Router bits used in a mill

[MrCrankyface]

I've used router bits several times in steel, sounds awful but just gotta feed in carefully so you don't shockload the bits.
Shanks have been between 8mm / 5/16" and 12mm / 1/2".
I usually use a collet chuck to grip them as close as possible to the head so the shank gets maximum support.

 

[ChazzC]

Go slower, avoid any chatter. But yeah, ½" shanks would be best.
I should add. I'm not saying router bits are a direct replacement for proper milling cutters, just that, in a pinch they can be used. EG, I wouldn't attempt to cut slot in steel with a ¼"shank ¾"wide router bit. But round over or chamfer an edge should be no problem.

I’ve used 1/2” shank Carbide radius router bits on Aluminum & Acetal with very good results. Speeds & feeds based on number of flutes, an advantage that router bits have is the ball bearing guide, making perfect cutter alignment much easier.

 

[Martin W]

Yes, they will work. Router bits are not as beefy as a machine cutter. Slow speeds and feeds will win the race. Not a router bit , but here is a link of a skilsaw blade used to cut a steel .
Martin
channel.

 

[BaronJ]

Most of my router bits are 1/4" shanks and that alone scares me.

I very often use router bits on mild steel particularly for angled or shaped cuts. I admit that I don't particularly care for the 1/4" shank ones but if you hold them with the shank well inside the collet they are ok. I much prefer the 1/2" shank cutters.