This should by rights be a question for the instructor of my milling class, but I forgot to ask him last night and I won't see him again till next week. So in the meantime I'll ask you guys:
We were squaring up some blocks of 4340 steel, using a face mill with carbide inserts, on a milling machine. The instructor told us not to use coolant; and indeed, now that I think about it, I don't think there even
*was* any coolant apparatus near the milling machines. As opposed to the bandsaw, which was constantly bathed in coolant, the grinders, and I think at least some of the lathes.
When does one use coolant? How come we got away with cutting the steel without any coolant? (The chips are blue; it certainly got hot enough, I guess.)
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1) You were using carbide, which can tolerate higher temperatures than HSS can tolerate.
2) You were using carbide which *cannot* tolerate the thermal shock of getting hot and then getting splashed with coolant.
Given that you were dealing with a face mill with (typically lots of )inserts, unless you could *keep* the inserts *flooded* with coolant, you would soon start to pop inserts, and a face mill uses quite a few, so consider the cost at perhaps $5.00-$15.00 per insert. (And even those which did not pop from thermal shock would be running into fragments of those which *did* pop, and thus likely be damaged as well.
So -- use coolant when:
A) The tool material which you are using would be degraded at the temperatures it would reach without coolant with the cuts which you were taking. Carbon steel would be the worst, then HSS, then Cobalt steel, then Carbide, and finally (probably) the ceramic inserts. (Or PCD (PolyCrystaline Diamond, as long as you are *not* cutting steel.)
B) The flood of coolant can carry away chips which would otherwise keep being recut, and impacting the finish on the workpiece. (especially a problem with pocket milling with an endmill.) Sometimes an air blast can accomplish this just as well, but beware of blowing chips into areas of the machine which can suffer from the chips, such as under the ways, or in bearings.
C) Thermal shock is *not* likely to damage the tools (carbide, probably ceramic, and perhaps others.) If you are using HSS or Cobalt steel, it is probably a good choice to use coolant, especially if you are pushing the machine.
D) You have a need to push the speed of machining -- e.g. for production cost/tight schedule reasons.
Note that mist coolant, or perhaps even the micro-drop coolant may or may not present the cooling shock problem. If the edges spend some time buried in the work and out of reach of the coolant, it is a risk with thermal-shock sensitive tool materials.
These are my opinions. Now we wait for others to add on.
The key phrase here is ....... "using a face mill with carbide inserts". Carbide insert cutters are generally run without coolant on both the mill and the lathe. They are designed to work 'hard' (i.e. large cuts and/or high feeds) and the chips should be a nice 'cobalt' or 'spring steel' blue colour. Light cuts will take off the cutting edge very quickly resulting in poor surface finish. A sure sign that the cutting edge has 'gone off' is if you are producing sparks or glowing red chips. Care should be taken to contain the flying chips when using carbide tipped tools as they are short, fly every where and are very hot. I have a scar under my left eye where a hot chip dropped 'behind' my safety glasses and lodged against my cheek (it left a nice burn mark in the glasses too!)
As to your coolant question, coolant is generally used when cutting with HSS cutters on steels to reduce the temperature at the cutting edge and to improve surface finish. Cast iron, brass and some types of aluminum are termed 'free cutting' and do not require coolant.
I do not understand the instructor's reasoning. All of my experience with this material indicates that in the annealed state a sharp tool and lubrication is absolutely essential, as this material will readily harden with heat and or friction. I cannot overstate the importance of keeping the cutter sharp. Tool drag WILL ruin your whole day. Please tell this group your instructor's reasoning. He must know something I don't. Steve