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A strong spindle connection is key to machining titanium

by Keith Wiggins

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 high torque at low rotational speeds.

Although all these advances add to greater productivity, the spindle connection often remains the weak link in the system. 

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.

Why restricting bending capacity is important
As mentioned earlier, 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  are designed with abundant torque at low rotational speeds. 

The spindle connection must provide torque and restrained bending capacity compatible with machine tool specifications and the requirements for higher productivity. It becomes obvious in end milling applications, where projection lengths are typically greater, the limiting factor is the spindle interface’s bending capacity. As an example, an indexable helical cutter with 250 mm (9.84 in.) projection from spindle face, 80 mm (3.15 in.) in diameter generates 4620 Nm (3407.5 ft lb) of bending moment and less than 900 Nm (663.8 ft lb) of torque. SMT

Keith Wiggins is leader, Tooling Systems Team, Global Product Management for Kennametal Inc.  |  |

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