Molds for making plastic lenses can be effectively machined with conventional polycrystalline and single-crystal diamond tools because the molds are made of relatively soft materials, such as electroless nickel. However, molds for making high-performance glass lenses, such as for digital cameras and sensors, are often made of hard cemented carbide.
In those applications, natural and synthetic single-crystal diamonds tend to split, or cleave, in one direction because of the nature of the crystalline structure. While sintered, PCD tools don’t cleave, they have problems consistently achieving the required accuracy and suffer from short tool life, in part because their metallic binder makes them not as hard as single-crystal diamond tools.
Those cutting tool drawbacks mean moldmakers require diamond grinding wheels to produce hard molds. This also creates challenges because the need to true wheels makes it difficult to achieve submicron-level form accuracy and nanometer-level surface roughness. Also, grinding small concave molds with small-diameter wheels means only a small number of abrasive particles inefficiently remove material.
Courtesy of A.L.M.T.
A comparison of the cutting edges of a monocrystalline diamond tool (left) and a nano-polycrystalline diamond tool after machining tungsten carbide on a CNC lathe using elliptical vibration cutting.
To overcome these obstacles, Sumitomo Electric Industries Ltd., which has head offices in Tokyo and Osaka, Japan, developed SumiDIA Binderless nano-polycrystalline diamond. Sumitomo Electric offers ballnose endmills and turning inserts tipped with the material, while Tokyo-based A.L.M.T. Corp., a Sumitomo Electric company, offers BL-UPC series tools, including aspherical surfacing tools, grooving tools and ball endmills.
The diamond material has a single-phase body synthesized by firmly and directly bonding diamond grains that are several tens of nanometers in diameter, the company reports. Sumitomo Electric produces the material under higher temperature and pressure conditions than when creating synthetic, single-crystal diamonds. The binderless, single-phase, nano-PCD material reportedly does not cleave and is harder than conventional PCD and single-crystal diamond material.
When applied on a CNC lathe using elliptical vibration machining, where a cutting tool is attached to a cylindrical support body vibrated by at least two piezoelectric drive elements, a BL-UPC tool with a -20° rake angle imparted a 7.4nm Ra surface finish, according to the company. The DOC was 2µm and the feed was 5 µm/rev. The workpiece was tungsten carbide containing 12 percent cobalt.
In addition to cemented carbide, suitable workpieces include hardened steel, glass and silicon carbide.
For more information about Sumitomo Electric Carbide Inc., Mt. Prospect, Ill., call (800) 950-5202 or visit www.sumicarbide.com.
Related Glossary Terms
- abrasive
abrasive
Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.
- computer numerical control ( CNC)
computer numerical control ( CNC)
Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.
- feed
feed
Rate of change of position of the tool as a whole, relative to the workpiece while cutting.
- grinding
grinding
Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.
- grooving
grooving
Machining grooves and shallow channels. Example: grooving ball-bearing raceways. Typically performed by tools that are capable of light cuts at high feed rates. Imparts high-quality finish.
- lathe
lathe
Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.
- polycrystalline diamond ( PCD)
polycrystalline diamond ( PCD)
Cutting tool material consisting of natural or synthetic diamond crystals bonded together under high pressure at elevated temperatures. PCD is available as a tip brazed to a carbide insert carrier. Used for machining nonferrous alloys and nonmetallic materials at high cutting speeds.
- rake
rake
Angle of inclination between the face of the cutting tool and the workpiece. If the face of the tool lies in a plane through the axis of the workpiece, the tool is said to have a neutral, or zero, rake. If the inclination of the tool face makes the cutting edge more acute than when the rake angle is zero, the rake is positive. If the inclination of the tool face makes the cutting edge less acute or more blunt than when the rake angle is zero, the rake is negative.
- single-crystal diamond
single-crystal diamond
Industrial-grade, natural diamond. Not recommended for cutting ferrous materials because it tends to react chemically with them and break down. Also not recommended for interrupted cuts in hard materials. Replaced by polycrystalline diamond in many applications. See diamond; PCD, polycrystalline diamond; superabrasive tools.
- tungsten carbide ( WC)
tungsten carbide ( WC)
Intermetallic compound consisting of equal parts, by atomic weight, of tungsten and carbon. Sometimes tungsten carbide is used in reference to the cemented tungsten carbide material with cobalt added and/or with titanium carbide or tantalum carbide added. Thus, the tungsten carbide may be used to refer to pure tungsten carbide as well as co-bonded tungsten carbide, which may or may not contain added titanium carbide and/or tantalum carbide.
- turning
turning
Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.
Author
Alan holds a bachelor’s degree in journalism from Southern Illinois University Carbondale. Including his 20 years at CTE, Alan has more than 30 years of trade journalism experience.
