Tooling and Strategies for Holemaking

By Mike Smith, Product Manager – Reaming, Boring and EPB

Holemaking is a fundamental manufacturing process, consuming nearly a third of all the time spent in metalcutting operations. Manufacturers make holes in a nearly an infinite number of sizes and levels of quality, ranging from loose-specification clearance holes to highly-toleranced holes engineered for critical applications.

Holes most often are made to accommodate mating parts, fasteners or shafts. The fit between the parts can be grouped in three very general categories: sliding or clearance fits, tight and secure interference fits, and transition fits in which a shaft is held securely but not so tightly so it cannot be removed from the hole.

Standards for quantitatively determining the class of fit for a hole cover nine characteristics, namely straightness, circularity, cylindricity, parallelism, perpendicularity, positioning, concentricity, diameter/dimensions/tolerances, and surface finish. For each characteristic, the hole’s actual dimensions are compared to a theoretical cylinder or standard dimension.

To produce the desired hole size and quality in the most cost efficient and productive way, it is crucial to apply the most appropriate tooling and machining strategies.

The selection process for holemaking tooling begins with determining a hole’s basic requirements, including its diameter and depth, whether it is a blind or through hole, its geometry in relation to the part, where the hole is positioned on the part, the desired surface finish and whether the hole includes steps or chamfers. Also influencing tool choice are the machinability characteristics of the workpiece material, the production volume required, machine tool and fixturing capabilities, and the availability of coolant.

The part manufacturer’s priorities play a large role in tool selection, including such issues as minimizing hole cost and tooling cost per part, speeding cycle times, and matching production requirements with shop capacity.

Different hole characteristics require different tooling configurations. For simple holes in solid material, drills are usually the first choice. Existing holes can be enlarged via turning and rough boring. Milling processes are often the most appropriate way to make complex or large holes. Depending on the quality desired, final dimensions and finish are produced via reaming and finish boring.

Crownloc Plus

Available holemaking tooling includes a wide variety of alternatives. When considering drills, for example, solid carbide versions enable efficient production of small diameter, accurate holes when applied on suitable machines. Solid carbide drills also can be reconditioned, reducing tool costs. For medium-diameter holes, “replaceable end” drills, such as Seco’s Crownloc® tools, provide high accuracy while offering the convenience of a replaceable drill point. When making larger diameter holes, inserted drills like Seco’s Perfomax® drill can be fitted with different insert grades to provide optimal performance in differing workpiece materials.

Hole finishing systems offer similar variety. To maximize hole quality in terms of class of fit, rough and fine boring heads such as those in the Seco A750 system offer high positioning accuracy. For control of hole geometry, boring heads provide the best results in regard to straightness. When surface finish is a major consideration, hole finishing tooling like Bifix®, Precifix™, Xfix™ reamers from Seco represent excellent solutions.

In many cases, production volume dictates the selection of holemaking tooling and process strategies. If a manufacturer’s goal is to minimize cost per hole in a high-volume situation, the focus will be on machine utilization, productivity, reduction of cycle times, and operational security of the long-running job. Those goals can be obtained by fine tuning cutting parameters and process steps with CNC programming written to produce the holes in the most efficient way. In choosing tooling, selection of drilling and reaming tools chosen for and dedicated to the particular job will provide reliable operation.

On the other hand, in small batch holemaking the emphasis is on flexibility. Tooling such as center-cutting endmills can provide versatility to handle a wide range of part configurations and workpiece materials. Advanced programming techniques including helical interpolation can enable a single milling tool to produce a variety of hole sizes and configurations. After the hole is milled, adjustable boring tools can provide the required tolerances and finishes.

In situations where a holemaking operation represents a bottleneck in the production flow, selection of the right tooling and procedures can overcome slowdowns that result from slow cycle times, poor chip control, less-than-rigid tooling, fixturing and machining equipment. Effective management of cutting speeds and feeds is essential in reducing cycle times while also avoiding vibration that can affect surface finish. High-performance holemaking tooling tailored to create specific hole requirements and provide top performance in certain workpiece materials will contribute as well.

Though not strictly a tool or a strategy, coolant delivery is a key element in nearly any holemaking operation. Coolant flow has three major functions. It enhances chip evacuation, always a crucial issue in holemaking. It also cools the tool and workpiece, which is essential because the heat of metalcutting in holemaking is concentrated deep in the hole. Finally, sufficient coolant flow lubricates the drill land margins, which are in direct contact with the wall of the hole and generate additional heat through friction. 

To channel the fluid directly to the drill cutting edges and margins and push the chips back out of the hole, it is always best to utilize holemaking tools with internal through-coolant capability. Where internal coolant delivery is not possible, the coolant flow should be directed parallel to the drill to ensure that as much of it as possible enters the cutting zone. It may be necessary to reduce cutting speeds by 20 percent compared to drilling with internal coolant.

The selection of the best tools and strategies for holemaking operations will facilitate productivity and boost cost efficiency in what is metalcutting’s most widely-used process.

About the Author
Mike is Seco's product manager for reaming, boring and EPB tool holders, which includes EPB’s line of rough and finishing boring heads. In his spare time, Mike enjoys spending time with his wife and two daughters as well as running when he gets a chance. Contact Mike at msmith@secotools.com.