Dealing with five axes when machining isn’t a simple process in any industry, but it gets especially interesting in the realm of aerospace. When we’re talking about subtractive manufacturing in general, you have to consider everything from your material and holding that work to your tooling and the toolpaths to get to your finished part. Adding more axes makes not only your toolpath and feeds-and-speeds choices more complicated, even choosing your machine can get complicated.
There are a lot of things to consider when looking for a 5-axis CNC machine for milling in the aerospace industry, and the options might not be as intuitive as you think.
Most folks in the world of manufacturing and engineering are often anchored in their tool choices by the bottom line – i.e. cheapest has a head start in the decision – but when considering a machine for aerospace manufacturing, price is often one of the lowest priorities.
Why 5-Axis Machining for Aerospace?
One of the biggest reasons 5-axis machining is so prevalent in the world of aerospace is machine setup. The parts often have complex geometries that don’t easily lend to 3- or 4-axis operations. While many of the parts might be possible without five axes, you’d have to continuously refixture your part. In other words, you could only machine one section of that part, take it off, refixture it, and machine your next section, and repeat, just to get the geometry correct.
A perfect example of this is NASA’s Orion bulk head, which is domed near the heat shield. It’s a big forged piece of aluminum and all the pockets are normal to the surface, so you wouldn’t be able to get those angles with any other operation.
Another key element of manufacturing in the aerospace industry is weight reduction – you’re building a machine that flies, after all. More often than not, the goal is to find the best way to get the most strength with the least amount of weight. This is a major factor in defining all the weird geometries in aero, and to that end, the need for 5-axis machining.
Defining Your Work Envelope
The aerospace industry has a broad array of parts, components, and structures that require varying levels of machining, molding, and even additive manufacturing. Defining your work envelope before buying a machine is vital to making the right choice.
Some machined parts are massive, like fuselage sections. Five axes (or even six or seven) are necessary for this type of machining, but the real catch is that the machine itself ends up being the size of a building.
In situations like this, every milling machine is custom-built for a specific part, and the machine is rarely reused after production of that particular part stops. Those machines are typically in operation for five to ten years, so once the part is no longer made, the technology has advanced so much that it’s more efficient to just upgrade to a new machine. Even if it costs $10M for just one of those new machines, you’re going to make it up in time savings and production.
There is plenty of work to be done with smaller work envelopes as well. 5-axis machines that aren’t made as one-off production machines are usually off-the-shelf tool room-style machines and are typically used in R&D centers of aerospace organizations. That’s where they’ll reconfigure the machines a lot to do different parts, like a SkunkWorks or Boeing’s Phantom Works. But, these off-the-shelf machine tools can also be used for production of smaller parts, like landing gear – which is still a large component that can reach the size of a decent office – but the principles stay the same.
When aerospace designers and engineers are designing parts that are smaller, most of the time, they aren’t terribly hard to manufacture; in fact, those parts are often designed to be made specifically in an off-the-shelf machine with some modular fixturing because they know the part will be manufactured in those types of machines – the designers know they have to design the parts to be manufactured on a pretty standard machine. The importance of designing for manufacturing can be see first-hand in these situations.
Another key element to defining your work envelope is considering workholding and the actual cutting process. Again, aerospace lends to very odd- or uniquely-shaped parts, which can be even harder to hold and then you have to machine those interesting features. Once you’ve considered your part’s size, how you plan to hold that part and the tools needed should be considered. If you’re dealing with cumbersome workholding or long tools, these elements need to be taken into account when determining the size of the machine.
The Cheapest Choice Is Rarely the Best Choice
When it comes to the aerospace industry, you’re looking for which machine can produce the tolerance of part you’re requiring, in the time that you want – this is very specific to the part or type of part that you’re creating.
Once you know your work envelope and the tolerances you need to meet, your field of choices gets reduced, but there’s usually still an array of choices.
The logical next step would be to look at the price. Since, at this point, you’re only looking at machines that fit your tolerances and your work envelope, comparing prices is the intuitive thing to do. But that’s wrong!
When the machine isn’t producing anything – when it’s going from one toolpath to another toolpath and not actually touching your part at all (rapids) – that time is huge. When you’re talking only ten seconds of off-part motion and you have another machine that can do it in two to three seconds, adding that up for a machining operation that takes four to five days for one part can come out to something like twelve hours of lost time (depending on the operations and your part, of course).
In this same realm, considering angular limits with different setups is important. When you’re dealing with five different axes of movement, the machine can get wrapped around itself when trying to get into various nooks and crannies of those odd-shaped parts. Determining what kinds of angular limits the machine has is key to making your rapids as fast as possible – if your machine needs to unravel itself from a weird angle on a part, that is all off-part motion.
Since we’re talking about production environments, time is always money. That’s the biggest thing, time is money.
So once you’ve determined machines that can handle your work envelope and tolerances, the next step to picking a machine comes down to speed. That’s why machine rapids are such a bragging point. Not just that companies can make parts faster, but that means that they are making more money.
In this same vein of thinking, determining which machines have better removal rates for the materials you’re planning on machining should weigh your decision. Price is of little concern when you can hog out big’ol chips in titanium.
For instance, you know that you have to produce a part in two days because the boss says that you have to manufacture so many planes by a certain time. So, that determines how fast a plane must get made. If the industry-best, fastest milling machine is going to take four days to machine the parts you need, then you’re going to buy two of them. That way you can have them always going and it reduces your time.
Cost is one of the last items with which to be concerned. The parts in the aerospace industry are so expensive that if you make a few parts with a machine, you’ve already paid it off. In fact, cost isn’t that big of a concern throughout the buying process.
The Last Major Factor – Uptime
Machines break down. You’re using metal tools to cut metal parts for long periods, eventually, something will wear out or break. That’s why determining what machine has the best uptime can be as important as metal removal rates. If your machine is down, you’re making no parts, which means no money – which is way less than if you just had a slow (yet reliable) machine.
Overall reliability and serviceability are key to determining your best-choice machine.
Having the ability to have spares to repair the machine without a service tech can certainly tip the decision. So, if several machine tools can all meet your criteria, then it comes to which machine is least likely to break down and/or if it does break down, which can be repaired the quickest. It’s all about time – and let that cliché phrase keep ringing in your ears, “time is money.”
In the end, every application is going to be unique, but there are four factors to consider in order before deciding on which 5-axis machine is right for your aerospace part.
- Which machines fit your application? Make sure it fits your work envelope, and then be sure to consider metal removal rates and rapids.
- Which machines have good serviceability? What are the wear-parts, how easily can the machine be serviced, and can your operator service the machine without a technician from the machine tool company?
- What is the overall reliability? Does the company offer a spares kit or elements like swappable spindle heads to keep your operation running if the machine breaks down?
- What do the different prices look like? While this is the last item on the list, it can still be a deciding factor if you have a list of machines that fit the rest of your manufacturing criteria