Strong Spindle Connections: The Key to Machining Titanium

Manufacturers have several spindle interface choices when making a machine tool investment. Choosing wisely to achieve and maximize cutting-edge performance is a must on materials such as titanium that are tough to machine and require considerably higher cutting forces. 

Building today’s modern aircraft, many engineers are switching to high-strength lighter alloys like titanium for component materials to increase fuel efficiencies. Machinists are challenged to maximize metal-removal rates to achieve production efficiencies, yet face low cutting speeds and considerably higher cutting forces. Machine tool builders must also provide greater stiffness and damping in their spindles to minimize undesirable vibrations that deteriorate tool life and part quality.

When machining tough materials like titanium, cutting speeds are relatively low due to thermal effects on cutting tools. In response, machine tool builders have improved stiffness and damping on spindles and machine structures over the years. Spindles have been designed with abundant torque at low rotational speeds. Although all these advances add to greater productivity, the spindle connection often remains the weak link in the system.

OVERVIEW OF EXISTING SPINDLE CONNECTIONS
The tool-spindle interface must withstand high loads and yet maintain its rigidity. In most cases it will determine how much material can be removed on a given operation until the tool deflection is too high or the onset of chatter is reached. High-performance machining is commonly characterized by the use of high feeds and aggressive depths of cut. With ongoing advances in cutting tools, there is a need for a spindle connection that makes the best utilization of available power possible.

Several different types of spindle connection have been developed or optimized over the last few decades. Due to a good cost/benefit position, the 7/24 ISO taper became one of the most popular systems in the market. Used successfully in many applications, limitations in its accuracy and speed prevent it from growing further. Generally, the taper starts to open up around 20,000 rpm, and if a system doesn’t have any interference fits, this is the point that the taper starts to lose contact on taper face contact tools and standard V-flange tooling moving within the spindle.

The advent of face contact represented a major step forward over the standard 7/24 taper.  The combination of face contact with 7/24 solid taper provides higher accuracy in the Z-axis, but also presents some disadvantages, namely the loss in stiffness at higher speeds or high side loads. Most of these tools in the market are solid, and the spindles have relatively low clamping force. Connection stiffness is limited, as radial interference needs to be kept to a minimum. Required tolerances to achieve consistent face contact are thus very tight, leading to high manufacturing costs.

The KV™ system from Kennametal, Inc. (Latrobe, PA) was introduced in the 1980s, which was a shortened version of CV tooling with a three-ball mechanism acting on a conical surface of a bore. Later versions were designed and sold with face contact. In 1985, Kennametal and Krupp WIDIA initiated a joint program to develop a universal quick-change system, now known as KM™ and recently standardized as ISO 26622. The polygonal taper-face connection known as PSC, now also standardized as ISO 26623, and the early 90’s HSK system started being employed on machines in Europe and later became DIN 69893, then ISO 121.