Page 63 - MetalForming April 2015

Types of Ballbearing Guide-Post Engagements
The first type of ball-cage engage- ment ensures that all ballbearings remain in constant contact with the guide post and bushing throughout the entire press stroke. In essence, the post, ball cage and bushing never dis- engage. This arrangement is ideally suited for high-speed, short-stroke dies.
The second type of engagement ensures that the ball cage never leaves the bushing; however, the guide post is designed to disengage the bush- ing at the top of each press stroke. This assembly is preferred when the elimination of pinch points is desired. An added benefit is that foreign mat- ter cannot enter the assembly and damage the components.
The third type of engagement is designed so that the component assembly totally disengages from the bushing. This type of engagement may be the only workable solution for long- stroke applications.
Heel Blocks, Post Guides or Both?
Dies that produce unbalanced form- ing or heavy side thrusts usually will utilize a boxed-heel construction. This can be done in place of, or in conjunc- tion with, guide pins.
Some engineers believe that when wear plates engage ahead of the guide pins, the wear will help bring a mis- aligned press slide into alignment before the guide pins engage the bush- ings. Here the wear plates provide ini- tial alignment and the guide pins and bushings provide the final alignment before any cutting or forming begins. Actually, this is a poor solution to a press-maintenance problem.
Other engineers prefer that guide pins engage the bushings before the wear plates make contact. The guide pins provide alignment while the plates absorb lateral forces during the work- ing portion of the die cycle. For this to work, however, the running clearance between the wear-plate surfaces must be less than that of the guide pins and bushings; otherwise the plates will not
counteract the unbalanced thrust. Instead, the guide pins will deflect under load before the wear-plate run- ning clearance is consumed. Proper design, construction and maintenance of this type of guide system are required for it to work effectively.
A primary advantage of guide-post- and-bushing over boxed-heel plate construction is that the pins and bush-
ings maintain pre-engineered running clearances with replaceable commer- cial components. When a self-lubri- cating bushing wears beyond its useful life, it can be replaced with a new one that works right out of the box. Wear plates, on the other hand, must be shimmed or ground to suit the appli- cation. In effect, they become custom components. MF
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