u/East_Bat9251

Hey everyone! I'm building a micro turbocharger and using it as an excuse to push the limits of a desktop CNC with a 4th-axis rotary attachment.

One thing I ran into: at the recommended cutting speed for 11SMn30 free-machining steel (~130 m/min), my TiAlN-coated end mills were dulling surprisingly fast. I ended up dropping the cutting speed to around 60 m/min, and tool life improved significantly.

The exact cutting parameters probably aren't that important, but for reference I tested setups like:

Setup 0 (Vc = 94m/min):
Tool D=2.5mm, 3 flutes
Ap = 0.2 mm
Ae = 0.8 mm
fz ≈ 0.01–0.015 mm/tooth
Vf ≈ 480 mm/min
MRR ≈ 0.08 cm³/min
12000 RPM

Setup 1 (Vc = 55m/min):
Tool D=2.5mm, 3 flutes
Ap = 0.2 mm
Ae = 1 mm
fz ≈ 0.01–0.015 mm/tooth
Vf ≈ 215 mm/min
MRR ≈ 0.04 cm³/min
7000 RPM

Setup 2 (Vc = 55m/min):
Tool D=2.5mm, 3 flutes
Ap = 1 mm
Ae = 0.3 mm
fz ≈ 0.01–0.015 mm/tooth
Vf ≈ 320 mm/min
MRR ≈ 0.1 cm³/min
7000 RPM

Am I doing something wrong here, or is this just the reality of a small machine with a ~200 W spindle?

Full build video in the comments if anyone's curious about the rest of the process.

https://www.youtube.com/watch?v=KOk96tyRNNA

Just to be transparent: the video is a collaboration with the CNC manufacturer whose machine I used. The engineering project itself is entirely my own hobby experiment. If sponsored content makes you uncomfortable, it's probably not for you 🙂

My hobby is building micro internal combustion engines. And now I’m building a turbocharger for one, for absolutely no practical reason. No real benefit to humanity, just engineering for fun.

The compressor wheel is 20mm in diameter. Because the air mass between the blades is so small, I need extremely high RPM to build any meaningful pressure. But at 300,000 RPM, the blade tip speed approaches transonic territory, and that causes serious problems.

Even below that threshold, the boundary layer on the blade surfaces can locally accelerate to Mach 1 at lower RPM, since local flow conditions around the blades are far from ideal. So I set myself a rough limit of 250,000 RPM, that's about 260 m/s tip speed, or roughly M=0.8 (not accounting for pressure rise inside the compressor). By my rough calculations, at that speed this turbocharger should be capable of around 0.2 bar of boost, even with the significant air consumption.

A few engineering decisions I made along the way:

Balancing: Since the rotating assembly is extremely light, I'll first test it without dynamic balancing just to see how severe the vibration actually becomes before investing time into a proper balancing setup.

Stress analysis: Back-of-envelope calculations on the compressor wheel gave me roughly 10x safety margin at max RPM.

Bearings: I went with standard zirconia ceramic bearings.

I managed to hold approximately 0.03mm clearance between the compressor blades and the housing.

Turbine nozzle: Since I can't accurately calculate the turbine inlet nozzle geometry, I made it adjustable. The exhaust gas gap can be varied from 0 to 1mm. I'll tune it experimentally.

The whole thing is roughly sized for 4-stroke petrol engines in the 20-30cc range.

If you've read this far, what are your thoughts? Have I missed anything obvious? I'd genuinely love to hear from anyone who has dealt with similar problems at this scale.

If you're curious about the machining process, I also documented the build in a YouTube video:

https://www.youtube.com/watch?v=KOk96tyRNNA

Just to be transparent: the video is a collaboration with the CNC manufacturer whose machine I used for the prototype parts. The engineering project itself is entirely my own hobby experiment. If sponsored content makes you uncomfortable, probably skip it 🙂

Thanks for reading!