refine it while blocking out other, perhaps better solutions. A designer should brainstorm for several good tooling alternatives, not just choose one path right away.
During this phase, the designer's goal should be adding options, not discarding them. In the interest of economy, alternative designs should be developed only far enough to make sure they are feasible and to do a cost estimate.
The designer usually starts with at least three options: permanent, modular, and general-purpose workholding. Each of these options has many clamping and locating options of its own. The more standard locating and clamping devices that a designer is familiar with, the more creative he can be.
Areas for locating a part include flat exterior surfaces (machined and unmachined), cylindrical and curved exterior surfaces, and internal features (such as holes and slots). The choice of standard locating devices is quite extensive.
Similarly, there are countless ways to clamp a part, using a wide array of standard clamping devices. For example, a workpiece can be clamped from the top, or by gripping its outside edge or an internal surface.
For preliminary drawings of the fixture, use several colored pencils. Often black is used to sketch the fixture, red for the part, and blue for the machine tool. Use isometric graph paper to keep the sketch proportional.
The exact procedure used to construct the preliminary design sketches is not as important as the items sketched. Generally, the preliminary sketch should start should start with the part to be fixtured. The required locating and supporting elements, including a base, should be the next items added. Then sketch the clamping devices. Finally, add the machine tool and cutting tools. Sketching these items together helps identify any problem areas in the design of the complete fixture.
4. CHOOSE THE BEST OPTION
The fourth phase of the tool-design process is a cost/benefit analysis of different tooling options. Some benefits, such as greater operator comfort and safety, are difficult to express in dollars but are still important. Other factors, such as tooling durability, are difficult to estimate.
In analyzing fixture costs, the emphasis is on comparing one method to another, rather than finding exact costs. Estimates are acceptable. Sometimes these methods compare both proposed and existing fixtures, so that, where possible, actual production data can be used instead of estimates.
To evaluate the cost of any workholding alternative, first estimate the initial cost of the fixture. To make this estimate, draw an accurate sketch of the fixture. Number and list each part and component of the fixture individually. Here it is important to have an orderly method for outlining this information.
For modular fixtures, total component cost should be amortized over the system's typical lifetime. Although somewhat arbitrary, dividing total component cost by 100 (10 uses per year, for 10 years) gives a fair estimate.
The next step is calculating the cost of material and labor for each tooling element. Once again it is important to have an orderly system for listing the data. First list the cost of each component, then itemize the operations needed to mount, machine, and assemble that component. Once those steps are listed, estimate the time required for each operation for each component, then multiply by the labor rate. This amount should then be added to the cost of the components and of the design to find the estimated cost of the fixture.
The total cost to manufacture a part is the sum of per-piece run cost, setup cost, and tooling cost. Expressed as a formula:
These variables are described below with sample values from three tooling options: a modular fixture, a permanent fixture, and a hydraulically powered permanent fixture.
Run Cost. This is the variable cost per piece to produce a part, at shop labor rate (material cost does not need to be included as long as it is the same for all fixturing options).
In our example, run costs for the permanent and modular fixtures are the same, while power workholding lowers costs by improving cycle time and reducing scrap.
Modular fixture: $4.50Setup Cost. This is the cost to retrieve a fixture, set it up on the machine, and return it to storage after use. The permanent fixture is fastest to set up, the power workholding fixture is slightly slower due to hydraulic connections, and the modular fixture is slowest due to the assembly required.
Permanent fixture: $4.50
Permanent hydraulic fixture: $3.50
Modular fixture: $240Lot Size. This is the average quantity manufactured each time the fixture is set up. In this example, lot size is 100 for all three options.
Permanent fixture: $80
Permanent hydraulic fixture: $100
Tooling Cost. This is the total cost of labor plus material to design and build a fixture. The modular fixture is least expensive because components can be re-used.
Modular fixture : $341Total Quantity Over Tooling Lifetime. This quantity is the lesser of 1) total anticipated production quantity and 2) the quantity that can be produced before the fixture wears out. The following results are obtained by evaluating the cost-per-part formula at different lifetime quantities.
Permanent fixture: $1632
Permanent hydraulic fixture: $3350
For a one-time run of 100 pieces, the modular fixture is clearly the most economical choice. If 10 runs (1000 pieces) are expected, the permanent fixture is best. For 2500 pieces and above, the power workholding fixture would be the best choice. This analysis assumes that all noneconomic factors are equal.
| Pieces | Modular | Permanent | Permanent Hydraulic |
| 100 | $10.31 | $21.62 | $38.00 |
| 1000 | 7.24 | 6.93 | 7.85 |
| 2500 | 7.04 | 5.95 | 5.84 |
| 5000 | 6.97 | 5.65 | 5.17 |
| 10,000 | 6.93 | 5.46 | 4.84 |
5. IMPLEMENT THE DESIGN
The final phase of the fixture-design process consists of turning the chosen design approach into reality. Final details are decided, final drawings are made, and the tooling is built and tested.
The following guidelines should be considered during the final-design process to make the fixture less costly while improving its efficiency. These rules are a mix of practical considerations, sound design practices, and common sense.
Use standard components. The economies of standardized parts apply to tooling components as well as to manufactured products. Standard, readily available components include clamps, locators, supports, studs, nuts, pins, and a host of other elements.
Most designers would never think of having the shop make cap screws, bolts, or nuts for a fixture. Likewise, no standard tooling components should be made in-house. The first rule of economic design is: Never build any component you can buy. Commercially available tooling components are manufactured in large quantities for much greater economy. In most cases, the cost of buying a component is less than 20% of the cost of making it.
Labor is usually the greatest cost element in the building of any fixture. Standard tooling components are one way to cut labor costs. Browse through catalogs and magazines to find new products and application ideas to make designs simpler and less expensive.
Use prefinished materials. Prefinished and preformed materials should be used where possible to lower costs and simplify construction. These materials include precision-ground flat stock, drill rod, structural sections, cast tooling sections, precast tooling bodies, tooling plates, and other standard preformed materials. Including these materials in a design both reduces the design time and lowers the labor cost.
Eliminate finishing operations. Finishing operations should never be performed for cosmetic purposes. Making a fixture look better often can double its cost. Here are a few suggestions to keep in mind with regard to finishing operations.
Machine only the areas important to the function and operation of the component. For example, do not machine the edges of a baseplate. Just remove the burrs.
Harden only those areas of the fixture subject to wear.
Grind only the areas of the fixture where necessary for operation.
Keep tolerances as liberal as possible. The most cost-effective tooling tolerance for a locator is approximately 30% to 50% of the workpiece's tolerance. Tighter tolerances normally add extra cost to the tooling with little benefit to the process. Where necessary, tighter tolerances can be used, but tighter tolerances do not necessarily result in a better fixture, only a more expensive one.
Simplify tooling details. Elaborate designs often add little or nothing to the function of the fixture. More often, a power clamp can do the same job at a fraction of the cost.
Keep the function and operation of a fixture as simple as possible. The likelihood of breakdowns and other problems increases with complex designs. These problems multiply when moving parts are added to the design. Misalignment, inaccuracy, wear, and malfunctions caused by chips and debris can cause many problems in the best fixture designs.
Reducing design complexity also reduces misunderstandings between the designer and the machine operator. Whenever possible, a fixture's function and operation should be obvious to the operator without instructions.
Once sketches and the basic fixture design have been completed, final engineering drawings, also called shop prints, are used in the toolroom to build the fixture.
The easiest way to reduce manual drawing time is by simplifying the drawing. Words or symbols should be used in place of drawn details where practical. All extra or unnecessary views, projections, and details should be eliminated from the drawing.
Drawing a complete clamp assembly, for example, adds very little to the total design. Simply showing the nose of the clamp, drawn in its proper relation to the workpiece and labeled with its part number, conveys the same information in a fraction of the time.
For drawings that require more detail, use tracing templates to reduce drawing time. These templates show most standard components in several views. If necessary, they may be enlarged or reduced on a copier to any scale needed for a drawing.
Once the proper tracing template is selected, simply slip it under the drawing sheet and align it with the drawing. When the template is properly positioned, tape it down and trace the component on the drawing sheet. Tracing templates save drawing time and improve the quality of the drawing.
Computers are rapidly replacing drawing boards as the preferred tool for preparing engineering drawings. Almost every area of design is affected by the computer. Computers, from large mainframes to micros, are becoming standard equipment in many design departments.
A standard tooling library often is used to add the fixturing components and elements to the drawing. Using a standard library in designing the fixture dramatically reduces drawing time. All components are drawn to full scale in a variety of views. Scaling down is best done in the final drawing, not when storing standard-component drawings. Storing a large fixture base at 1/4 scale does little good, because all components will have to be 1/4 scale to fit on it. For ease of use, all components should be stored at full scale. Each component can be called up from the library and placed on the drawing where it is required.
A CAD system also can be useful during the initial phase of the workholder design as numerous tooling options are developed. CAD is sometimes faster than sketching by hand, especially when detailed cost estimates are required.
Once drawings have been thoroughly checked, the next step is actually building the actual fixture. During the building stage, the designer should make sure the toolroom personnel know exactly what must be done when making the fixture. By periodically checking with the fixture builder, the designer can help eliminate any possible misunderstandings and speed the building process. If there are any difficulties with the design, the designer and builder, working together, can solve the problems with a minimum of lost time.
After the fixture is completed and inspected, it should be tested. The fixture is set up on the machine tool and several parts are run. The designer should be on hand to help solve any problems. When the fixture proves itself in this phase, it is ready for production.
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A very useful post. All the points are explained clearly & understandable. Great source of information about Jig and fixture machining & mechanical engineering. We also offer design services for machining, welding, assembly & checking fixtures. Our objective is to reduce labor and also improve quality, accuracy & precision. Thanks for this article.
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