Five Key Factors Determine Holemaking Outcomes

Drilling is the most common of all machining applications. There are as many ways to drill as there are holes to be produced. Optimizing those applications starts with knowing the desired outcome. What are the key criteria — quality, Geometric Dimension and Tolerance (GD&T), throughput, cost per unit or tool life? Or maybe you just want a simple, standardized process that you can easily repeat. Once you know what you’re trying to achieve, then it’s time to examine the factors that influence the process.

Five basic factors to consider, outlined below are material, machining parameters, holders, tool design, and coolant, and all influence one another.

1.) Material: Once you have established your objectives, part characteristics are the next critical issue to your tooling decision, starting with the material to be machined — type, treatment, hardness, surface condition. Soft, non-ferrous materials have quite different properties than hardened materials and superalloys, and proper tool designs are to be chosen based upon each of the ISO material grades.

2.) Machining Parameters: Ensuring optimum cutting data is crucial to controlling heat, formation of the chip and tool life. Speeds (Surface Footage Per Minute, SFM) and feeds (Inches Per Revolution, IPR) impact tool life through heat and pressure.

3.) Holders: There are three basic holder types – mechanical, hydraulic or heat shrink. All are available with different interface options depending on the machine spindle. In some cases, collets are used to standardize holder options and size down to the tool shank size instead of requiring additional holder options. In most cases, the shank tolerance of the cutting tool should be h6, which is considered standard. Mechanical holders normally use a set screw using a torque wrench to locking specifications and eliminate the risk of the tool coming loose. With a mechanical holder, it’s important to verify runout. Hydraulic holders have a specific shank length to be held for the distance inside the holder. If the tool gets pulled out too far, the internal sleeve can become distorted over time as the hydraulic bladder gets clamped down. This type of holder has mass that reduces residual harmonics. Heat-shrink holders are commonly used and create a very rigid assembly. While providing particularly good runout characteristics, the rigidity can in some cases create chatter in the part on longer tools.

4.) Tool Design: Matching the carbide substrate, coating, and coolant hole placement to the needs of the workpiece is what modern metallurgy is all about. When done correctly, forces on the cutting edge are minimized, chip breaking is controlled and evacuation is complete, resulting in smooth finishes and controlled tool life. Proper tool geometry maximizes part quality as well as tool life. Each application determines the optimal design for point geometry, clearances, coatings and margin widths – the part of the drill geometry located along the outer diameter trailing the cutting edge. Margins are needed to stabilize the drill in the hole to provide support behind the cutting edge and the workpiece, helping produce a truer hole by essentially burnishing. Single margins are common and multiple margins are used for part and material-specific requirements to accommodate surface finish, hole location, and bore size criteria. Advanced tool coating options are also available to maximize tool life along with the number of times a tool can be reconditioned. Overrunning a tool will create excessive margin wear and reduce the number of times a tool can be reconditioned. The area between the cutting edges is referred to as a flute. Straight-fluted tools can maintain proper hole diameter, less drift, and achieve good part surface finish. However, workpieces that feature intersecting bores can pose challenges to chip evacuation, that can be better addressed using a helical tool. Straight flute tools can be used in deep-hole applications, but helical tools are preferred depending on the diameter to depth of cut ratio. Looking at the actual operation: Will machining be done from the solid? Will there be through holes or blind holes (possibly cored), or any interrupted cuts?

5.) Coolants: There are two general types of coolant delivery systems. Flood coolant has no through-the-spindle supply and is not ideal for the cutting edge. Whereas through-the-spindle coolant flows through the internal channels of the cutting tool. Today, more applications use through-the-tool, and when combined with proper geometries, the part and cutting edge stay cool while chips flush up the flute channel to evacuate continuously. Coolant pressure, especially on deep hole drilling where the diameter-to-depth ratio is great, must be set properly. If it is too low, chips can get packed together during machining leading to tool breakage. Using correct coolant pressure and proper clearances on the cutting tool aids in making smaller chips and preventing material build-up on the cutting edges.

There are three typical types of coolants used in machining:

• Straight oil, or neat oils, provide the best lubrication and tool life.
• More environmentally friendly semi-synthetic coolants.
• Water soluble coolants.

There are numerous factors to consider in selecting the right tool. To improve best practices for your holemaking, work with a tooling solution expert to engineer an optimal solution.

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