August 14, 2003
In this era of global warp speed and virtual reality, calculating the deep draws of progressive dies or the springback of metal is performed by simulation software instead of the earlier trial-and-error method. These software programs essentially replace the artistic methods of diemaking with scientific formulas.For more than a decade tool engineers have been designing parts, progressive die sequences, and dies and detailing cutter paths as a continuous process (see Figure 1) using computer-aided design (CAD).
Progressive die design used to be considered as just a road map for the diemaker during the build process. First the form stations were cut and given to the assembly department to be mounted on the die shoes. Later the die was taken to the press, where tryout began. To perform the tryout, the diemaker would hit a part and troubleshoot the tool for rips in the part where metal did not flow correctly or wrinkles where too much metal gathered. This process was labor-intensive and required many hours of press time.
3-D solid modeling was used to design this progressive die.
Today diemakers begin building by running simulations on the part without webs attached. The simulations are done for the forming stations of the die, such as draw stations, flange stations, and restrike stations. Two tools in the software, the forming limit diagram (FLD) and the thickness plot, then are evaluated to determine if the process can produce a part without fractures or wrinkles.
The FLD shows points plotted on two curves that are created from the material properties. All points should fall into the area below the first curve (see Figure 2a, 2b). Points above the first curve and below the second are in the marginal zone, areas of the part that are prone to fractures. For parts in this zone, any variation in the process—such as material property changes, thickness changes, or even a change in lubrication—could cause part failure. Points above the second curve are fractures in the part.
The thickness plot shows where material is thickening or thinning and is in danger of wrinkling or tearing. When the FLD shows no fractures and the thickness plot shows no wrinkling or tears, the process will produce a good part.
In the past a diemaker developed the webs while he was troubleshooting the die in the press. He experimented with different web configurations to make the part feed through the die, but because these tryout strips consisted of only two or three parts, he was unable to prove that the web would have enough strength to push the part the entire length of the tool.
Today stretching of the carriers is simulated. A web is taken out of the CAD design and set up in the simulation with locator pins in the pilot holes. The locator pins lock that area of the web so that it will not move during the simulated press stroke.
The remaining webs react by stretching as they will in the real press, which shows where clearances are needed in steels for the movement of the stretch webs. They must have adequate stretch to allow the part to form, but enough strength to advance the part through the tool.
After the tool was altered so that it produced a part without ripping or wrinkling, the diemaker moved on to the next step, which was developing the trim line. To do this, he had a part laser-cut to the shape he believed would make a finished part and then ran it through the die. After the part was finished, the diemaker checked the part and made adjustments before laser cutting another part with the changes needed to get the trim correct.
This process required several iterations, all of which took even more time. Also, laser cutting the parts introduced errors into the process. The laser-cut webs usually were only three parts long, so the die was not fully loaded and therefore would not always produce a consistent part. Laser cutting of the metal annealed the edges, which caused some forming operations to succeed during tryout, but they failed when the part was formed using the cutting steels.
With today's forming simulations, the blanking and trimming stations are developed virtually. The finished-part geometry is projected onto the simulation result, and the software reverses the forming process, producing a part that becomes what is cut for the final trim steels.
Virtual tryout allows the whole tool to go directly into machining and assembly (see Figure 3). With this process, the diemaker knows that the die can produce the part when it is in the press. The diemaker then adjusts the part to bring it into tolerance.
In the past dies were fully assembled and tried out before any indication arose of a problem area that required a tool steel coating to correct. While simulation software cannot tell which areas of a tool need to be coated, it can provide hints, such as showing thickening material in a corner, which could be an indication of a high-pressure area in the tool.
In an area such as this, a coating often is required to prevent the detail from galling. If a coating station is required, the coefficient of friction can be adjusted and the simulation recalculated to show how the material will react with a tool coating. By changing the friction, the software also can show the effect of not using lubricant on the die.
Simulation software can accurately calculate how much pressure will be required to hold the part for each of the forming stations, thus allowing an accurate estimate of the number of nitrogen cylinders that are required. This is important because if the pressure is too low, the binder may gap, which might cause uncontrolled material flow, thinning, or wrinkles in the part. On flange stations, if the pressure is incorrect, the center of the part may bow upward, which causes springback in the flange walls (see Figure 4).
While simulation software will not tell the diemaker how to build the tooling, it will accurately predict the result of the chosen process and help the designer develop a robust process to produce the part.
When coupled with a solid modeling CAD program, the software allows a virtual construction and tryout of the die in the computer before any steel is sent to the CNC machining center. This allows for the complete assembly of 95 percent of all tools before they make the first hit, greatly reducing the amount of tryout time.
A forming simulation cannot detect interference between details in the tool, nor does it find areas where the part is colliding with the adjoining die station. These are jobs for the solid-modeling CAD software.
CAD can check for clearance between adjacent details, as well as check the part against any detail that it may come in contact with. This is a critical step in the design of progressive dies and saves countless hours on the shop floor. Making sure that all clearances are correct before machining reduces the remachining of details and the number of times that the diemaker must assemble and tear down the die.
If a part has a feature such as a tight corner or an emboss needs to be stretched into the middle of the part, the simulation will show if the part can be manufactured as it is designed. When several different attempts have been made to produce the part and none of them is successful, data can be shown to the customer for discussion and recommendations on how to alter the design so that a good part can be produced.
Like any other technology, simulation software has some limitations:
Terry Swan is vice president of technology and design with Tomco Tool & Die Inc., 807 Edna St., P.O. Box 404, Belding, MI 48809, 616-794-1640, fax 616-794-3640, firstname.lastname@example.org, www.tomcotool.com.
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