How cutting fluid works (newbie question)

Very new at this stuff. I was drilling a hole in some bronze the other day and was using cutting fluid, backing the drill out frequently to clear the
chips and paint some more cutting fluid on, and I began to wonder if the cutting fluid was even getting to the actual cutting edges of the drill bit. Seems like the fluid would get wiped off instantly by the chips, so is the cutting actually metal-on-metal with the fluid just cooling and lubricating the drill bit and hole so that the chips slide out more easily, or is the cutting fluid supposed to actually act right at the cutting edge of the drill bit?
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John Snow
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John, The answers are, "Yes" and "Yes". The cutting fluid does two things: It cools the material at the cutting edge (chips too) and its lubricious quality aids in chip flow away from the area of the cut. Bob Swinney

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Robert Swinney writes:

You forget, "smells good, too".
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Hitch wrote:

Cutting fluids serve a variety of puropses, depending on the fluid and meat being cut.
1) A cutting fluid reduces cutting forces. For reasons not well understood, an appropriate cutting fluid actually makes it easier to peel metal. The fluid penetrates the grain structure of the metals surface, somehow making it easier to separate the grains. This is analogous to a reduction in "surface tension" of the metal, and related to microscopic crack propigation.
2) The fluid lubricates the cutting tool. This reduces friction, especially between the tool and the chip, and also between the tool and the cut surface. Lessened friction means less heating of the tool, and lower cutting forces.
3) The fluid cools the tools cutting edges, preventing loss of 'temper', and allowing them to stay sharp longer (especially important with carbon-steel tools).
4) The fluid serves as a barrier, preventing chips from friction-welding to the tip of the cutting tool. Such a lump of metal stuck to the tool changes its effective shape, and hence degrades its cutting properties.
5) In the case of drills, taps, reamers, milling cutters and similar fluted tools, the fluid keeps chips from sticking in the flutes, lessening clogging of the tool.
6) When machining most plastics, and low melting-point metals, the fluid cools the workpiece, and prevents localized melting near the cutting edges of the tool, with corresponding workpiece deformation, and possible sticking of the cutting tool.
7) In a production situation, cutting fluids are applied in large quantities, often under pressure, to flush chips away from the cutting tool.
8) Reduce or eliminate oxidation (if this is a problem).
Dan Mitchell ==========
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I wrote:
"Cutting fluids serve a variety of puropses, depending on the fluid and meat being cut."
Obviously I meant "METAL being cut" ...
Must be time for lunch! :-(
Dan Mitchell ==========Daniel A. Mitchell wrote:

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Ha. I didn't even notice it! ;o) Guess I'm used to raednig tpyoes . . .
But meat... I s'pose that'd be the infamous lard oil, no? Good for cutting everything from its original pork source to HOGging through O1 steel! ;-)
Tim
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Thanks, Daniel, that's a helpful rundown.
However, it doesn't answer one thing, which is related to the OP's original question: how can it do all those things, when it seems like it gets wiped away or evaporates almost instantly?
I understand how it works in an industrial situation where the workpiece is being flooded by coolant. But I've also seen recommendations to "brush some cutting oil onto the tool before making the cut", or dribble a few drops onto the workpiece from a hand oiler, or the like. I've always wondered how/whether that works.
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Brushing or "dribbling" a few drops: How does it work? Not very well, but defintely better than none at all.
My personal theory is that it serves to lubricate the margins of the dirll which are in costant contact with the side of the hole, along with helping with chip removal, etc.

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but
Well, in theory, the drill isn't in contact with the hole sides. Twist drills are not made straight, contrary to popular opinion. They, like machine reamers, have a small amount of taper towards the shank so they do not drag on the margin, which, in both cases, is circularly ground, unlike end mills, which are intended to cut on the flutes. That, plus the fact that drills tend to cut slightly oversized (not always) equates to a drill that doesn't make contact, but in practice they do, at least at intervals. Part of the problem with drilling is that due to poor design, drills don't exactly cut everything in their path. The point, where the web resides, tends to just deform and more or less get pushed away. That accounts for a large part of resistance one experiences when pushing drills through tough metals. A split point, or a pilot hole that is the size of the drill web helps reduce drilling pressure, and often eliminates the fine particles that can be problematic. Again, not always. A lot depends on the job at hand and many other circumstances.
Back to the oil and how a drill fits the hole, the bits of metal that flake off from the drilling operation play a part in how well chips evacuate the flutes. By oiling occasionally, the small bits tend to keep moving instead of getting friction welded to the drill and hole, so even the slightest lubrication plays a big role in how well the project turns out.
If you go back far enough in time, before 1,1,1, trichloroethane was removed from TapMatic for ferrous metals, you'd see that the smallest amount of lubrication made a huge difference in performance, both in tapping and in reaming. In spite of the fact that the fluid was thinner than water, it made a significant difference in outcome. It often spelled the difference between broken taps and tapped holes. Don't discount brush lubrication. It works, just not as well as flood cooling. That, by the way, is one of the messiest of operations when applied to a drill press. Imagine those long chips flinging oil. Been there, done that. Luckily, on a gang drill set up for coolant.
Harold
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Harold & Susan Vordos wrote:

I've found this to be especially true with aluminum.
Ted
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It is surprising how little oil it takes to reduce the friction. When using a cut off tool in the lathe, you can tell the difference between not using any oil and just brushing a thin film on the cutter. I usually use a lot more than that, but the effect of thin film can be observed.
Dan
Daniel, that's a helpful rundown.

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Dan Caster wrote:

A 'flood' of cutting flood is mostly for cooling, or for washing away the chips. It's effective, but messy. Mists use less fluid, but have their own problems with degrading air quality, etc. A blast of COLD air can also be useful for cooling and chip removal, but has no other 'cutting fluid' effects.
Only a small amount of the correct fluid is needed for lubrication, or for altering the mechanics of metal separation.
Much of the fluid is carried away on the chips, so it must be replenished as consumed, but a full flood is not really needed to get considerable benefit. Even a few drops can often make a BIG difference.
Dan Mitchell ==========
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Daniel A. Mitchell wrote:

I trip it on using a large hypodermic and a #16 needle. Obtain from a rural pharmacy or a vet supply. There was an article in HSM some years ago on building an electronically controlled, solenoid actuated pump unit that would do this. It's on the to-do list but ....
Drop-at-a-time is much cleaner and less mist-ifying <G> than either flood or mist.
Ted
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I think it's imprortant to remember that even a few molecules of oil at the interface can effect the results. So the original poster, who thought that brushed on or even flood coolant would not reach the exact point of the cutting tool might be mistaken.
As you say, I've found that the effect (reduced cutting forces, improved finish) seems to last a bit longer than the observed film of fluid on the workpiece. It takes longer than I would expect for the cutoff tool to start complaining again, for example.
Jim
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wrote:

Some cutting fluids are liquid until subjected to the high pressure at the cutting point where they become solid. So there is an actual barrier between the tool and the work. A very thin barrier. The coatings on cutting tools can be quite thin also. Just so many millionths of an inch. But applied properly to the tool will lengthen tool life and lower cutting forces considerably. ERS
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I think the OP's point was that, if the tool metal is in perfect contact with the work, how can fluid get there? If it coats the tool, liquid TiN in other words, that's a different story.
Tim
-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms
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I'm not an expert on this, but I can think of a couple of ideas.
1) it's not.
2) grain boundary diffusion.
Jim
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If it does indeed solidify under pressure, then it might stick to the cutter in preference to the work, or something, and allow some flow, and so forth. But grain boundries doesn't seem plausible, as oil doesn't leak out of your car's castings! Not counting splitting of course, which I can easily visualize, having whittled on wood before.
Tim
-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms
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For oil to leak out of a casting, the spaces between the grains have to be continous over the entire thickness of the casting, so the fluid can percolate, right?
In the case we're discussing, the behavior near the cutting edge of a metalworking tool, the distance scale is much much smaller - more like a thousanth of an inch or less, vs a half or quarter of an inch.
If you made an engine casting with a two mill wall thickness, you *would* have to worry about oil percolation through the grain boundaries.
Jim
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Tim Williams wrote:

Cast iron is indeed porous to oil. Many old castings are soaked with it!
Dan Mitchell ==========
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