Deep Hole Drilling Demands Precise Coolant Control

Coolant is so critical to the deep hole drilling process that today’s state-of-the-art deep hole drilling systems control it much the same as they would a machine’s spindle or axes. Careful management of coolant pressure, filtration, temperature and flow rate is key to optimizing deep hole drilling processes. This requires programmable, infinitely variable flow-based control capability integrated into the deep hole drilling machine itself. The result is a system with the adjustability necessary to ensure there is never more pressure in the coolant system than is required for effective chip evacuation and precise drilling.

For many years, the most advanced coolant delivery system beyond flood types were through-spindle/through-tool coolant systems. Then the advent of high-pressure coolant systems operating at around 1,000-plus psi changed the coolant technology landscape with especially effective tool cooling as well as efficient chip evacuation for most conventional machining operations. Drilling applications, mainly those using twist drills, were a main driver of the development of high-pressure coolant systems, in particular deep hole drilling applications where depth-to-diameter ratios are typically 10:1 and beyond.

However, as coolant pressures increase, so too does the need for proper filtration and temperature control. When considering 1,000-plus psi systems, 20- to 50-level filtration is needed to keep pumps from failing, and in most instances, high-pressure coolant systems will require a chiller to regulate coolant temperature. While most shops stop with these systems, even for demanding drilling applications, filtration and coolant alone fail to address one of the most important variables in high-pressure coolant use, which is flow rate.

Shops often have no idea how much coolant their system delivers or should be delivering. Typical flood coolant systems, for example, provide about 10-gpm to around 40-gpm flow rates, depending on the system. However, much higher volumes are needed to evacuate chips in drilling operations as holes become larger in diameter and/or deeper. When using larger gundrills or BTA tooling, for instance, the required coolant flow can range from 50 gpm to upwards of 75 to 350 gpm for hole diameters as big as 10 to 12" (25.4 to 30.5 cm). Conversely, flow rates for small-diameter deep holes may need to be only 2 gpm but with much higher pressure levels. A 0.040" (1.016-mm) diameter hole, for example, might require coolant pressure as high as 3,000 psi.

Because there is an exponential factor involved, when hole diameter increases slightly, area/metal removal increases significantly. Consider the difference between a 1" (25.4 mm) diameter hole and a 1.5" (38.1 mm) diameter one—a 50 percent increase in diameter. The resulting area amounts to 0.79 in3 (12.95 cm3) for the 1" hole versus 1.77 in3 (29.01 cm3) for the 1.5" hole, a 100 percent increase. Doubling the hole diameter from 1 to 2" equates to four times more area, and four times more material to evacuate out of the hole. In other words, shops should assume that even a slight increase in hole diameter will warrant a change in coolant parameters.

Despite this, most coolant systems offer very little flexibility. Flood coolant systems, for example, have no flexibility at all—the coolant is either on or off. Through-spindle coolant systems may include relief settings or M codes that provide low-, medium-, and high-pressure settings, but these are insufficient for shops that need truly optimized coolant delivery.

Enter infinite variability coolant control. This technology allows shops to start a hole at perhaps 400 psi, then as the drill progresses deeper, increase that to whatever level is needed to maintain flow rate for effectively evacuating chips. However, this is a difficult process to dial in manually. If a flow rate is too low, chips will remain in the hole and could eventually break the drill. Too much flow can create excessive pressure, which, in turn, generates unwanted forces that can hinder drilling accuracy.

For successful and consistent performance, deep hole drilling machine OEMs such as UNISIG will engineer coolant systems and controls into the actual design of a machine from the start to ensure complete integration. This allows controls to provide immediate in-process feedback for extremely granular levels of coolant adjustability that put exactly the right amount of coolant at the cutting edge at all times.

The system works by implementing process feedback in the control system, which makes immediate coolant adjustments to prevent tool breakage. The feedback from the control also guides operators in optimizing deep hole drilling coolant flow and pressure, and once these parameters are determined, they can be used to repeat the process over and over.

Additionally, coolant pressure and flow feedback from the process itself is used to detect broken tools. For instance, if the application’s coolant pressure suddenly drops, that can indicate a broken tool. And right at that moment, the process can be stopped and the tool replaced. This is especially useful when drilling extremely small-diameter holes where the lighter drill loads make even a slight change in force difficult to detect. But by monitoring coolant, operators can determine the health of the tool and whether or not chips are being evacuated. And it can help further optimize parameters such as feeds and speeds for better chip control in those instances.

In addition to coolant systems and controls, coolant reservoirs are also specifically engineered for deep hole drilling machines by OEMs like UNISIG. While flood and through-spindle coolant systems for machining centers might have 20- to 50-gal coolant reservoirs, those with deep hole drilling systems will vary to accommodate the speed and flow of the coolant. Depending on the size of the deep hole drilling machine, coolant reservoir capacities can run up to 3,000 gal and be paired with a 350-gpm capacity filtering unit.

Flood, through-spindle and high-pressure coolant systems do make the occasional high-performance drilling operation possible on conventional machining centers. However, when drilling operations are performed every day and involve much deeper holes at 10, 20 or even 40:1 ratios and beyond, a dedicated deep hole drilling machine is needed. And the best types are those engineered with the coolant system as an extension of the machine, much like a spindle or axis is part of the machine. Highly engineered coolant systems and controls from OEMs like UNISIG allow shops to reliably and accurately drill thousands of holes on a daily basis—and at depth-to-diameter ratios over 100:1—with minimal operator intervention, if any.

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