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Why Does Cutting Fluid Work?

ptsmith

Cast Iron
Joined
Feb 15, 2018
I know it does work, but I don't understand why. Say you brush some cutting fluid on the surface of the part you're turning in a manual lathe and start cutting. The point of contact that's doing the cutting has virtually no contact with the surface, and the point of cutting contact is under tremendous pressure. So how does cutting fluid migrate into this high pressure zone?
 
Look at the slow motion videos, it's lubricating the stationary material the cutter is 'rubbing' on as well as helping to lubricate the material building up in the flute as the cutter is doing it's thing.
 
In addition to the above explanation, sulphur containing lubricants chemically react with the freshly exposed metal surface to chemically form metal sulphides. These sulphides are extreme pressure lubricants, just like tungsten disulphide and molybdenum disulphide are extreme pressure lubricants. Not sure what the mechanism for chlorine based lubricants is, but they work at lower temperatures than sulphur and then overlap the lower end of sulphur's temperature performance and then sulphur takes over lubrication as the temps increase.
 
Although the fumes from cutting oil seem to darken my 5C collets and the nose of my Hardinge HLV when it gets hot enough to smoke :angry:. I suppose that means that the stuff is pretty chemically active when hot.

Note to self:- try wiping a bit of way oil/grease/paraffin wax on the affected parts next time instead of polishing the stain off...
 
I'm a simple-minded simpleton, but I always thought it had little to do with the actual cutting, but mainly reduced friction and thereby reducing heat, galling, sticking etc. on all the surfaces (including chip) that were not actually doing the cutting. Some of the explanations above are interesting... guess I better get educated!
 
I did not present my question clearly. Let me try again.

Cutting fluid is on the surface of the part being machined. The actual cutting happens below the surface. There is tremendous pressure between the cutting tool edge and the material being removed. How does the cutting fluid get from the surface to the leading edge of the cutting tool?

Think of peeling an orange that's covered in grease. The finger that's under the orange skin is shielded from the grease and could peel the entire orange without getting any grease on it.

close-up-super-slow-motion-shot-of-mild-steel-on-a-lathe-131618.gif
 
Well, that's what I was trying to say. I don't think it does affect the cutting zone directly. Or, at least I didn't think so until reading this thread. Don't know now.
 
I know it does work, but I don't understand why. . .

Good question. Reasons I've been told cutting fluids work, which can act in combination:

1) By the surface-active effects noted several times above. Essentially making metal atoms more willing to leave one another.

2) By lubricating the interface between the chip, cutting edge (insert), and workpiece. Lots of energy lost in that, so lubricating reduces the hp of the lathe lost to friction. In addition, may inserts fail by crater and flank wear -- both much larger surfaces than the point you're worrying about. Lubing the flank and crater area reduces energy lost to friction and also puts less heat into the tool.

3) By directly cooling the tool (coolant/cutting fluid), increasing it's life. Can also go the other way with interrupted cooling and thermal shock.

4) Another factor is limiting workpiece temperature, reducing thermal expansion effects and improving accuracy.

FWIW, the more HP that can go into that small plastic deformation area (not wasted elsewhere in the cut), without breaking down the cutting tool, the faster material can be removed. Higher heat in that small shear area eases plastic deformation. At least that's my understanding.
 
does making something slippery and so one item slides easier across another really need explaining ??

His questions involves why making something slippery on the outside improves tool life etc when the cutting tool and material interface is on the inside
 
Just a guess, but what the OP is describing (brush some cutting fluid on the surface of the part) does sound like it "shouldn't" work, but I think it does because it lubricates the material and tool, to a certain extent. Flood coolant on the other hand is different. If you are constantly spraying a tool/part with coolant, there is always a fresh supply to the cutting edge and material, and to 'both' edges of materiel and tool engaged in the cut.... right?? :confused:
 
To some extent putting any fluid on will reduce the temp, energy going somewhere to make all that smoke.... this is the shit that people spend a decade and write a doctoral thesis about to really understand.
 

Guys that raced RC cars back when they had brushed motors would coat the armature commutators with Sharpies before truing them up in small comm lathes made specifically for that purpose. RC car racing converted to brushless motors maybe 15 years ago, so the Sharpie thing has been around for a long time.

I only heard about this a year or two ago. Apparently they debated which color Sharpies worked best. I was skeptical for the same reason I started this thread. I forgot about that. I've had this question for longer than I remembered.
 
Just a guess, but what the OP is describing (brush some cutting fluid on the surface of the part) does sound like it "shouldn't" work, but I think it does because it lubricates the material and tool, to a certain extent. Flood coolant on the other hand is different. If you are constantly spraying a tool/part with coolant, there is always a fresh supply to the cutting edge and material, and to 'both' edges of materiel and tool engaged in the cut.... right?? :confused:

Yes that's the question I'm asking.

When I started this thread yesterday I had side milled a piece of 6061 dry. A very light cut. Then I wiped on a very small amount of WD-40, nowhere near enough to have any cooling effect, at least I wouldn't think it would, and made another pass. There was a noticeable improvement in surface finish.

It seems counter intuitive to me.

Some of you are saying it effects the way the chip comes off I guess? That seems counter intuitive as well.

Then you have the dry coatings from gnadoc's link that can't possibly have any cooling effect or migrate to the tool cutting tip.

I'm more confused than ever. LOL!
 
Chips come off hot, if they are cooled quickly, they become brittle and break. Lubrication also is on the cutting tool, so chips don't stick to it. This becomes really obvious when Machining Aluminum. In the example the OP offered, "side Milling Aluminum". The chip sticks to the Endmill, rolls around and recuts shit and junk. That's why the surface finish is so obviously different.

R
 
Any test involving WD-40 = "junk Science"

Try flood coolant directed properly under hi-pressure to get it where it's needed, and see what proper coolant can really doo.
 
I thought of this this morning:

Lubricated parts such as lead screws require a gap so lubrication can penetrate. No such gap exist where the tool cutting edge meets the part. And the lead screw and nut mating surfaces are stationary, giving plenty of time for lube to penetrate, not zipping by at hundreds/thousands of feet per minute.

That thought leaves me convinced there's no effing way cutting fluid gets to the cutting edge.
 
When turning steel, the chip comes off in a stacked series of slip wedge planes, like a deck of cards that you might hold one edge down and pry the pile upwards and they slip past one another as you bend the pile. Look inside the curve of a chip formed by a lathe or a drill or even a mill, and you can see this serrated appearance inside the chip. The outer curve of the chip gets a polish from rubbing on the tool surface.

This chip formation is discontinuous (fractures many slip planes at a high speed) as seen in that slow mo video of a turning tool.

So the lubricant on the surface of the metal gets sucked into the slip planes by capillary action, lowering the friction involved so that it happens more easily with less pile up. Some of the lube may also get to the actual cutting edge of the tool and help with friction there as well. All told, with less pressure build up, the bull dozing effect of the cutting edge comes closer to a scraping action. It is predominantly built up edge (chip welding) on the top of the tool that makes a crummy finish, as then the geometry of the edge more or less is covered up by this blob of material that comes and goes, screwing with the finish and final dimension. You can also see this happen in that video, right on the tip of the tool.

I've never really seen any benefit to applying lubricant to a very heavy cut, other than to get the heat out so that the tool can survive.
 








 
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