Page 13 - Metalforming Magazine April 2022

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.
Unfortunately, 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
compensate for timing issues inside 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 per- formance and less die damage. MF
t
A
B
.050-in. Deep Groove
X = A + B + (low limit of t) minus 0.010 in.
Set Block Height “X”
Pad
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 (1 mm) to 0.060 in. (1.5 mm). 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 condi- tion could be caused by any number of reasons 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
Fig. 2—Establishing set-block height for solder checks—calculate the inside shut height for this die by summing the pad thickness (A), die thickness (B) and the low limit of material thickness (t).
Tooling by Design
Pad
Approx. 20-30-deg. Tangent to Radius
R
Back Angle
Impact Area
Heel block not shown
Fig. 3—This figure represents a bending station with a form block (or an insert)
Rt
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designed for simple removal in the press to facilitate adjustments. After setting the shut height, forming pressure can be increased or decreased to adjust for springback by adding or removing shims under the punch.