Designing Tools to Avoid Failure
Tools and machine parts made from tool steels are often subjected to high stress in operation. These parts also have a certain amount of internal stress as a result of their fabrication and heat treatment. When these stresses, either singly or in combination, exceed the strength limits of the steel, cracking, breaking or warping of the part results. Many fully hardened tool steels, particularly highly alloyed types, can withstand relatively high compressive loading, but only limited tensile loading. Tool engineers should seek to minimize tensile stresses through proper design and use of support tooling so as to permit use of the highest performance die steels on crucial components. When required tooling designs must involve significant tensile stresses, then selection of a tougher tool steel with reduced wear resistance, most likely one of the shock resisting grades, is advised.
Common Errors in Tool Design
- Use of sharp corners.
- Failure to use fillets or adequate radii.
- Presence of non-uniform sections in tooling causing variation in stress distribution in service as well as variable quenching rates during hardening.
- Use of improper clearance between punch and die edges.
- Tool designs involving excessive unit stresses or overloading during operation. Tools should be redesigned to operate at a lower unit stress.
Sensitive Tooling Designs
If sharp corners and variable sections cannot be avoided in the design of a part the use of an air hardening die steel is essential for greatest safety in hardening. Cracking and/or distortion are more apt to occur on such sensitive sections when liquid quenching is employed during hardening.
Proper Tool Clearance
Tool clearance is the distance between adjacent punch and die edges. In general the press load required for a given operation decreases as clearance increases, so tools are more highly stressed with a small degree of punch and die clearance. Enlarging clearance from 5 to 10% of stock thickness usually will improve tool life. Although the finish of the sheared edges of parts may improve with small clearance, tool life will be shortened. Breakage due to misalignment may also result.
While acceptable clearance is often 10% of the stock thickness, this subject is debatable since many variables besides stock thickness influence clearance, including stock material, hardness and surface (scale condition and finish) and the required finish on the shear cut.