Chipped washer die with 0.05 mm clearance on 1.6 mm CRCA
A progressive die blanks washers from 1.6 mm cold-rolled mild steel, outside diameter 50 mm, hole 20 mm. Clearance was ground at 0.05 mm per side on both punch and die. After about 5,000 strokes the punch edges chip, burr height on the slug reaches 0.15 mm, and stripping is heavy enough that the stripper springs are being compressed solid. The die runs on a 40-tonne press at 60 strokes per minute; the shear strength of the strip is about 300 N/mm2. The tool room has a surface grinder and a wire EDM available, and the die block is 60 mm thick. The customer wants 50,000 parts from this tool before the next service.
1. Calculate the blanking clearance you would specify and explain why the existing 0.05 mm per side is causing chipping and heavy stripping.
Read the worked answer
For 1.6 mm cold-rolled mild steel the usual blanking clearance is 5-8% of thickness per side: 0.08-0.128 mm. The existing 0.05 mm per side is about 3%, well under that band.
With too little clearance the fracture cracks started from the punch edge and from the die edge do not meet. The slug is torn rather than sheared, so the burr grows, punch load rises, and the strip grips the punch - which is why the stripper springs are bottoming. Chipping follows because the cutting edges are being loaded in compression and rubbing rather than cleanly shearing, and the heat and pressure build up at the edge. Recommend 0.10 mm per side (about 6%), so the total gap between punch and die is 0.20 mm, applied consistently around the profile.
2. Check the press capacity for this job and comment on the 60 strokes per minute.
Read the worked answer
Shear length is the full cut perimeter: pi x (50 + 20) = 219.9 mm. Cutting force F = L x t x shear strength = 219.9 x 1.6 x 300 = 105,600 N, about 10.8 tonne-force.
The 40-tonne press has ample capacity for the cut itself. Two things still need checking. First, stripping force is typically 5-10% of cutting force, so about 0.5-1 tonne, but the spring preload when the stripper is compressed solid at bottom dead centre adds to the load and may be what is overloading the springs and the press. Second, at 60 strokes per minute with a 10.8 tonne load the flywheel energy demand is continuous and high; check the press's rated energy per stroke, not just its tonnage, and check the shut height so the stripper is not being bottomed out.
3. Give the repair and setting-up sequence, and how you would establish a service interval instead of the arbitrary 5,000 strokes.
Read the worked answer
Strip the tool and record the chipped edges. Regrind the die opening and punch faces on the surface grinder, removing at least 0.5 mm to get past the cracked layer, then re-establish 0.10 mm per side. If the die profile is too damaged, or the clearance change must follow the profile, wire-EDM the die opening to the new size and make a matching punch.
Setting: check punch-to-die alignment with blue marking and a feeler at four points; check stripper spring free length against solid height and replace springs that have taken a set; check guide-pillar clearance; confirm the die height and shut height. Run 50 pieces and measure burr (target under 0.05 mm) and slug quality.
Service interval: instead of 5,000 strokes, inspect the edge every 2,000 strokes and plot burr height against stroke count. Regrind when burr reaches about 0.05 mm or edge wear is visible. With the correct clearance, 50,000 parts between major services should be achievable, with perhaps two light regrinds in between.