Alex-
You need to get some understanding on how to set your speeds and feeds for various materials and cutter types. One way is to use one of the PC or online based speed/feed calculators. I use the rules of thumb below to get me close, and dial in from there on how the machine is responding.
Figuring your SFM (Surface Feet/Minute, which will determine your RPM)
SFM with HSS endmills
Stainless Steel 40
Mild Steel 100
Brass 300
Aluminum 400
With carbide endmills, multiply those settings by 3 as a starting point, so:
SFM with Carbide Endmills
Stainless Steel 120
Mild Steel 300
Brass 900
Aluminum 1200
Then:
RPM = 4 x SFM/Diameter
Now to find your feed, first calculate your chip load. A reasonable starting point for the chip load is to divide your endmill diameter by 200.
Chip Load = Diameter/200
Then to calculate your Feed Rate:
Feed Rate= RPM x Num of Teeth x Chip Load
So with your 1/4" HSS endmill in steel, your RPM should be:
RPM = 4 x 100/.25 = 1600
Your feedrate should be:
Feed Rate = 1600 x 2 x .25/200 = 4 ipm
This a starting point, I usually crank down a little from these recommended settings to see how the machine responds.
However in your last post you stated that you changed to a carbide endmill. You should recalculate the feed and speed for that endmill. Carbide endmills don't last long if they are underfed, which is what you are doing with the last feed and speed you mentioned.
As the other poster mentioned, a 4 flute endmill would be better for steel. In particular, I find the "Hanita" style variable flute carbide ones work really well on Tormach and BP sized machines. I get mine from
www.maritool.com and
www.lakeshorecarbide.com .
As far as your maximum depth of cut, on smaller machines this is often determined by the available spindle HP you have. However if the machine is up to it, I use the following guidelines for max radial and axial cut (borrowed from Stan Dorfeld):
Slotting: Cut Depths
6061 Aluminum, Brass - 1/2 endmill diameter
7075 Aluminum - 40% endmill diameter
Mild Steel - 30-35% endmill diameter
Stainless Steel - 25% endmill diameter
Rough Profiling Tool Overlap: 70% endmill diameter or less
Finish Profiling Tool Overlap: 3% endmill diameter
Good luck-
Paul T.