Die Science: Tricks of the stamping trade

I recall a TV show from many years ago called “Breaking the Magician's Code: Magic's Biggest Secrets Finally Revealed.” A masked magician would first perform an illusion or trick and later reveal how it was done. Diemakers have been portrayed as magicians for decades. When a problem occurs at the press, the toolmaker or die technician is called out and asked to perform their tricks of the trade.

Today I get to be the masked magician! But instead of making an elephant disappear or turning a beautiful lady into a lion, I’m going to make slug pulling disappear and make wrinkles and splits magically evaporate before your very eyes! But keep in mind that although these tricks often are very effective, some are only temporary solutions at best. Others are used to collect data for more permanent corrective action. But in any case, they work.

Slug Pulling Tricks

When a pierce punch creates a hole, it also produces scrap, called the slug. When the slug sticks to the face of the punch during withdrawal or comes out of the button or lower matrix, it is called slug pulling.

Slug pulling can be a serious problem in a stamping operation. If a slug falls off the punch and onto the strip or part, it can damage parts and the die. Keeping the slug down in the matrix or, better yet, pushing it out of the die completely is the desired scenario. One of the most common ways to eject slugs, other than using a spring ejector, is to put small metal or carbide deposits on the inside of the matrix. This can be done using a small arc welder or a spark deposition machine. The small deposit or barb of metal on the inside of the matrix creates slug interference, preventing the slug from pulling up with the punch.

A reverse-taper, bell-mouth button is another option. Most die buttons have a bell-mouth taper machined into them, with the hole diameter increasing toward the bottom of the button. In the reverse-taper button, the hole in the matrix gets slightly smaller as it nears the clearance opening. This holds the slugs in compression in the matrix. Keep in mind that for most piercing operations, 0.0005 to 0.001 inch is more than enough taper; if the taper is too great, compression can cause the matrix to split.

Split Prevention Tricks

Splitting during drawing or stretching is the result of too much localized stretching and thinning of the metal. To reduce the amount of thinning, the metal must flow inward: More flow equals less stretch. Lubricants reduce the friction between the die and the sheet material, greatly affecting flow. To simulate a super-lubricant in a drawing operation, use a thin film barrier lubricant. Garbage bags and Saran Wrap® work well, but high-strength grocery sacks work best.

Place the plastic material on both sides of the blank, either on the entire blank or on areas where you want to increase flow. Using this special lubricant reduces the friction and may reduce or eliminate splitting or necking problems. Although some products such as kitchen sinks and bright stainless steel parts are made using these films, this procedure typically should be used only as an experimental process to collect data for permanent tool changes.

Wrinkle-reduction Magic

Wrinkles are the direct result of uncontrolled metal flow combined with compression, or the result of too much material flowing to an area that does not have a geometry that can consume this excessive amount of material.

Traditionally, draw beads are used to reduce the amount of metal flowing into a specific area of the part’s geometry. Draw beads reduce the amount of metal flowing into a die cavity by forcing it to bend and unbend before it enters the die cavity. To simulate the effects of a draw bead, place grit cloth or sandpaper on one or both sides of the blank in a specific area. The abrasiveness of the grit cloth and the increased pressure exerted on the blank simulate the draw bead effect and help reduce wrinkling in the product area.

Once you determine that limiting the metal flow will reduce or eliminate the wrinkling, you can add a permanent draw bead to the problem area on the blank holder. But be careful: Excessive use of abrasives is likely to cause die erosion. Do not use this method if the blank holder bottoms out, as it could cause the blank holder to break. And don’t use it on coated dies.

The Magic of Physics

If you ever attended one of my public training seminars, chances are that during one of the breaks you saw me perform some card magic. And chances are I revealed the trick. I’m an amateur magician, yet I don’t believe in magic. I believe in physics. Magic never once fixed a die or improved part quality. I hope I’m not expelled from the diemakers’ secret magic society for revealing these tricks of the trade.