Coolant: why you need it.

Everybody hates coolant. It gets dirty and filter changes cause downtime. It can smell bad and it costs a lot of money to dispose of properly. So why do we use it? Some machining operations just can't be done without it. The most dramatic example that I can think of is in drilling small holes, such as drilling 304 stainless 0.750" deep with an 0.080" diameter drill.

Without coolant, this operation takes 45 seconds per hole with a tool life of approximately 100 holes. With high-pressure coolant, it drills a hole in 2 seconds with tool life over 8,000 holes.

Every application isn't this dramatic, but who wants to give up even a 10 percent advantage in these competitive times? So here is a brief understanding of how coolant works and how to take care of it.

Cooling

Some people think that cooling is the only function of coolant. It is a major part of the process, but it isn't very well understood in the industry. How much coolant do you need? It's a pretty simple equation, if you believe that matter and energy can not be created or destroyed. Each horsepower that you put into the cut is equal to 746 watts of energy. If that energy becomes heat, and the conversion to heat is 90 percent efficient, etc., you can calculate how much coolant you need at the point of cut. The rule of thumb is that you need 0.5 gallons per minute per horsepower. If a machine has 20 horsepower, you should equip it with at least a 10 gallon-per-minute coolant system.

Lubrication 

The coolant lubricates the tool as it cuts. So what concentration do you need? As the amount of tool surface area in contact with the workpeice increases, so does the need for lubrication. With a single-point turning tool with a relatively small contact area, 5 percent is probably okay. As you increase contact area with a drill, concentrate levels must go up to at least 8 percent. A reamer may need 10 percent. The tool with the most surface area in contact with the workpiece, a tap, needs the most lubrication at 12 percent. In the example above, a coolant concentration of 9 percent was used. Lowering the concentration to 5 percent might reduce the tool life by 90 percent. These numbers are good starting points but you have to remember that there are hundreds of coolants on the market all with different characteristics.

Chip control/removal

Using coolant to remove chips is misunderstood. It is commonly thought that the force of high-pressure coolant breaks chips. It doesn't. Cooling shortens the primary shear zone and thins the chip as it forms causing it to curl much more aggressively and break. More important, the force of the coolant blasts the chips away from the tool and prevents damage. The increase in drill life in the example above can only happen when the chips are moved away from the cutting and are never recut.

Filter the particles out

Dirty coolant that is full of abrasive metallic fines destroys tools. Each particle that gets caught between the tool and the workpiece causes damage. Imagine the effect of throwing a handful of sand into a car engine. That's more or less what happens with dirty coolant. In the drilling example above, the coolant holes in an 0.080" diameter drill are very small and unfiltered coolant would clog them almost immediately. Damage is much more destructive than wear. Cheap, easy ways to keep coolant clean have just not been available until now. Standard bag filters are cheap but they let lots of particulate bypass at the top ring seal of the bag. Cartridge filters generally do a better job of preventing bypass but they are expensive. Centrifuges can be very effective but they generally have high maintenance requirements or high cost that can only be justified on a central coolant system.

There are two new technical improvements that solve these problems. The first is a flexible seal for bag filters that eliminates 90 percent of the filter bypass (see illustration). It utilizes a standard bag but gives you the effectiveness of a cartridge filter without the cost. Filters that rely on a bag or a cartridge still require lots of maintenance; you have the regular, messy job of changing and disposing of the filter element.

The latest technological improvement in this area is the high efficiency media-free filter. This works as kind of a reverse centrifuge. The impeller that causes the fluid to spin is solid, with only a small gap between it and the housing. A centrifuge uses centrifugal force to move the particles to the outside of the bowl, and the longer and faster you spin the fluid, the better the separation. With the new technology, the particles start at the outside of the housing. To get into the clean coolant stream, they would have to move against the centrifugal force created by the spinning liquid. The result is better filtration than a bag filter with continuous high volume and totally automatic sludge disposal. Tests have shown this filter to be 98.75 percent effective at removing 2 micron and larger particles at a 30-gallon-per-minute flow rate. This new system is an order of magnitude better than the previously available technology at a cost that allows it to be part of an individual machine coolant system.

Water quality can have an impact

The water quality in your area can have a tremendous effect on the performance of your coolant. Hard water can keep the water and concentrate from mixing properly. Soft water can make some coolants foam. The pH of the water can also have an effect. The black stain (inter granular corrosion) that is often a problem on some aluminum alloys like 7075 is caused by pH levels over 8. Most coolant companies will test for you and make specific recommendations. They are your best source of information on water quality issues.

Give it a rest (sump time) but not too long

Today's small coolant sumps often don't have enough capacity and keep the coolant agitated all the time. Coolant that has a tendency to foam can often benefit from a period of relative stillness that allows the foam to break down. This is called "decanting." A larger sump is almost always better than a smaller sump. On the other hand, it isn't good to let coolant sit for long periods of time without moving it. Keeping even a small pump running over a weekend will prevent problems like "Monday morning stench."

There is lots of information available on this subject and if you have any questions about this article, coolant, filtration, or high pressure you can email me at [e-mail]. ChipBLASTER Inc., www.rsleads.com/412tp-240