I wanted to update everyone on my steel-cutting adventure with the Onefinity Journeyman.
Tl;dr: If you are patient (i.e. can tolerate slow feeds), you CAN cut steel!
First, the bad news:
I have used small 2mm and 6mm carbide endmills to keep surface speeds lower (and broke them).
While I still have many parameters to tune the one I’ve used successfully are veery sllowww feeds–not for time-sensitive production runs.
How slow? My successful settings are clearing material @ 50mm/min in 1mm depth passes with an 8mm carbide square end mill. (This is made a lot slower by the free non-commercial version of Fusion 360 makes all movements at cut speed. This is remedied with a subscription.)
The next biggest problem I’ve encountered is chatter, so mass in the table, a firmly attached CNC and an extra-rigid spindle attachment are all your friends (I’d use a steel plate tabletop if plate steel wasn’t so expensive). As it stands, my tabletop is a thick layer (about 43mm) of plywood.
And now the good news:
It works! I am now milling mild steel with the Onefinity Journeyman successfully.
I ran my 8mm carbide end mill last night and made 8 successful cuts. I’ll be making another 12 today.
The steel looks great–no black-and-blue chips (and I’m still cutting dry) and the roughing passes look great.
I worked with @PwnCNC to find an inexpensive 4-pole 80mm motor which could mill steel. It’s a G-Penny 2.2kW water-cooled unit. Peak torque isn’t super high at 0.9Nm, and minimum speed for peak torque is a high 12k RPM, but running the unit at 9k RPM gives consistent speed (i.e. sufficient torque to not vary speed or stall during cuts) without being so fast as to burn up the steel stock.
Daniel’s VFD supports sensorless vector control, a feature which seems to be important for lower RPM, higher torque milling. It also supports 800Hz operation, which means you can continue to mill your higher-speed materials at 24k RPM with the very same spindle! (I like it when I can have my cake and eat it too! 
)
Earlier in the thread, I promised to share specifications of the motor, if I got any:
For additional $$$, you can get more powerful spindles which will give you better (higher) feeds and more torque at lower speeds–for details, please see some of @Aiph5u’s comments in this thread.
From the CNC machine’s perspective, it seems plenty rigid so far (needs more mass). The weakest link is the 80mm spindle holder. A more rigid design for steel there would give excellent bang for the buck. Those are two areas I may explore in the future, but for now, I am unblocked, and consider the experiment to be a success. 
The above pictures show 5mm notches cut into mild steel. I have gone as deep as 10mm so far with the same performance. This has all worked well enough that soon I’ll be testing cutting 28mm diameter holes all the way through 13mm plate steel with 2mm 45° chamfers (building a welding fixture table). Wish me luck! 
Thanks to everyone in this thread and especially Daniel @ @PwnCNC for his stellar support and low-cost solution, which has saved me over a thousand dollars.
If you are interested in milling steel, let me know if you have any questions, and I’ll do my best to help you out!
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