STAT engineering design

Press Killers: Off center loading and snap-through shock

Press design in recent years has improved significantly. It was not uncommon in older presses to only be tolerant of 10% reverse loading (snap through shock), and not at all tolerant of off center loads.

It is now common that new presses are built with long slides, guided with zero-clearance rollers and high pressure lubrication. They are often also equiped with hydraulic shock dampers that can reduce snap-through shock by 80% or more.

Nonetheless, off-center loading and snap-through shock can be greatly mitigated by good die design, and it should be because even a brand new robust press will remain precise for much longer if the dies are designed, built and maintained properly.

Blanking, notching, and piercing operations, result in reverse tonnage forces, otherwise known as snap-through shock. Sometimes these forces can be quit high and easily exceeding what the press was built to tolerate. This shock force does not dissapear at once, but creates oscillating forces that gradually dissapate. In a high speed press operation, these oscillations can overlap the following stroke which creates further damaging oscillations at even greater magnitudes ( see Aida article on this subject.)

Whether a die is going into a press with reverse tonnage damping or not, the issue of snap-through shock should be addressed at the design stage. A designer will typically calculate the tonnage requirements of a die, and if a die is going to require, say, 100 Tons of blanking pressure, a snap-through shock approaching 50 Tons could be anticipated, and this is excessive for any press. First of all, every press should have or be retrofitted with hydraulic damping or some other method to arrest this damaging release of energy. However these forces can be managed completely in the die itself.

Every designer knows, or should know that staggering the cutting punches by about a third of the stock thickness can reduce snap-through shock by a considerable amount. By spreading out the work, the amount of work done is spread out over time, and the resulting shock energy release is diminished in magnitude in the same manner. Besides timing adjustments such as staggering the punches, if punch shear is properly applied to the punch or die face, the shock is reduced further, even though the required cutting pressure is increase by the amount of bending done to the slug or part. Punch shear and staggered punch timing should be standard practice on every die, however, there is still substantial amount of snap through shock left to deal with. The remaining uncompensated for energy release should be arrested with hydraulic or elastomeric dampers. Elastomer or urethane dampers are very effective and less expensive than hydraulic dampers for in-die use. Hydraulic dampers are better used as permanent items mounted to the press bolster. However, elastomer dampers can also be placed in such a way as to help balance the die by adding load strategically for that purpose.

OFF CENTER LOADS

Progressive dies and transfer dies typically do most of the cutting operations on the incoming end of the die and most of the forming operations on the outgoing side of the die. This means that most dies of this sort are overloaded on one end or the other, unless steps have been taken to balance the die. Cutting forces are fairly straight forward to calculate but bending and forming forces are not so easily determined. There are bending/forming formulas that give us a place to start but these formulas do not take into consideration, coining or other features that are often used to compensate for spring back. The only way to get a decent estimate of pressures required is to do a computer simulation or to wait until you tryout the die and discover what the tonnage meter reveals.

The designer does calculate the pressures required to make the part so that stripping pressures can be calculated, etc. It is a good idea at that time to also calculate the load center location. Many press manufacturers are now boasting that their presses are virtually immune to off-center loading but I don't care how they do it, off-center loading to any significant degree is going to created stresses in the slide guidance components that will, sooner or later, become a cause for maintenance or repair. An off-center load means that the ram is being subject to a tipping force, and so is the die. If this tipping force is not completely compensated for, the price will be paid in reduced punch/die life, lower part quality and higher die/press maintenance.

A well designed die will be able to make a part to print, which is the primary concern, but also be able to continue making parts to print by not presenting the press with work to do that it was not designed for. You cannot rely on guide pins/bushings to compensate for a sloppy, misaligned press and you cannot expect a press to endlessly stand up to off-center loads and snap-through shock without failure. If at die tryout time, the tonnage meter shows a variance of more than 0.5% imbalance, the die should be modified to compensate for this imbalance before it goes into production. And don't forget, sometimes such adjustments can be accomplished by timing the cutting punches differently - an easy and economic solution - but then it is also necessary that the die maintenance team maintain this timing, but that is another problem.