The PF4 and PF5 chipbreakers on most of the Walter inserts I sent have a minimum depth of cut of .004" per side, or .008" on diameter. While that light of a dpeth of cut probably wouldn't fly in a stringy material like 1018 cold rolled steel, it'll work well in 4140.
Sometimes it's best to just not use coolant at all. Carbide can be sensitive to thermal shock, resulting in micro cracks at the cutting edge. This kills insert life. Also, coatings such as TiAlN and AlTiN do work better if the coating is allowed to take some heat. Too slow and cool can result in material building up on the edge of the tool. When it builds up too much, the force can cause catastrophic insert failure at worst or a horrible finish at best.
Sometimes it's best to just not use coolant at all. Carbide can be sensitive to thermal shock, resulting in micro cracks at the cutting edge. This kills insert life. Also, coatings such as TiAlN and AlTiN do work better if the coating is allowed to take some heat. Too slow and cool can result in material building up on the edge of the tool. When it builds up too much, the force can cause catastrophic insert failure at worst or a horrible finish at best.