Building and setting the die

A die is a stack of plates, blocks and punches held in a die set and clamped into a press, and it works only if every height in the stack is right. The punches must enter the die openings by the right amount, the pilots must enter the strip before the punches touch it, the stripper must meet the strip first, and the press must close the die to exactly the height it was built for. Setting a die is the process of making all of that true in a particular press, safely, and proving it with the first parts.

This chapter describes the die set and the materials of MP-40’s die, builds up its shut height from its plates, sets the die in a press step by step, inspects the first parts, and ends with a die set to a shut height it had lost to regrinding.

The die set

A die set is a pair of heavy plates, the upper and lower die shoes, joined by hardened guide pillars in the lower shoe running in bushes in the upper. Its job is to keep the punches aligned with the die openings to within a fraction of the clearance, stroke after stroke, independently of the press’s own guidance. The lower shoe is clamped to the press’s bolster and the upper shoe to the ram.

Pillars and bushes come in two kinds: plain bushes, with a sliding fit and lubrication, and ball-bearing bushes, with a cage of balls running on the pillar with no clearance at all. Ball-bearing sets give the closest alignment and suit small clearances and high stroke rates; plain sets carry side loads better. MP-40’s 0.15 mm clearance and 120 strokes a minute suit a ball-bearing set with four pillars, which also resists the twisting a single off-centre load would cause (Chapter 7).

The cutting parts are mounted on the shoes:

• the die block, or die inserts, screwed and dowelled to the lower shoe;
• the punch plate, holding the punches by their heads, with a hardened backing plate behind it so that the punch heads do not sink into the softer upper shoe;
• the spring stripper, hung from the upper shoe on stripper bolts, with its springs in pockets (Chapter 5);
• stock guides on the die block to steer the strip, and a stop or the pilots to set its position.

Screws hold parts together; dowels locate them. A die plate held only by screws can shift under the cutting load, and the clearance moves with it.

Steels and hardening

The cutting parts are made of hardened tool steel, chosen for the balance between wear resistance, which keeps edges sharp, and toughness, which stops them chipping. The Library’s chapter on choosing tool steels sets out the families and how they are matched to blanking dies, and the steel supplier’s data and the heat treater’s advice govern the grade, hardness and treatment for a specific die.

Hodgson’s 1903 book describes a very different practice, where dies of carbon steel were hardened by quenching and then tempered by reheating until the surface showed a particular oxide colour, a straw colour for the lower die of tools cutting thin brass and tin, with a table of colours for different tools. The principle is the same today: hardening makes the steel hard and brittle, and tempering trades a little hardness for toughness. What has changed is the control. Modern tool steels are hardened in controlled atmospheres or vacuum to specified temperatures and tempered, often more than once, to a specified hardness, which is then checked with a hardness tester rather than judged by colour.

Two design habits protect a die in hardening: avoiding sharp internal corners and abrupt changes of section, which crack, and leaving enough material around openings near the edge of a block, which otherwise distorts. MP-40’s die openings are well inside a 32 mm block, and its corners are radiused.

MP-40’s shut height

The shut height of a die is its overall height when closed at the bottom of the stroke, from the underside of the lower shoe to the top of the upper shoe. The press must be able to close to that height. For MP-40, as an illustration, the stack from the bottom is:

• lower die shoe: 50 mm, so the die block sits on a face 50 mm up;
• die block: 32 mm, so the die face, where the strip sits, is at 82 mm;
• punch entry into the die at the bottom of the stroke: 1.0 mm, so the punch tips are at 81 mm;
• punch length, from tip to head, flush with the top of the punch plate: 63 mm, so the top of the punch plate is at 144 mm;
• backing plate: 10 mm, to 154 mm;
• upper die shoe: 50 mm, to 204 mm.

The die’s shut height is 204 mm. The stripper, its springs and the stock guides fit within the stack and do not add to it, but they must be checked for room: with the die closed, the stripper sits on the strip at the die face and its springs are compressed by 9.5 mm (Chapter 5).

The open height is the shut height plus the press’s stroke: with a 60 mm stroke, the die opens to 264 mm, and the punch tips rise 59 mm clear of the die face. The strip line, the height of the die face above the bolster, is 82 mm, and the feeder is set to that.

Figure 1MP-40's die stack at the bottom of the stroke

Part	Thickness or length, mm	Height of its top, mm
Lower die shoe	50	50
Die block, die face at its top	32	82
Punch tips, 1.0 mm into the die	entry 1.0	81
Punches, flush with the punch plate	63	144
Backing plate	10	154
Upper die shoe, the shut height	50	204

Pilots before punches

The pilots must enter the holes and pull the strip into position before the stripper clamps it and before any punch touches it. So the pilots project beyond the stripper face, and the stripper beyond the punches.

This is an illustration. In MP-40’s die, the stripper face stands 1.0 mm below the punch tips at rest (Chapter 5), and the pilot noses project 3.0 mm below the stripper face. Coming down, the pilots enter the strip’s holes 3.0 mm before the stripper meets the strip, and 4.0 mm before the punches reach it. If the feeder is still gripping the strip as the pilots enter, the pilots cannot move it, so the feeder must release the strip, or run its rolls loose, for the moment the pilots enter. The companion book deals with that timing at the press.

Setting the die

Die setting puts hands and tools inside the press, and it is where many press injuries happen. WorkSafe Victoria’s guidance on power presses calls for chocks to stop the ram falling during maintenance, inching controls for setting and cleaning, interlocks and isolation, and several test runs before production; the press’s own manual and the shop’s procedures govern the details. The sequence below is an illustration of the order of work for MP-40’s die, not a safe system of work for any press.

1. Isolate and block. Stop the press at the top of the stroke, isolate its energy sources, and fit a safety block between the ram and the bolster before anyone reaches in.
2. Clean and check. Clean the bolster and ram faces and the die’s shoes, because a chip under a shoe tilts the die. Check the die’s tag for its current shut height.
3. Raise the ram. Set the press’s shut height adjustment above the die’s shut height, so that the die cannot be closed solid on the first stroke.
4. Place the die. Slide the die into position with its centre-of-load mark on the ram’s centreline (Chapter 7) and clamp the lower shoe lightly.
5. Bring the ram down. Remove the safety block, and inch the ram down to the bottom of its stroke. Adjust the shut height down until the ram face just meets the upper shoe, then clamp the upper shoe to the ram.
6. Clamp and recheck. Clamp the lower shoe fully, inch through a full stroke and check that nothing binds.
7. Set the entry. With a strip in the die, inch strokes and reduce the shut height in small steps until the punches cut cleanly and push the slugs and blank through, then stop; the target is 1.0 mm of entry, not more.
8. Set the feed and pilots. Set the feed length to the 52 mm pitch and the pilot release, and run single strokes until the strip has filled the die.
9. First-off inspection. Measure the first finished plates before running continuously.

Figure 2Order of work in setting MP-40's die

1. 1Isolate and blockpress stopped at top, energy isolated, safety block fitted
2. 2Clean and checkbolster, ram and shoes clean; current shut height read from the die tag
3. 3Raise the ram, place the dieadjustment above 204 mm; centre-of-load mark on the ram's centreline
4. 4Bring the ram down and clampinch down; upper shoe clamped to the ram, then the lower shoe
5. 5Set the entry in small stepsstrip in the die, inching strokes, until a clean cut at 1.0 mm entry
6. 6Set feed and pilot release52 mm pitch; single strokes until the strip fills the die
7. 7First-off inspectionbefore continuous running

Clamping

A die is clamped to the press with bolts and clamps in the bolster’s and ram’s T-slots, or with quick die-change clamps on presses that change dies often. Whatever the method, the clamps must hold the shoes down against the full cutting load’s reaction and against the sudden release at breakthrough, which tries to lift the lower shoe and drop the upper.

Clamps work best close to the shoe’s edge and close to the cutting area, with the clamp’s heel packed to the same height as the shoe so that it presses square. A clamp bearing on an overhanging part of a shoe bends the shoe; a clamp with its heel low tips and loosens. Clamping bolts are tightened to the press maker’s or clamp maker’s figures, and checked again after the first few strokes, when the shoes have bedded down. MP-40’s die uses four clamps on each shoe, two on each side, and the die-setting sheet shows where they go.

First-off inspection

The first plates from every set-up are measured against the drawing before the press runs continuously, and recorded. For MP-40, the check is:

• outline: length and width, against 79.90 to 80.10 by 49.90 to 50.10 mm;
• holes and slot: diameters and widths, against their +0.10/0 tolerances;
• positions: hole centres from the outline, against ±0.15 mm;
• burr: height at the outline and around the holes, against 0.10 mm, and which faces;
• flatness: the plate laid on a surface plate, rocking or gaps checked with feeler gauges;
• edges: the zones of the cut, read as in Chapter 1, for signs of wrong clearance or blunt edges.

The shop also keeps the last plate of each batch, labelled with its stroke count. Comparing it with the first plate of the next batch shows what the die has done between set-ups, which is where Chapter 9’s sharpening decisions come from.

The error: the shut height on the old tag

This is an illustration. After its first regrind, MP-40’s die had 0.15 mm ground off its punches and 0.15 mm off its die block face, so its closed height for 1.0 mm of entry had fallen from 204.00 to 203.70 mm. The toolmaker did not update the die’s tag, which still read 204 mm, and did not shim the punches or the die block back to their original heights.

The setter set the press to 204 mm from the tag and, in a hurry, skipped step 7. The punches now entered the die by only 1.0 − 0.30 = 0.70 mm. On the first strokes the slugs were not pushed fully clear of the die land, and at the tenth stroke a slug pulled back up on a hole punch and was struck into the next plate. The plates also showed partly attached blanks, hanging by a thread of metal at one corner, because the outline was only just breaking through.

The setter stopped the press, inched the shut height down 0.30 mm in steps until the cut was clean, and quarantined the plates made so far. The fix was procedural: after every regrind, the toolmaker either shims the punches and die block back to their original heights or records the new shut height on the die’s tag and in the die’s log, and step 7 is never skipped, because the tag is a starting point and the cut is the proof.

Figure 3Die setting checks before continuous running

• Do: Press isolated and blocked before reaching insafety block between ram and bolster
• Do: Die on its centre-of-load mark23.44 mm from the die's middle
• Do: Shut height approached from above in small stepstarget 1.0 mm entry
• Do: Pilots enter before the stripper and punches3.0 mm and 4.0 mm ahead
• Do: First plates measured against every drawing requirementoutline, holes, positions, burr, flatness
• Avoid: Shut height taken from an out-of-date tag0.70 mm entry; a slug pulled and struck

Exercise: shimming after regrinds

After three regrinds, each removing 0.15 mm from the punches and 0.15 mm from the die block face, what is the die’s closed height for 1.0 mm of entry if nothing is shimmed? What shims restore the original 204 mm, and what happens to the stripper springs in each case?

Worked answer. Each regrind removes 0.30 mm of height in all.

• After three regrinds: 204.00 − 3 × 0.30 = 203.10 mm.
• Shims of 0.45 mm under the punch plate’s punches, or under the backing plate, and 0.45 mm under the die block restore the original heights and the 204 mm shut height.

If nothing is shimmed, the press is set to 203.10 mm and the punches are 0.45 mm shorter relative to the stripper, so at the bottom of the stroke the springs compress 0.45 mm more than designed: 3,000 × 0.45 = 1.35 kN more on the press. If the punches are shimmed back, the stripper’s relationship to them is restored and the spring force is unchanged. Shimming keeps the die’s numbers as designed, which is why most shops prefer it, up to the point where the shims are thick enough to need replacing with a solid spacer.