Indexable or solid – which design concept of a rotating cutting tool is better? As in many subjects of technology, there is no absolute answer to this question. However, a definite answer does exist if the advantages and disadvantages of both concepts are considered according to specific conditions.
An assembled tool carrying removable indexable inserts, a concept that has become common in industry since the 1960s, 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.
Indexable insert T890 with complex shape
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 utilisation 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.
At the same time, an indexable tool has definite disadvantages. Firstly, accuracy is lower compared with a solid cutter. Secondly, the tool diameter cannot be relatively small (for example, less than 8mm to 10mm). 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 aluminium, which require a sharp edge, additional edge grinding needs to be performed.
The main advantage of a ground solid tool is its high precision: In average one quality grade higher than that of an indexable cutter. A solid tool cannot be indexed but it is suitable for regrinding.
Like 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 25mm in overall length. This type of tool demands significantly more tool material and it takes more time to manufacture such a tool by grinding. These constraints lead to a substantially 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, for drilling a hole of 3mm in diameter a solid drill will be used. Aside from this dimensional aspect, the following principles characterise correct tool selection.
For heavy cuts (usually rough or semi rough), featuring significant cutting force and power consumption, an indexable tool is the preferred solution. If an operation features light cuts and demands high accuracy and surface finish, a solid tool is required.
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, represented a second wind for solid tool use by opening up new options in rough machining. 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.
Drill with exchangeable carbide head (tool shown in centre)
Tool manufacturers recognised the advantages of combining both solid and indexable concepts into a single design to meet the latest developments. Iscar’s popular Multi-Master and ChamDrill tool families are representative of this beneficial combination. Both lines feature tools with exchangeable cutting heads made from solid carbide. In the Multi-Master tool range, which was introduced in 2001, a cutting head can be mounted in different bodies, and a body can carry different heads. This ‘indexable solid’ principle enables over 40 000 possible tool configurations.
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, whether solid or indexable, more and more demanding.
For further details contact Iscar South Africa on TEL: 011 997 2700 or visit www.iscar.com