Toolholders for high-power cutting

A cutting tool and its toolholder should behave as one solid, perfectly balanced unit but obviously this is never the case. They抮e two separate tools united only by clamping pressure at their interface, where the various forces of metal cutting tend to uncouple them. These forces can sometimes be strong and multi-directional especially in high-powered milling with tools. However, different experts have different definitions of what might be called high-force machining. Applications normally requiring higher gripping forces are first-operation cuts, or applications involving higher-strength workpiece materials, such as hardened workpieces and the common aerospace materials. Higher cutting forces are also found during secondary finishing operations where the end-user is trying to reduce cycle time by increasing the depth of cut, width of cut, or table feed. Especially in a cost-conscious industry like automotive, toolholders are under pressure from the higher feeds and speeds asked of them. You also have the new specialty cutting tools with a high helix that are made to remove material faster-all of this plays into how you抮e going to hold onto. Tougher workplace materials increase the forces that tend to pull helical carbide cutting tools out of the holder. This applies especially to companies that do a lot of high-helix-angle end milling with 揺xotic" materials, such as Inconel and titanium that are used in the aerospace arena. When milling these materials with tool helix angles of 45?or 60?the end mill acts like a screw, pulling itself into the workpiece and out of the toolholder. Likewise, roughing out mold steel requires exceptionally high gripping torque on the tool. But in talking about 揾igh force" situations generally, it may be better simply to focus on the size of the cutting tool and the tool pressures it experiences. End mills or drills at or above 3/4-inch (19mm) in diameter, cutting at the highest pressures. The workpiece material is not critical, because the softer the material is, the faster you can go. Thus, high forces can result even when cutting aluminum at a speed of four times as fast as that employed when cutting steel. Toolholders don抰 get much credit when they supply sufficient clamping forces under pressure but they may get the blame when a tool slips or breaks. Often the source of a problem lies elsewhere, for example with dull cutting tools, tools with poor shank tolerance, improper insertion of the tool into the toolholder, poor workpiece fixturing, or the use of an old machine with a loose spindle or inadequate power. Today's machine tools are lighter, less rigid, and have smaller main-spindle motors and axis-drive motors. This has changed the method of machining from taking maximum depths and widths of cut at lower feed rates, to taking lighter depths and smaller widths of cut at much higher feed rates. On the bright side, this often means that the clamping forces generated by the holder are much less important today than they were in the past. Still, the choice of toolholder remains important, and there's a debate about which styles provide high clamping forces along with the best overall benefits for an operation. Mechanical, shrink-fit, and even hydraulic holders offer advantages for various kinds of tough cutting situations. There's often a tradeoff between clamping force and other positive attributes, such as low runout, vibration, and cost. Consider basic end mill holders with side setscrews for engaging a flat in the tools. For secure toolholding in many situations, there's still no better holding method than this traditional, cheap, if you will, solution. But the user may sacrifice: rigidity and stability during cutting. Higher-end toolholding that relies on friction for maintaining clamping force, such as the shrink-fit approach, is popular, but there are prosnike sb