The first step in designing any jig or fixture is a thorough evaluation of its functional requirements. The goal is to find a balanced combination of design characteristics at a reasonable cost. The part itself, processing, tooling, and machine-tool availability may all affect the extent of planning needed. Preliminary analysis may take from a few hours up to several days for more-complicated designs.
To design a workholder, begin with a logical and systematic plan. With a complete analysis, very few design problems occur. Workholder problems occur when design requirements are forgotten or underestimated. No specific formula or method works for every design, but the designer can employ a deliberate and logical system in the initial planning and design.
Tool design is essentially an exercise in problem solving. Creative problem solving can be described as a five-step process: 1) Identifying and defining the problem; 2) Gathering and analyzing information; 3) Brainstorming for alternative solutions; 4) Choosing the best solution; 5) Implementing the solution. This five-step process adapted to jig-and-fixture designs is shown in Figure 2-1.
DEFINING REQUIREMENTS
The first step in the tool-design process should be to clearly state the problem to be solved, or needs to be met. These requirements should be stated as broadly as possible, but specifically enough to define the scope of the design project.
The new tooling might be required either for first-time production of a new product, or to improve production of an existing part. When improving an existing job, the goal might be greater accuracy, faster cycle times, or both. Tooling might be designed for one part, or an entire family.
Tool design is an integral part of the product-planning process, interacting with product design, manufacturing, and marketing. To reach an optimum solution, all four of these groups need to work together concurrently.
GATHERING AND ANALYZING INFORMATION
In the second design phase, all data is collected and assembled for evaluation. The main sources of information are the part print, process sheets, and machine specifications. When collecting this information, 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 product-design department for pending part revisions.
An important part of the evaluation process is notetaking. Complete, accurate notes allow the designer to record important information. All ideas, thoughts, observations, and any other data about the part or tool are then available for later reference. It is always better to have too many ideas about a particular design than not enough. Good notes also minimize the chance that good ideas will be lost.
Four categories of design considerations need to be taken into account at this time: the workpiece, manufacturing operations, equipment, and personnel. A checklist is shown in Figure 2-2.
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 workholder design.
Workpiece Considerations
Workpiece specifications are usually the most-important factors and have the largest influence on the workholder's final design. Typically these considerations include the size and shape of the part, the accuracy required, the properties of the part material, locating and clamping surfaces, and the number of pieces.
Operation Considerations
These considerations include the type of operations required for the part, the number of operations performed, the sequence of operations, inspection requirements, and time restrictions.
Equipment Considerations
Equipment considerations control the type of equipment needed to machine, assemble, and inspect a part. Often the available equipment determines whether the workholder is designed for single or multiple parts. A process engineer sometimes selects the equipment for required functions before the tool designer begins the design. Still, the tool designer should verify equipment choices 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 machine tools, inspection equipment, scheduling, cutting tools, and general plant facilities.
Personnel Considerations
Personnel considerations deal with the end user, or operator, of the equipment. Most special tools are designed to be used by shop personnel, so the design of any workholder must be made with the operator in mind. The first and most-important consideration in this phase is safety. No tool should ever be designed without complete safety in mind.
Additional factors typically considered in this category are operator fatigue, efficiency, economy of motion, and the speed of the operation. The designer must also know and understand the general aspects of design safety and all appropriate government and company safety rules and codes.
DEVELOPING SEVERAL OPTIONS
The third phase of the tool-design process requires the most creativity. A typical workpiece can be located and clamped many different ways. An important strategy for successful tool design is brainstorming for several good tooling alternatives, not just choosing 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 only be developed far enough to make sure they are feasible, and to do a cost estimate.
Brainstorming for Ideas
The designer usually starts with at least three options: permanent, modular, and general-purpose workholding, as seen in Figure 2-3. 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.
There is seldom only one way to locate a part. Options 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. For example, a workpiece can be clamped from the top, by gripping its outside edge, or gripping an internal surface. The choice of standard clamping devices is also very broad.
Design Sketches
For preliminary sketches of the tool, one good idea is to use several colored pencils. Often, black is used to sketch the tool, red for the part, and blue for the machine tool. The different colors allow you to see, at a glance, which areas of the sketch show what part of the assembled unit. Another idea: use graph paper to keep the sketch proportional. Either plain or isometric graph paper works well for most design sketches.
The exact procedure used to construct the preliminary design sketches is not as important as the items sketched. For the most part, the preliminary sketch should start with the part to be fixtured. The required locating and supporting elements should be the next items added, including a base. The next step is to sketch the clamping devices. Once these elements are added to the sketch, the final items to add are the machine tool and cutters. Sketching these items together on the preliminary design sketch helps identify any problem areas in the design of the complete workholder.
CHOOSING 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. Cost analysis is sometimes more of an art than a science.
Workholder-cost analysis compares one method to another, rather than finding exact costs. So, even though the values used must be accurate, estimates are acceptable. Sometimes these methods compare both proposed tools and existing tools, so, where possible, actual production data can be used instead of estimates.
Initial Tooling Cost
The first step of evaluating the cost of any alternative is estimating the initial cost of the workholder. Add the cost of each element to the labor expense needed to design and build the jig or fixture.
To make this estimate, an accurate sketch of the tool is made first. Each part and component of the tool is numbered and listed individually. Here it is important to have an orderly method to outline this information. Figure 2-4 shows one way to make this listing. The exact appearance of the form is unimportant; only the information is important.
The next step is calculating the cost of material and labor for each tool element. Once again, it is important to have an orderly system of listing the data. First list the cost of each component, then itemize the operations needed to mount, machine, or assemble that component. Once these 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 the cost of design to find the estimated cost of the workholder.
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 ten years) gives a fair estimate.
Cost Comparison
The total cost to manufacture a part is the sum of per-piece run cost, setup cost, and tooling cost. Expressed as a formula:
The following example shows three tooling options for the part in Figure 2-3: 1) a modular fixture; 2) a permanent fixture; 3) a permanent fixture using hydraulic power workholding. Each variable in the cost equation is explained separately below.
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.
Setup 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.
Lot Size. This is the average quantity manufactured each time the fixture is set up. In our example, lot size is the same for all three options.
Initial Tooling Cost. This is the total cost of labor plus material to design and build a fixture (as explained in the previous section). The modular fixture is least expensive because components can be reused, the permanent fixture next, and the hydraulic fixture most expensive.
Total Quantity Over Tooling Lifetime. This quantity, the last remaining variable, is the lesser of 1) total anticipated production quantity and 2) the quantity that can be produced before the tooling wears out. The following results are obtained by evaluating the cost-per-part formula at different lifetime quantities:
For a one-time run of 100 pieces, the modular fixture is clearly the most economical. If ten runs (1,000 pieces) are expected, the permanent fixture is best. For 2,500 pieces and above, the power workholding fixture would be the best choice. This analysis assumes that all noneconomic factors are equal.
IMPLEMENTING THE DESIGN
The final phase of the tool-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.
Guidelines for Economical Design
The following guidelines should be considered during the final-design process. These rules are a mix of practical considerations, sound design practices, and common sense. Application of these rules makes the workholder less costly, and improves its efficiency and operation.
Use Standard Tooling Components. The economies of standardized parts apply to tooling components as well as to manufactured products. Standard tooling components, readily available from your industrial supplier, 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 workholder. For the same reason, virtually 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.
Labor is usually the largest cost element in the building of any workholder. Standard tooling components are one way to cut labor costs. Always look for new workholding products to make designs simpler and less expensive. Browse through catalogs and magazines to find new products and application ideas. In most cases, the cost of buying a component is less than 20% of the cost of making it.
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 tool 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 Unneeded Finishing Operations. Finishing operations should never be performed for cosmetic purposes. Making a tool look better can often double the cost of the fixture. 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 tool. For example, do not machine the edges of a baseplate. Just remove the burrs.
- Harden only those areas of the tool subject to wear.
- Grind only the areas of the fixture where necessary for the operation of the tool.
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 tool with little benefit to the process. Where necessary, tighter tolerances can be used, but a tighter tolerance does not necessarily mean a better tool, only a more expensive tool.
Simplify Tooling Operation. Elaborate designs often add little or nothing to the function of the jig or fixture. Complex mechanical clamping systems, for example, are often elaborate and unneeded designs. More often, a power clamp could do the same job at a fraction of the cost. Cosmetic details are another example of little gained for the money spent. These details may make the tool look good, but seldom justify the added cost.
Keep the function and operation of a workholder 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 tool designs.
Reducing design complexity also reduces misunderstandings between the designer and the machine operator. Whenever possible, a workholder's function and operation should be obvious to the operator without instructions.
Manual Drawings
Once sketches and the basic workholder design have been completed, final engineering drawings can be prepared. Copies of the engineering drawings, also called shop prints, are used by the toolroom to build the workholder.
Manual drawing is the process of constructing engineering drawings by hand on a drawing board. The easiest way to reduce 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. This cuts the time spent drawing details that add little to the meaning of the drawing.
Drawing a complete clamp assembly, for example, adds very little to the complete design. Simply showing the nose of the clamp in its proper relation to the workpiece, along with specifying its part number, conveys the same information in a fraction of the time.
Computer-Aided Design
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 microcomputers, are becoming standard equipment in many design departments.
A standard tooling library, shown in Figure 2-5, is often used to add the fixturing components and elements to the tool drawing. Using a standard library in designing the workholder dramatically reduces drawing time. All components are drawn to full scale in a variety of views. Each component can be called up from the library and placed on the drawing where it is required.
A CAD system is also sometimes useful during the initial phase of developing numerous tooling options. Computer-aided design is sometimes faster than sketching by hand, especially when detailed cost estimates are required.
Building and Testing the Workholder
Once drawings have been thoroughly checked, the next step is building the actual workholder. During the building stage, the designer should ensure the toolroom knows exactly what must be done when making the tool. By periodically checking with the toolmakers, the designer can help eliminate any possible misunderstandings and speed the building process. If there are any difficulties with the design, the designer and toolmaker, working together, can solve the problems with a minimum of lost time.
After the tool is completed and inspected, the last step is tool tryout. The workholder 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 tool proves itself in this phase, it is ready for production.
