Drilling and Hole Machining | Challenges and Solutions for Efficient Hole Machining

Challenges in Drilling Holes

The challenges in drilling holes include the following:

・Poor accuracy
・Tool breakage

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Poor Accuracy

In hole drilling, two types of accuracy are required: "hole position accuracy" and "dimensional accuracy". Deviations in hole position or dimensional inaccuracies can affect the assembly and fitting of parts, so even simple hole drilling requires caution. It is necessary to check whether the surface to be machined is perpendicular, and if it is not, centering or face machining needs to be performed. Causes of poor accuracy include drill runout and wear.

For drilling holes with strict tolerances, finishing processes using reamers are performed for precision holes with tight tolerances.

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Tool Breakage

Compared to milling tools, drills are more prone to breaking, and in automated machining such as with machining centers, they can cause machine stops due to alarms, requiring caution. Drill breakages can be classified into four types, each with different causes:

・Spiral shaft breakage: Occurs as a result of forcing further axial pressure even when the drill is not advancing.

・Tip breakage: Occurs as a result of hitting a hard object and losing a piece.

・Shaft breakage: Occurs as a result of repeated force on the neck under a stepped shaft.

・Tip groove bottom breakage: Occurs when drilling beyond the effective length or after prolonged use.Factors contributing to tool breakage include chip clogging and overload due to vibration.

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Countermeasures for Challenges in Drilling Holes

To improve issues of poor accuracy and tool breakage in drilling, the following measures are available:

To Improve Poor Accuracy:

Perform center hole or pilot hole drilling.

Drilling a center hole as a pre-process improves the bite of the drill, preventing accuracy issues caused by wobble or misalignment upon contact with the workpiece. Center holes are also effective for drilling holes on slopes prone to misalignment. Furthermore, when using long drills for deep hole drilling, drilling a deeper hole known as a guide hole can stabilize machining accuracy.

Select a tool holder that suppresses wobble.

To minimize drill wobble, selecting the right tool holder is crucial. For high-precision requirements, considering tool holders with excellent mounting accuracy and rigidity, such as shrink fit chucks or hydraulic chucks, is necessary.

Adjust cutting conditions at each point considering drill wobble.

Drilling involves three elements: "rotation speed," "feed rate," and "amount of feed." By adjusting cutting conditions at each point, you can improve poor accuracy.

1. Insert the drill at a low rotation speed until about 1mm above the bottom of the guide hole.When entering the drill into the guide hole, reduce the rotation speed or stop. By minimizing wobble caused by rotation, you ensure the drill is inserted securely into the guide hole.
2. Increase the rotation speed to the recommended cutting conditions while machining with coolant.After inserting into the guide hole, increase the rotation speed to the recommended cutting conditions. For deep holes, use drills with coolant holes to eject coolant from the tool tip, facilitating chip removal.
3. Slow down when withdrawing and quickly pull out.After machining, reduce the rotation speed again and quickly withdraw the drill. As the drill exits, the load is significant, so be cautious of tool breakage.

Common Challenges and Considerations in Hole Drilling

Consider the Machining Process for Deep Hole Drilling with L/D over 10

L/D (Length to Diameter ratio) is an indicator of "hole depth," calculated as L (hole depth) ÷ D (tool diameter). The difficulty of machining changes even if the hole depth remains the same but the drill diameter changes. When L/D exceeds 10, it becomes "deep hole drilling," necessitating careful consideration of drill type, cutting conditions, and chip evacuation, in addition to normal hole drilling procedures.

Create Paths Considering Drill Tip Deflection as Rotation Speed

IncreasesThe risk of tool breakage increases when the drill bites into the metal surface at high rotation speeds upon workpiece contact. Especially, long protruding drills used in deep hole drilling require caution due to significant deflection. In such cases, perform pilot hole drilling with a drill of high rigidity and then switch to a drill for deep hole drilling. After entering the pilot hole bottom at a low rotation speed, increase the rotation speed to the recommended cutting conditions and proceed with drilling.

Design Processes Considering Deflection at the Time of Workpiece Contact

Drills become less rigid and more prone to deflection as they get thinner and longer. Especially, deflection at the time of bite can lead to tool breakage and decreased accuracy, requiring caution. To prevent deflection, it is crucial to use drills of high rigidity in a staged process: center hole → pilot hole → deep hole.

Summary of Challenges and Countermeasures in Drilling Holes

This article has explained the challenges encountered in drilling holes with drills and the key points to keep in mind. Hole drilling is often one of the final processes in machining, where failure is not an option. Therefore, selecting the appropriate drill based on the required accuracy, diameter, and depth of the hole is crucial.