Check fixtures help, but understanding specifications and tolerances is the first step
July 15, 2008
Verifying that tube was bent correctly is not as simple as it sounds. Bending specifications and tolerances aren't cut-and-dried, but are open to interpretation. The fabricator, the end user, and the check fixture designer might have three different perspectives on specifications and tolerances. Achieving a consensus is critical for designing and manufacturing a check fixture.
As the manufacturing environment becomes increasingly competitive, fabricators become increasingly focused on productivity, which often takes one of two forms: produce correct parts without rework and prevent the scrapping of good parts. For tube fabricators, an attribute fixture is an increasingly important part of this focus. As the pressure increases to squeeze more value out of every expense dollar, tube fabricators rely increasingly on hard fixtures, partly because these tools provide in-process control as well as final audit verification.
With a properly designed and constructed fixture, tube fabricators require little discussion or training to confirm the dimensional accuracy of the tube, whether it is a simple part or a complex assembly of several tubes. If the tube fits into the cavity, the part is correct; if it does not fit, it is not correct. It is as simple as that. The simplicity of a check fixture transcends language differences or cultural dissimilarities. The staffs of many factories and fabrication shops are multicultural and multilingual. No translation is required when fabricators use a hard fixture.
A check fixture is a quick-to-use, easily understood tool that ensures the part is built to the customer's requirements. A properly built fixture not only gives the operator enough information about the part so he has confidence in what he is building, but also helps him prevent building a poor-quality part. This is accomplished without the involvement of supervisors, a quality team, or any discussion.
The operator should be able to read the fixture and quickly determine a part's conformity to the specification. A properly designed fixture empowers the employee to separate good parts from bad parts regardless of the complexity of the tolerance scheme and associated features.
These days tube fabricators increasingly use fixtures upstream in the fabrication process, which prevents discovering a large quantity of finished parts that have to be scrapped. Using a simple, inexpensive fixture immediately after a tube bender can do quite a bit to control scrap rates.
Another advantageous location is immediately after any step that heats the part, such as welding or brazing, which may lead to distortion. In addition to catching errors early in the process, establishing proper control throughout the process can result in two additional benefits: a less expensive final audit fixture and faster results with greater confidence.
Like many elegant, simple-looking products, a check fixture is not simple or easy to manufacture. A fabricated tube's geometry is based on a sophisticated and often misunderstood mathematical model, so developing a suitable check fixture is much more complex than simply cutting a groove in some material.
The first step in developing a check fixture is developing a consensus regarding the tube's geometry (see sidebar).
The second step is to gather the relevant information. This step seems obvious, but the fact is that often drawings are incomplete or paper drawings conflict with CAD models. It is critical to determine which source is the master, and it must be complete.
The third step is designing the fixture, which consists of two parts: determining the style and specifying the material.
Style. The style, or type, is the first consideration. The best style to confirm geometric conformance is a rigid surface that replicates the tube run with all tolerances, including tolerances that change throughout the tube path. This is called a full-contour fixture (seeFigure 1).
While a full-contour fixture controls the path from one end of the part to the other, other styles are available and may be advantageous for financial or process reasons. An L-style fixture is similar to a full-contour fixture, but has one side removed at the maximum tolerance (see Figure 2). A third design, the pin style, supports the base and controls the run with a series of dowel pins located just inside the tangents of the straights on either side of the tube (see Figure 3).
Material. Designers have several material choices, such as wood, metal, and composite tooling board.
A wooden fixture, machined from a solid laminated plank rather than a series of small blocks, is satisfactory and dimensionally stable. Metal fixtures are stable and durable, but they are expensive, cumbersome, and require long lead-times. Machining the fixture body from a less expensive material, such as composite tooling board, and using metal inserts in critical areas is another option.
Fixtures are intended to answer one question: Does the product conform to the dimensional specifications and tolerances?
A well-designed and -constructed fixture makes the process run smoothly; a poorly conceived and executed fixture can result in a lengthy and nightmarish journey, which mainly is the result of a misunderstood concept of the foundational tube geometry. Often the problems surrounding rejected parts are a result of foundational differences of interpretation regarding the tube's geometry in terms of specifications and tolerances.
Many tubes have critical functions—power steering lines, fuel rails, and jet engine components—and incorrectly fabricated tubes can have serious consequences. For example, a part failure in a military jet engine can have catastrophic outcomes: a downed pilot, a crashed airplane, and a failed mission.
It is often said that fixture design is an exercise in controlling tolerances. A carefully designed fixture not only encourages the equipment operator to produce a good part, but forces him to.
What are a given tube's specifications and tolerances? The answer to this question is not as simple as it would seem. No single description, definition, or standard governs manufacturing specifications and tolerances. Although it seems that these should be straightforward and definite, specs and tolerances are open to interpretation. The fixture manufacturer, fabricator, fabricator's customer, and end user all might have different perspectives of the finished tube's geometry.
Does the customer believe that the tolerance zone around a control point is a sphere or a cube? Does the customer believe that the projection of the centerline should be taken to the control intersection points, regardless of the depth of bend, or does the customer qualify the part by using the tangents at the ends of the straight sections? These issues may seem minor, but if the fixture designer does not understand the tolerance scheme, function of the tube, and other acceptance or rejection criteria, you likely will find out that your customer's quality team does not consider these issues to be minor.
It is critical to build a consensus on these issues before attempting to build a check fixture for the part. The burden of understanding these issues and creating an agreement that satisfies all parties involved rests on the shoulders of the fixture design-and-build team. A lack of agreement can result in many hidden invoices. In a worst-case scenario, nearly everything stops: Truckloads of parts sit at the loading dock; production grinds to a halt; and the only sign of life is in a conference room, where an interminable discussion of the nuances of tube geometry drags on. A properly designed check fixture accompanied by a certified inspection report prevents this sort of situation.
The fixture designer often leads the way in developing a single, comprehensive view of the tube's dimensions in terms of specifications and tolerances. This doesn't mean he has all the answers; his knowledge and expertise vary with his education and experience. At the very least he should ask questions about various interpretations of tube geometry to create awareness; with more experience, he should acquire an understanding of the fabricator's and end user's questions, concerns, and perspectives; with even more experience, he can answer their questions, resolve their concerns, and bring all parties to an agreement on the tube's geometry.