Page 12 - Metalforming Magazine April 2022

Tooling by Design
By Peter Ulintz
Die Shut Height—Not Always a Single Setting
SSetting the press shut height cor- rectly 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.
When 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. of the part being formed. The practice of ‘kissing’ set blocks may prove insufficient.
Difference
Set blocks may be round, square or rec-
tangular. 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 estab- lished 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.
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His background includes tool and die making, tool engineering, process design, engineering management and advanced product development. As an educator and technical presenter, Peter speaks at PMA national seminars, regional roundtables, international confer- ences, and college and university programs. He also provides onsite training and consultations to the met- alforming industry.
Peter Ulintz
Technical Director, PMA pulintz@pma.org
Unfortunately, many die setters have been trained to set the die shut height by bring the ram down (with the die sta- tions fully loaded) until the upper die just touches or ‘kisses’ the set blocks. This is usually 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.
When 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 rep- resents 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 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
Fig. 1—The inside shut height of the die is established by adding together the form- punch height, the draw-pad height in the home position and the minimum thickness
10 MetalForming/April 2022
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