Ever wonder why two CNC-milled parts can look so similar but perform so differently? Here’s a secret: it’s not just about the machine—it’s about the material. Yep, the stuff you start with matters… a lot.
At Advanced Precision Machining, we’ve seen first-hand how picking the right material can either save the day or lead to expensive re-dos (let’s not talk about that one crazy stainless steel job that dulled three sets of tools). Trust me, it isn’t always as simple as reaching for whatever’s in stock.
In this guide, I’ll walk you through the ins and outs of CNC milling materials, what makes some materials more “machinable” than others, and why a little knowledge goes a long way in material selection for CNC machining. Whether you’re new to CNC, puzzled by material specs, or just want to know if aluminum is really that much easier than steel, you’re in the right place. Let’s dive in!
Material Properties and Their Impact on Machinability
Alright, so what actually makes a material good—or not so good—for CNC milling? Let’s break down a few key CNC material properties. Picture these like the “personality traits” of your raw material, and trust me, each one makes a difference in the shop.
Hardness: The Tough Cookie
Think of hardness as how stubborn your material is. Hardened steel or tough titanium? That stuff fights back. It wears out your tools, slows everything down, and can even cause headaches with chatter or poor finishes. But, sometimes, you need that durability—like in aerospace parts or high-wear gears.
On the flip side, softer materials like nylon or mild steel are kinder on tools and let you ramp up the speed. It’s like the difference between cutting a carrot and chopping through a coconut shell.
Tensile Strength: Pulling Power
Here’s where things get interesting. Tensile strength is about how much a material can take before it snaps when you pull on it. High-tensile materials—think stainless steel—put up a fight. You’ll need extra muscle (and sharper tools) to keep up with the cutting forces. If you’re not careful, you might even get snapped tools or messy chip breakage.
Ductility: “Bend, Don’t Break”
Ductile materials, like aluminum, bend and stretch instead of shattering. That’s good for surface finish but can make chips get long and stringy (ever pull a huge tangled aluminum chip from a project?). Brittle stuff, like acrylic, can snap or crack if you push it too far—definitely not fun.
Thermal Conductivity: Handling the Heat
I love aluminum because it’s basically a heat sponge—it pulls heat away from the cutting area, protecting both your part and your tools. Some plastics and stainless steels, though, are like thermal blankets. They hang onto heat and can warp or even melt during high-speed machining. That’s a recipe for all sorts of weird surprises.
A Quick Look: How Properties Affect Machinability
| Property | High Example | Low Example | What Does It Mean for Machining |
|---|---|---|---|
| Hardness | Hardened Steel | Nylon | Hard stuff = more tool wear, slowness |
| Tensile Strength | Titanium | ABS Plastic | Higher strength = tougher to cut |
| Ductility | Aluminum | Acrylic | More ductile = longer (messier) chips |
| Thermal Conduct. | Aluminum | Stainless Steel | Low = more heat trouble, higher risk of warps |
Let’s not forget elasticity (how springy the material is) and grain structure, which can mess with surface finish if you’re chasing perfection. And yes, we’ve all had a part that just wouldn’t hold still because it kept flexing during the cut!
If you want a deeper dive into these properties and what they mean for your project, check out our full Colorado CNC Milling Guide.
Common Materials Used in CNC Milling
So, what’s out there? At the end of the day, your choice of material is a balancing act between performance, cost, and good old-fashioned machinability.
Metals
Aluminum
Aluminum is the go-to for a reason. It machines like butter, is perfect for prototyping and production, and it’s resistant to corrosion. If you want a fast turnaround and a great all-around performer, start here. It’s like the trusty pickup truck of CNC materials—reliable and ready for anything.
Steel
Steel is strong and durable—great for heavy-duty parts. Mild steels aren’t hard to work with, but crank up the hardness and you’ll need specialized tooling. Gears, shafts, and structural supports often call for steel. Just keep a few extra endmills on standby!
Stainless Steel
If there’s a material people love to hate (but still have to use), it’s stainless steel. Less machinable thanks to its toughness and tendency to “work harden,” but if you’re building medical devices or anything that’ll see moisture, it’s worth it.
Titanium
Machining titanium isn’t for the faint of heart (or the under-prepared). It’s strong and light—fantastic for aerospace or medical parts—but eats tools for breakfast. If you need titanium, make sure you’re working with a shop that knows its stuff (like us at Advanced Precision Machine).
Side-by-Side Metals Comparison
| Metal | Strength | Hardness | Machinability | Ballpark Cost |
|---|---|---|---|---|
| Aluminum | Good | Low | Excellent | $ |
| Steel | High | Medium | Good/Fair | $$ |
| Stainless | High | Medium | Challenging | $$ |
| Titanium | Very High | High | Difficult | $$$ |
Plastics
ABS
ABS is popular for prototyping and sturdy, lightweight parts. It cuts cleanly and leaves a great finish—plus, it’s lower cost than metals.
Acrylic
Acrylic looks fantastic when polished and works well for signs and optical parts. It’s brittle, though, so be gentle and keep your feeds and speeds steady.
Delrin (Acetal)
Delrin is a machinist’s dream for low friction, high-wear applications (think bushings and gears). It machines clean and holds tight tolerances with ease.
Nylon
Tough and wear-resistant, nylon does well for functional prototypes or parts that need a little flex. It does absorb moisture, though, so beware of warping over time.
Side-by-Side Plastics Comparison
| Plastic | Strength | Machinability | Typical Use |
|---|---|---|---|
| ABS | Good | Excellent | Prototypes, housings |
| Acrylic | Moderate | Good | Displays, lenses |
| Delrin | High | Excellent | Gears, bushings |
| Nylon | High | Good | Functional parts |
Composites
You’ll find carbon fiber-reinforced plastics (CFRPs) in cutting-edge applications. These materials are strong, stiff, and super light, but they’re also abrasive—so tool life can take a hit. Oh, and don’t forget the dust; proper ventilation is a must!
Wood
Yes, CNC can mill wood! Hardwoods like maple or oak make incredible custom pieces. Just remember, wood can move and chip unpredictably. It’s not as precise as metal or plastic, but if you want character, it can’t be beat.
Machinability Ratings and Selection Criteria
Ever see machinability ratings and wonder what they really mean? You’re not alone! These are basically “ease-of-use” scores for machinable materials, with standard steel set at 100%. Something like free-machining brass can be up to 200%, while tough stuff like titanium is way down the scale. Higher numbers mean easier, faster, cheaper machining.
How Do You Pick?
Let me break it down with a few simple questions you can ask yourself (or your machinist):
- What does the part need to do? If it needs to survive harsh chemistry or extreme wear, choose high-performance materials, even if they’re tougher to machine.
- What’s your budget? Easier-to-machine materials usually bring costs down. Sometimes, though, only titanium or specialty steel will get the job done.
- How fast do you need it? Quick prototyping? Aluminum or ABS will save you both time and money.
- Any special requirements? Medical, aerospace, or food-grade? That’s where stainless or titanium shine.
And hey, don’t forget about lead time. The most machinable material in the world won’t help if it’s stuck in backorder.
Conclusion
Choosing the right material for CNC milling isn’t just a technical checkbox—it’s a foundational decision that can make or break your project’s quality, budget, and lead time. From the toughness of titanium to the user-friendliness of aluminum and ABS plastics, every material brings its own set of benefits and challenges to the table. Maybe you’ve had a project run smoothly because the right material made machining a breeze, or perhaps you’ve lost hours (and tools) battling with something that just wouldn’t cooperate. Either way, material selection truly is where expertise shines.
Ready to get started, or want to troubleshoot a tricky material challenge? Explore our in-depth Colorado CNC Milling Guide here, or reach out for a consultation with our experienced team of CNC machinists. Let’s make your next project a success—one expertly chosen material at a time!
by Gerry Dillon
Gerry Dillon is a co-founder, current owner and certified CNC machinist at Advanced Precision Machining (APM), a full-service machine shop located in Longmont, Colorado. Gerry has over 30 years of precision milling and machining experience under his belt.