Every part of every product has a job to do, and how well it does that job depends on several factors, starting with what it’s made of. Whether it’s a fin in a jet engine, a heat shield, or the hood ornament on a car, the metal in it is selected to meet certain requirements. You don’t need a master’s degree in metallurgy to make that call; you just need to know what questions to ask.
You need to know the environment your part will be working in.
• High (and maybe Low) temperature
• Exposure to water/salt/corrosive chemicals
• Flexing/pressure/vibration
• Finishes
Metal Fatigue
Parts that are exposed to loads, pressure, and vibrations can give way seemingly without warning. One shortcut to circumvent this is the Maximum Service Temperature of the metal. CNC-machined metal parts can deform if they get too close to their melting point. That’s known as the Yield Point. But they can also fail without ever reaching the Yield Point. Creep is the elastic deformation of a part under stress. It can deform when in use, but when taken out of service and measured, everything is fine. This Creep grows with use until the unit truly fails. Choosing a metal with an MST well above what your part will see goes a long way toward avoiding this kind of failure.
Strength
Strength in metals can be measured as tensile, compressive, or impact, and every metal has a different profile among the three. For example, some brass can have a higher yield strength than titanium, but titanium will have a higher tensile strength. There’s no shortcut to this one; this is one of those areas where mechanical engineers really earn their keep.
Hardness
Hardness is specifically the resistance to penetration. Chromium is ridiculously hard, which is one reason it’s widely used in alloys. By the same token, you can find bronze alloys that are nearly as hard as steel or titanium. At the highest end, you’ll find refractory metals like Tungsten and Tantalum. Hardness matters when choosing CNC machining materials. The metal wears down CNC machine tools. It has to be milled more slowly and tools replaced more often, driving up machining costs, so bear that in mind.
Freezing
Cryogenics comes up once in a while, and knowing the cold limits your part will have to endure can be important. Carbon steel, for example, becomes too brittle for most uses not much below zero. If you’ll be operating below -325°F, take a look at 304 stainless or aluminum.
Polishing
Parts may need to be polished on wearing surfaces, or for decorative reasons. Both are valid, and it’s something to be considered. Titanium, for all its strength, doesn’t polish well. Stainless polishes well, as does brass, but brass will need some sort of clear coat to protect the finish.
Finishing
The surface finish may be central to marketing your product, or it may have more to do with durability. Steels -other than stainless, of course- will need some sort of rust protection in most cases. If your parts will be powder coated, surface finishes may be slightly more forgiving than an aluminum part that will be anodized, since anodizing will show every tiny defect.
In the end, figuring out CNC machining materials involves dozens of metals and alloys and a lot of different criteria. Not all criteria are factors in every choice. It’s simpler when you focus on the ones that apply to your situation. It’s even simpler than that when you drop the selection process in the lap of a qualified expert as we have at Ben Machine. We have the experience and knowledge to help you determine exactly which metal will meet your specific criteria and do it economically, which probably makes calling us the real decision you need to make.
