Bahadır Coşkun 030070008 (7th Week)

Hard Turning (25.03.2011 01:23)

The clear attraction to hard turning (Figure 1.8) is the possibility of eliminating grinding operations. However, for many shops, the process of repeatedly turning parts that are harder than 45 HRC to grinding-level accuracies is still unclear. Moreover, the economics of such a process is not well understood as efficiency of the process and cost per unit depend on many parameters, varying from one shop to another. A properly “dialed-in” hard-turning process can deliver surface finish of Ra 0.4 – 0.8 μm, roundness of 2–5 μm, and diameter tolerance of ±3–7 μm. Such performance can be achieved on the same machine that “soft” turns the part prior to hardening, maximizing equipment utilization. However, some shops misstep by initially using the wrong (that is, less expensive) tool insert for the application. Others may not be sure if their machine possesses the rigidity to handle the highly dynamic thrust component of the cutting force that can be twice that of a typical turning operation.

Though a material of hardness 47 HRC is hard turning’s starting point, hard turning is regularly performed on parts of hardness 60 HRC and even higher. Commonly hard-turned materials include tool, bearing, and case-hardened steels. Although Inconel, Hastelloy, Stellite, and other exotic materials are often considered as falling in the category of hard turning [34], it is not correct as their hardness is much less than 47 HRC and thus the mechanism of chip formation and process requirements including tool materials are considerably different.

(Davim J. P.,Machining of Hard Materials, 2011, p. 15)

Hard Boring (25.03.2011 01:23)

Boring, also called internal turning, and reaming are used to increase the inside diameter of an existing hole. The original hole is made with a drill, or it may be a cored hole in a casting. Boring and reaming achieve three prime objectives: (a) sizing – boring and reaming bring the hole to the proper diametric accuracy with a tight tolerance while achieving the required surface finish; (b) straightness – boring and reaming straighten the original drilled or cored hole; (c) concentricity – boring and reaming make the hole concentric with the outside diameter within the limits of the accuracy of the workholding fixture. This unique set of objectives is not normally achieved in grinding so it is logical to use hard boring and/or reaming as the finishing operation after the work material has been hardened.

If distortion and size variation as a result of heat treatment places unreasonable constraints on the “soft stage” machining, increasing its tooling cost, hard boring will provide a cost-effective, scrap-reducing alternative. The greater the distortion (due to part asymmetry, for example) and the length-to-diameter ratio, the greater advantages of hard boring.

(Davim J. P.,Machining of Hard Materials, 2011, p. 16-17)

Hard Milling (25.03.2011 01:23)

Throughout the last few years, hard milling has captured the attention of manufacturers around the world. These manufacturers are typically focused on the mold and die industry where materials such as P20, H13, W5, S7, and others are commonly cut. Traditionally, core and cavities from these materials are manufactured in the hardened state using electrical-discharge machining. Through the years, new technologies have been developed where these materials can be, in most cases, machined directly into hardened material using new toolpath processing techniques to form hard milling. These materials can range from 45 HRC to as hard as 64 HRC. Advanced moldmakers have realized that adopting new technology can be one of their keys to survival against global competition.

Digital drives that can handle fast acceleration/deceleration provide good contouring accuracy while helping to minimize cutting-tool wear. Spindles should provide flexibility, offering high torque at low speeds and high power over a large speed range.

Mold shops use three general types of hard milling tools: solid carbide endmills, indexable carbide inserts, and, most recently, ceramic indexable inserts. Each of these tools has its strengths and weaknesses depending upon the application. Solid carbide endmills are usually precision ground, coated, and quite expensive. The second type of hard milling tool is a cutter with indexable carbide inserts. In most cases the carbide grades and geometry of these inserts are not designed well for hard milling, and they do not offer optimal tool life or productivity in hardened materials.

The third type is ceramic indexable inserts, more specifically, whisker-reinforced ceramic inserts. The benefits of using a system of cutters with indexable ceramic inserts include faster cycle times and a reduced number of operations per part. A full line of cutters for hard milling with whisker-reinforced ceramics enables a shop to rough out a part from a solid hardened block – including face milling, pocketing and profiling with indexable inserts – and finish it in one setup.

(Davim J. P.,Machining of Hard Materials, 2011, p. 17-18)

Sintered Carbide (Hardmetal) (25.03.2011 01:23)

Sintered carbide tools, also known as hardmetal tools or cemented carbide tools are made by a mixture of tungsten carbide micrograins with cobalt at high temperature and pressure. Tantalum, titanium or vanadium carbides can be also mixed in small proportions.

Therefore two main description factors define a hardmetal grade:

• The ratio of tungsten carbide and cobalt. The latter usually ranges from 6 to 12 % and it acts as binder. Cobalt has a high melting point (1493 °C) and forms a soluble phase with tungsten carbide grains at 1275 °C which helps to reduce porosity.
• The grain size, thus micrograin grades include particles smaller than 1 ìm, and submicrograin are smaller than a half of a micron; the smaller the grain, the harder the hardmetal. Hardness increases with the reduction in binder content and tungsten carbide grain size, and vice versa, with values from 600 to 2100 HV.

Hardmetal tools are manufactured in two forms:

• Integral tools: they are manufactured by grinding a raw hardmetal rod, obtaining an endmill, a ball-endmill (Figure 2.3) or a drilling tool. The main advantage is the perfect balance of these rotary tools, but the main disadvantage is their high price, taking into account that only a little and very specific zone of the tool is worn by the cutting process. Several resharping of each tool are possible.
• Inserts: small pads with special geometry made with hardmetal, but they are fixed on toolholders made of steel. Turning tools and big milling discs use this configuration, which implies a rapid substitution of worn inserts.

(Davim J. P.,Machining of Hard Materials, 2011, p. 36)