Cutting tool challenges such as the ability to avoid chatter, deflection and minimize runout are equally determined by the rigidity, concentricity and balance of the toolholder. There are numerous choices for toolholding, and no one set answer for all applications. Among these are collet style, those that use a (arbor) set-screw, hydraulic holders and of course thermal shrink style.

When considering HSM, balanced tooling is a must, and the highest amount of natural balance possible only serves to help the overall cause. Collet chucks are acceptable, but only if they are precision ground to avoid natural run-out.

For example, when looking at “D Type” collets (precision ground all over), you get completely static clamping, meaning it will not expand at high rpm. In contrast, if you were to use ER collets, there is a larger natural runout and as rpm rises, your clamping will actually decrease as centrifugal forces cause the collet to naturally open slightly.Hydraulic chucks offer excellent transmittable torque and near perfect concentricity, so this style is an ideal choice with only one drawback—the units can be relatively bulky and can limit practical use in certain applications.

Among all of the toolholding choices, thermal shrink fit stands alone above the rest as the most accurate and rigid, making it the best choice for max rpm HSM applications. Shrink-fit systems consist of a machine with a thermal induction coil and a choice of cooling station configurations, and a range of special holders that can shrink and ‘unshrink’ a virtually unlimited number of times.

The process focuses heat to the nose of the holder to expand the inside diameter of the holder and allow the cutting tool of choice to slip inside. As the nose cools, it contracts and becomes one with the tool. Balancing and presetting also can be accomplished. Unlike any other method of holding, shrink fit allows the highest torque transfer, accuracy of 3 micron at a 3X length to diameter ratio and flexibility to reach areas where other tools would simply be impractical.

There has been much debate about the different taper types, and arguments can be made for each as they all have their strengths. Judging a back-end solely on repeatability, weight and capability for high speed, would lead us to accept that HSK is the most ideally suited for HSM; however, this can change depending on shop conditions and several other factors, so all must be considered during evaluation.