by Andrei Petrilin
Tips for selecting the right attributes in cutting tools
Indexable or solid – which design concept of a rotating cutting tool is better? To answer this question, consider the advantages and disadvantages of both concepts according to specific conditions.
An assembled tool carrying removable indexable inserts requires cutting capabilities only from one of its components – the insert. The cutter body acts as a holder for inserts of a specific shape produced from different hard-to-machine tool materials (for example, various cemented carbide grades, cubic boron nitride (CBN), cermet etc.), while the body itself is made mainly from steel.
The inserts can differ in their chip forming surface, to generate the necessary cutting geometry. Clamping the insert, which features the geometry and material suitable for cutting the workpiece, in the body results in an optimal cutting tool for the workpiece. The insert possesses several cutting edges. If one edge is worn, it is simply replaced by indexing the insert by means of rotation or reversing. The indexable principle ensures cost-beneficial utilization of the tool material.
The insert is formed by powder metallurgy technology to produce the unique shape of the chip forming surfaces, whereas obtaining this shape by other technology methods is extremely difficult or even impossible, and an exceptionally strong cutting edge capable of standing up to heavy loading.
An indexable tool has disadvantages, however. Accuracy is lower compared with a solid cutter and the tool diameter cannot be relatively small (for example, less than 8-10 mm or .315-.375 in.). Reducing the diameter leads to diminishing the size of all assembly components, including the insert and its clamping elements (usually a screw), which have a natural dimensional barrier. In addition, the insert cutting edge is strong, but not as sharp as that of a solid tool. For machining soft materials, like copper, commercially pure titanium or aluminum, which require a sharp edge, additional edge grinding needs to be performed.
The main advantage of a ground solid tool is its high precision: on average, one quality grade higher than that of an indexable cutter. A solid tool cannot be indexed, but it is suitable for regrinding.
an indexable cutter, a solid tool also has dimensional limitations that relate to the tool cost. As opposed to the indexable concept, the solid tool cannot be relatively large in diameter; usually the diameter of the solid tool does not exceed 25 mm or 1.000 in. – or in overall length. This type of tool demands more tool material and takes more time to manufacture. These constraints lead to a higher tool cost. By contrast to the indexable tool, the cutting edge of the solid tool is sharper but less strong.
The machined surface dimensions may dictate which concept should be applied to an operation. For example, a solid drill is used for drilling a hole of 3 mm (0.12 in.) in diameter. Aside from this dimensional aspect, the following principles characterize correct tool selection:
- for heavy cuts (usually rough or semi rough), featuring significant cutting force and power consumption, use an indexable tool
- for light cuts, high accuracy and surface finish requirements, use a solid tool
The past few years have seen a dramatic change in this logical and traditional concept. The search for new solutions to improve productivity, combined with advances in machine tool engineering has engendered efficient cutting strategies and appropriate machines. A significant number of modern machines have less power but far higher speed drives and advanced computer numerical control units for high speed machining, performed by a small diameter tool moving at optimal trajectory for constant tool loading. This step, together with progress in regrinding and recoating technologies, opens up new options in rough machining for solid tools.
Advances in tool materials have increased the hardness level of machine workpieces. Today, for example, solid carbide endmills, operated by high speed milling technique, are capable of successfully cutting hard steel up to HRC 65.
Tool manufacturers recognize the advantages of combining both solid and indexable concepts into a single design to meet the latest developments.
So which concept is better? The industry requires both types of cutting tool, depending on technology processes. The ratio of indexable tools to solid and “indexable solid” tools in today’s market is estimated at 1:1, which indicates how cutting tool development is progressing in both directions. But technology advances and improvements in processing will make tool requirements more and more demanding. SMT
Andrei Petrilin is a technical manager at Iscar Tools.
