Tools to Succeed in High-Speed Machining

Machining at higher speeds can save time and money. But like driving down the highway, increasing the speed means increasing the inherent risk. A tool holder rotating at 35,000 rpm poses more danger than the same tool holder rotating at 8,000 rpm, based on kinetic energy alone. If a high-speed operation goes awry, there isn’t much time to act, and typically the part, workholding or machine bears the brunt of the damage, not to mention potential danger to the operator. Nevertheless, with great risk comes great reward, and those who master machining at high speeds quickly reap the benefits of increased productivity. If necessity demands running at higher speeds, choose the proper tooling and cutting strategies or you may be doomed from the start. Spindle speed and balance The first step is knowing the maximum speed of your spindle. This is the absolute ceiling when it comes to speed. The next inclination is to look at the cutting tool. This means trading in high-speed steel tools for carbide, or carbide for super hard materials like PCD or CBN. Increasingly hard cutting tools largely translate to increasingly fast cutting speeds, and today’s modern cutting geometry designs have made possible previously unthinkable speeds. But many ignore the interface between the two: the tool holder. Machining speed, besides depending on the physical limitations of the machine tool and cutting tool, will largely be governed on how well balanced the tool holder is. Balance is usually the focus when talking about high-speed tool holders because it offers the greatest opportunity for improvement, but keep in mind the maximum speed will only be as fast as the “weakest link” allows. If you have a 12,000 rpm spindle, inserts that allow cutting at 9,000 rpm, but the tool holder is only balanced for 8,000 rpm, then 8,000 is the max rpm. The faster you spin, the more exaggerated mass irregularities become, and speed harmonics present in the system will become increasingly excited. This translates into vibrations which, in turn, cause poor runout, surface finish, scrapped parts and equipment damage. Symmetry is your best friend at high speeds. Shrink fit holders, hydraulic chucks and high-performance collet chucks are typically the best examples of well-balanced tool holders because, by design, they’re symmetrical. They’re also generally slim – most of the mass is concentrated near their centerline. Balancing act When it comes to fine boring tools, balancing can be a serious issue. Single-point finish boring heads are fundamentally unbalanced because of asymmetry, and special care is required to ensure they perform as expected. To say bores are tolerance-sensitive is an understatement; this unbalance can easily scrap a part. Running a boring head as fast as it can go should always be the goal, however, unbalance can be introduced to the system. An unbalanced boring head can be deceptive because it will frequently bore your diameter within spec. Yet, upon further inspection, you will find an oval-shaped bore rather than a circle-shaped one. Circularity/cylindricity is a common GD&T (geometric dimensioning and tolerancing) callout for shafts and holes, so boring head balance is crucial for those situations. At the very least, standard boring heads are usually pre-balanced for a diameter somewhat near the middle of their diameter range. If it’s a rigid, reasonably short bore, this will suffice. When higher speeds and longer boring bars are needed, consider auto-balancing heads. These heads are well-balanced throughout their diameter range; the head that automatically adjusts a counterweight as the diameter is adjusted.