Oil and cutting fluid when drilling metals

I'm curious if others here use just any oil when using mill end bits or drilling metals, or if you use cutting fluids. And, is cutting fluid really something special or is it just rebranded oil?

When I took a machining class many years ago we used cutting fluid that had a very familiar smell. "That sure smells like WD-40 to me." Though the tins were just labeled "Cutting Fluid." Ever since then when drilling metal I have used whatever spray lubricant or penetrant has been sitting around.

Maybe real cutting fluids have a higher flash point so that you are less likely to start a fire?

When Grandad died we inherited his mill, lathe, cutters and accessories, a load of different metals and several ancient tins of trefolex.

When I'm cutting threads (by hand, I'm not that advanced!), I always reach for the trefolex, broken taps are not fun!

For drilling, turning and the bandsaw we use a some cutting oil (no idea what). For most of my drilling I haven't noticed any difference in the quailty of cut between the proper oil, nothing or wd40. All I have noticed is that using oil keeps the bit cool.

WD40 is a pain because as you spray it at the tool it just bounces off, the cutting oil does a much better job of staying where I want it. It would be interesting to try chainsaw oil as it's designed to stick to the chain.

I'm travelling now but when I get back home I'm tempted to compare some different oils.    

Long time go an experienced metal engraving artisan told me to use animal suet. Probably, because it's most thermally stable from all animal fats. I also use coconut. I just don't like the smell of industrial oils on the things I fabricate. Also cleaning them on many parts will be an arduous project.

I suspect cutting fluid is mostly to protect the bit from excessive heat, which could ruin its temper. Good bits are not cheap. (All my bits are cheap, which tells you something.)

WD-40 says that it is made of 50% mineral oil.

Thomasnet says that "Cutting oils are generally compounds of mineral oil with the addition of animal, vegetable, or marine oils to improve the wetting and lubricating properties. Sulfur, chlorine, and phosphorous compounds, sometimes called extreme pressure (EP) additives, provide for even greater lubricity.

So probably the cheapest way to get some cutting oil is to use straight mineral oil.

On drill bits, I don't notice much difference between oils unless boring a very deep hole.

I too have noticed WD-40 makes an excellent cutting fluid, which does not make sense as we are told it was not intended to be a lubricant, and many things like fishing reels and guns can be damaged using it on them. This is much more noticeable on a lathe. What is unusual (and desirable) about WD-40 is that it seems to really pull itself into the area being cut far more than any other oil I have used. I have yet to find a generic version of it that does the same. I don't know what chemical in it causes it to work so well, but it has been unique in my experience.

It was know a long time ago that one of the very best cutting oils in existence was lard. It's cheap, works extremely well, non-toxic, renewable. It would have been more common were it not for two drawbacks. It goes rancid in time, and the smell of burnt metal mixed with burnt lard is quite nauseating to many people.

I suspect cutting fluid is mostly to protect the bit from excessive heat, which could ruin its temper.

Yes, it does prevent heat buildup in addition to lubrication. I have even used water in a pinch, but it's not as good as oil. Though most all metal cutting bits made in our lifetime are air hardening and do not have a temper to ruin, they still become weaker/softer at higher temperatures. Changes in temperature also change the sizes of the tool and work, which is not good. Especially in cases like machining things like titanium, it can make a drastic difference. Titanium is a poor conductor, poorer than steel anyway, which means the heat from cutting will be absorbed more by the cutter than the workpiece.

There are many factors that go into cutting fluids, as evidenced by the plethora of them available. Cost, flammability, longevity, stability, toxicity... There are different types for different metals and different situations. It is a much deeper subject than typically meets the eye, but I use whatever is on hand and makes sense without obsessing over it too much.

Anything is better than nothing.
I like a pill bottle of Vaseline for dipping taps into.
I have used high temperature grease as well.
When I was on the job in construction we were expected to make the bits last, so using oil made a lot of sense.
Typically we used what ever the  conduit threading machine used, and that was cheap motor oil.

Bees wax is what I use on my burs, bits and jewelry saw blades. A block of paraffin at my bandsaw because I sometimes cut wood on it and don't want oil on the wood. Disc grinder flap wheels used on aluminum don't fill up up with aluminum when hit with wax before use.

Robert Ray wrote:Disc grinder flap wheels used on aluminum don't fill up up with aluminum when hit with wax before use.

Whoa, great idea! Thanks.

Home shop machinist here (milling machine and lathe). The right fluid does make a difference.

I keep a spray bottle filled with WD-40 for cutting aluminum.

I keep another spray bottle with a 10% strength of a water-soluble cutting oil like Kool Rite and water. This is good for removing heat when cutting steel and adding some lubricity.

I keep a small bottle of good tapping fluid for tapping threads.

I have a little aquarium pump/ 5 gal bucket setup for water-soluble oil flood coolant but I almost never use that since it makes a mess. It is good for flushing out chips when milling deep slotting or pockets.

Cast iron and brass are often cut dry.

Da nial, I believe you are on the money.
I was going to write virtually what you have.
Lubrication, cooling and swarf removal are all different aspects that different tasks require.
If you use the best one each time, it will cost you less in the long run.
IE turning bits need watery coolant, drills may need one or the other depending on speeds and diameters involved.
Tapping is usually slow speed and lubrication  is best, do you move it backwards  every revolution, to cut the swarf?

I use an atomizer spraying a mist powered by low pressue air and water soluble oil when using my lathe not quite as messy as oil and a brush that I started with.