Look Ahead: Stiff competition: General Industry Coverage

Stiffness is a must in precision machining. Traditional machine tools depend on massive structures and large beds and footprints to enhance stiffness. But the massiveness that ensures predicable stiffness all the way to the end of the cutting tool also ensures that the machine itself isn’t easily moved. For example, a 5-axis machine able to drill accurate 50mm (1.97″) holes in titanium could weigh as much as 20 tons (18,184 kg) and have a 3-sq.-m (9.84-sq.-ft.) footprint.

Robot arms used in assembly and other operations are much easier to move, but lack the necessary stiffness needed for precision machining. Robot arms and machine tools traditionally work as a series of serial links between the base and the end effector, which, in the latter’s case, is the cutting tool. The more links, the less stiffness.

The goal of parallel kinematics machines (PKMs), which have been in development since the mid-1990s, is to provide the stiffness necessary for highly precise machining without as much mass as conventional machines. PKMs have parallel rather than serial links between the base and the working end. The links that control the X-, Y- and Z-axis movements of the end effector aren’t connected serially, with one stacked on the other. Rather, they each have their own connection to the end effector, which allows improved stiffness with much less mass.

Three composite-constructed, parallel-sequenced arms are separately linked to the machining head of the XMini. Image courtesy of Exechon Enterprises.

(Imagine four acrobats standing on each other’s shoulders. The one at the bottom has to be strong, but no matter how strong he or she is, the acrobat at the top isn’t going to want to thread a needle up there. Now imagine three acrobats on the ground, working together to hold the fourth one up as if they were the legs of a stool. That’s a rough approximation of serial vs. parallel kinematics.)