Optimization of Mission Critical Joints in Bolted Machine Tool Structures

In general, precision machine tools consist of a number of structural components, usually castings machined and bolted together to very tight and precise tolerances. Machine bolts are used to prevent the contact surfaces from separating or sliding relative to each other. The issues critical in the design of these precision bolted joints include tensile, compressive, and lateral stiffness, and stability of the joint under different types of load. In machine tool design, the shape and tightening force in these joints are usually evaluated based on two counteracting requirements: (1) maintain sufficient stiffness provided by the joint at the cutting (load) point, and (2) allow the joint to slip (breakaway) in the event of a machine crash; in this case, the bolted joint works as a fuse preventing damage and, thus, protect critical/expensive components in the machine and/or avoid extensive repairs. Field data from machines running production have shown that satisfaction of the two criteria, presented above, using conventional methods of bolted joint design does not always assure that the stability requirements are met. This data shows that in the range of loads that do not exceed the maximum force allowed in the machine, there might occur permanent lateral displacements in the joint. These displacements accumulate during normal operations under repeated loads and the machine looses alignment without obvious instantaneous slippage in the joint. This paper discusses an approach that gives a qualitative and numerical evaluation of the joint shape, position of the bolts, tightening force, and load that it can withstand without compromising the joint integrity while still providing an effective breakaway for the protection of critical components.