Peter Ulintz Peter Ulintz
Technical Director

Die Shut Height—Not Always a Single Setting

March 31, 2022


Setting the press shut height correctly during the die-change process proves critical for part quality and prevention of damage to the die and the press. Set blocks usually mount inside of the die assembly to assist with fine shut-height adjustments. Most dies feature two to four set blocks, but small dies may house a single block due to limited space inside of the die.

Design Fig. 1When dies require parallels, set blocks must be positioned directly over parallels to support any excess force applied to the set blocks by the upper die. This often is referred to as ‘over-hitting the die.’ Parallels under the set blocks reduce the likelihood of a permanently bent or distorted die shoe.

Set blocks may be round, square or rectangular. The overall height may be incorporated into a single block or divided between an upper and lower block, with the setting surface completely flat or containing a machined groove.

Set blocks sometimes are referred to as stop blocks. I discourage use of the term ‘stop blocks’ as it creates a false sense of security on the shop floor—implying that accurate shut-height adjustments are not critical because the ‘stop blocks’ will maintain proper shut height. 

A Single Shut Height May Not Suffice

The inside shut height of the die shown in Fig. 1 is established by adding together the form-punch height, the draw-pad height in the home position and the minimum thickness of the part being formed. When part thickness is greater than minimum, the upper will not reach the set block when the lower pad reaches its home position, creating a difference in shut height. 

Unfortunately, many die setters have been trained to set the die shut height by bring the ram down (with the die stations fully loaded) until the upper die just touches or ‘kisses’ the set blocks. This usually is accomplished by placing a thin film of grease on the top of the set block and looking for it to transfer to the upper die half.

Design Fig. 2When running thin metals with a total thickness variance of 0.002 in., the shut-height difference may not be much of an issue. However, when running 0.100-in.-thick hot-rolled steel (per ASTM A1011), expect a thickness variance of as much as 0.016 in. In this case, bringing the ram down onto the set blocks results in significant tonnage increases, which may greatly exceed the calculated tonnage for that die. A press without a tonnage monitor provides no way to know if press capacity is being exceeded. Even for material procured to half-standard tolerance (0.008-in. variance), this still represents a considerable amount of overhitting on the die, especially when forming stainless steels and higher-strength steel grades.

Solder Check Methods

Today, many dies incorporate a solder-check feature—a machined groove in the top of the set block—to aid setup personnel in making fine shut-height adjustments. The die setter places a small length of 1⁄8-in.-dia. solder in the groove, strokes the press with all of the die stations loaded, removes the flattened solder from the set block grooves and measures its thickness. Some companies simply require the flattened solder thickness to measure a couple of thousandths inches greater than the groove depth to assure that the press does not generate false tonnage by hitting on top of the set blocks. However, this does not account for any variance in stock thickness.

Design fig 3Unfortunately, no industry standard describes how to establish the set-block height, the depth of the solder-check groove or procedural steps for making final shut-height adjustments. Fig. 2 provides one option that allows a die setter to set the die shut height relative to material thickness. 
The inside shut height for this die is the sum of the pad thickness (A), die thickness (B) and the low limit of material thickness (t). The height of the set block should be less than the calculated shut-height dimension to assure that the set blocks do not contact the upper die shoe (eliminating false-tonnage readings). This height reduction is arbitrary, but can range from 0.002 to 0.020 in.

I used 0.010 in. for the height reduction in Fig. 2, making the formula for the set block height:

A + B + (low limit of t) minus 0.010 in.

The groove depth, also arbitrary, typically ranges from 0.040 to 0.060 in. In this example I use 0.050 in.

Establishing the Shut Height

With the die stations fully loaded, place solder in the set-block grooves, stroke the press and measure the solder thickness. With the material thickness at its low limit, the flattened solder thickness should equal 0.060 in. As material thickness increases, so should the thickness of the flattened solder. For example, if material thickness measures 0.008 in. above the low limit, then the flattened solder should measure 0.068 in. with a correctly set shut height.
For dies with more than one set block, comparing solder measurements with the other set blocks can indicate an out-of-parallel condition in the die. This condition could be caused by any number of issues including improper die timing, excessive die-tipping moments, a bent or out-of-parallel ram or other press-related issues.

Springback Compensation

With a correct die shut height for the material thickness in the die, personnel have no reason to adjust the shut height until the material thickness changes. Far too often the die shut height is altered to compensate for timing issues inside of the die, such as a stencil hitting too hard or too light, or a sprung-open or overbent flange. After correctly setting the die shut height, never adjust the ram up or down to increase or decrease pressure for stencils and forms. These timing issues must be corrected by means of shimming. 

Fig. 3 shows a bending station with a form block (or an insert) designed for simple removal in the press to facilitate adjustments. Now, forming pressure can be increased or decreased to adjust springback by adding or removing shims under the punch. 

The correct die shut height and internal die timing helps to ensure quality production parts, improved press performance and less die damage. MF

Industry-Related Terms: Bending, Case, Die, Dimension, Flange, Form, Forming, Ram, Shut Height, Stroke, Surface, Thickness, Tolerance, Transfer
View Glossary of Metalforming Terms

Technologies: Tooling

Comments

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Deryl M. THU, SEP 8, 2022 @ 8:33 AM

I have a question regarding shut height during die design, how is the form-punch height & the draw-pad height determined? I understand the height of the form of the part, but how is it determined on how much material is beyond that?


Peter U. FRI, SEP 9, 2022 @ 2:38 PM

Hello Deryl, For progressive die applications I suggest the pad to be slightly higher than the draw punch. When running thin materials, this may be a few thousandths of an inch. For thicker materials, it could be 0.020���-0.030��� The objective is to be high enough so the blankholder is exerting enough pressure to control material flow before the punch contacts the sheet. Make sure the pressure pad is preloaded with enough pressure at the point of contact. You don���t want the pad set too high where the progressive die strip could catch on the edge of the draw pad (above the punch), resulting in a misfeed. Even with a single hit die or transfer die application you want to minimize the pad height. Hitting high in the press stroke results in higher contact velocity, which can lead to pad bounce.

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