Drilling with high pressure and high volume coolant
Everybody wants a ‘silver bullet" to solve problems, drilling with high pressure coolant is the current "flavor of the month". It isn’t magic but it’s an excellent tool that helps to control some major elements of your process and of course process control is everything in manufacturing.
The need for high pressure and high volume coolant in drilling became apparent when gun drills came into use over 100 years ago. The essence of the problem (then and now) with standard low pressure coolant systems is that so much heat is produced that the coolant boils away before it can reach the chip tool interface where metal is actually cut. The super heated steam forms a barrier that low pressure coolant can’t penetrate. Effective cooling does not occur and there is little real lubrication provided. Unfortunately the vapor barrier that forms is not powerful enough to keep chips from falling back into the chip/tool interface and causing damage. Properly applied high pressure and high volume coolant prevents this vapor barrier from forming by causing a localized pressure increase. So much liquid is forced into the cutting zone that heat is removed and no vapor can form because of the pressurization. A great deal of FORCE is required to achieve this pressurization, and this mass of liquid provides lubrication and flushes chips away from the cut . With properly applied high pressure coolant all of the problems I have just mentioned can be minimized and you can have real control of your process, damage from heat and chips is eliminated, tools can cut until they wear out. High pressure coolant keeps the temperature low, changing the way metal is cut. Tools last a long time, chips can’t weld, metal can be cut at much higher surface speeds, damaging chemical reactions do not occur at low temperatures. With a properly designed high pressure and high volume coolant and the appropriate tooling system, surface speed can be increased a minimum of 30%, with some operations improving by 300%. High pressure coolant also provides lubricity by blasting lubricating fluid between the chip and the cutting edge at hundreds of miles per hour. Combined with much lower temperature, this increased lubricity often causes surface finishes to be twice as good. With conventional coolant the cutting edge comes up to a very high temperature as it enters the cut, and stays hot until it finishes the cut and is exposed to an extreme thermal shock as the coolant quenches the exposed tool. Most cutting tool companies recommend wet turning but they ask you to mill dry even when the carbide, coating, and work piece material are the same. With turning there may be only 3 or 4 passes per minute and therefore only 3 or 4 thermal shocks. With milling every time the cutter makes a full rotation each insert gets hot and is quenched, with conventional flood coolant a face mill running at 1000 RPM subjects every insert to a 1000 damaging quenches every minute. Drilling falls somewhere in between, with a thermal shock ocuring every time the drill pulls out of the cut in a peck cycle. This rapid cycling between high temperature and quench can be more damaging than heat or wear. Tooling manufacturers recommend dry milling because they believed that the continual heat and chip damage is better than thermal shock damage. You don’t have to make this tradeoff with a better engineered system. Lets also answer a frequently asked question, yes you do have to have a drill with coolant holes (oil hole drill). Without coolant holes in the drill the application of coolant is difficult to control, the operators individual preference, experience or mood is completely responsible for the process. As a result the coolant is often poorly applied. To just aim some snap together plastic beads somewhere in the direction of the tool and hope it stays in the same general position is not a process. Some coolant manufacturers claim that in "normal" operation the coolant doesn’t even hit the tool 40% of the time. This haphazard system introduces a huge amount of process variation. High pressure coolant is a fantastic tool but it is only part of the process. Improperly directed high pressure coolant is like a soldier who hopes to win a war by randomly pointing his rifle in the air as he fires. In fact if you aim high pressure coolant down the flute of a drill the way you would with low pressure coolant you can force the chips back into the bottom of the hole and cause premature drill breakage. DRILLING fits into the category of "contained coolant" where you are machining in a confined area like a drilled hole, or tapping a blind hole. Here your intention is to pump enough coolant through the tool to completely fill and pressurize the hole. The point of this is to eliminate any possibility of vapor forming so you never get the high temperatures normally associated with metal cutting. As mentioned earlier this is the same principal that is used in gun drilling and the pressure required is essentially the same. Pressures in the 1000 PSI range are generally adequate, but higher pressures may be needed for particularly difficult applications. There is a general misunderstanding of the way pressure relates to drilling efficiency. The pressure that matters is really back pressure. Take two .375" drills as an example, a high speed drill typically has much larger coolant holes than some (but not all) solid carbide drills. Much more coolant will pass through the drill with large coolant holes and cause much more flow, chip removal and pressure in the hole. Three factors determine the required pressure, the coolant hole size in the drill, the open area of drill flutes and the pressure of the coolant entering the back end of the drill. Sometimes a solid carbide drill will have very small holes compared to a high speed drill. As a result a coated high speed drill will often work better than a poorly designed solid carbide drill with smaller coolant holes. In the table below you will see that a drill with .030" holes will only pass 1.43 GPM at 1000psi but when you increase the coolant hole size to .060 the same size drill would pass 5.94 gpm. Remember when you double the I.D. of the coolant hole you get 4x the flow.
| Pressure@ | Drill dia. | Hole dia. | GPM | Hole dia. | GPM |
| 1000 PSI | .375 | .060 | 5.94 | .030 | 1.43 |
| 1200 PSI | .375 | .060 | 6.28 | .030 | 1.57 |
| 1500 PSI | .375 | .060 | 7.04 | .030 | 1.76 |
| 2000 PSI | .375 | .060 | 8.11 | .030 | 2.03 |
| 20 PSI | .375 | .060 | .81 | .030 | .020 |
Chips cause unpredictable damage, in general the longer the chips the harder it is to control and the more damage they cause. Long stringy chips wrap around drills, fill the bottoms of holes, catch on the chucks, cause mechanical problems with loaders and in many cases require manual removal. Broken chips that can fall away, or that can be blown out of the cutting zone with coolant force and away from the part and tool are almost always more desirable. Many people don’t understand the difference between wear and damage. Wear is a predictable part of any mechanical process. Damage on the other hand is random, producing the same bell shaped curve that any random event with enough samples must produce.