Is there a PCD/PCBN insert in your future?

Manufacturing Engineering, Jul 1998 by Huddle, David

Ignore sticker shock PCBN and PCD cutting tools can boost productivity

Polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) inserts could be your best means of increasing productivity despite their high cost. In the past, PCD and PCBN cutting tools were difficult to cost-justify, unless they were essential for the machining job.

Today, improvements in quality and reliability make these tools, although still pricey, competitive in many machining applications in the automotive, aerospace, and medical equipment industries. More rigid machines and tooling setups enable manufacturers to take full advantage of the potential for improved productivity offered by PCD and PCBN inserts. Also, having more cutting-tool options gives manufacturing engineers an opportunity for cost-effective productivity improvement in various machining applications.

New, thicker PCBN solid inserts with larger grain size possess improved wear and impact resistancethe key to effective machining of materials like cast irons containing less than 10% ferrite content. Such performance improvement is especially important when machining alloy cast irons for automotive applications. Previously, manufacturers had to grind these castings.

In roughing operations on alloy cast irons, tools must withstand interrupted cuts due to surface cracks, sand inclusions, and other surface discontinuities inherent to the casting process. Good wear resistance comes into play in finishing operations, where parts containing 28-30% chromium have a hardness between RC 68 and RC 72.

Solid PCBN inserts provide multiple cutting edges on two sides, reducing insert cost per part produced. A cross-section 3/is" (4.76-mm)-thick-up from the traditional 1/8" (3.17 mm)-makes it possible to hold the inserts securely in standard carbide-insert toolholders using shim seats.

PCBN inserts also come in full-face and tipped types. The full-face type has a complete PCBN top face sintered onto a carbide substrate, and provides multiple cutting edges on one side only. These inserts are less expensive than solid PCBN inserts. The tipped style contains a small PCBN segment brazed onto one corner of a carbide insert, providing either a single cutting edge or double cutting edges. Most PCBN inserts used today are tipped. Both full-face and tipped PCBN inserts come in industry-standard sizes and, like the solid insert, can be used in the insert pockets of standard toolholders and milling cutters.

Harder Parts Preferred

Inserts made of PCBN work best in hard-part machining applications. In practice, the low end for part hardness falls at about Rc 45. Machining softer parts using PCBN produces insert cratering.

In roughing operations, maximum depths of cut using solidstyle PCBN inserts range from about 3/16" (4.76 mm) for white iron and other hard, high-chromium irons to about 1/4" (6.4 mm) for unalloyed, "clean" cast irons. Finishing speeds range from 350 to 400 fpm (107-122 m/min) on high-chromium irons to as high as 7000 fpm (2134 m/min) on gray cast irons. All PCBN operations require the use of very rigid tooling, work fixturing, machine spindles, and machine tools.

Appropriate PCBN finishing speeds vary dramatically depending on the work material, material hardness, and part size and shape. For example, you can achieve a cutting speed of 600 fpm (183 m/min) at a 0.020" (0.51-mm) depth of cut on hardened steels in the range of Rl 60-62. A feed rate of 0.002-0.004 ipr (0.05-0.1 mm/rev) typically produces a surface finish of about 8 rms. Harder materials require faster speeds, but speeds greater than about 650 fpm (198 m/min) result in excessive wear. You can attain much higher speeds on plain cast irons.

Tipped PCBN inserts are economical and reliable for a wide range of roughing and finishing applications, but some applications require a solid or full-face insert. Rough-boring cylinder liners with casting flash in the middle of the part is a good example. With a tipped insert running at exactly the right depth of cut, the flash can eventually displace the PCBN segment. The stronger solid or full-face PCBN insert eliminates this problem. You can also regrind both solid and full-face insert styles, which extends tool life and helps offset their higher initial cost. The most cost-effective applications for solid inserts should allow the use of cutting edges on both sides.

Currently, the largest growth area for PCBN inserts is in hard turning-finish turning of alloy steel automotive engine components such as gears, shafts, and bearings with hardnesses between Rc 60-65. Traditionally, manufacturers ground these parts to obtain very tight dimensional tolerances and a fine surface finish. Hard turning gives the same results using a CNC lathe. Priced at about half as much as a CNC grinder, the lathe also costs less to maintain.

Hard turning operations use lower-cost PCBN tipped inserts because of the light depth of cutin the range of 0.012-0.015" (0.3-0.38 mm). In tests at Carboloy, machining hardened steel components (RP 60-62) using tipped inserts consistently produced surface finishes of 0.2 pm or less.