The rule of thumb for high-speed metal machining tooling designs is to maximise space for chip evacuation.
by Team MT
Aluminium alloys can be machined rapidly and economically. It is their complex metallurgical structure that allows one to do so, besides the fact that their machining characteristics are superior to those of pure aluminium. Machining of metals and alloys requires the microconstituents to be non-abrasive as they have a beneficial effect.
There are several concerns to machining of metals. The primary tooling concerns when machining aluminum are: minimising the tendency of aluminum to stick to the tool cutting edges; ensuring there is good chip evacuation from the cutting edge; and ensuring the core strength of the tool is sufﬁcient to withstand the cutting forces without breaking.
Similarly there are three elements in tool design that interrelate to minimise these concerns. They are materials, coatings and geometry. If these three elements do not work together, successful high-speed milling is not possible. It is imperative to understand all three of these elements in order to be successful in the high-speed machining of aluminium.
Similarly, other metals such as tungsten, cobalt, platinum, uranium, etc. call for high expertise in machining. With density comparable to gold and uranium and almost twice that of lead, tungsten is a stellar choice for use in counterweights and ballast, as well as radiation shielding, ballistic penetrators, vibration-damped tooling, and sporting goods such as golf clubs. It is one metal where many applications exploit tungsten’s singular qualities and that is in the aerospace industry. This is because tungsten as a metal is compact, and where precisely-positioned concentrations of mass contribute to aircraft stability and smooth operation of flight controls and engines. The parts are typically alloys formed from powders of tungsten, nickel and iron and then sintered. These tough, stable, abrasive alloys can present machining challenges.
The SMEs and MSMEs who machine tungsten and made a conscious decision to focus on machining hard and heavy metals have built capacity to maintain both aluminum and heavy-metal machining capability.
For instance, an MSME based out of Bangalore machines hardened steel, tungsten and titanium for an assortment of aerospace parts. It has cut tungsten for Lockheed-Martin. The parts are called ballast, and the company uses them as counterweights to balance the aircraft. It has also made tungsten parts for helicopter rotor heads.
Ambrish Nasit, assistant manager, technical sales support, Jyoti CNC Automation, says, “The present day market demands higher accuracy and timely delivery with economical operation. Most customers aim for optimum cycle time, least tooling cost, yield for upgradation, reduced setup to finish job and higher machine utilisation along with accuracy, surface finish, process capability and manpower requirement. Products with automation and technological tooled-up solutions are often demanded by users in metal machining field.”
Making a go of machining heavy metals as a specialty is not a decision to be taken lightly. Those involved in the machining of such metals must remember that it dictates the choice of every major factor in the machining process. To machine tungsten alloys, for example, an organisation will need a machining centre with exceptional rigidity and high torque at low spindle speeds. Indexable inserts and other cutting tools have to have exceptionally sharp cutting edges. Workholding fixtures need to have vibration resistance and repeatability, and quick-change capability is a definite plus. Of all the choices mentioned above, the most important to consolidating commitment to heavy metal machining is acquiring a horizontal machine centre (HMC) specifically suited to this task. The high-end HMCs offer rigidity, high torque at low spindle speeds, high-pressure coolant delivery and other features.
For instance, most machines from Jyoti’s product basket are technological advanced and come with innovation aligning with market and future needs thus providing the best solution for metal cutting. With multi-tasking machining already a buzz in the world of metal cutting, Jyoti CNC offers high tech models like Tachyon, TS 120, ATM 160, K3X8 Five, KX Series, KXG, MU Tech, MX Series and MTX.
In general, high hardness will lead to poor machinability, however many other factors affect machinability. Factors affecting machinability include tool material, feeds, speeds, cutting fluids, rigidity of the tool holding device, and the microstructure, grain size, heat treat condition, chemical composition, fabrication methods, hardness, yield and tensile
strength of the work piece.
Among the many different metals available, some are easier to machine than others. Many years ago, a system was developed to rate the relative ease or difficulty of machining various metals. These ratings are called “Machinability Ratings” (MR) and they provide a starting point for understanding the severity of a metalworking operation.
Machinability ratings are used when selecting tool material, feed rates, coolants, cutting oils, and machine speeds for metal cutting and grinding operations. The machinability rating of a metal takes the normal cutting speed, surface finish and tool life attained into consideration. These factors are weighted and combined to arrive at a final machinability rating. Machinability ratings are “relative” ratings. Another point here is that pure metals tend to adhere to the working surfaces of the cutting tool and give high friction and high tool-wear rates. For example, when pure iron is machined, cutting forces and tool wear rates are almost as high as those experienced when some of the very tough alloy steels are machined. Nasit says, “Machine selection is the prime important for economical machining which lead to optimised utilisation. Reducing non-cutting parasitic time is big challenge to get economical operation. Automation with productivity option, TPM compliance and efficient application solution can do wonder for cost-conscious metal machining.”
To provide desired properties in metals, they are sometimes put through a series of heating and cooling operations when in the solid state. A material may be treated to reduce brittleness, remove stress, to obtain ductility or toughness, to increase strength, to obtain a definite microstructure, to change hardness or to make other changes that affect machinability. Nasit adds, “With allied technology advancement high speed machining (HSM), 5-axis contouring capability and turnmill multi-tasking solution are for complex shape machining. For hexagonal shape part producing polygon turning is suitable. The combination of relative revolution of component and tool revolution makes it possible to have polygon cutting. Special tooling and speed ratio can give different combination of polygon on cylindrical surfaces.”
Chemical composition of a metal is a major factor in determining its machinability. The effects of composition though, are not always clear, because the elements that make up an alloy metal, work both singly and collectively. Certain generalisations about chemical composition of steels in relation to machinability can be made, but nonferrous alloys are too numerous and varied to permit such generalisations.
Machining of metals has become advanced and more machine shops are looking at making a business of it. With CNC machine makers launching specific models for this sector, the business of machining metals has certainly picked up in recent years.