Hold fast: General Industry Coverage

High-speed machining is an amorphous term, but can generally mean anything from 10,000 rpm to, at least theoretically, 500,000 rpm or higher. At high speeds, even the slightest toolholder movement or runout can drastically reduce tool life, spindle life, surface finish and part quality.

There is no one correct HSM toolholder solution; essentially, one that is balanced or balanceable and offers sufficient clamping force and minimal runout can do the job. Four of the most common options are shrink-fit holders, collet chucks, hydraulic chucks and endmill adapters.

Shrink-fit plays a prominent role in HSM because it has virtually no moving parts, making it the easiest holder to balance, noted Jack Burley, vice president of sales and engineering at toolmaker BIG Kaiser Precision Tooling Inc., Hoffman Estates, Ill.

Rego-Fix powRgrip collet chucks are designed for HSM. Image courtesy Rego-Fix Tool.

“If you make a shrink-fit toolholder precisely concentric, do a lot of profile grinding on the outside, perform the necessary work on the inside and accommodate for any symmetrical imbalances—such as drive keys for HSK or chip holes—you can have a very repeatable product for balancing,” he said.

Justin London, sales engineer for Briney Tooling Systems, Bad Axe, Mich., agreed that shrink-fit toolholders are the first choice for HSM applications. “If you were to take an imaginary centerline and split the toolholder down the middle from end to end, what you would want to see is symmetry on both sides so that balance adjustments will be minimal,” he said.

However, a shrink-fit system is not without its drawbacks—namely, it is expensive and tool changes can be cumbersome. Some shops cannot justify the up-front cost of the toolholders and the induction-heating system, London said, while others want to change tools in process, something shrink-fit toolholders are ill-equipped for.

In addition, shrink-fit holders have limited contraction after being heated, added Burley, so the tool and toolholder-bore tolerances determine the amount of interference—essentially, the amount of clamping force.

“If you have a toolholder on the high side of the hole tolerance while the tool itself is made to the low side of the tolerance, you will have the least amount of interference when you put the tool in,” he said. “When you get into high speeds, you’re competing with centrifugal force trying to force the bore to expand. When the bore gets bigger, you have even less interference, and that can cause tool slippage or pullout.”

Shrink-fit toolholders are often the first choice for high-speed operations. Image courtesy Briney Tooling Systems.

Many toolholder manufacturers compensate for this with thicker outer walls, reducing the influence of tolerances on overall clamping effectiveness, Burley noted.

When performing HSM, one alternative to a shrink-fit holder is an ER collet chuck.