Hi David,

I don't really understand cutting fluids either.

Though, they do seem to work.

One important aspect is surely cooling the face being worked.

Flood coolant/lubricant also helps to keep the cutting face clear

of swarf and chips, thereby delivering a cleaner cut.

But, then, why do certain materials need to be machined dry ?

I, too, would be interested in any enlightenment.

ATB,

Daniel

Cast iron is machined dry since the carbon in the iron acts as a lubricant. Unlike steel it is not chemically combined with the iron. I suspect the zinc in brass may act as a lubricant (or may be it is just a weak material). When it comes to bronze a cutting fluid is certainly required.

I use the same cutting fluid, a neat cutting oil, for all metals except cast iron and brass. It works. Obviously I use a tapping paste when cutting a thread with a tap or die (again not on cast iron and brass).

JA

Soluble and neat oils are different, and have different purposes. Soluble oils are intended to cool with a little lubrication, whereas neat oils are intended to lubricate with a little cooling.

With carbide tooling it's a moot point as to whether cutting fluid is needed. I don't use it for general turning and milling, except for parting off. I use soluble oil for drilling with HSS tools and on the horizontal mill with HSS cutters. I also use it on the CNC mill irrespective of tooling, mainly to wash away the swarf. Anything other than flood coolant is a waste of time, especially if the primary purpose is cooling.

Whether fluid is needed depends upon the chipping characteristics of the material. For long chipping materials, like steel, as the chip is sheared it is deflected (creating a pressure point) by the top of the tool before curling up and possibly breaking. The fluid helps to cool and lubricate the chip as it passes over the tool increasing the life of the tool. For short chipping materials, like brass and cast iron, the chips come off as individual particles so don't exert the same pressure across the top of the tool. In addition, for cast iron, swarf plus oil creates a sticky mess.

Andrew

Building on Andrew's answer, cutting fluids have at least five different purposes, not necessarily all at the same time.

1. Lubrication of the tool tip as it penetrates.
2. Reducing frictional heat as swarf slides away over the tool's flanks
3. Keeping the tool cool by removing heat by evaporation or flooding. The edge lasts longer, which is important with HSS
4. Helping swarf slide away by lubricating it.
5. Washing swarf away by flooding the job.

The main purpose of cutting fluid during hand-tapping is lubrication. Waste of time flooding the job with suds because tapping doesn't get hot, and because swarf is trapped in the hole anyway. Conversely, heavy roughing cuts in steel with HSS is much more about removing heat and swarf rather than lubrication, hence flooding with cheap suds is a good idea.

Carbide worked at optimum speed and feed gets rid of heat and swarf differently. The tip carves into the work at high speed, deliberately hotter than HSS can cope with to soften the metal, with a chip breaker designed to spray hot chips away from the work. No swarf ribbons. Most of the unwanted heat is carried away by chips being physically thrown away from the job and violently thrown chips don't jamb the cutter. In this scenario, flooding with suds is counter-productive because it hardens the job by cooling it, concentrates unwanted heat at the tool tip, and stops chips flying away. It's also messy. HSS works the same way on short cutting materials like Brass and Cast-iron. (Wot Andrew said!)

In amateur hands at pedestrian speeds, carbide is good news because many long cutting metals don't need cutting fluids either, carbide is cleaner because it can take the heat.

Another danger, HSS benefits from cutting fluids dripped on or splashed with a brush. Bad for carbide because thermal shock is likely to crack it. Carbide and HSS aren't identical, both perform best when used appropriately, including choosing the right cutting fluid.

In practice in a slow home workshop, cutting fluid isn't mission critical. Apart from Brass & Cast-iron; I always use neat cutting oil when threading and sawing; I always put paraffin or WD40 on Aluminium because it tends to weld to HSS and Carbide. Apart from that I mostly cut dry with carbide, unless there's a finish problem. Rather rarely I flood cool my milling machine when removing a lot of steel with either carbide or HSS. The flood cooling system hasn't paid for itself because it's mostly unnecessary in my workshop.

Likely sheer prejudice, but I always use neat oil rather than water-emulsions because I fear rust. Probably complete nonsense, because suds users don't moan about it damaging their machines, other than the collecting tanks of old machines.

Dave

Edited By SillyOldDuffer on 09/06/2020 10:34:17