Questions with Michael Minton

1. What factors should be considered when selecting a spindle?

Two key components for consideration are the workpiece material and the physical size of the workpiece. Medical components are often made from difficult to machine materials. Small workpieces that do not have larger surface areas may only see a small benefit from higher spindle speeds. To realize a benefit from high-speed spindles, the workpiece needs features that allow the higher feedrates associated with high-speed spindles to be achieved.

2. What strategy should be used for determining the max spindle speed required?

It’s important to evaluate both the raw material and the finished part conditions. Determine the percentage of roughing for the overall process. Look at the number of small tools required and calculate RPM from surface speed to determine the max RPM of the tool. Typically, small tools require movements that are usually short. Therefore, to reduce cycle time and take advantage of the higher spindle speeds, the component should have features allowing the machine to achieve the desired feedrate.

3. What are the drawbacks with higher spindle speeds?

Higher spindle speed options tend to be more expensive and typically provide less torque and radial stiffness in the spindle itself. This is because higher speed spindles use smaller bearings and internal components. The higher the speed the less mass you want rotating. This reduces the amount of side load you can apply to the tool which may increase cycle time during roughing operations, negating the advantage of the higher speed spindle.

4. What other machine options should I consider for high-speed spindles?

Controlling or limiting thermal growth of the spindle is a major concern. Some high-speed spindles may provide some methodology to address spindle thermal growth. Another critical consideration is motion control, especially in high-speed machining and five-axis applications. The motion control system manages the acceleration/deceleration of the motion as it moves across the workpiece. To realize the advantages of the higher spindle speed, maintaining part integrity and surface finish, motion control capability is required. Makino’s proprietary Super Geometric Intelligence (SGI.5) is especially useful when machining complex, three-dimensional, freeform surfaces, and high-definition features typical of the medical market.

5. What solutions exist to protect my high-speed spindle investment?

The best way to protect your spindle is to understand its capabilities and how to properly utilize it within those capabilities. It’s important to have balanced tool assemblies, G2.5 or better, and to calculate the bending moment of the tool assembly to ensure you do not exceed the load capacity of the bearings. A system such as Collision SafeGuard (CSG) can protect the spindle from damage in real time. CSG uses models of the machine, workpiece, fixture and tooling in a virtual machine environment, using actual work and tool offsets entered in the control. Monitoring and looking ahead, this system will stop the machine, protecting the spindle table and other critical machine components.

High-speed spindles can offer advantages when applied properly. Extended testing over a multitude of medical component applications have found that a 20,000 RPM spindle will provide the right balance of roughing capability and the high-speed finesse required for medical manufacturing, yielding the best possible surface finish and cycle time.

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