by Jim Barnes
Benefits and pitfalls of cutting exotic metals
Many shops avoid working with exotic metals such as titanium and superalloys. High material costs and notorious challenges in machining make some owners shy away from unfamiliar processes. It does not have to be that way. Any competent machine shop can make these parts profitably, with the right tools, processes, systems and attitudes.
Step one: ask yourself why you are getting involved with these materials in the first place. Are you pursuing a new market or just trying to fill an unusual order?
The strategy for every shop will be different, depending on the products they make, their size, the equipment they already have and their vision for the future, notes Dr. Gabriel Dontu, R&D director, Heritage Cutter, which includes Brubaker, Data Flute and Weldon brands. A big operation will focus on long-term, strategic investments. “For smaller shops that focus on local work, their investment should be more along the lines of flexible toolholding and tools that favour versatility versus performance, as long as the machine tools will cover the required power, stability, etc.”
For the first timer, expensive new materials and tooling may create a fear factor. “Under that kind of pressure, they revert back to what always worked for them in the past,” says Tim Martin, vice president, business development, international sales, LMT Onsrud LP
The tooling suppliers have a stake in seeing you succeed and considerable resources are available. You can avoid preliminary, time-consuming, expensive errors just by asking questions.
“The technical specialist is going to try to help you get within a narrow test range and avoid major mistakes,” explains Dontu. “I don’t tell customers that I have a silver bullet for them… The number of variables in these applications is so large that there are no prescribed formulas that would give you a definite answer for your situation.”
According to Martin, “you sit down with the production people, the tooling people, the programmers and even some of the management people… You help them to identify the best way to approach a stable production environment solution for the workpiece. You’re talking about tool path, programming… you’re really getting down into the details.”
Taking the heat
Apart from exceptional mechanical characteristics, one constant in working with these materials is poor heat conductivity. “All the super alloys are designed to keep their properties at the same level at high temperatures. For that reason, they don’t absorb a lot of heat,” says Dontu.
You need to understand what Dontu calls the material’s thermal regime. “Basically, that’s the heat distribution in the cutting area. How much heat will be generated and where will it go? As temperatures get higher, the qualities and properties of your tool suffer… thus, in some cases, the application of coolant is paramount. For example, in machining of titanium alloys with super-hard materials, there is at least a factor of two in
tool life with and without coolant,” says Dontu.
High-pressure and through-tool coolant offer advantages where the volume of parts justifies the cost. As well, these systems can improve chip evacuation. “All those materials work-harden quite readily. And once they work-harden, you’re really going to have problems machining them,” notes Ed Mulvey, technical specialist., Horn USA, Inc.
Keeping up to date on tooling capabilities is a good investment. “Tooling has come such a long way. It’s an ebb and flow,” says Mulvey. “The tooling gets ahead of the machine tools and then the machine tools catch up and get ahead of the tooling. In the last five years, the advances in tooling have been phenomenal.”
New coatings, new carbides, new kinds of CVD/diamond and new kinds of CBN have changed the game. “At one time, I would never have thought of machining something at 60 Rockwell or harder with carbide,” says Mulvey. “Now, there are coatings and tools that do this easily.”
Be realistic about the machines in your shop. “We often get involved with machine tool issues,” says Martin. “The machine tools may be too light duty. A key component in this is the toolholder. The machines often use older, tapered-shank type tooling, more in the smaller sizes rather than in the medium or large sizes,” says Martin.
The cutting forces required make a secure connection between the machine and tool even more important. “The fewer connections you have between the insert in the toolholder and the toolholder and the machine, the better,” says Mulvey. “A lot of times, people want us to make modular tools for really tough applications. We don’t recommend that. One problem with one connection can make a really big difference.”
“In a small shop, you can sometimes machine with older machines… The slack can sometimes be compensated for with the right toolholders and high versatility tools,” says Dontu.
In-machine probing can help you build some predictability into your workflow, adds Mulvey. If your control has a tool-management system, use it. “You can program it to detect the change in torque on a drill, or check the cycles of the tool.”
The devil is in the details of feeds and speeds. “One of the focus areas is the relationship between the material removal mechanism specific to these materials and the demands on the tool, in other words, how are you taking the material off the workpiece? What does that do to your tool,” asks Dontu. “It’s a balancing act, finding enough energy to cut yet not break the tool.”
Using recommended parameters is crucial. “Sometimes, [users] do not run the tools at the correct feeds and speeds. They are often very, very conservative. That causes breakage, excessive tool wear and bad finishes,” says Martin.
“A lot of the new coatings need the heat in the chip to produce lubricity. If you’re not used to those coatings and you run at the same feeds and speeds that you’re used to, that coating is not going to help you,” says Mulvey.
Assess your processes
Successfully machining exotic materials is not all hardware and machining parameters. “People need to rethink their processes and QA procedures in terms of machining exotic materials,” as well, says Dontu.
The high value of the material is driving a movement toward near-net shape machining. “There’s a significant shift from roughing and high material removal applications to finishing applications and high precision applications,” says Dontu. That can change the workflow in the shop.
“A lot of the time when you’re dealing with
these kinds of materials and components, you’re usually working to very tight tolerances. Your QA protocol and procedures may need to improve,” says Martin. Customers have high expectations for components like these and the ability to document the process is important.
A shop with the technology, people and processes in place to cut exotic materials can cut anything. Meeting this challenge will make you stronger as a shop, even on simpler materials.
You need a systems approach. “It’s not only the cutting tool – it’s the fixturing, the machine tool, the coolant and how you apply it. Everything has an influence,” says Dontu.SMT