Many competitive companies aim at reducing their manufacturing cost by reducing the cutting time, setting time and the rejection levels. In order to reduce the cost per component they are ready to invest in more productive machine-tools and tooling. As cost pressure increases, the requirement for high productivity that Polycrystalline Cubic Boron Nitride (PCBN) delivers becomes increasingly important. The properties of PCBN make it possible to exceed a level of performance beyond the capabilities of cemented carbides and ceramics. Polycrystalline cubic boron nitride (PCBN) tool materials represent a revolutionary advancement in cutting tool technology. For the manufacturer, the benefits are: increased productivity, longer tool life, better surface finish, and the ability to hold closer tolerances.
What is PCBN?
Polycrystalline Cubic Boron Nitride (PCBN) is a purely man-made product. It is not found naturally in any form and its unique properties of high hardness (second only to diamond), its ability to retain its hardness at elevated temperatures as well as its inertness to iron, make it an ideal cutting tool material for machining hard and abrasive ferrous workpiece materials.
PCBN consists of selected cBN grits which have been bound together using ceramic binder under high pressure, high-temperature synthesis to form a homogenous material blank. The ceramic binder in PCBN performs a similar function to cobalt in the manufacture of tungsten carbide, in that it also acts as a binder or glue to bond the hard cBN particles together.
How is PCBN made?
The manufacturing of PCBN cutting tool materials is carried out by high pressure/high-temperature synthesis. The process begins by taking soft friable hexagonal boron nitride powder (hBN) and subjecting it to pressures in the region of 75 Kbar and 2000°K. In this process, the hBN is converted into finely sintered particles of cubic boron nitride (cBN). The cBN is then graded into size and depending on the PCBN grade being produced a specific distribution of cBN grit sizes and mixed with the appropriate binder or catalyst. The powder mix is encapsulated in a capsule and subjected to similar pressure and temperatures that were used to produce the CBN grit. After synthesis the capsule is disassembled and the Polycrystalline Cubic Boron Nitride (PCBN) disc is processed flat and round by diamond lapping and grinding. The disc is then ready to be cut into the appropriate blanks for manufacture into PCBN inserts.
The composition and properties of PCBN are tailored to optimize performance in the targeted areas. The main variables are % cBN content, cBN particle size, binder type and format. Based on above, CBN grades are available for machining materials like hardened steels, cast irons, sintered irons and super alloys.
Increased grain size in the latest solid PCBN grades has significantly improved their performance capabilities. The larger PCBN particle size provides both better wear resistance and impact resistance. That’s a key to productive machining of materials like cast irons with less than 10% ferrite content. In roughing operations on parts of this type, good impact resistance is essential to withstand the cracks, sand inclusions, and other interruptions left from the casting process.
How PCBN Works?
The effectiveness of PCBN in machining hardened steel components in the range of 45 to 70Hrc is due to the exploitation of PCBN’s high hot hardness and chemical stability at elevated temperatures through the deliberate generation of heat in the cutting zone, which in turn softens the workpiece material
The self-induced heat generation at the cutting zone is estimated to be in the region of 700-800°C, and is enough to reduce the hardness of the material in contact with the cutting tool. The heat induced soft cutting means that the PCBN is not in contact with the workpiece in its hardened state, thus giving PCBN longer tool life compared with other cutting tool materials.
When finish machining, due to the small depth of cut, heat generation in the shear cutting zone is reduced. To compensate for this, PCBN materials have been designed with increased contents of ceramic binder. The binder acts as an insulator limiting heat dissipation from the cutting zone, thereby improving tool life in finishing applications. These PCBN grades are generally known as ‘Low cBN content PCBN” and they typically have cBN contents not greater than 60% by volume.
The machining of hardened steel and hard irons (45 to 65HRc) with PCBN is very dependent on the heat generation in the cutting zone. During milling the cutting edge is out of cut for at least 50% of the time during each revolution of the cutter. To compensate for the time PCBN is out of cut during milling, cutting speeds are generally increased by 50% when compared with continuous turning recommendations. Milling with PCBN at the lower cutting speed (which is advised for continuous turning) will result in reduced tool life, as the cutting tool interruptions associated with milling result in reduced heat generation at the cutting zone.
When milling grey cast iron, the material is relatively soft but abrasive. Therefore, compared to hard steels and hard irons, the presence of heat at the cutting zone is less important and consequently recommended cutting speeds for milling are similar to continuous turning.
Coolant should not be used when machining with PCBN. Using coolant severely reduces PCBN’s performance, or more likely, result in thermal shocking of the insert, which invariable results in PCBN breakage.
Edge preparation – A key to success
The success of machining with PCBN strongly influenced by inserts edge preparation. To get good tool life, the cutting edge of PCBN insert must be reinforced with the proper edge preparation. These can range from a small hone for finish, T-land for heavy roughing. Combined lands and hones may also be used. A general guideline for proper edge preparation of PCBN inserts is to keep the angle of the land proportional to the hardness of the work material, since the reinforced cutting edge will redirect cutting forces away from the edge. By providing proper edge preparation, we can increase and strengthen the cutting edge.
There are three broad areas of applications for PCBN:
- High-speed machining of grey cast irons
- Elimination of grinding (hard part machining)
- Roughing of difficult to machine materials
PCBN grades are developed for machining hardened steels, pearlitic cast iron, hard iron and super alloys for turning, milling, boring & drilling applications. While machining these materials with PCBN, substantial reduction in production costs can be achieved. PCBN inserts outperform ceramics when machining pearlitic grey cast iron. Capable of high stock removal rates and environmental benefits, PCBN offers many advantages compared to grinding. The composition and properties of PCBN are tailored to optimize performance in the targeted areas.
PCBN covers wide range in Solid CBN, Solid CBN with hole, full top and tipped PCBN inserts to machine all materials. Both the full-face and tipped PCBN insert styles are of industry-standard size. Like the latest solid style, they can be used in the insert pockets of standard carbide tool holders and milling cutters.
Coated PCBN improves the resistance to chemical attack and reduce the crater wear. PVD coating increases the tool life, particularly in finish machining applications. Coated PCBN inserts also helps in detecting the wear on the cutting edges.
PCBN wiper inserts are available which combines the advantages of high feed capability and high surface quality.
As cost pressures increase on the industry, the requirement for the high productivity that PCBN delivers will become increasingly important. Further development of new PCBN materials into the new millennium is therefore inevitable and will further accelerate the use of PCBN and assist industry in achieving the required performance goals.
Rajesh Gupta is Engineering Graduate with over two decades of experience in machine tool industry. He is Deputy GM of Seco Tools India at Milling and Advance Materials department and directly responsible for some of the innovations at the company.
Image Courtesy: Seco Tools India Pvt Ltd