I’m going to be switching between various structural plastics, aluminum, steel, and even wood from time to time. Plastics and wood I can cut without any coolant without a problem. For steel I imagine there’s no compromise there… flood coolant every time. For aluminum though, there appears to be some flexibility – sometimes coolant… sometimes not. I’m trying to understand the limits of cutting aluminum without coolant. Are there any rules of thumb on feed speed, depths of cut, mill types, lengths of time, etc.? Do these change for various grades of Aluminum?
I would say the opposite. You can cut steel dry but you cannot cut aluminum dry, it will heat up and clog the flutes of the endmill. Ive cut steel dry without issue i have not attempted aluminum. But i cut both with flood coolant now and havent had any issues.
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Really? That is surprising to me. I thought: harder metal, higher temps.
To an extent. More knowledgeable people will have to chime in but the chips should take away a lot of the heat, when steel heats up it doesnt get gummy but aluminum will get soft and stick in the flutes of the endmill. So its important to stop the material from heating up.
Some will say coolant reduces the life of carbide tooling as well. And it may, but flood coolant 1/4 inch 3 flute carbide at 60 ipm gets me a great surface finish on aluminum. I tried doing langmuires speeds and feeds chart with a 2 flute and the finish was ass and loud. It was an altin coated endmill as well. I tried uploading a video but for some reason it says it isnt formstted correctly
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I cut aluminum with a strong air blast to get chips away from the work. I’ll mix in a small amount of coolant droplets if I am working in a tight bore or other spot where chip evacuation is a problem. My machine doesn’t have flood coolant and that hasn’t been a problem.
I use a vacuum when cutting wood or plastic so that the fine dust doesn’t end up everywhere.
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You are correct, steel chips are very spicy. Steel is often run dry because much of the heat leaves with the chip (good). Also, with coatings such as AlTiN, the elevated cutting temperatures also help form a thin aluminium oxide layer on the coating that protects the tool.
Another reason AlTiN tools are frequently run dry is that poorly applied flood coolant (e.g. flood from one direction only) can repeatedly heat and quench the cutting edge, causing thermal shock that can shorten tool life.
But then you might wonder, if thermal shock is so harmful and AlTiN performs best at high temperatures, why is flood coolant so often recommended when milling stainless steel and titanium?
Well, long story short, the heat doesn’t leave with the chip, so what was a good thing in steel becomes too much heat at the cutting edge. This excess heat makes stainless and titanium much more likely to weld to the tool and accelerates tool wear. In these materials, using proper flood coolant to remove that heat is more beneficial than trying to keep the coating hotter, with little to no thermal cycling downside as long as you’re truly flooding it.
At least, that’s how I understand it. Stuff made no sense to me at first lol
1. Steel & AlTiN Coatings: Keep it Dry
I cut steel dry about 98% of the time. You do not want to run flood coolant on AlTiN-coated tools. The extreme temperature swings cause thermal shock, which cracks the coating and causes it to flake off, ruining the tool.
Instead, AlTiN relies on heat to activate. At around 1450°F (800°C), the aluminum in the coating reacts with oxygen to form a micro-thin layer of Aluminum Oxide. This sapphire-hard ceramic layer acts as a thermal shield, protecting the carbide core and drastically extending tool life. Running flood coolant prevents the tool from reaching this optimal activation temperature and causes thermal fracturing.
2. AlCrN Coatings & Stainless Steel (SS)
With AlCrN (Aluminum Chromium Nitride) coatings, you can run coolant, but it should generally be reserved for full-width slotting or machining Stainless Steel. While AlCrN has better thermal shock resistance than AlTiN, using coolant will still slightly reduce its overall tool life compared to running it dry.
However, Stainless Steel requires coolant for two critical reasons:
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Work Hardening: SS work-hardens rapidly under heat and friction. If your surface footage (SFM) is too high or you lack proper lubrication, a soft, easy-to-cut stainless can instantly jump to 40+ HRC mid-cut. If that happens, your programmed feeds and speeds are out the window and your tool is toast.
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Thermal Warpage: Stainless steel has poor thermal conductivity and a high coefficient of thermal expansion. Any heat buildup will cause the part to warp severely.
3. Aluminum & Titanium: Flood Them
Aluminum is incredibly gummy and will weld itself to the cutting edges—even on bright (uncoated) or DLC (Diamond-Like Carbon) tooling. Because of this, I almost always run flood coolant on aluminum. If I am running a lighter, low-load toolpath, I might use a mist/MQL system, but I almost never cut aluminum with a dry air blast.
Ultimately, Stainless Steel, Aluminum, and Titanium all require heavy coolant—the more volume and pressure, the better.
4. Hardened Tool Steels (45+ HRC)
For hardened tool steels over 45 HRC, completely turn off the coolant. Instead, drop your cutting speed down to 150 to 200 SFM and use a strong air blast to clear the chips. Introducing coolant to hardened steel machining guarantees immediate thermal micro-cracking on your carbide edges.
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