If you're like most CNC machine shops, you're always looking for new sources of income. Quite often this means seeking customers in industries that are new to you, which usually requires a bit of research before you can successfully compete. It also means taking a hard look at your capabilities and deciding if your current equipment is up to the task. If your list of target industries includes aerospace, here is some critical information to help get you started.
The aerospace industry includes civil, commercial and military aircraft that require a vast number of components, many of which are machined from metal. In almost every case producing these parts is unlike making parts for consumer goods, automotive, construction and many other industries. The materials tend to be more exotic, the tolerances are often stricter, and the traceability requirements more stringent. All of which makes sense if you are machining components that are part of a highly sophisticated product subject to unimaginable stress.
Not All Parts Are Created Equal
Aerospace part families are broadly categorized into 3 groups:
This category includes bulkheads and other fuselage parts, wing components and sub assemblies, spars, brackets, door assemblies, trusses and others. These parts are mostly machined from aircraft grade aluminum and titanium, and most are prismatic. They range in size from quite small to very large, requiring CNC machines with corresponding table sizes and strokes.
Most machining is done in a 3-axis environment, however some require 4th and 5th axis positioning and contouring capability. Most of these parts start from billet or castings, requiring extensive pocketing to remove as much weight from the work piece as possible without sacrificing strength. To minimize cutting time, most manufacturers rely on high-speed machining principles, and often use horizontal machining centers (HMC) because of their chip control capability and rotational pallet control that minimizes part-handling time.
These include pumps, actuators, valves, landing gear components and wheel assemblies, to name a few. These parts require highly precise, exacting processes and quality control because of their importance in the control and reliability of the aircraft. Some of these parts are machined from billet, requiring 5-axis contouring capability. Valve spools and other round components are turned on conventional, high-accuracy and Swiss-style lathes. Final sizing may also require external and internal grinding processes.
This critical category involves the production of all stationary and rotating components found in turbine and reciprocating engines. These parts are machined from high temperature alloys such as Inconel® and others. Most are very complex shapes requiring both turning and milling from expensive castings and forgings, and have long cycle times, sometimes approaching days of machining.
Some parts end up as very thin-walled structures requiring sophisticated fixtures and support during multi-sequence machining. Typically these parts are machined using traditional vertical and horizontal machining centers, and multi-function horizontal and vertical turning centers. Part sizes run from a few inches to over 10 feet in diameter. Manufacturing these components requires careful process and quality control, 100 percent final part inspection, and strict traceability.
Milling Aluminum Aerospace Parts: VMC or HMC?
Because many aerospace components are machined from aluminum alloys, which may be a new material to your shop, you need to consider a number of factors when deciding what kind of machine tool best suits your purpose. First, of course, you must determine the correct machine table size and stroke requirements. Next, you may wish to consider the differences between a traditional vertical machining center (VMC) and a horizontal machining center (HMC). HMCs provide superior chip control, automatic pallet indexing and rotation (B axis) as part of the basic design.
Although typically more expensive than vertical machining centers, productivity gains often outweigh the increased capital investment in an HMC. If you don't expect to have enough new work to justify an HMC, consider moving work now performed on older VMCs onto a new HMC. You'll experience better tool life, improved process stability and increased part quality among other improvements.
Another important consideration, especially if you machine close-tolerance parts, is acquiring an HMC that is thermally stable by design. Otherwise you may find your operators chasing size because of machine warm-up and changes in plant temperature. Machine tools that feature thermal stability by design will reduce scrap and free up operators for other tasks.
Critical CNC Machine Considerations
Regardless of whether you choose a vertical or horizontal machining center, there are a number of important features you should consider:
Spindle Horsepower And Maximum RPM
Aluminum alloys for the most part are free cutting in comparison to harder materials, however aluminum will consume a surprising amount of horsepower at aggressive speeds and feeds. Therefore the spindle should be capable of speeds in excess of 12,000 RPM with a horsepower rating sufficient to accommodate your production needs.
Another consideration is selecting the best spindle nose for your application. CAT 40 spindles are commonly used with spindle speeds up to 15,000 -- 20,000 RPM. CAT 50 spindles are popular for direct drive and geared spindles with higher horsepower and torque ratings, and maximum speeds less than 15,000 RPM. The CAT 50 spindles are also well suited for larger diameter drills, end mills over 1.0" diameter and tooling that generates high axial thrust loads, such as extended length end mills and larger diameter face mills. HSK spindles are used across a wide range of spindle speeds, particularly in high RPM applications greater than 20,000 RPM.
Additionally you should give careful thought to spindle horsepower/torque characteristics when matching a machine to your needs. For example, if you're machining other materials such as steels or high temperature alloys like titanium, you'll require different spindle performance characteristics than those applicable only to aluminum. And just because a CNC machine is designed for high-speed operation does not mean it has the spindle torque, axis thrust and overall machine rigidity necessary for difficult to machine materials.
Tool Changer Capacity
Make sure the tool magazine has enough capacity to support your current and future needs. Very few vertical machining center tool magazines are expandable, although some horizontal machining center magazines systems are.
Through Spindle Coolant
At minimum a machine tool used to make aerospace parts should be prepared for through spindle coolant. This typically means that the spindle draw bar is gun drilled with a coolant hole, and additional components, such as the rotary union and associated hardware, are either included or easily added in the field without major machine modifications. If the machine is thus prepared, make sure the kit is capable of 1,000 psi. If not, you can't use the hardware for high-pressure applications. In today's competitive market it's best to equip the machine at the outset with a 1,000 psi through spindle coolant system.
Consider the addition of a flood coolant system to assist in flushing chips away from the work envelope. In some cases this feature requires additional coolant tank capacity, and the chip conveyor must also have the flow rate capacity to support the additional coolant flow. So it's best to specify it as part of an OEM package.
Some machines come standard with a general-purpose chip conveyor while others may offer some type of coolant filtration. General-purpose hinge belt-type conveyors may work for your application, but coolant viscosity along with chip thickness and shape could cause the chips to adhere to the conveyor belt, eventually clogging the chip conveyor. Some conveyors include air knives or brushes that can, in certain applications, alleviate this problem but often create other housekeeping issues. The best solution is to specify a chip conveyor capable of filtering aluminum chips as part of the OEM package.
CNC Control Considerations
This is an extensive topic but here are some key issues to consider when purchasing a new CNC machine tool.
Axis Drive Expandability: The control must accommodate additional axis drives when added in the future. This is especially important on 3-axis vertical machining centers. Most controls are capable of a 4th axis positioning or contouring expansion, typically used for a rotary table. Some controls can accept a 4th axis contouring, and positioning 5th axis. Certain controls are capable of simultaneous 4th and 5th axis contouring expansion.
Control Processing Capability: Today's CNC controls have significantly greater speed in comparison to controls made just a few years ago. However, 3-axis simultaneous contouring programs can pose demands on standard CNC controls that decrease machine performance. Simultaneous 4 or 5-axis programming definitely places additional demands on CNC control performance.
The machine builder determines the standard hardware and software content of the CNC control. It's very common to see the same brand and model control on various makes of machines. The control hardware and software content will vary depending upon the machine tool builder's specification, and what may be standard content on one brand of machine may be optional on another, even though both have the same control.
Software options such as extended part program look ahead, contour control, support for NURBS programming and others, enhance the CNCs computational speed in demanding, multi-axis environments thus enabling faster machine execution.
State-of-the-art CAD/CAM packages support the output of sophisticated cutter path generation such as tricordial milling. These programming techniques can greatly enhance metal removal rates while increasing cutter life. Part programs range from mostly point-to point to circular interpolative, placing increased demands on the CNC and drive package. Employing techniques such as these at elevated speeds and feeds can become constrained if the machine tool system is not capable of quickly handling this intensive code.
Various types of tool and work piece probing are available, and they can present a number of challenges. For example, you must preset tool lengths off-line or tool probing may accelerate tool length offsetting. Tool probe sensors can also take up valuable vertical machining center table space.
Spindle-mounted part probes can be used to check for part presence, determine zero point offset amounts and perform part size inspection, all of which adds cycle time. However the benefits should outweigh the additional in-process time. Additional control software may be necessary to support probing.
For sophisticated work on aerospace components it's mandatory to network the CNC control. Ethernet connection via 10-T base is widespread. Plus, many machines and accessories now have remote diagnostic capability via Internet access, which decreases, and in some cases, eliminates the need for on-site service calls. User remote monitoring is also gaining widespread popularity.
USB ports support the use of standard computer accessories along with specialized wireless gauging devices and other shop related USB devices.
USB connectivity provides an easy way to integrate these types of peripherals.
Pallet Shuttle Devices
If you have decided on a vertical machining center, consider the addition of an external pallet shuttle device. These units greatly reduce the amount of spindle down time associated with part loading and unloading, and work holding changeover. Including this accessory with the machine order will make the cost part of the capital acquisition. If you want to acquire this accessory at a later date, you may want to equip the machine with the necessary interface, such as extra M codes, at the time the machine is purchased.
High Temperature Alloys
Those manufacturers who are accustomed to working primarily with more common materials such as carbon steels and non-ferrous alloys may be unaware of the additional demands machining high temperature alloys place on tooling and CNC machines.
For example, these heat-resistant alloys may require both significantly lower cutting speeds and greater tool pressures. The reality is that machine tools and tooling that perform well on more commonly machined metals may not be up to the task of roughing and finishing exotic alloys. To determine if your equipment is capable of working on aerospace parts made from high temperature alloys, consider the following factors:
Choosing the right tooling is essential when working with aerospace materials. Even under ideal conditions tooling is subject to accelerated edge wear due to the hardness and toughness of high temperature alloys. This, of course, can dramatically reduce tool life, interrupt machining cycles and lead to quality problems. That's why, if you plan to use general purpose CNC machine tools for aerospace operations, you should work closely with a cutting tool supplier who has experience with and understands the nature of machining these exotic metals.
The right cutting tools and edge preparation have a major impact on tool life and surface finish. Additionally, advanced machine programming techniques such as trochoidal milling and taper turning during rough machining can help extend tool life.
High Pressure Coolant
The spindle speeds and higher tooling pressure necessary for machining high temperature alloys produce significant heat that can distort the workpiece if not properly controlled. The best way to eliminate the thermal problem is by applying the optimum amount of high-pressure coolant to the cutting area. Many quick-change cutting tool holders include through-tool coolant orifices that precisely deliver high-pressure coolant to the tool tip. This configuration eliminates the need for custom coolant lines.
High pressure coolant flooding is also necessary to help remove chips that can damage the workpiece or slow down the machining cycle. New variable pressure coolant systems allow users to automatically vary the coolant pressure output with signals from the machine tool or via pre-programmed settings to apply the optimum coolant amount and pressure (up to 2000 PSI) for each machining operation.
Is The Machine Tool Capable?
Probably the most important consideration for newcomers dealing with high temperature alloys is recognizing whether or not their existing CNC machines can handle the demands of machining exotic alloys. Quite often otherwise acceptable machine tools lack the rigidity, spindle power or axial thrust necessary to drive cutting tools at the appropriate speed and feed rates. The fact is, machining exotic aerospace materials requires machine characteristics that far exceed those needed for less demanding work. Here are some of the key capabilities to look for:
Spindle Power. Contrary to popular belief, spindle power is not, necessarily, all about more horsepower. As important as horsepower is, it's also critical that the spindle motor has adequate torque.
In most cases machining high temperature alloys requires slower spindle speeds that potentially keep the spindle motor operating at RPMs in a constant torque range, thus prohibiting the motor from turning fast enough to develop full horsepower. Therefore the constant torque rating is more important than horsepower, and because the motor design and drive train are what determines torque and horsepower, a greater horsepower rating is not necessarily better.
If you experience spindle overload while drilling high temperature alloys, check the spindle torque chart to determine if you are machining in the constant torque or full horsepower range of the spindle motor. You'll likely find that your machine lacks the constant torque necessary for these cutting conditions.
Axis Thrust. Although most CNC machine programmers aim for the fastest possible traverse rates to reduce non cutting time, it's important to consider if the increased speed comes at the expense of axis thrust. As with spindle power, greater horsepower may not be the answer. The thrust rating of the axis is, in fact, more important than the servomotor horsepower when drilling high temperature alloys. A servomotor with a smaller horsepower rating driving a finer pitched ball screw may actually develop greater thrust than a larger servomotor driving a coarse pitched ball screw. So check the machine tool's specifications carefully.
Machine Rigidity. Because heat resistant materials create high cutting forces, a machine tool must be constructed so that it is capable of damping the resultant vibration resonance that causes chatter.
To determine if a machine tool will perform adequately under the high thrust loads associated with machining high temperature alloys, look for appropriate casting design, way construction, ball screw size and mounting, and spindle bearing size and type. All of these factors contribute to the rigidity necessary to machine aerospace components from these alloys. Moreover, machine rigidity greatly influences cutting tool life and, because the life of a tool cutting high temperature alloys can often be measured in minutes rather than hours, having a rigid, stable CNC machine is critical for maximizing tool life.
Choosing The Right CNC Machine
As you see, there are many facts to consider when machining exotic alloys for aerospace applications, so it's vitally important to have the right machine tools and accessories. That's where the Gosiger team can help. With more than 90 years of machine tool experience, the Gosiger engineers and technical specialists can help you determine the kind of equipment that precisely fits your aerospace applications.
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