Progressive die stamping: how a coil becomes a finished part

What a progressive die does

A progressive die is a single tool with several stations. The strip advances by one fixed pitch on every stroke, and each station performs one part of the job. The first stations usually pierce pilots and rough out the outline. Later stations add bends, forms, holes and embosses. The last station cuts the finished part free of the carrier.

The part is never removed between stations. That is the defining feature. Because the strip stays in the tool, the die can hold the relationship between features made at different stations, and the press can run continuously. The trade-off is that the strip needs a carrier strong enough to move the part through every station without stretching or twisting.

A typical station sequence

The exact sequence depends on the part, but most progressive dies follow a similar order. The principle is to make locating features first, then cut and form around them, and to keep the strip stable until the part is complete.

1. 01

   Pilot and notch

   Pilot holes are pierced first so every later station locates on the same features. An edge notch may be added to set the strip position.

2. 02

   Rough outline

   The outer profile is partly cut, leaving a carrier and enough material to keep the strip rigid.

3. 03

   Pierce and lance

   Holes, slots and windows are cut. Lancing may separate a tab without removing it from the strip.

4. 04

   Form and bend

   Bends, ribs, embosses and dimples are added. Bends that would trap the strip are usually left until the part is nearly free.

5. 05

   Trim and separate

   The carrier is cut and the finished part falls through the die or is blown clear. Scrap is chuted away.

Strip layout: where the cost is decided

The strip layout is the top view of the coil with every station drawn on it. It shows the feed pitch, the strip width, the carrier, the bridge between parts, the pilot positions and the scrap. A good layout uses the material efficiently and keeps the strip strong enough to feed. A poor layout either wastes material or produces a strip that buckles, twists or fails to locate.

Carrier design is the part most often misunderstood by designers. A centre carrier runs down the middle of the strip and is trimmed at the end. A side carrier runs along one edge and is simpler but uses more material. A double side carrier is stiffer and suits wider parts. The carrier has to survive every bend and every cut, so its width and the bridge to the part are set by the material and the part weight, not by a fixed rule.

Coil handling is part of the layout. A wide coil needs a straightener and a feed that can move the mass without slipping. Coil set, the residual curvature left by the mill, has to be removed before the strip enters the die, or the part will feed at an angle and the pilot will fight the strip. The feed pitch has to match the die exactly, because a small error accumulates over a run and shows up as a damaged pilot or a miss-hit.

• Feed pitch: the distance the strip moves per stroke, set by the part length plus the bridge
• Strip width: part width plus the carrier plus edge scrap
• Pilot holes: at least two, placed to control rotation as well as position
• Bridge width: enough material to carry the part without tearing
• Scrap: designed so slugs fall clear and cannot be pulled back into the die

How the die is built

A progressive die sits in a press with an upper shoe and a lower shoe. The punches are mounted on the upper shoe and the die block is in the lower shoe. Guide pillars and bushes keep the two halves aligned. A stripper plate holds the strip down as the punches withdraw, and pilots enter the strip before the cutting tools engage.

The punch and die clearance is set by the material and thickness. Too much clearance gives a torn edge and a large burr. Too little clearance raises the force and wears the punch. For mild steel the clearance is often around 5 to 8 per cent of thickness per side, but the toolmaker will set it from the material data and the required edge. The die block is usually hardened tool steel, and the punches may be coated or made from a tougher grade where wear is high.

Volume, cycle time and die cost

A progressive die is a fixed investment. The tool has more stations, more punches and more maintenance points than a compound die, so it costs more and takes longer to build. In return, the cycle time is short and the part is complete when it leaves the press. The economics work when the annual volume is high enough to spread the tool cost and when the design is stable enough that the die will not be rebuilt after the first year.

The crossover is not a single number. It depends on the number of stations, the part size, the material and the press. A simple bracket with four stations and a small terminal with twelve stations are different projects. The useful comparison is total cost at the expected volume: die plus trial parts plus maintenance reserve plus material plus press time, divided by the good parts produced. Compare that with the same calculation for a compound die, laser cutting or machining.

Coil width is a purchasing decision as well as a tool decision. A non-standard width may need a slitting charge and a longer lead time. If the strip layout can use a standard width, both the material cost and the availability improve. The same applies to the material grade: a standard gauge and temper is easier to source than a special mill order.

Failure modes in the first trial

The first trial is where the strip layout meets the press. Most problems show up as a dimensional drift, a mark on the part or a stoppage. The common ones are worth checking in a fixed order, because each one can look like another.

Diagnose the feed first, then the pilot, then the slug. A miss-hit looks like a tool problem but is often a feed problem. A slug pull looks like a dirty strip but is often a clearance or a vacuum problem. Fixing the cause is cheaper than replacing the punch and losing a shift.

• Slug pulling: a cut slug is dragged back into the strip and embosses the next part
• Miss-hit: the strip is not advanced fully, so a station cuts in the wrong place
• Pilot damage: a pilot hits a mislocated hole and bends or breaks
• Galling: material picks up on the punch or die and scores the part
• Strip buckle: the carrier is too weak or the feed is misaligned
• Bend springback: the angle is open or closed after the punch releases

Commissioning and handover

Commissioning is the step between a tool that works on the bench and a process that runs in production. The supplier should run a strip with the production material, at the production speed, and record the first, middle and last parts. Check the strip feed, the slug clearance, the pilot engagement and the scrap path. A die that runs at half speed during the trial may fail at full speed when the strip starts to vibrate.

The handover pack should include the die drawing, the strip layout, the station list, the setup sheet, the spare punch list and the maintenance schedule. It should also say which features are adjusted at setup and which are fixed by the tool. That information is what keeps the process stable when the die is moved to another press or another shift.

What to send before the die is built

Send the current drawing revision, the native CAD file, the material grade and thickness range, the annual volume, the release size, the finish and the inspection level. Add the critical dimensions and the burr direction. If the part is still in development, say which features are fixed and which can move.

Ask for the strip layout, the station list and the die maintenance plan with the quote. Those three documents show whether the tool has been designed for the part or priced from a similar job. They also give the quality team a basis for the first article, because each feature can be traced to a station.