Determining when a custom toolholder is a better option

When it comes to selecting toolholders for drills, taps, endmills, shell mills and other round cutting tools, there is virtually an unlimited number of options.

A machine’s spindle will dictate the required toolholder shank. The most common shanks are steep-taper ones, such as BT and CAT, and short-taper HSK shanks, which typically perform better in high-speed applications.

All images courtesy of T.M. Smith Tool International.

The toolholding end is where the choices begin to grow, depending on the application and the operation being performed, such as drilling, tapping or milling. The type of cutting tool clearly identifies what other toolholders a user will need. For example, when tapping, a user can choose whether a rigid holder or a compensating (tension/compression, radial floating) holder is right for the application. This choice, in turn, determines the type of holder and, possibly, the adapters that will be required.

Frequently, machine shops must get creative with toolholder selection because of the part design or fixturing challenges.

One Choice Stacks Up

Issues like a cutting tool’s ability to access the workpiece and tool reach can lead operators to rethink toolholder selection. As a result, they may consider custom holders instead of the standard holders they have on the shop floor.

It has become commonplace to “stack up,” or combine, standard holders in an assembly that enables a cutting tool to get where it needs to be. Figure 1 shows a single-piece toolholder next to an unassembled and assembled two-piece toolholder.

Stacking makes sense, particularly because a range of suitable standards are often on hand. The practice involves combining different types of holders, and possibly extensions, to replicate what would otherwise be accomplished with a dedicated, or special, toolholder.

Figure 1. A single-piece toolholder (left) next to an unassembled (center) and an assembled two-piece toolholder that performs similarly.

Consider a long-reach drilling application. Figure 2 shows the choice between a single-piece holder and a typical stack-up holder. The single-piece holder is an HSK 100A, 12mm (0.472″) shrink-fit holder with a 360mm (14.173″) projection. A holder like this features a custom, extended length with a slim-nose design to maximize rigidity. It is dual-plane-balanced to a G2.5 tolerance at the customer-specified rpm to reduce vibration and chatter. The holder has balance screws in the nose that allow the user to finely balance the holder after installing the cutting tool. Users can expect a tight tolerance of 0.0001″ (3µm) TIR from a custom holder like this.

The assembly on the bottom of Figure 2 is comprised of two holders: a standard HSK 100A 1.25″ (31.75mm) endmill holder and a 1.25″ straight-shank collet extension. Both the holder and extension in this assembly meet industry-standard tolerances. The holder has a tolerance of 0.0005″ (12.5µm) TIR, and the collet extension has a tolerance of 0.0002″ (5µm) TIR. The connections are reliable and meet the required projection and nose configuration.

With both items being readily available from many sources, the stack-up holder can be assembled on an as-needed basis without having to order and wait for a special to be delivered. The lead time for a special can be several weeks.

Stack-Up Downsides

Despite saving time and money, there are some downsides to stack-ups that are worth mentioning. As previously noted, the custom holder has a single tolerance of only 0.0001″ TIR. Keep in mind that any time two or more holders are combined, tolerance stack-up occurs. In the example with the TIR of 0.0002″ and 0.0005″ for the holder and collet extension, respectively, the tolerance stack-up causes more total runout, possibly beyond an acceptable amount.

Another critical issue that comes into play along with runout variance is rigidity. Tool and holder rigidity impact repeatability, part-surface finish and cutting tool life. Surface finish is usually driven by the part’s print requirement.

Poor tool life is the hidden enemy of productivity and can be a profit killer. Too often, tool life is ignored. High tool wear is simply considered “the way it is” to make the part. Skilled operators and toolholder and cutting tool suppliers often uncover root problems and propose options that can lead to dramatic improvements in performance.