This component includes both the toolholders and the cutting tools where certain standards must be taken into account in order to be successful in HSM applications. The specified standards pertain to balance, concentricity and spindle connections.

  Balance
Every expert in the die/mold industry has his or her own definition for HSM. As a general guideline HSM can be defined at 8,000 rpms or more at 100 ipm or faster. While there are many exceptions to this definition, the basis for this explanation deals directly with an issue of balance. Balance dictates how evenly weight is distributed to prevent centrifugal forces from causing excessive vibration. Repeated machining tests have shown that balance is critical above 8,000 rpm. Unbalanced tooling above this speed will cause excessive vibration diminishing spindle life, decrease quality in surface finish and reduce tool life as much as 35 percent.

  In fact, doubling spindle speed increases the force from unbalance by four times. Tripling the speed will increase the force from unbalance by nine times. In other words, if $100,000 were spent on tooling, $35,000 would be virtually thrown away by running unbalanced tooling. For these reasons, all toolholder assemblies should be balanced to the ISO g2.5 balance specification. It is important to make sure that this specification is adhered to the individual's maximum spindle speed capacity. In other words, any holder can be declared balanced if no corresponding spindle speed is called out along with it. The question to ask is not 'Is it balanced?' but rather, 'To what speed is the holder balanced?'

  Concentricity
Concentricity and balance go hand-in-hand; however, they do not refer to the same thing. Concentricity measures how closely the toolholder aligns the tool to the centerline of the spindle. A lack of concentricity, when one tooth is taking a bigger bite than the other, is commonly referred to as runout.

  Runout at a lower speed is not critical, as the cutting load in conventional machining is so great that vibration is inconsequential. In high-speed applications, where lighter depths-of-cut are being used, runout will lead to excessive chatter and uneven tool wear. For example, most coatings range in thickness from three to six microns, or roughly a thickness of 0.0003 inch. With this in mind, it is easier to understand how even 0.0005 inch of runout can lead to premature tool failure.

  Taper Tolerances
One of the most important relationships in the HSM process is the interface between the spindle and the toolholder. Large variances in fit between the toolholder and spindle socket will lead to fretting, which is seen by the discolored ring around many toolholder shanks in shops around the world. This commonly accepted ring is a result of vibration in the spindle. While it is impossible to achieve a perfect fit where no fretting occurs, it is very feasible to reduce the degree to which it happens. Therefore, a maximum taper tolerance of AT3 is recommended. The AT tolerance simply dictates a specific tolerance to which the holder and spindle must be held within.

  The majority of machine spindles are held to an AT2 tolerance, while the majority of holder's range from AT4 to AT5 taper tolerances. An important fact to keep in mind is that AT tolerances are cumulative, thus a machine tool with a tolerance of AT2 plus a holder with an AT tolerance of AT4, at maximum material condition, can have a tolerance as large as AT6. For this reason, it is critical to have a holder system that has the tightest taper tolerance possible.                   

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