What Is Progressive Dies?

Metal stamping shops run progressive dies when they need to make thousands of the same part. The strip feeds off a coil, goes through the die, and a finished part drops out the back. Every hit of the press does all the operations at once.

I see guys call them prog dies on the floor. Some old timers still say gang dies but that term fell out of use around the 80s.

The Basic Idea

You got a strip of material. Could be .020 cold rolled, could be .062 stainless, whatever the job calls for. The strip indexes through the die one pitch at a time. Station 1 might punch your pilot holes. Station 2 punches a slot. Station 3 puts a lance in. Station 4 does your first form. And so on down the line until the last station cuts the part off the carrier.

The carrier strip is what holds everything together. You need enough material on the sides to keep the strip from flopping around. Too skinny and the strip walks. Too wide and you waste material. Most jobs I see run carriers between 3 and 6mm depending on stock thickness.

Why Pilots Matter

Pilots locate the strip before the press closes. Without good pilots you get progressive error. That means each station adds a little bit of misalignment until your last few operations are way off.

Round pilots work fine for most jobs. Some shops use diamond pilots on the first hit to pull out any slop in the feed. I worked at a place in Michigan that swore by bullet nose pilots but honestly I never saw much difference.

Your pilot to hole clearance runs maybe .0003 to .0008 on the diameter. Tighter than that and you score the holes. Looser and you defeat the purpose.

Station Sequence

There is no magic formula here. You rough out the sequence based on what features need to go in first.

Pilots always go early. Holes before forms. Lance before bend if the lance is near the bend line. Cutoff goes last.

Sometimes you need idle stations. Say you got a form that needs clearance underneath and there is no room with the adjacent punch. You skip a station. Costs you some strip but solves the interference.

Compound stations do two things at once. A pierce and a form on opposite sides of the strip for example. Saves pitch length but makes the die more complicated to build and maintain.

Strip Development

This is where guys screw up. You got to unfold the part and figure out how much material you need. The blank size has to account for bend allowances. Different shops use different K factors. I use .42 for air bends on mild steel under 90 degrees. Goes up from there.

Once you got your blank worked out you figure the pitch. Part length plus web between parts. The web needs to be enough that you do not get distortion when you punch near the edge. Rule of thumb is 1.5T minimum. Most progressive jobs I see run 2T to be safe.

Strip width is blank width plus two carriers plus trim. On narrow parts you might run two out or three out to get better utilization. That means multiple parts across the strip width.

The Feed

Most progressive die work uses air feeds or servo feeds. Roll feeds work too but they slip on oily stock. The feed has to be timed to the press. If the feed advances while the pilots are still in the holes you tear up the strip.

Feed length accuracy matters more than people think. On a die running 300 strokes per minute a .002 feed error every hit adds up fast. By the time you are halfway through a coil you are out of tolerance.

Misfeeds happen. Could be oil on the rolls. Could be a coil with tight spots. Could be the loop control acting up. Most dies have misfeed detection. A sensor checks for the pilot hole in the right spot. If it is not there the press stops.

Tonnage

Press tonnage has to cover all your cutting and forming forces plus some safety margin. Add up the perimeters of all your cuts and multiply by material thickness and shear strength. That is your cutting tonnage. Forming tonnage depends on the bend length and the V die opening. There are charts for this.

I have seen guys size the press at 1.5 times calculated tonnage. That accounts for dull punches and hard spots in the material.

Reverse tonnage matters too. The snapthrough when the punch breaks through the material. Shock loads like that wear out presses and crack die shoes. Shear on the punches helps spread out the load.

Punch and Die Clearance

Clearance is not the same for every material. Soft aluminum maybe runs 6 percent of thickness per side. Cold rolled steel closer to 8 percent. Stainless up around 10 or 12 percent.

Too tight and you get secondary shear on the cut edge. Looks like a double break line. Also wears out your punches faster. Too loose and you get rollover and heavy burr.

I check clearance with a wire gauge. Or you can blue up a test piece and look at the break. You want maybe one third shear and two thirds break for most applications.

Die Materials

Punches and die buttons are usually tool steel. D2 is common. Lasts a decent amount of time on mild steel. M2 high speed for abrasive materials or long runs. Carbide for really high volume or stainless that eats everything.

Die shoes are A36 or cast iron. Some shops go with aluminum die sets on light duty work. Saves weight when you are swapping dies on a small press.

Strippers can be hardened tool steel or bronze. Bronze works good when you need the stripper to contact formed features without marking them up.

What Goes Wrong

Slug pulling is the big one. The slug sticks to the punch face and comes back up. Next hit you got a slug between the punch and die and something breaks. Polished punch faces help. So does vacuum through the die button. Some guys swear by spring loaded ejector pins but they add complexity.

Punch galling shows up on stainless and aluminum. The material welds to the punch. Coatings help. TiN or TiCN. Some shops use Lubricool or similar die lubes. Chlorinated lubes work great but nobody wants to deal with the disposal anymore.

Carrier strip breaking usually means your web is too narrow or your pilots are too tight. Or someone ran the press too fast. The strip gets work hardened as it goes through and eventually cracks.

Maintenance

Sharpening intervals depend on the job. Mild steel you might get 100000 hits between sharpens. Stainless maybe 30000. Silicon steel for motor laminations even less.

When you sharpen you got to keep everything even. Take .002 off the punches and .002 off the die block. Otherwise your shut heights get out of whack and your clearances go bad.

Pilots wear too. Most shops replace pilots when they are down .001 or so on the diameter. Past that you start seeing location problems.

I have run dies past 10 million hits on easy jobs. 2 or 3 million is more typical before you need significant rework. Carbide tooling lasts longer but costs more up front.

Compared to Other Methods

Single hit dies do one operation. You hand transfer the blank between dies. Slow but cheap tooling.

Transfer dies use mechanical fingers to move individual blanks through a series of stations. Works for bigger parts where the strip would be too heavy. Or parts with deep draws that need multiple reductions.

Progressive dies make sense when you got volume. The tooling costs more but you make it back on cycle time. Below maybe 50000 pieces a year you probably go with single hit. Above a few hundred thousand progressive usually wins.

Where They Get Used

Electrical contacts. Terminal pins. Brackets. Spring clips. Motor laminations. Lead frames for semiconductors. The little tabs that hold batteries in place. Shielding cans for RF applications.

Anywhere you need a lot of the same small to medium sized part in sheet metal there is probably a progressive die making it.

Medical parts run in progressive dies too. Surgical staples. Implant components. Those jobs have more documentation and tighter process controls but the die technology is the same.