Machinists have long relied on the use of coolant to extend tool life, machine to tight tolerances consistently, and produce fine finishes. If a flow of coolant over the tool and part fails to produce the desired results, and the expected improvements do not materialize, the answer, typically, is to use even more coolant. And when still more needs to be done, many shop owners turn to through-coolant tooling.

It hasn't always been possible for shops to use coolant as liberally as they do today. Usage was limited when shops were predominantly equipped with manual machines. Manually operated machine tools require the operator to remain near the actual machining process. Because of this, coolant has to be applied sparingly to keep the work area clean and dry.

But it's 1999, and CNC machine tools are in full force almost everywhere you look. With the movement of the cutting tool completely programmed, an operator no longer has to remain near the machine to monitor the progress of jobs. His or her main responsibilities are to load and unload parts.

Exposure to coolant is further limited by the sheet-metal or high-strength-plastic enclosures that surround most modern CNC machine tools. These enclosures keep chips and coolant within the work area, allowing the operator to stay safe and dry while applying a heavier and more constant volume of coolant. Continuously bathed with coolant, the tool can make chips at peak productivity and efficiency.

Getting the most out of today's CNC machines, coolant, and tooling requires a concerted effort on the user's part. This is especially true when it comes to applications involving through-coolant tools. Those who fail to follow established guidelines risk catastrophic tool failure and poor workpiece quality.

Through-Coolant Delivery Systems
The first through-coolant tools that most shops use are drills. And since the principles guiding their usage applies to other through-coolant tools, we'll limit our discussion to drills and drilling operations.

With a through-coolant system,coolant is usually pumped through the center of the machine spindle and out the hollow toolholder that has been designed specially for this purpose. A flood-coolant system, on the other hand, directs the flow to the outer surfaces of the drill. When a through-coolant tool is connected to a properly equipped spindle and toolholder, the coolant flows through the drill's internal ducts and out of the tool's tip.

Some machining centers that do not route coolant through the spindle utilize a coolant-inducer system. It transfers coolant via a special port fitting located on the side of the machine-spindle nose. These systems require the use of special toolholders with integrated extension arms that fit over the port fitting. When a toolholder is placed into the machine spindle, the extension arm completes the connection and coolant can pass freely from the spindle port to the internal coolant ducts within the tool itself. These types of toolholders are usually compatible with most machining centers' toolchangers.

Through-coolant tools can be used with machine tools equipped with simple flood-coolant systems, although the setup can prove cumbersome. To route coolant through the tool, a toolholder with an internally channeled, rotating collar must be used. Using a flexible tubing extension with the appropriate fittings, the collar is connected to the machine's existing piping line. The piping extension then connects directly to the rotating coolant collar on the toolholder. In order to maintain adequate coolant pressure for satisfactory through-coolant results, all coolant must be directed through the rotating collar.

Changing a tool requires the operator to manually disconnect the flexible coolant line from the toolholder. This can make tool changes awkward and lengthy. But collars do provide a viable alternative to flood coolant for operations that require the drilling of numerous holes with the same tool or when tool changes are not part of the machining process.

Flood-Coolant Approach
To compare the performances of through-coolant and flood-coolant applications, we must first examine the purposes coolant serves in a drilling operation.

Coolant is applied to control heat and chips. Extremely high temperatures cause the tool edge to break down or deform and can lead to inconsistent part quality. Also, chips that are not removed from the cutting area can become impacted so tightly in the cut that they seem to almost congeal.

The application of flood coolant is probably the most widely used method for controlling heat and chips and for providing lubrication during machining processes. Its chief benefits are its versatility—it works with numerous cutting operations—and its ability to dissipate heat and clear chips from the cutting zone. Flood coolant used for shallow-hole drilling prevents heat buildup, which can cause galling (smearing and pitting) and complete seizure of the tool (when friction prevents the tool from rotating in the hole).

In addition, flood coolant can prevent the recutting of chips in many applications. When a blind hole is being drilled, flood coolant may not be able to push the chips aside or lift them out. The chips that are left in the hole end up being "pinch-cut," which leads to broken and chipped cutting edges and premature tool wear.

Hole depth can also inhibit flood coolant's performance. As the drilled hole becomes deeper, less and less coolant reaches the drillpoint at the cutting zone. Before the drillpoint becomes excessively hot, it must be raised from the hole and into the flow of the coolant so that the heat dissipates. While the drill is raised, coolant runs into the hole. This action helps clear some chips, but as the hole becomes deeper, flushing becomes completely ineffective.

In some cases, peck drilling with flood coolant actually exacerbates the problem. This happens if the coolant pushes already evacuated chips back into the hole. The alternating hot and cold temperatures can also cause continuous fractures at the cutting edges of carbide tools. And even at its most effective, peck drilling causes valuable time to be lost each time the tool is raised from the hole.

As the hole becomes deeper, it becomes more difficult to control heat and flush chips. Excessive heat at the drill tip can cause galling, which eventually will result in the part becoming welded to the drill's flutes and body. A drill with a galled tip and material welded to it is likely to cut a hole that is somewhat oversize and of poor quality. Uneven welding can produce effects similar to that of an off-center point.

In addition, the chances of tool deflection increase dramatically. If the oversize hole is still within tolerance or there is enough material left to allow boring or reaming, the part may still be salvaged. But a drill that has deflected from its true position often goes undetected so long that it ruins the part.

Despite these drawbacks, flood coolant offers some degree of temperature and chip control. A conventional drill and an ample supply of coolant generally do a good job. But this is not optimal machining. To achieve high productivity, a shop must do more than produce a steady flow of work. It must keep the cost of making those parts low in relation to the value of dollars generated. When a conventional drilling operation becomes a bottleneck, holding back productivity and efficiency, it may be time to advance to through-coolant tooling.

To the Point
Through-coolant drills provide coolant flow and coolant pressure directly to the point of contact. The coolant flushes chips away from the cutting zone and moves them along the drill flutes until they exit the hole. The key factors here, however, are coolant volume and the pressure that the system can deliver. Standard coolant pumps provide adequate chip-flushing pressure to enable most through-coolant drills to outperform conventional twist drills.

Through-coolant drills are specially designed to eliminate the long, stringy chips associated with conventional drilling. But to take advantage of the benefits a through-coolant drill can offer, the operator must use a feed rate high enough to force the chip to curl and break.

Programmers and operators who have previous experience with through-coolant drills are familiar with the higher speeds and feed rates that these tools demand. First-time users, however, tend to shy away from the recommended high starting speeds. Such parameters can seem downright scary. Their caution typically causes novices to start out at much slower speeds. Unfortunately, this can lead to poor results right from the beginning.

Operators must guard against overconfidence as well. Ignoring insert wear and failing to maintain coolant levels and pressure can have disastrous effects on through-coolant tooling. An interruption in coolant flow or a substantial drop in pressure can cause an immediate meltdown of the workpiece or the cutting tool material at the cutting zone, relegating an expensive tool to the scrap heap.

System Demands
As the demand for increased productivity has risen, so have the abilities of cutting tools. Today's tools can cut at speeds and feed rates that were unheard of just seven to 10 years ago. In many cases, these high cutting rates are made possible through the use of high-pressure coolant pumps equipped with sophisticated monitoring and filtering systems.

Few question the productivity benefits of high-pressure, through-coolant tooling. However, users must ensure that all of their machining systems can handle the demands of high-pressure coolant. Enclosures must prevent substantial amounts of coolant from leaking into the areas surrounding machine tools. And cutting rates shouldn't be so high as to cause excessive wear and tear on the tool, damage the machining center's spindle bearings or the turning center's indexing turrets, or cause internal seals to fail.

Users of high-volume, high-pressure coolant systems must also consider the additional costs associated with handling and disposing of the extra coolant they'll use.

However, for shop owners who think through-coolant tooling might increase their present productivity levels, tooling manufacturers will gladly give them all of the guidance necessary. Toolmakers' success depends on the success of their customers.

About the Author
Jerry Arpaio has more than 28 years of experience in the metalworking and manufacturing industries. He is currently the North American product manager for two Italian machine tool manufacturers, AVM Angelini/AVM Machine Tools Inc. and Costruzioni Mechaniche Caorle S.P.A./Caorle Corp.