|A hardened H-13 block (54 Rc) is machined into a finished cavity, accurate to within 1 micro of the CAD model. |
Zero-stock machining — also called “netshape” or “negative-stock” machining — incorporates hard-milling and highspeed machining techniques to produce components without extra stock. While that might appear to be the goal of all machining processes, zero-stock machining further distinguishes itself by producing components that do not require additional polishing or subsequent processes. Savings are both in the machining process and by eliminating additional processes. And quality is improved by eliminating manual or secondary processes.
Machining to net shape is particularly advantageous to moldmakers because it eliminates time-consuming electrical discharge machining and manual or secondary operations — such as polishing, fitting, spotting, finishing and assembly operations — so it speeds up and reduces the cost of making a mold.
However, successful zero-stock machining requires that the machine tool, programming and tooling are all used in harmony.
|Examples of multi-layer TiAlN coatings. |
And, zero-stock machining constitutes a departure from traditional methods and requires a change in mindset as much as change in technology.
Machine tools must be stable and rigid and provide high precision, spindle speeds of 18,000 rpm or greater and feedrates of at lease 500 ipm. Efficient software must be programmed to produce accurate and optimized tool paths to minimize machining time. In 5-axis machining, the software must be programmed to orient the cutting tool to the workpiece at the optimum angle of attack.
If any of these elements is selected or applied improperly, the process will be ineffective.
Effective, high-speed cutting
High-speed cutting — the foundation of zero-stock machining — is the process of machining materials at cutting speeds of 5 to10 times faster than conventional machining with highly accelerated and precise rapid movements.
High-speed cutting uses spindle speed to take relatively light depths of cut at accelerated feedrates to efficiently remove material. The process can be defined as machining at a cutting speed that is fast enough to create a level of friction, or released as heat, that is sufficient to melt the material in the shearing zone — the exact point where chips are separated from the workpiece.
The heat-resistant characteristics of carbide, combined with optimized flute geometry and small chip size aid in rapid chip evacuation to keep the area immediately surrounding the shearing zone relatively cool.
Due to the high spindle speeds, cutting tools must possess inherently good balance characteristics.
For example, tools with straight plane shanks are better balanced than tools with Weldon-style flats. For finishing operations, tools should have an even number of flutes for smooth, even cutting. Most importantly, end mills should include micron tolerances on the shank and cutting diameter to minimize vibrations to maximize tool life.
Economical, effective dry machining
Dry machining is preferable for machining hardened steel. It eliminates the danger of thermal shock that can occur when relatively cold fluid comes in contact with the relatively hot cutting tool. This drastic temperature change can cause cracks or micro-fissures on the tool’s cutting edge, leading to premature tool wear.
Dry machining can be augmented with a cool air blast to keep the cutting area cool and help facilitate chip evacuation.
Also, dry machining is economical, not only because it eliminates the cost of coolant, but also because it eliminates the cost of coolant disposal that can exceed the initial cost for the coolant. It is also a good ecological option for the environment as a whole, and because coolants can present health hazards such as allergic reactions, respiratory irritations and poisoning.
Successful hard milling
The generally accepted definition of hard milling is the machining of metals with a hardness at or above 52 Rc that cannot be efficiently machined with conventional high-speed steel milling cutters.
Effective hard milling requires cutting tools that are made from harder materials such as carbide, cermet or cubic boron nitride. Carbide is the cheaper, more durable option.
Important cutting tool characteristics to achieve
• Tools made of micro-grain or sub-micrograin carbide materials.
• High-performance titanium aluminum nitride (TiAIN) coatings for optimal tool performance and tool life.
• And, application-specific cutting geometry. Typically, the harder the workpiece the more negative the rake angle — zero degrees to -3 degrees — and the slower the helix angle — 30 degrees to zero degrees.
Tooling and coating considerations
Just as cutting tools are essential for hard milling, they are central to zero-stock machining.
This is an area in which changing the machinist’s outlook is important.
The machinist must recognize the importance of the cutting tools used in this process. While the role of cutting tools may be minimized or overlooked in traditional machining, they are critical in zero-stock machining because they are the single interface between the machine tool and its programming elements and the finished workpiece.
As with hard milling, cutting tool performance in zero-stock machining depends on the tool coating and proper cooling, although, unlike hard milling, lubrication also may be required.
Shops that are considering using zero-stock machining should evaluate cutting tools as carefully as they would the machine tool and programming elements.
It is important to evaluate different cutters — which often is possible to do through vendor test programs — to determine the best tool for the application. Tools cannot be evaluated effectively on the basis of price alone. As cutting tool technology progresses, additional features, such as edge prep, sophisticated geometries, state-of-the-art coatings and taskspecific carbides, will add costs, but the additional costs can be offset by the value added in tool life and performance.
Carbide tools are made from composite materials consisting of a relatively soft bonding agent, cobalt (Co) and of carbides of tungsten (WC), titanium (TiC), tantalum (TaC) and niobium (Nb) that provide hardness. Through a sintering process, the cobalt material is liquified under extreme heat while the carbide, with a much higher melting point, remains solid. Once cooled, the result is a matrix of the cobalt bonding agent and the brittle carbide particles into a solid body.
Carbides are available with various grain structures including nano, sub-micro, micro, fine, medium, coarse and extra-coarse grain. Preferred tools for modern machining are made from micro or sub-micro grain carbides.
Through various compositions of cobalt and carbides and different carbides and grain sizes, a multitude of hardness and toughness properties can be achieved. A balance must be struck between the carbide tool’s hardness and brittleness. As the hardness increases, carbide becomes more brittle. For hard milling, tools with the harder carbides should be used; for high-speed cutting in mild steel, less hard, more resilient carbides will provide optimum tool life.
Physical vapor deposition (PVD) coatings are recommended for tools used for hard milling and high-speed cutting, and therefore the zero-stock machining process.
In a typical PVD process, electrodes made from titanium and aluminum are introduced into a vacuum chamber and bombarded by an electrical current, or arc, that vaporizes the electrodes and releases charged electrons. Nitrogen gas pumped into the vacuum chamber under high heat forms a plasma of gas and electrons that is attracted to, and deposited onto, the carbide as a hard, thin film such as TiAlN.
These coatings can be applied in a single layer, in multi layers or in alternating layers of different coatings.
Coatings play an important role relative to productivity and tool life, ultimately affecting the cost-effectiveness of the moldmaking process.
In general, coatings decrease tool wear due to added resistance to friction and heat. A coating’s lubricity — measured as a coefficient of friction — helps prevent cold welding and minimizes cutting forces. In turn, production costs are decreased due to longer tool life, higher cutting speeds and improved surface quality of workpieces.
However, in some cases, coatings can be detrimental.
For example, a relatively thick coating can have a negative influence on surface quality and the sharpness of the cutting edge.
|Editor’s note: This article based on information supplied by Stephen Jean, Emuge Corp. (www.emuge.com) milling products manager. Photos and illustrations courtesy of Emuge Corp.|
Also, while multi-layer coatings can reduce the spread of cracks caused by frictional heat or thermal shock, it is possible in the PVD process for electrons to clump together and form “droplets” within the layers. These droplets increase the surface roughness of the coating, inhibiting chip flow. To combat this problem, manufacturers of quality end mills perform a surface treatment within the flutes, similar to polishing, that substantially improves the coating’s roughness.
Also beneficial are edge-preparation treatments, performed prior to coating, where the cutting edge is honed to minimize or remove the microscopic grind lines left by the manufacturing process.
The resulting smoother surface provides a better base for coating adhesion and less opportunity for cold welding and or material build-up on the cutting edge.