Page 11 - Metallforming Magazine December 2020

am not aware of any coefficient to apply to the force calculations that would account for this. Personally, I would not use the coefficient from the online calculator without a better understanding of its purpose and how it was derived.
Regardless of which formula used, this job falls outside of the capacity of your press. One reason is that you must also add in the force of the blankholder, which typically amounts to approxi- mately 30 percent of the drawing force. Because you are drawing relatively thick material, the required blankhold- er force could be as low as 20 percent. That puts you at around 451 tons using the online formula, and assumes that nothing in the die will hit home or bot- tom out, or that the blankholder pres- sure will not have to increase.
Secondly, if this is a mechanical press and not a hydraulic press, you will not have enough tonnage or press energy to carry out the deep-drawing process, even if the calculated tonnage falls below the rated press capacity.
Mechanical-press capacity is rated slightly above bottom dead center (BDC). In the accompanying tonnage graph, the 600-ton rating for this press occurs at approximately 1⁄2-in. above BDC. Deep drawing requires working the material high in the press stroke where derated tonnage comes into play. In this example, a 600-ton press has approximately 280 tons available at 2 in. above BDC.
Drawing Aluminum
Question: We have a job that uses 0.05-in.-thick 1100-0 aluminum. We use a 12.5-in.-dia. blank to draw a 4- in.-tall, 8-in.-dia. dome with a 1.5-in. flange (trimmed later). This is per- formed in a single draw.
This job has been running for nearly 60 years. We had been successfully drawing this part using a lubricant that is no longer available. We substituted with another product that tended to separate and not perform as well. We experimented with other lubricants but had minimal success.
This die uses an air-cushion blank-
holder to hold the flange during form- ing. We can reduce the pressure and make some parts that do not crack, but the flange is too wrinkled to use.
Response: A follow-up conversation with the reader revealed that some test- ing was conducted using thin, plastic food wrap and a very thick oil additive, with little success. The plastic wrap would shred and tear and became dif- ficult to remove after forming.
The stamper decided to use a lightly oiled trash bag, as it is thicker and can stretch farther without tearing. The trash bag also serves as an ideal lubri- cant for troubleshooting because it pro- vides a very low friction coefficient. If the part can be produced with the oiled trash bag, then we know that the mate- rial can make the part and that lubri- cation is the problem. If the part fails when using the oiled trash bag, then the material must be addressed because no better lubrication option exists.
Because making a good part also was sensitive to blankholder pressure,
I suggested measuring the thickness of the blank material and making some shims that measure approximately 0.005-in. greater than the material thickness. The shims are to be placed on the blankholder, outside of the blank periphery. The blankholder pres- sure now can be increased to suppress the wrinkles without placing excessive pressure on the blank and causing the part to fracture. If wrinkling continues to occur, the shims can be ground down in 0.001-in. increments until just enough restriction is provided to sup- press the wrinkles but still allow free material flow into the die cavity.
Facing a problem in your shop? Be sure to approach it as a system of inputs and outputs, not simply as a die issue. Too often we try to solve material, lubrication and machine- related problems as die issues. Some- times these problems can be compen- sated for in the die but doing so only prevents us from finding the root cause.
MF
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