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?
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 metal?s 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 tool?s 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).
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.
((((((((((((((((((((((((((((((((((((()))))))))))))))))))))))))))))))))))))) Well there are lots of variations, mostly it adds sulpher to the mix. When sulpher is present in the steel it cuts easier, as in high sulpher steels. Chlorine is another additive that works at a lower temp than sulpher oils. However the job you described may have been helped better by a spray of cool mist. Bronze especially in deep hole drilling is very troublesome without the mist.
Chlorine oil... cut max 570 E.F Houghton Best for stainless and bronze Costs a little...does a lot... HTH BeeVee
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.
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.
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.
Logic says that it doesn't reach the very cutting point. It reaches just about everything else tough (including your clothes). The likely benefit is that it helps the chips/swarf slide against the surfaces above the cutting point easier, where it bends, thereby reducing pressure on the points. With deep holes, I find that the surface tension actually makes the chips stick in the flutes, but the advantages outweigh that disadvantage.
But cutting-force analysis shows that lubricants sharply reduce cutting force in many cases.
This subject has been studied in some depth, beginning with research at Carnegie-Mellon Univ. around 50 years ago, and continuing with studies by Dr. Eugene Merchant at Tempe Univ., and elsewhere. If you talk to one of the top engineers at Sandvik, Kennametal, or one of the other biggies, they can direct you to the research info. There also are several knowledgable people at Univ. of Ohio, Perdue, and some other universities where they do academic research on metalworking manufacturing.
There are several mechanisms by which cutting lubricants lower cutting forces, alter the geometry of chip formation, improve finish, and so on.
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.
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
What logic? Remember all it takes is a monolayer to affect how the cutting happens. That's not much, and right near the cutting there's a storm at work all the time.
Also, when you say 'the very cutting point' you are missing the issue that cutting doesn't just happen at a single location in space. The deformation, chip separation, and chip flow really do occupy a larger volume than just a single point.
So the real question is, how much of the cutting process is devoid of even a monolayer of cutting fluid, if the entire rest of the tool is coated or flooded?
Consider that temperatures get hot enough to vaporize the oil - which means that now there is oil vapor permating the work as well.
In many, if not most, cutting situations, the 'cutting point' doesn't actually do the cutting anyway. This is obviously true of negative rake or zero rake tools, which work more by fracturing the work in front of the tool, but is less obviously true of positive rake tools also. There, the tool acts more like a wedge, prying material out of the work. The fracture in the work still occurs some distance (a SMALL distance to be sure) in front of the cutting tool. The face of the cutting tool tip (point) may not even be in contact with the work (at a microscopic level), with the chip first contacting the tool part way up the rake slope. The chip is 'peeled' upward, tearing the material AHEAD of the so called 'point'. This is the region in which the cutting fluid may aid in material failure and separation.
The lower face of the cutting point will ride on the work behind the cut. Considerable friction (heating) can still occur there, as well as at the point of chip contact. As has been stated, even a thin layer of fluid at those points can substantially reduce friction, heating, and chip 'welding' problems.
"Cutting fluids play a significant role in machining operations and impact shop productivity, tool life and quality of work. The primary function of cutting fluid is temperature control through cooling and lubrication [Aronson, et al., 1994]. A fluid's cooling and lubrication properties are critical in decreasing tool wear and extending tool life. Cooling and lubrication are also important in achieving the desired size, finish and shape of the workpiece [Sluhan, 1994]. A secondary function of cutting fluid is to flush away chips and metal fines from the tool/workpiece interface to prevent a finished surface from becoming marred and also to reduce the occurrence of built-up edge (BUE)."