Die Science: Fundamentals of drawing
Columnist Art Hedrick goes back to basics about drawing, offering a comparison of drawing and casting, an explanation of flow and control.
Once a week, it seems, I receive a print in my e-mail containing the following question: Can this part be made? In general, if the part has a great deal of contour or shape to it, a drawing operation will be required to make it.
Even though the term draw also can mean a method of deformation in which metal is both compressed and stretched simultaneously, draIwing is a loosely used term referring to an operation containing many different forms of metal deformation. In fact, certain types of draw dies produce parts that don’t have any draw deformation in them.
Drawing is advantageous compared to other forming operations in that it can form very tall, wrinkle-free, highly sculptured parts out of a large variety of both ferrous and nonferrous metals while maintaining a quality surface finish. Some typical parts made in draw dies are car doors; hoods; oil pans; oil cans; filters; and numerous components used in the automotive, appliance, construction, hardware, and medical fields.
Drawing Versus Casting
Drawing a part, as opposed to casting it, typically reduces part cost, improves production rates and part quality, and lowers overall production cost. As a result, a large percentage of parts previously made as castings have been converted to drawn stamped parts, and many product designers consider drawing to be an economically sound, viable option. In fact, part design is the main limiting factor that determines whether a part can be drawn or if it needs to be made in an alternative process.
Quite simply, if the design for a sheet metal stamped part dictates that the metal undergo stretch or compression or that it maintain a high-quality surface finish, drawing usually is the best process for the job.
Flow and Control
In drawing, not to be confused with stretching, the flow of flat sheet metal into a specific shape is controlled. In other words, the basic purpose of drawing is to create as much of the part shape as possible through the displacement of surface area.
The key words in the definition of drawing are flow and control.
Flow, in its simplest form, describes what the metal must do so it can restructure into a specific shape. Picture in your mind a large sheet of rubbery plastic being formed over a given shape, liquefying not under heat but under pressure. Steel and other ferrous and nonferrous metals react to forming in much the same manner.
Control of that flow—making it go where you want it to and not where you don’t—is one of the key principles of drawing operations. Control of the metal flow is what differentiates a drawing die from any other stamping die. The flow of the metal is controlled by a pressure-loaded plate called the drawing pad or binder, along with the pressure system underneath the pad, which comprises a series of gas springs, a manifold, or an alternate pressure system.
When drawing is used to form a part that is deeper than its minimum part width, the process is called deep drawing. An example of a typical deep-drawn part is an oil filter.
Successful deep drawing depends on many factors:
- Material type, thickness, and mechanical properties
- Blank size and shape
- Part geometry
- Press speed (ram speed)
- Draw radii
- Draw ratio
- Die surface finish
- Die temperature
- Draw bead height and shape
- Binder pressure
- Binder deflection
- Standoff height
The material used in deep drawing must have the mechanical ability to take the part shape without rupturing. For this reason, you need to pay strict attention to the material type and thickness. Press speeds must allow time for materials to flow.
Die surface finishes and lubricants are important because they can reduce the coefficient of friction, allowing materials to slide through tools more easily. In most cases, the dies are polished to a mirror-like finish.
A draw bead is like a speed bump for the metal—it forces the metal to bend and unbend prior to flowing into the cavities and over the punch. Increasing binder or draw pad pressure will exert more force on the material, creating a restriction to flow. This helps reduce wrinkling and increase the amount of stretch occurring in the part.
The draw ratio—the relationship between the size of the draw punch and the blank—is among the most important elements to consider when attempting to deep draw a part. In fact, it is one of the main reasons that so many stations are required to make tall, small-diameter parts. If the blank is too far from the punch, the metal most likely will stretch and could fail. Reducing the blank size will cause the metal to flow.
STAMPING Journal is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping.