The Science Of High-Pressure Coolant
High-pressure coolant is the new 'hot' technology, with approximately a 500 percent increase in use in the last 10 years.
High-pressure coolant is the new “hot” technology, with approximately a 500 percent increase in use in the last 10 years. You have all heard of or seen the phenomenal results that are possible with high-pressure coolant. Unfortunately, most of the manufacturers that buy high pressure don’t really know how to get the most benefit out of it. There is even some confusion about what pressure qualifies as “high pressure.” When I refer to high pressure, I mean at least 1,000 psi. This article will give you a brief outline of the science-based rules for using high pressure.
Technique Isn’t Science
When I ask, “how does this process work,” I almost always get a technique answer, such as, “set it to seven, and if that doesn’t work, then try seven and a half.” Science is different; it allows you to know what is going to happen before you spend your money. This article will look at metal cutting as a physics or chemistry problem that must be analyzed with standard scientific understanding.
The rules of the universe apply to all of us, even if we are in manufacturing. There seems to be some sort of strange belief that trial and error is the only way to develop a process in our business. This can then result in your own personal theory that is outside traditional scientific thinking. While there is a chance that you may discover a new understanding of the laws that govern the universe, it’s one chance in many millions. The problem with this line of thinking is that it is expensive and often doesn’t produce the best results.
If you can’t explain what is happening with first-year university science, you don’t understand it. When we send a probe to Mars and get back stunning pictures, the people who designed the equipment have used theories with names like Newton, Bernoulli or Einstein. Standard scientific theories that have proven formulas that can predict the outcome of a physical interaction. Our businesses should also have these predictable scientific methodologies.
How Much Coolant?How much coolant do you need for your new CNC machine? Typical experience-based answers are “as much as you can get,” “whatever comes with the machine is OK” or “it will look right.” The scientific answer (short version) is 0.5 gpm per horsepower. Using standard scientific understanding that is universally accepted, we know that each horsepower of energy that you put into the cut is by definition equal to 746 watts. If you are using 10 hp, then you are inputting 7,460 watts (10 × 746) into your cut. What happens to the energy that you input into the cut? We all know that it becomes heat and for this simple calculation, we can use a constant of 90 percent:
|1 watt = 3.41 Btu; 7,460 watts × 3.41 =
25,438 Btu converted at 90 percent = 22,894 Btu
People who design radiators or home heating systems calculate the volume of water that is needed to absorb and move a Btu of heat. There are many factors in the calculation: how hot the metal is, how cool the fluid is, what percentage of the fluid contacts the metal and for how long. Without going through the many calculations, we can make certain assumptions about an average process with properly directed coolant, and the rule of thumb answer, with a 25 percent safety factor, is 0.5 gpm per horsepower. Thus, if you use 10 hp in a cut, you need 5 gpm of coolant to achieve the high-pressure effect.
How It Works
High pressure creates a localized pressure increase that eliminates the formation of vapor. The force of the liquid, the result of both mass and acceleration directed at the point of cut, does the job.
|Force = Mass × Velocity2
14.7 (feet/second) × Square Root of PSI
You will notice that pressure is not a part of this familiar equation. It is related to velocity, but not on a one-to-one basis. If you increase pressure by 100 percent, you only get 40 percent more force. If you increase volume by 100 percent, you get the full 100 percent more force.
Always increase volume; never increase pressure unless you must. Why would that be necessary? Simple mechanics: remember that all of the coolant must hit the chip tool interface. If your target area is small and you are using lots of horsepower in the cut, you may have to fit more liquid into a small area. As you increase pressure, the same amount of coolant will fit through a smaller and smaller orifice. You must increase the pressure until all of the coolant required to remove the heat you are generating fits into the target area. (Orifice tables are available at www.chipblaster.com to tell you how much coolant will fit through an orifice at a given pressure.) The best strategy is to keep the pressure constant and vary the volume based on the size of a drill or the horsepower used in the cut, only increasing pressure when absolutely necessary.
Drilling presents another situation: The pressure that counts is the back pressure created as the fluid exits the hole. The rule here is 10 gpm per inch of drill diameter. A 0.500-inch diameter drill needs 5 gpm to achieve the high pressure effect; a 0.250-inch drill needs 2.5 gpm.
The most frequent problem is coolant holes that are just too small. The drill must be capable of passing enough coolant, or it just won’t perform properly. Check the hole size against an orifice chart or do a bucket test: With the drill stationary, direct the coolant into a bucket for 30 seconds and measure the volume that you collected.
The next most common problem is that the coolant holes are in the primary relief. This happens most often in small drills. The clearance is often only 5 degrees, and that doesn’t allow space for the coolant to get out of the holes. In use, the bottom of the hole blocks them. For proper function, at least 50 percent of the hole must be in the secondary grind. Some drill manufacturers still don’t understand this simple relationship between the hole size and positioning. You have to make sure—it can make the difference between 100 holes and 10,000 holes. If you don’t have a large enough coolant system to maintain full pressure, some misinformed suppliers suggest welding the holes in the end of the drill closed and drilling them smaller so that the gauge on the coolant system will show full pressure. Never, ever do this; it is conceptually wrong.
Normally, the drilling rule and the horsepower rule give you the same answer. If there is a conflict between them, use the higher volume.
Coolant concentration is one of the least-understood elements of most processes. What concentration should you use? Is 5 percent the right number? Or 6 percent? Tribology is the study of lubrication, and we are all familiar with one of its principal ideas: The more surface area that is in contact, the higher the need for lubrication. A plain bearing requires more lubrication than a roller bearing, and a roller bearing needs more lubrication than a ball bearing.
Cutting tools follow the same rules; the greater the tool surface that is in contact with the workpiece, the higher the need for lubrication and, therefore, coolant concentration. A single-point turning tool has the least amount of surface contact, and 5 percent concentration may be OK. A drill has the entire point and the margins in contact and always requires 8 percent minimum concentration. A reamer has more surface contact and may need 10 percent. The tool with highest need for lubrication is a tap. Its 60-degree cutting edges give it the highest surface contact of all commonly used tools.
You don’t have to spend weeks fighting a job with poor tool life or bad finishes. You can all remember a job that ran terribly until you increased the coolant concentration and everything settled down. How much money, time and credibility were lost? Look at the tool surface in contact with the workpiece and you can determine the proper coolant concentration before you have a problem. For example, increasing concentration from 5 percent to 8 percent can often increase drill life by 1,000 percent or more. Have you ever discovered that somehow, all of those expensive German reamers that took 14 weeks to get have all been destroyed in one shift?
Why is tool life so good with high-pressure coolant? You have always heard that from the moment you turn a machine on, it is wearing itself out. That is true, and the way we can describe this is with a wear line (see graph, above). High initial wear is called break in, then we get a period of long stable, predictable wear and then wear typically accelerates near failure. This is the pattern that you see on a finish insert that has no chip problems, a wheel bearing or the break shoes on your car. Almost all mechanical devices follow some form of this wear pattern.
Damage is different. Chips falling back into the chip tool interface cause damage. This is a randomly occurring event, and we all know that any group of random events, when plotted, gives us a normal distribution. These are two very different shapes that reflect very different modes of tool failure. The force of properly applied high-pressure coolant removes the chips from the cutting zone so that they never contact the tool or workpiece to cause damage. If you look at a process and say, “I normally get 30 or 40 parts per insert, but sometimes the tool breaks at two and sometimes I get 60 parts,” then you have described a normal distribution. It is possible to plot tool life data and know what the inserts look like without having to actually see them, because wear and damage are fundamentally different modes of failure. High-pressure coolant allows the tools to wear out instead of being beaten to death.
Why Do It?
High-pressure coolant fixes heat problems, chip problems and poor lubricity. How much benefit will you get by using it? Problems with heat, chips or lubricity will be functionally eliminated. The bigger the problem, the greater the benefit. For example, drilling with low-pressure coolant almost always has problems with chip damage. The tool and the chips are confined in the hole, and as the chips make their way out, they are recut at the drill point and grind between the margins and the side of the hole. This causes poor hole finish and short tool life. If the drill has the right sized holes and you have enough volume and the proper coolant concentration, the results are nothing short of spectacular. I do seminars around the world, and I ask everyone in attendance to estimate the time it would take to drill a 0.125-inch ± 0.001-inch diameter hole 1.300 inches deep in 1018 steel. The average result for each group is almost always between 45 and 60 seconds per hole. This is the average over thousands of intelligent people in our industry. The hole can be made with high pressure coolant in 1.2 seconds with an expected tool life of at least 4,000 holes. Exotic materials such as titanium that are very heat sensitive can be machined at much higher speeds by controlling the temperature of the process.
High-pressure coolant is becoming a normal part of most machine tool quotes, but in our experienced-based industry, it is not yet fully understood. The scientific understanding now exists to improve the productivity of many metalcutting operations by orders of magnitude. As our industry comes out of a 3-year slow period and we begin to replace older equipment with new machine tools, they will probably have high-pressure coolant capabilities. To stay competitive, you will have to learn how to use this tool.