Tech Front: Hard Turning Finishes Strong with PCBN
Recent advances in PCBN cutting tool technology have increased the acceptance of hard turning as a finishing operation of critical surfaces, according to Seco Tools Inc. (Troy, MI). PCBN tool inserts, combined with the latest plunge turning technique, are said to produce as good or better tolerances than conventional grinding for finishing steel parts hardened to Rc 45–64.
The economics of hard turning are fairly well known. CNC lathes are less capital-intensive than grinders (one-third to one-half the cost) and extremely flexible in terms of machine capabilities and tool change time. Lathes take up less space, avoid messy government-regulated chip disposal and reclamation issues, and involve simpler tool maintenance. PCBN tool inserts can be quickly indexed to a new edge or removed and replaced with new inserts and don’t require truing or dressing to maintain the cutting profile.
According to Seco, the turning process consists of small depths of cut, yet estimates of reduced machining time are as high as 60% for conventional hard turning, and 90% for the latest plunge-turning techniques. For increased productivity in conventional side turning and profiling, cutting tool manufacturers have engineered PCBN inserts with a wiper geometry that permits increased feed rates without sacrificing surface integrity.
Seco studies have shown that by using the right combination of insert nose radii, feed rate, or the new wiper insert technology, hard turning can produce a better surface finish than grinding. Performing multiple hard turning operations in a single chucking as opposed to multiple grinding setups also contributes to a high degree of accuracy.
Seco’s solid insert grades, CBN060K, CBN100, and CBN300, provide use of all edges similar to conventional carbide inserts, and have long edges, not common to PCBN inserts. Due to their solid construction, the PCBN inserts offer higher wear resistance and lower cost per edge. Comparing the solid PCBN inserts to various tipped inserts, cost per edge savings ranged from 25 to 208%.
An added advantage is found in the fact that CBN060K and CBN100 make plunge turning possible because of the properties of the grades and the long usable cutting edges. Plunge turning is a highly productive form of finish hard turning that uses the entire cutting edge or a portion of the cutting edge to create an orthogonal cut.
The most critical properties of ground or hard-turned components are dimension (diameter), roundness, straightness, bearing area, surface roughness and residual stresses. For the first four properties, plunging can achieve the following tolerances for a gear wheel or bearing ring: diameter ±0.0006" (0.015 mm); roundness ±0.00015" (0.004 mm); straightness ±0.00015" (0.004 mm); bearing area 90% deep 0.00008" (0.002 mm).
In conventional turning, which generates a continuous groove, surface finish is largely determined by insert nose radius and feed rates, and, to some extent, also cutting speed and depth of cut. In contrast, the surface quality of plunge-turned components is mainly dependent on the quality of the cutting edge. As a result, plunge-turned components display low residual stress in the axial direction, and sealing properties (as with a bearing surface) are equal to or better than ground surfaces.
Using solid PCBN inserts, these stress patterns remain constant, even when large batches of parts are machined. In another test, in which Seco examined the physical properties of 250 plunge-turned parts, the residual stress was found to be significantly lower than the stresses caused by grinding. Part quality is also affected by the distribution of residual stress in the tangential direction. Still in another test of 250 parts, both grinding and plunging produced tensile residual stress at the surface, but the thickness of the affected zone was significantly thinner on plunge-turned components.
Plunge turning also was found to reduce the occurrence of white layers, because it reduces flank wear on the tool edge due to shorter cutting time per machined surface. This also generates lower cutting forces and less friction, which results in less heat exposure to the surface of the workpiece.
The problem of white layer generation exists in both grinding and hard turning. Although they are commonly associated with residual tensile stress at the surface, white layers may also indicate residual compressive stresses. Either way, the cause of white layers and the effects they have on the finished workpiece aren’t entirely understood, although tool wear could be a contributing factor. ME
For more information from Seco Tools Inc., go to
www.secotools.com, or phone 248-528-5200.
CNC Puts Precision
into Gear Production
A high-performance 11-axis CNC system from NUM Corp. (Naperville, IL) provides complex control for the Grono 250 gear honing machine from Sicmat SpA for post-hardening fine-finishing. The Grono 250 uses an innovative honing wheel with external teeth and an application-specific CNC program to provide accuracy in gear shaving that eliminates the need for shaving and grinding stages.
Until recently, Sicmat specialized in machines that used shaving technology to obtain the necessary finish to medium and high-quality gears for the automotive industry. Shaving generally was performed before the workpiece was case hardened by heat treating. Any heat-induced deformation had to be corrected by grinding or honing. Sicmat’s objective was to develop a high-power honing machine that would help gear manufacturers accelerate production throughput by reducing the number of process stages.
Until now, gear-honing automation has used highly specialized machine tools, comprising circular ring-type assemblies with teeth cut in their internal face. These tools are expensive and time-consuming to set up. Sicmat believed that by using a honing wheel with external teeth, the accuracy of the process could be improved to such an extent that it could replace gear shaving in many applications with the advantages of lower tooling costs and simpler setup.
Needing high mechanical rigidity for accurate and repeatable results, Sicmat chose to base its new honing machine on the proven mechanical platform of its Raso TP 250 gear-shaving machine, a platform with exceptional stiffness and resistance to vibration. Also, the physical layout provides excellent accessibility for operation and maintenance, and features vertical workpiece positioning to simplify integration with other production line automation.
The NUM Flexium CNC was selected and provides eight motion axes control, plus another three on an associated robotic loader. The motors of all 11 axes are driven by NUMDrive C servodrives, and the overall system is equipped with two NUM MDLL 3050 regulated power supplies. Each power supply is rated at 50-kW continuous and uses regenerative braking to maximize efficiency. The main machining axes are operated by direct-drive motors, while the honing wheel and workpiece axes are driven by high-torque motors synchronized and controlled by NUM application-specific software.
Every stage of the honing process on the Grono 250 is automated. In normal use, the case-hardened workpiece is removed from the production line conveyor by a robotic handler/clamp unit which identifies the type of gear by checking its outside diameter before transferring it to an integral pre-process measurement station. The workpiece is synchronized with a secondary gear, then driven into mesh and rotated through a complete revolution. During this time, the displacement between the two axes is measured continuously to ascertain how much stock material needs to be removed from the workpiece, and these data are fed to the NUM Flexium CNC.
After measurement, a robotic handler transfers the workpiece to the honing stage, where it is initially indexed before being run up to speed and synchronized with the abrasive-toothed honing wheel. The honing wheel is then driven progressively into mesh with the workpiece. The two axes operate in a unique master-slave configuration that has zero delay of the slave which required NUM to create a second master for the workpiece motor. Controlling the speeds of both these axes precisely and fractionally--varying one relative to the other—facilitates fine adjustment of the honing process. As soon as the honing process is complete, the workpiece is disengaged from the honing wheel, spun to remove coolant, and then transferred back to the production line by the robotic handler.
Grono 250 machines are capable of spindle speeds of 7000 rpm. A 10,000-rpm spindle speed is being developed for even tighter process control. The Grono 250 was introduced at IMTS. Sicmat is represented in the US and Canada by Star SU LLC (Hoffman Estates, IL). ME
For more information on NUM Corp., go to www.num.com, or phone 630-505-7722; on Star SU LLC/Sicmat, go to
www.star-su.com, or phone 847-649-1450.
The new six-spindle CNC automatic production lathe, the MS16C from Index Corp. (Noblesville, IN), is built on a multispindle modular system. Aimed at the cam-drive segment of the multispindle market, the MS16C deploys form tool slides as well as full CNC tool slides plus backworking spindle to provide part-drop times of less than 3 sec.
The MS16C is designed to provide a stable platform for high-precision work from bar to 16 mm in a compact footprint. Each spindle offers independent infinitely variable speeds to 10,000 rpm. With smart NC drives, each driven tool can be synchronized with the C axis of the spindles to permit milling and polygon turning.
The machine features liquid spindle cooling, resulting in a higher power density in the spindle carrier, permitting high loads and high speeds without excessive heat generation. Liquid cooling also contributes thermal stability and energy recovered from the heated cooling fluid can be profitably used in the plant.
The X-axis slides provide high dynamic stiffness with steel-on-steel, matched box-type guideways, for high-precision machining. Z-axis slides are quill-type with hydrostatic bushings. The five form tool slides work in the X axis and feature W-grooves that make for easy adjustment of the tools. Backworking is accomplished with a Z-axis synchronized spindle. Up to three backworking tools may be used plus a cut-off tool, for more machining in a single rapid cycle.
The standard version of the MS16C includes 24 CNC axes, five forming slides in the X axis, two drilling slides, three compound slides with X/Z axis, a cut-off slide with backworking and X/Z axis, and a Synchron spindle in the Z axis. The machine can also be configured with 26 CNC axes, five forming slides, five compound slides, a cut-off slide, and a Synchron spindle for backworking.
Applications for the MS16C range from the auto industry to medical device manufacturing, and with short setup time, it can also produce small batches economically. The MS16C multispindle was introduced at IMTS. ME
For more information on INDEX Corp., go to www.indextraub.com, or phone 317-770-6300.
AM Targets Larger
Industrial Metal Parts
Additive manufacturing continues to enjoy substantial growth and notoriety as the process which has proven itself in prototyping, producing fixturing, and short-run production is gaining traction for longer part runs in both plastics and metals. The Ex One Company (Irwin, PA) has positioned its additive manufacturing technology for the industrial sector. “As a result of that decision our build platforms are much larger than what is routinely available in AM equipment and our build speeds are much higher,” explained David Burns, president and COO. ExOne’s new M-FLEX 3-D printing system uses a printhead to distribute binder into layers of powdered materials that are then put into a sintering furnace. “It’s a two-stage process that allows us more volumetric output, because we are racing a print head over the powder, which is faster than lasers or welding particles.” Materials used are principally metals like stainless steel infiltrated with a bronze binder, though tungsten, glass, ceramics, and silica sand for casting molds can be processed.
The M-FLEX model, one of four sizes, features a build chamber of 400 × 250 × 250 mm and build speeds of three seconds per layer and is well-suited for either prototypes or short runs of multiple and/or custom parts. The M-FLEX system includes a printer, recycling equipment, printing materials, and requires a sintering furnace.
“The volumetric output of the M-FLEX machines per unit time has made it competitive in mid-range production runs,” said Burns. “The parts that are well-suited have some level of complexity like pumps and gears that rotate around a center point and are usually too difficult to make using traditional processes. With AM it’s pretty straightforward. We have good penetration into the pump business with both fluid and gas transmission, and a lot of traction in aerospace, where volume and complexity make aerospace a logical choice for ExOne technology.”
ExOne both runs its own production service centers and sells and ships equipment, with each segment accounting for about half of its business. “We have five production service centers around the world, and our intention is to expand the number of service centers in the next 18–24 months. Our equipment has gone into OEMs or Tier 1, but we believe that the M FLEX price point will make it attractive to job shops, as well,” Burns said.
The largest ExOne applications to date have been sand molds produced in the MAX platform that has a build chamber measuring 1.8 m × 1 m × 700 mm. ExOne also has directly metal printed a pump housing that is 660 × 305 × 305 mm. “Each new generation of printers is designed to print more volume at a faster rate, as we continue to improve the return on investment for our customers,” said Burns. ME
For more information on The Ex One Co.,
go to www.exone.com, or phone 724-765-1328.
This article was first published in the November 2012 edition of Manufacturing Engineering magazine. Click here for PDF.