Mastering Aluminum on a Hobby CNC: Feeds, Speeds, and the DLC Secret
Introduction
For many makers, the transition from cutting wood to cutting aluminum is the ultimate level-up. It turns your CNC router from a woodworking tool into a rapid prototyping machine. However, it is also the source of the most frustration: broken bits, ruined workpieces, and the dreaded sound of chatter.
The good news? Your machine is capable of it. The secret isn't a more expensive machine; it’s understanding the physics of the cut. In this guide, we’ll break down exactly how to machine aluminum successfully, starting with the one tool upgrade you absolutely need.
The Secret Weapon: Why You Need DLC Coated Endmills
If you take nothing else away from this article, let it be this: Stop using standard carbide bits for aluminum.
I literally broke over 50 standard carbide bits before I figured this out. Endmills get expensive fast when you break that many!
DLC (Diamond-Like Carbon) bits can be slightly more expensive if you buy the professional industrial versions, but you don’t necessarily need those for a hobby machine. Amazon offers some very inexpensive options; however, you need to be careful. There are many "imposter" bits that claim to have this coating but are just standard carbide.
Do not be fooled. Real DLC bits are unmistakable from regular carbide as they have a distinct rainbow-colored shine to them (see the image above).
Here is a link to get you started on the right path. (Note: This is not an affiliate link; it is just a recommendation for the specific type of bit you need).
1/4" DLC Coated Endmills
These are the bits I personally use. I have only broken one so far, and that was user error (I accidentally hit the bit while removing my finished workpiece). I have another bit that has been used at least 50 times for deep pockets and slots, and it cuts just as well as the day it was brand new.
Avoid the "Gum"
Gum is bad for your teeth, and it is terrible for your flutes.
Aluminum is a "gummy" metal. As it heats up, it wants to stick to things. When you cut aluminum with standard uncoated carbide, the aluminum creates friction, generates heat, and microscopically welds itself to the cutting edge of your bit. This is called "built-up edge" (BUE).
Once aluminum sticks to your bit, the flutes clog, the bit can no longer eject chips, and snap—your endmill breaks.
The Solution: DLC
This coating creates an incredibly hard, slick surface with a very low coefficient of friction. Think of it like a non-stick frying pan for your CNC. The aluminum chips slide right off the flute rather than sticking to it. This keeps heat down and allows you to run faster and longer without clogging.
Understanding Flutes: 1 vs. 3
Before we get into the settings, we need to understand the tool geometry.
What is a Flute?
A flute is more than just the sharp cutting edge of the endmill; it is also the deep groove (or valley) that runs up the spiral of the bit. While the sharp edge slices the material, the groove is responsible for chip evacuation—carrying the waste material up and out of the cut zone.
How Flutes Affect Speeds and Feeds
The relationship between flutes and feed rate comes down to Chip Load (the thickness of the slice each tooth takes). Every time your spindle makes one full rotation, every flute on the bit hits the material.
The Golden Rule: As you add flutes, you must increase your feed rate to prevent rubbing.
The Comparison
Single Flute (1-Flute):
Characteristics: Has the largest "valley" for chips to escape. Excellent for preventing clogging in gummy aluminum.
Feed Rate: Slowest. Since there is only one cutting edge hitting the material per revolution, you don't need to move the machine as fast to take a bite.
Three Flute (3-Flute):
Characteristics: High core strength (less deflection) and superior surface finish. However, the valleys are smaller, so chips must be cleared instantly.
Feed Rate: Fastest. Because three cutting edges hit the material every rotation, you must move the machine 3x faster than a single flute.
Bit Selection: Why 3-Flute DLC is King
While many beginners start with single flute bits to prevent clogging, experienced machinists know that for the best surface finish—especially on pocket floors and walls—a 3-Flute DLC Coated Endmill is the superior choice.
Why choose 3 flutes over 1?
Surface Finish: With three cutting edges hitting the material per revolution, you get a significantly smoother wall finish and fewer "scallop" marks on the floor of your pockets.
Strength: A 3-flute bit has a thicker core than a single flute bit, making it more rigid. This reduces deflection, which is critical when accuracy matters.
The DLC Factor: Normally, 3-flute bits risk clogging in aluminum on hobby machines. However, the slick DLC coating allows chips to slide out easily, giving you the benefits of 3 flutes without the welding risk.
Speeds and Feeds: The Recipes
Because every machine is different (Shapeoko vs. Onefinity vs. custom builds), I have provided two recipes below. Choose the one that matches your bit.
Recipe 1: The 3-Flute DLC Success (Recommended)
For a 1/4" (6.35mm) 3-Flute DLC Endmill
RPM: 18,000
Feed Rate: 80 - 100 inches per minute (IPM)
Note: We move fast here to keep the chip load healthy!
Depth of Cut (DOC): 0.015" - 0.030" (Depending on machine rigidity—start shallow!)
Plunge Rate: 20 IPM
Key Takeaway: Don't be afraid of the higher feed rate. With three flutes, if you move too slow, you aren't biting into the aluminum; you're just burnishing it, which creates heat. Trust the geometry of the bit and keep it moving!
Recipe 2: The Single Flute Baseline
For a 1/8" (3.175mm) Single Flute DLC Endmill
RPM: 18,000
Feed Rate: 35 - 45 IPM
Depth of Cut (DOC): 0.010" - 0.015"
Plunge Rate: 10 IPM
Chip Evacuation: Don't Recut Your Trash
This is the silent killer of endmills. If you cut a chip, and that chip stays in the slot, your bit will hit it again on the next rotation. Recutting chips creates massive heat spikes instantly.
Vacuum: A dust boot is often not enough for deep slots because it doesn't have the velocity to lift heavy metal chips.
Air Blast: You need a directed stream of air pointing right at the bit. This blows the chips out of the cut zone immediately.
Lubrication: If you don't have a misting system, a manual spray of WD-40 or Denatured Alcohol helps prevent chip welding, but consistent air blast is arguably more important.
Work Holding: Vibration is the Enemy
Wood is forgiving; aluminum is not. If your workpiece vibrates even microscopically, it will cause "chatter," which leaves a terrible surface finish and destroys tool life.
The Tape & Glue Method
For plate aluminum, the "Blue Tape and Super Glue" method is often superior to clamps. It holds the entire bottom surface area of the stock, preventing the center of the sheet from lifting.
Blue Tape and Super Glue
Place a strip of blue painter's tape on your workpiece.
Place a matching strip on your waste board.
Apply CA (Super Glue) on one strip and a spray of accelerant on the other.
Stick them together for an instant, rigid bond.
Clamps
If using clamps, keep them close to the cut area, but ensure your clearance height (Z-Safe Height) is set high enough to avoid crashing into them.
Holding Tabs: The Safety Belt for Your Parts
One of the scariest moments in CNC machining is the "loose part" phenomenon. If you cut a part completely free from the stock material while the spindle is still spinning at 18,000 RPM, two things happen:
The part vibrates instantly, ruining the beautiful edge finish you just worked so hard to get. The part catches the spinning bit and becomes a dangerous projectile that can take out your cat, dog, or fellow humans.
Unless you are using the Tape & Glue method, you must use holding tabs for any profile (cut-out) operation.
Best Practices for Aluminum Tabs
Tabs in aluminum are different than tabs in wood. You cannot easily sand them off, so placement is critical.
Use "Triangular" (3D) Tabs: In your CAM software, choose Triangular/3D tabs over Square ones. This allows the machine to ramp up and down smoothly without stopping, leaving a cleaner surface finish on the rest of the wall.
Placement: Never place a tab on a corner. Always place tabs on the longest, flattest straight edges of your part so they are easy to file off later.
Size: Aluminum is strong. A tab Height of 0.040" - 0.060" and a Length of 0.125" is plenty to hold the part.
Conclusion
Cutting aluminum doesn't require industrial equipment; it requires the right recipe. By switching to DLC endmills, keeping your chips clear, and trusting your feed rates, you can turn your garage CNC into a precision manufacturing tool.
Have you tried cutting aluminum yet? Join one of our local Great Meets Maker Groups to share your projects or ask for help with your settings!.