Shrink-fit toolholding is a simple concept—an induction coil is adjusted and fits over the top of the toolholder. The induction coil heats the toolholder end of the shrinker, expanding the inside diameter, which opens the engagement bore (IDs of shrink-fit toolholders are smaller than the shank diameter of the cutting tool). Then the tool shank slips into the heated toolholder. Once the tool cools, the toolholder shrinks around the actual tool and there is uniform pressure and concentric gripping strength around the surface of the tool shank.
“The advantage of shrink clamping is not only is it accurate and quick, it is consistent,” explained Michael Colyer, regional sales manager for Zoller. (Ann Arbor, MI). “If you use a manual system you might get slight differences. With shrink-fit you are obtaining consistency no matter who sets it.”
Another advantage of shrink-fit toolholding is the availability of the shrink chuck. Most major manufacturers are making shrink-fit toolholders, and they are available in different configurations for many applications. Also, there is usually less chance of pullout with shrink-fit clamping since there is higher torque. Plus, there is more rigidity because there is more surface contact between the tool and the bore of the holder.
“A standard induction machine covers 80% of shrink-fit applications,” said Colyer. “For titanium machining, the wall of the holder is much thicker and a standard induction system doesn’t put enough electricity in the holder to heat it up.
When machining titanium you need more rigidity in the tool than what a standard toolholder gives you, so the wall thickness of the toolholder is increased. Zoller’s solution for that is in the stronger induction coil that will heat the holder up properly.”
Zoller sees shrink-fit used in tight-tolerance automotive, aerospace, medical and general machining applications in order to achieve minimal tool runout and higher machined part accuracies. “The automotive industry uses shrink-fit holding systems a lot because of the consistency of setting the tool,” explained Colyer. “Now the manufacturer gets the consistency and setup they are looking for across the manufacturing plants—it’s all about reducing the variables. When we can reduce the stick-out tool length as a variable, then every tool is set the same, creating consistency.”
With Zoller’s induction system, the stick-out length of the tool can be set within ±10 µm. In many induction systems, this is a manual process and may not be as accurate. Zoller offers two different models of the induction system—the powerShrink, an inductive shrink-clamping unit for HSS and carbide tools, and the redomatic, a heat-shrink and tool setting machine all in one.
Shrink-fit toolholding offers several benefits, according to Colyer. “The No. 1 benefit is the accuracy [low tool runout] obtained when shrinking the tool into the holder, and when you shrink a tool into place you get more contact between the tool and the holder, which reduces the amount of runout,” he said. “The No. 2 benefit is you are getting a higher gripping force, so the tool is less likely to pull out of the holder. There is more contact between the tool and the holder, creating a higher torque value.”
Also, the availability of shrink-fit toolholders in slim profiles is beneficial, especially for moldmaking since there is no exterior clamping unit, such as a set screw or something that must clamp the tool in place. “With a slim profile there is less material around the holder itself—you can get into tighter places without interference between the workholding part and toolholder,” explained Colyer.
The fourth benefit is a more balanced tool. “When shrink clamping is used in high-speed applications, where spindles are running more than 15,000 rpm, the toolholders themselves are coming out more balanced, so you have less strain on the machine and more accuracy in the machining application,” said Colyer.
The fifth benefit is a reduction in setup time. “With shrink clamping, it takes less than 10 seconds to shrink a tool into a holder,” said Colyer. “This is significantly less time then what it takes to manually lock a tool into a holder.”