Stamping calculation: Calculating tonnage in progressive stamping

Q: How do I calculate press tonnage in progressive stamping?

A: You have to review the total amount of work being done by the press at each progression (station) in the die. This must include the skeleton scrap, parts web carrier, pilot hole punching, spring stripper pressures, strip lifter pin pressures, nitrogen pressure pads, driven cams, and final web scrap cutting, as well as the piercing, drawing, forming, bending, coining, and stenciling stations. Once you have recorded the load for each station, you add them together to get the total tonnage needed for the press.

Energy. To size a press, you must do calculations for both tonnage and energy, because you can have enough tonnage but not enough energy. Insufficient energy is a common cause of press jams at bottom dead center (BDC).

To get an accurate calculation, you must convert all of the values to the same unit of measurement, (inches, pounds, and tons). This prepares the data for the second calculation: the required energy in inch/ton.

Metal Properties. You must know the coil material properties—shear strength and tensile strength in pounds per square inch (PSI), the material thickness in inches. When getting into high-speed, high-strength materials and draw calculations, the ultimate tensile strength must be known.

Progression Layout. With a part that has 15 or more progressions, I recommend that you have a progression strip layout on paper that you can color-code and mark the loads at each station. That way, you know you have not missed anything that will create an additional load. The strip layout also helps you to calculate the die's position in the press, which enables you to balance the loads for both the press and die. Doing so shows maximum die life per regrind, as well as maximum die life for optimal part quality.

To calculate the balance of the die to the press, you have to take moments about the right-to-left centerline of the die at each station. This will indicate the die's out-of-balance condition. Then, by repositioning the die's centerline with the press's centerline, you can improve the die-to-press balance.

It is not uncommon for this to require several pages of calculations and several hours of work.

In most press shops I visit, it is a common practice to position the die as close as possible to the feed. This is incorrect. More often than not, you can obtain a better press and die balance, as well as part quality, by moving the die away from the feed.

Note that the material shear distance is the distance from where the punch compresses the material to the point of fracture. This is visible on the edge of any stamped part.

Loads can be reduced further by putting shear on the punches up to full material thickness or steeping the punches. But this has to be done in a scientific manner, not by guesswork.

The calculation for drawing is similar to that used for blanking except the ultimate tensile strength of the coil material is used because the sides of the shell or cup are in tension during the drawing operation.

Do not forget to add the blank holder pad pressure which is dependent on material thickness, plus pressure of any spring strippers, strip lifter pins, nitrogen pressure pads, driven cams, or scrap cutters.

Final Steps During the Stamping Calculation

After you calculate the required die tonnage, it is important to determine:

1. Where in the press stroke the die load (tonnage) is applied from bottom dead center (BDC) of the press stroke. From this you can calculate the press energy requirement.
2. The distribution of die tonnage over the press bed and slide areas for press deflection. Often small dies are placed in a very large bed area.

The press tonnage rating and deflection is based on the die load being equally distributed over two-thirds of the press bed area.

Then the last two checks are:

1. Review any limitations of slide velocity at any of the die stations, especially the draw station. This calculation will allow you to set the speed of the press to get maximum productivity and part quality.
2. Review any off-center loading of the die to the press. (To obtain maximum die and press life)