Strip Layout for Progressive Dies: Material Utilization, Pilots and Carriers

The strip layout is the plan of a progressive die: it shows how the part develops from the flat strip, station by station, until it is cut free. It fixes the material cost per part, the number of stations, the size of the die and the press it needs. Mistakes made here cannot be repaired later by good die detailing. This article goes through the decisions in the order they are normally taken.

The terms

Step 1: Develop the flat blank

Unfold the part to its flat shape using the bend allowance for the material and radii. Mark on it:

• the grain (rolling) direction limits for each bend;
• the burr side required by the drawing;
• holes and edges with tight tolerances relative to each other;
• features that can only be made after forming (holes close to a bend, for example).

Step 2: Choose the orientation

Try the blank on the strip in several positions: lengthwise, crosswise, angled, and nested in pairs. Judge each by four criteria.

The orientation with the best utilization is not always the best choice. A layout that saves 3 % of material but needs cams to form sideways may cost more over the die's life.

Step 3: Set the web and edge margins

The webs must be strong enough for the strip to be fed and piloted without buckling, and wide enough for the scrap to be cut cleanly. Typical minimum values:

As a rule of thumb the web is 1 to 1.5 times the sheet thickness, with an absolute minimum of about 1 mm. Increase the values for soft materials, for long blank edges running parallel to each other, and for hand-fed strips.

Step 4: Calculate pitch, width and utilization

• Pitch = blank length in the feed direction + web
• Strip width = blank width across the strip + 2 × edge margin (+ carrier width, if the carrier is outside the blank)
• Utilization = blank area × parts per pitch ÷ (pitch × strip width)

Worked example. A blank 40 × 25 mm with an area of 880 mm², in 1.0 mm steel.

Layout A saves 0.30 g per part. Over one million parts that is 300 kg of steel. It also has the shorter pitch, which means a shorter die and a faster feed. Layout B would be chosen only if the grain direction or the carrier required it.

Most single-row layouts fall between 60 and 80 % utilization. Two-row or nested layouts can exceed that, at the cost of a wider die and a part that leaves the die in two orientations.

Step 5: Plan the pilots

Feeders are not accurate enough to position the strip for precise work. The pilots do the final positioning.

1. Pierce the pilot holes in the first station and use them in every following station that needs accuracy.
2. Place pilots as far apart as possible, ideally on both sides of the strip, to control position and rotation.
3. Prefer holes in the carrier or in the scrap. Use holes in the part only if their tolerance and appearance allow slight marking.
4. Let the pilots enter before any punch touches the strip. The pilot nose should project beyond the stripper face by at least the sheet thickness.
5. Release the feed just before the pilots enter, so the strip is free to move into position.
6. Size the pilot a few hundredths of a millimetre smaller than the pierced hole: about 0.02–0.05 mm for precise work, more for general work.

Step 6: Choose the carrier

The carrier must be stiff enough to push the strip through the die without buckling, and its connection to the part must be placed where the final cut leaves an acceptable edge.

Step 7: Sequence the stations

Step 8: Check the layout before design continues

Common mistakes