Shut height, the bolster and fitting the die

A die that is strong enough for a press, and within its energy, still has to fit in it. The press must close to the die’s height at the bottom of its stroke, open far enough at the top to feed and unload, carry the die’s footprint on its bolster and ram, and let parts and slugs fall away through the bolster where the die drops them. These are dimensions, not forces, and they are checked with a tape and a drawing. They are also the checks most often skipped, because they seem too simple to get wrong.

This chapter defines shut height, adjustment and daylight, fits both of Press 4’s dies to the candidate press, sizes a riser for MP-40’s short die that allows for a lifetime of regrinds, places the bolster’s drop-through opening, and ends with parallels that held the die up and blocked its blanks.

Shut height, adjustment and daylight

Three dimensions describe the space a press offers a die.

• Shut height is the distance from the top of the bolster to the underside of the ram at the bottom of the stroke. It is usually quoted with the ram’s adjustment fully up, which gives the greatest shut height. Adjusting the ram down reduces it.
• Adjustment is how far the ram’s screw or wedge lets its height be changed. The smallest shut height is the greatest less the adjustment.
• Daylight, or open height, is the distance from bolster to ram at the top of the stroke: the shut height plus the stroke.

A die fits if its own shut height, measured closed, falls within the press’s range with some margin at each end, and if the press’s stroke opens it enough for the work.

Press 4’s candidate has, as an illustration:

• greatest shut height 300 mm, adjustment 60 mm, so a range of 240 to 300 mm;
• stroke 60 mm, so a greatest daylight of 360 mm;
• bolster 800 × 500 mm, with T-slots and a central opening that can be machined to suit.

Fitting AB-75

AB-75’s wiping die is 265 mm high closed. That is 25 mm above the press’s minimum and 35 mm below its maximum, near the middle of the range, with room for the die to be reground or shimmed either way. Open, the upper die rises 60 mm above its closed position, which gives the loader room to place a blank in the nest and take out the formed bracket, whose upstanding leg is 50 mm high once it has sprung free of the die.

The opening is far more than the 6 mm WorkSafe Western Australia’s guidance mentions as a gap small enough to keep fingers out, so AB-75’s tool area is a danger zone on every stroke, and the press must be guarded to protect the loader (Chapter 9). A press job can only be made inherently safe by its opening when the opening is tiny, as on some coining and assembly work.

Fitting MP-40

MP-40’s progressive die is 204 mm high closed (from the companion book’s die stack). That is 36 mm below the press’s minimum shut height of 240 mm. The press cannot close down onto it: even with the ram adjusted fully down, the punches would stop 36 mm short of the die.

The usual fix is a riser, or sub-plate: a thick, flat, parallel steel plate bolted to the bolster under the die, raising the die’s effective height. Its thickness has to allow for everything that changes the die’s height over its life.

This is an illustration. The die loses 0.30 mm of height at every regrind unless it is shimmed back, and the companion book allows for 20 regrinds, a total of 6.0 mm. The press’s frame also stretches about 0.22 mm under the die’s load, which the setter takes up by adjusting the ram down a little further. And the riser should leave the ram’s adjustment some room at both ends, so that the setter is never at the end of the screw.

• With a 40 mm riser, the die stands at 244 mm new: 4 mm above the press’s minimum. After 6.0 mm of unshimmed regrinds it would be 238 mm, 2 mm below the minimum, and the press could no longer close it.
• With a 50 mm riser, the die stands at 254 mm new: 14 mm above the minimum and 46 mm below the maximum. After 6.0 mm of regrinds it is 248 mm, still 8 mm above the minimum.

The shop specifies the 50 mm riser. A riser that only just fits on the day the die is new is a riser that will need replacing when the die is half worn, usually on a day when nobody remembers why.

Figure 1MP-40's die height on Press 4's candidate, with two risers

Riser	Die height new, mm	After 6.0 mm of regrinds, mm	Margin above the 240 mm minimum, worn
None	204	198	−42, does not fit
40 mm	244	238	−2, does not fit when worn
50 mm	254	248	+8

Setting the shut height

On the day, the setter sets the ram’s adjustment so that the punches enter the die by the right amount under load, which the companion book describes in detail. Two press-side points matter here.

• Approach from above. The shut height is set high first and brought down in small steps with strokes on inching, so that the die is never closed solid by surprise. A die closed solid on a mechanical press near the bottom of the stroke meets the full force of the flywheel, and the ram can stick on the bottom.
• Allow for the frame’s stretch. The press’s frame stretches about 0.22 mm under MP-40’s load (Chapter 5), so the punches enter about 0.22 mm less under load than they do when inched through empty. Setting the entry by the cut, not by a dial on the adjustment, takes care of it.

The ram’s adjustment is locked after setting, and the setting is recorded on the job’s setting sheet, with the riser used.

The bolster and the drop-through opening

MP-40’s die blanks each plate at station 3 and drops it through the die block, the lower die shoe, the riser and the bolster into a chute below. The opening through each of them must be large enough for the plate to fall freely and must be in the right place.

The right place is not the bolster’s centre. The companion book found MP-40’s centre of load 75.44 mm from station 1, and the die is set with that point on the ram’s centreline. Station 3 is at 104 mm from station 1, so it sits 28.56 mm downstream of the press’s centre, along the feed. The drop-through opening in the riser and the bolster is centred there, not at the middle of the bolster.

Its size follows from the plate: an 80 × 50 mm blank, plus clearance for it to fall at a slight angle, suggests an opening of about 100 × 70 mm through the riser and the bolster, with the die shoe’s own opening slightly smaller, so that each opening below is larger than the one above and nothing catches on a ledge. The piercing slugs at station 1, 75.44 mm upstream of the centre, need their own smaller opening.

Figure 2Where MP-40's openings fall on the bolster

Standard heights and quick die changes

A shop that runs many dies in one press gains by making their shut heights the same. If every die for Press 4 is built, or fitted with a riser, to stand at one standard height, say 254 mm, the setter does not need to re-adjust the ram at every change: the ram is left at the standard setting, and only the final entry is fine-tuned. Standard die heights go naturally with standard clamping points and a standard feed line height, and together they turn a die change from a fitting job into a routine.

This is an illustration. MP-40’s die is changed about twelve times a year, once per batch. If standardising its height, its clamping and its feed line cut each change from about 1.5 hours to about 0.5 hours, the shop would save twelve hours of press and setter time a year on that die alone, about A$1,320 at A$110 an hour, and proportionally more across all the dies Press 4 will run. The same time saved is press time made available for other work.

Quick die change equipment goes further: hydraulic clamps that grip standard clamping edges at the push of a button, rollers or arms that slide dies in and out on the bolster, and carts that bring the next die to the press before the last one is removed. They pay where dies change often; for a press that changes a die once a shift, standard heights and clamping are usually enough.

Die cushions

Some presses have a die cushion under the bolster: a pneumatic or hydraulic cylinder that pushes up through pins passing through holes in the bolster and the lower die shoe. It provides a pressure pad’s force from below, for blank holding in drawing dies and for pads in forming dies, without springs in the die itself, and it can lift the formed part out of the die at the end of the stroke.

A cushion’s force acts on the ram for as long as the die keeps it compressed, so it adds to the die’s load exactly as Chapter 3’s pressure pad would have done, and it must be checked against the force curve and the flywheel’s energy in the same way. Its pins also need clear holes through the bolster and any riser, which is one more reason a riser is a machined plate and not a pair of parallels. Press 4’s candidate does not have a cushion, and neither of its two jobs needs one; it is listed in the specification as an option for future forming work, with its holes left blank in the bolster for now.

The ram face and its parallelism

The upper die shoe is clamped to the ram’s underside, and the ram face must be parallel to the bolster within the press maker’s tolerance, or the die’s punches enter unevenly before the frame has deflected at all. On a new press, parallelism is part of the acceptance check (Chapter 10); on an old one, it is worth measuring with a dial indicator on a parallel block at several points across the bolster, ram at the bottom, before blaming a die for an uneven burr.

The ram’s face and the bolster’s top must also be clean and flat where the shoes sit. A chip or a burr under a die shoe tilts the die by far more than a press’s frame ever will.

The error: parallels over the drop hole

This is an illustration. The 50 mm riser plate was on order when MP-40’s die first ran on Press 4. To fit the die in the meantime, the setter put it on two 50 mm parallel bars from the toolroom, one under each long edge of the lower die shoe, running along the feed.

The die ran for 30 strokes. Then the press’s tonnage monitor stopped it at its high limit. The blanks, dropping through the die shoe at station 3, had landed on the bolster between the parallels instead of falling through its opening, which the parallels did not line up with; after 30 strokes the pile had reached back up into the die. The 31st blank could not leave the die block, and the punch had struck down onto a stack of two.

The monitor had done its job: the press stopped at about 330 kN, below the frame’s rating, and the die was undamaged. The parallels had also left the die shoe supported only along its edges, with the blanking station’s load carried across a 200 mm span; the shoe was checked and found unharmed, but a thinner one could have bent.

The setter’s sheet now includes the riser by its part number, and the shop’s rule for any die that drops parts or slugs through the press is that whatever sits under it, riser or bolster, must have its openings aligned with the die’s and must support the die shoe under its cutting stations. Parallels are for dies that drop nothing.

Figure 3Fitting checks before a die is set in a press

• Do: Die height within the press's shut height range, new and fully worn254 mm new, 248 mm worn, on a 50 mm riser
• Do: Stroke opens the die enough to feed and unload60 mm for AB-75's 50 mm leg
• Do: Die footprint within the bolster and ram, clear of the frameabout 400 × 300 mm on 800 × 500 mm
• Do: Drop-through openings aligned with the die's, stepped larger downwardblanks 28.56 mm downstream of centre
• Do: Die shoe supported under its cutting stationsriser plate, not edge parallels
• Avoid: Parallels over the drop holeblanks piled back into the die in 30 strokes

Exercise: a die from another press

A die from another customer’s job is 286 mm high closed, and is expected to lose up to 4.0 mm in regrinds. It drops nothing through the press. Does it fit Press 4’s candidate without a riser, and how much adjustment margin does it leave at each end, new and fully worn?

Worked answer. The press’s range is 240 to 300 mm.

• New, at 286 mm: 46 mm above the minimum and 14 mm below the maximum.
• Fully worn, at 282 mm: 42 mm above the minimum and 18 mm below the maximum.

It fits without a riser at every stage of its life. The tighter margin is at the top when the die is new, 14 mm, which is enough for setting; a die more than about 295 mm high would leave too little room for the setter to approach the shut height from above, and would be better on a press with a larger shut height.