Connectivity
The real subject of this section could be titled “connectivity between
structural elements” — for example between a spindle and the frame
or bed of a lathe. There are two basic methods of connection of
structural elements-kinematic design and elastically averaged design.
The features of each are as follows:
kinematic design — deterministic, less reliance on manufacturing
of the components, limit to the load capacity and stiffness.
elastically averaged design — nondeterministic, heavier reliance
on the particulars of the manufacturing process, no limit
on the load capacity and stiffness
Elastically averaged designs are referred to as over-constrained.
As such they can suffer from the problem illustrated in Figure 4.6.
They are also susceptible to warping due to uneven thermal expansion.
On the other hand, kinematic designs have concentrated loads
at points and can experience local deformation. As a general rule, it
is preferable to replace excessive surface and line contact with point
constraints. Figure 4.7 shows a kinematic three point support of the
machine tool in Figure 4.6 eliminating the problem of machine base
distortion due to uneven foundation support. While the machine may
not stay level, it will not see any distortion of its frame due to the
foundation support. We now look at kinematic design in more detail.