Watching for Weak Links When Integrating a New Machine (Part 3 of 3)

When purchasing a machine, many manufacturers will select a spindle interface based upon either what they are already using in their operations or whatever will result in the shortest possible delivery. At first glance, establishing a standard interface within the facility seems to provide a benefit through the consistency it offers. However, this is not always the case. As a general rule, when demanding applications are being run, the convenience of standardization quickly takes a back seat to overall capabilities. When working with difficult materials, long overhangs, or aggressive metal removal rates, attention should be focused on pull force and bending stiffness.

The most common spindle interfaces are the conventional ISO 7/24 steep taper (CAT, BT, DIN with or without flange contact surface), HSK, and Capto. Each of these is an ISO standard. The 7/24 options feature greater tapers than are found on the HSK and Capto. Due to the mechanical properties of the holders, the shallower the taper, the more it will lock itself into place and resist pull force. The old Morse taper provides a perfect example of this, as it was completely self-locking. Additionally, the 7/24 interfaces make use of a pull stud, which has a relatively small diameter thread. Compared to the other two options, the 7/24 interfaces offer weaker resistance to pull forces, despite appearing strong in a visual inspection.

The HSK spindle interface offers a shallower taper that increases clamping forces. It also integrates a draw bar instead of a pull stud. This offers a substantial improvement over the 7/24 options. The Capto system also uses a draw bar, but further increases resistance to pull forces by implementing a tapered polygon design. By using a polygon instead of a circle, torque is transmitted via the entire toolholder instead of just the drive key. The chart on this page shows the maximum pull forces that the various spindle interfaces can withstand.

In addition to resistance to pull force, bending stiffness should be considered when selecting a spindle interface. This is especially vital for aerospace manufacturers working with applications that require long tools. When looking at the 7/24 options, the 7/24 taper with flange contact surface clearly provides increased interface stiffness due to the additional contact between the interface and toolholder.

The hollow taper design of Capto and HSK utilizes a draw bar that allows for a greater pull force. The total amount of pull force applied depends upon the draw bar mechanism and the cross-sectional area of the coupling itself. In the latter of these, Capto provides an advantage in clamping force through its strong cross section. This edge is further bolstered through its utilization of unique gas spring clamping, a system that provides superior clamping forces and cycle life when compared to traditional mechanical belville washers. The chart on the following page demonstrates the results of a study conducted by Aachen University in Germany, demonstrating the bending behavior of 7/24, HSK, and Capto interfaces under a range of cutting conditions.

Selecting a spindle interface with the proper capabilities in terms of pull force and bending stiffness will have a dramatic effect on an aerospace manufacturer’s operations. Increasing tool stability allows for more aggressive machining, improving productivity and profitability. It also reduces vibration, achieving increased part quality and process security.

Looking back at this column and the two preceding it, the topics were selected based on two factors. First, they are frequently lost in the shuffle when a manufacturer is evaluating the myriad of options on a new machine tool purchase. Second, if the wrong option is initially chosen in one of these areas, correcting it can range from being very costly to practically impossible. As a result, these aspects of the machine are well-deserving of consideration up front.

While these columns have attempted to shed light on a few key machine capabilities that are frequently overlooked, there is an even more important message to take away. With the incredible pace of today’s technological development, better means of production are being created on what seems like a daily basis. In such an environment, information is vital to making the best possible machine tool investment. This can only be accomplished by identifying the parties with the most relevant expertise and partnering with them throughout the purchasing process.

From the machine tool builder to the cutting tool manufacturer, the software developer, and the producer of automation components, each of the suppliers a shop works with possesses unique knowledge that can contribute to a manufacturer’s bottom line. By working closely with the relevant parties, a shop can greatly increase its competitive advantage and potential for success.


Sandvik Coromant
Fair Lawn, NJ
coromant.sandvik.com/us