High-pressure coolant delivery is necessary to cool the workpiece and the tool, to provide lubrication between tool and workpiece (chiefly by lessening weld action and built-up edge formation on the cutting and supporting areas), and to carry chips away from the cutting area along the flute to the chip box. Cooling action dissipates both the external heat of friction and the internal heat of deformation. Lubrication between the workpiece and the drill contact areas facilitates cutting action by lessening the natural tendencies toward pressure welding (adhesion and/or diffusion wear) or heat welding (built-up formation on the contact surfaces). To effectively carry chips away, the coolant should posses a sufficient combination of viscosity and velocity. Improper selection of this combination causes chip plugs in the flute that lead to an increase in torque and probable drill breakage.

Coolants

Straight Oils are generally recommended for gundrilling. Compared to water-soluble coolants, they cause significantly reduced tool wear, yield better surface finishes, and generally improve the accuracy of drilling. EP (extreme pressure) additives of Sulfur to provide Anti-weld characteristics, which resist “pick-up” or BUE, and Chlorine, which provides additional lubricity to reduce adhesion. We recommend the composition include Sulphur (2.5-3.5 %) for high alloy steels and heat treated cast irons, and Chlorine (3.5-5 %) for light ferrous materials (up to 7% for high nickel steels such as Inconel), and 10-14% Fat.

Low to moderate viscosity (SSU) aids in good heat dissipation and good load carrying capacity. It also reduces the risk of pump starving when cold starting, improves the efficiency of filters, and reduces the amount of oil carried off with the chips. The kinetic viscosity should generally not exceed 20-30 cSt/20degrees C. In exceptional cases, 45 cSt/20degrees C can be used.

Recommended ranges of Coolant Viscosity, Pressure and Flow rate

Hole Diameter	Viscosity (SSU)	Pressure (PSI)	Flow rate (GPM)
.055-.125	60-70	1200-1500	.5 - 1
.140-.375	75-90	800-1000	1 - 4.5
.390-.750	75-120	400-800	5 - 10
.750-1.00	75-150	300-400	10 - 14
Over 1”	125-200	200-300	14 - 35

Water Soluble Oils (Emulsions ) are more economical and can be used for non-ferrous metals and high machinability steels under light cutting conditions. They are not recommended for tougher steels. EP additives will help prevent material welding to the tip (BUE), and they should contain film strength enhancers (animal and vegetable fats) to reduce friction and wear. Concentration of less than 10 to 1 reduces film strength and creates vapor pockets at high tool load areas.

Synthetics

These water-based fluids are easier on the environment but harder on the gundrill. Should not be used in deep holes as they lack the necessary film strength.

Filtration

Because the coolant collects and circulates considerable quantities of both coarse and fine chips, it must be carefully purified in the interests of both tool life and hole quality. Poor filtration can lead to increased coolant temperatures and rapid failure of the coolant pump. It also causes premature failure of solenoid valves, leaking servo valves, and bearing failure in the coolant inducer. We recommend using cartridge filters which should separate particles in the range of 15-20 microns for precision holes and in the range of 20-30 microns for normal holes. Drilling cast iron requires rough filters, magnetic drums, or rolled media, followed by a bag type or woven media polishing filter.

Temperature

The temperature of the coolant defines to a large extent its cooling, lubricating, and transportation abilities. This is particularly important with oil-based active coolants. About 100 to 120 o F. is generally recommended as the maximum temperature of the coolant. It can often be maintained by circulation through a heat exchanger or even by installing a fan to blow across the surface of the coolant reservoir. When precise holes are to be drilled, refrigeration systems may be necessary.

Reservoir

The coolant reservoir should be sized at ten times the maximum gpm of the pump unit, and should be baffled to slow the speed of the coolant back to the pump, allowing the small chips to settle out. Another way to look at it, is to circulate the coolant about 6 times per hour. You should be able to drain any water that has found its way into the coolant tank via a bottom drain in the tank.

Pumps

Piston type pumps are the most expensive. Pressure compensated variable-volume Vane pumps require less power consumption, and Centrifugal pumps are only good for large holes requiring large flow rates with less than 50 PSI pressure.

Most gundrill machine manufacturers use variable displacement pumps with a bypass, but a system that would control the pressure in order to deliver a constant flow rate would be a better choice.

Pressure vs. Flow Rate

The controlling factor in effective chip evacuation is the coolant flow rate. Pressure can be adjusted to assure the proper flow rate. Different size and shaped orifices in the gundrill tip (e.g. round hole, kidney hole, twin hole) will deliver different flow rates at a given pressure setting. Likewise, the point grind and the feed rate as well as the length of the gundrill can alter the flow rate for a given pressure setting. Finally, overloaded filters and/or chips packing in the flute will also reduce the flow rate. While the pump appears to be putting out constant pressure, the coolant is being diverted through the bypass, robbing the gundrill of the flow rate it needs. Given these circumstances, we strongly recommend that all gundrill machines be fitted with flow meters on each spindle, and that they be monitored. What you don't know, can hurt you!

Proper selection of the coolant and its delivery parameters will increase production, improve hole quality, and can as much as double tool life, significantly reducing the cost per hole.

Chlorinated olefins break down chemically under the influence of high heat and pressure near the cutting zone to form a solid lubricant film between the ferrous work piece and the tool. While this same phenonemon takes place in drilling alluminum, there are other non-chlorinated lubricants that work just as well with aluminum.

The downside is that chlorine may theoretically accelerate the process of stress cracking, though it is difficult to prove either way.