Machining Guide: Cutting Fluid Tips for Holemaking Operations

For starters, many shops upgrade their machine tools and other machining technology without considering whether they’re using the best possible cutting fluids. Filling the sump with the sulfur-based oils or water-soluble fluids of yesteryear are a little like putting unleaded fuel into a high-performance sportscar—it might run for a while, but you'll eventually run into performance problems. 

Cutting fluids experts will tell you that replacing these outdated cutting fluids with modern, premium grades of semi-synthetics helps to reduce costs and improve productivity. This is mainly due to these fluids’ high-lubricity esters, which eliminate the need for chlorinated paraffin or sulfur-based EP compounds. 

These esters create a tight boundary layer around oil droplets, keeping them intact during metalworking operations. It also enhances their lubricity and thus reduces wear on cutters and machinery. And because they're less likely to break down under pressure at today’s higher cutting speeds and feedrates. Finally, and just as important is their greater friendliness to workers and the environment alike. 

Today’s cutting fluids offer other advantages as well. Since they’re more efficient at lubrication, less concentrate is needed when mixing up a fresh batch of coolant. This cuts operating costs while limiting the amount of “food" for bacteria lurking in any machine tool sump, resulting in a nicer-smelling shop and no “Monday morning stink.” 

With that, they tend to reduce the amount of time spent on fluid maintenance and sump cleaning. And since these high-ester fluids increase the emulsion’s water content, they enhance its cooling ability, a benefit that’s critical during holemaking. 

That brings us to maintenance. Here again, failing to change the oil in your sportscar regularly will shorten its lifespan, and cutting fluids are no different. This isn’t meant to suggest that tossing out a sump full of tired coolant is always needed. For one, doing so is expensive, not only to refill the tank but to also dispose of the waste fluid. 

This is why many shops have invested in a coolant management device that makes sump cleaning easier and greatly prolongs coolant life. Several brands exist, but these “Sump Doctors” make fluid maintenance about as difficult as vacuuming the living room rug—just wheel it over, plug it in and get sucking. Dirty fluid goes in one end and clean, filtered coolant ready for another few months of metalworking comes out the other. And when it's finally time to recycle these cutting fluids (an inevitable fact of machining life), they are much easier and less costly to dispose of if first sent through the cleaning device just described. 

In between these routine cleanings, cutting fluids should be checked regularly (every week is a good idea) and monitored for hardness, chloride levels, and PH with test strips or an electronic meter. Concentration levels should also be measured using a Brix-style refractometer, with “make-up” fluid mixed accordingly to bring the sump back to a 9% to 12% (on average) ratio of concentrate to water. And always be sure to ask the cutting fluid manufacturer for concentration level recommendations and mixing advice. 

It's also critical that CNC lathes and machining centers are equipped with belt or tube-style skimming units to pull out the tramp oils that all sumps accumulate. Budget permitting, shops should consider installing an inline filtration unit to remove small metal fines. This will extend the life of the CNC and its cutting tools, and is a must with our next bit of investment advice: high-pressure coolant, or HPC. 

HPC is just as its name describes—a high-pressure auxiliary pumping system that pushes coolant through the cutting tool at 300-psi or more (often way more). This action removes heat from the work zone while blasting away chips and swarf, usually breaking them up in the process. As one might expect, HPC significantly improves tool life and supports feeds and speeds much higher than is possible with the standard pumps found on most machine tools. 

As noted, a filtration unit will be needed to avoid damaging the pump. But because this also removes the microscopic bits of metal and dirt generated during any machining operation, tool life and part surface quality are further improved. Here again, be sure to check with the manufacturer for proper sizing advice. 

Of course, using high-pressure coolant during drilling operations requires that you use the correct drill. Forget high-speed steel (HSS) or cobalt Jobbers drills; forget Silver and Deming (S&D) or any other old-fashioned bit not outfitted with through-the-tool coolant holes. Instead, shops must use one of the following: an indexable insert drill, a modular replaceable-tip drill, or a solid carbide drill designed for HPC use. 

The common theme here is carbide, which provides cutting speeds at least four times that of HSS, more if the tool has been coated with TiC, TiCN, AlTiN, or any of the other high-performance, often material-specific coatings available on the market today. 

When coupled with through-the-tool HPC, productivity during holemaking operations is significantly increased, especially when one considers that there's no longer a need to "peck" the tool for chip-clearing. These advantages help explain why all leading cutting tool manufacturers—Kennametal included—have stopped producing HSS drill bits in favor of solid carbide and carbide-tipped tools. 

There’s one more topic to cover before closing this cutting fluid discussion, and that’s the possibility of eliminating cutting fluid altogether. It’s called minimum quantity lubrication, or MQL. It works by applying a small, metered amount of lubricant directly to the cutting zone, reducing friction and therefore heat. And thanks to the pressurized air carrying this lubricant, chips are efficiently removed. 

The advantages are clear. All the hassle and expense of traditional cutting fluids is replaced by a simple, environmentally friendly alternative. Yes, special equipment is required, as are tools with an MQL-capable interface, but here again, it’s one that Kennametal and several other cutting tool suppliers can provide.