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What Is U Drill

U drills are drilling tools that contain hard alloy cutting inserts. They are characterized by convenience of use and high cost-effectiveness. To facilitate the machining of various materials, we provide U drills of different types and sizes.

Application Background Of U Drill

U Drill

Compared with solid drills, U drills are tooling that performs hole machining by combining inner and outer cutting edges formed by a center bit (inner edge) and peripheral bits (outer edge). This structural characteristic determines the irreplaceable advantages of U drills over other drilling tools.

Advantages Of U Drill

  1. The difference between a U drill and a regular drill is that a U drill uses peripheral cutting inserts and a center insert. When the inserts wear out, they can be replaced directly without resharpening. Using indexable inserts is more material-saving than solid carbide drills, and insert consistency makes it easier to control workpiece dimensions.
  2. U drills have better rigidity and can use very high feed rates. The maximum machining diameter of a U drill is much larger than a regular drill, up to D50~60mm. Of course, due to how inserts are mounted, U drills cannot be made too small.
  3. For different materials, only the insert type and grade need to be changed. This is more convenient than solid carbide drills.
  4. Hole accuracy of U drilling is generally better than solid carbide drilling, with better surface finish, especially under insufficient lubrication conditions. U drills can also correct hole positional accuracy, which solid carbides cannot do.

Advantages Of U Drill In CNC Machining

  1. U drills can drill holes on surfaces with inclination angles of less than 30° without reducing cutting parameters.
  2. After reducing cutting parameters by 30%, U drills can perform discontinuous cutting such as drilling intersecting, through, or counterbored holes.
  3. U drills can machine multi-step holes and cored holes, ream, chamfer, and eccentric drill holes.
  4. Chip formation during U drilling is mostly short and broken chips, which can be safely removed via internal cooling, without needing to clean-cutting residues off the tool. This improves workpiece processing continuity and reduces processing time.
  5. When standard bore-to-diameter ratios are met, no chip evacuation is needed during U drilling.
  6. U drills are indexable tools. When inserts wear out, no resharpening is required – only replacement, which is more convenient and cost-effective.
  7. U drilling produces holes with smaller surface roughness values and tolerance ranges, substituting some reaming operations.
  8. U drilling does not require pre-drilling center holes. Blind holes have a smoother bottom surface, eliminating the need for bottoming drills.
  9. Adopting U drilling technology reduces the number of machining tools needed and increases tool life by 10-15 times compared to regular drills since U drills use solid carbide-tipped head inserts. Each insert also has four cutting edges, allowing easy edge changing when worn to further save resharpening and replacement time and improve efficiency by 6-7 times on average.

Techniques For Using U Drill On CNC Machine Tools

U Drill
  1. U drills require high rigidity of the machine tool and tool-workpiece centration, so they are suitable for use on large power, high rigidity, and high-speed CNC machine tools.
  2. When using U drills, select inserts with good toughness for the center insert, and sharper inserts for the peripheral inserts.
  3. For different materials, select inserts with different chip slot shapes. Generally for small feeds, small tolerances, and larger bore-to-diameter ratios, use inserts with smaller cutting forces; whereas for roughing, large tolerances, and smaller bore-to-diameter ratios, use inserts with larger cutting forces.
  4. Always consider the machine spindle power, U drill clamping stability, coolant pressure, and flow rate during use, and control chip removal to avoid affecting hole surface roughness and dimensional accuracy.
  5. Ensure the U drill center coincides with and is perpendicular to the workpiece surface during clamping.
  6. Select suitable cutting parameters according to the workpiece material.
  7. During trial cutting, do not carelessly reduce feed rates or lower speeds to avoid breaking or damaging the U drill insert or tool.
  8. When inserts show wear or break during use, analyze the reasons carefully and replace them with more tough or wear-resistant inserts.
  9. When machining multi-step holes, start with larger holes and then process smaller holes.
  10. Ensure sufficient coolant pressure to flush chips during U drilling.
  11. Do not mix up inserts for the center and periphery of U drills, otherwise, the U drill shank may be damaged.
  12. When drilling with a U drill, workpiece rotation, tool rotation, or simultaneous rotation can be used; but linear feed is commonly used with workpiece rotation.
  13. Consider the machine’s capability during CNC machining and appropriately adjust cutting parameters, usually lowering speeds and feeds.

Common Problems With U Drill In CNC Machining

  1. Insert breakage or damage occurs too quickly, increasing machining costs.
  2. Abnormal screaming sounds during machining, indicating an abnormal cutting state.
  3. Machine tool vibration affects machining accuracy.

Important Points For Using U Drill

U Drill
  1. Pay attention to the correct installation direction – which inserts faces up, down, inside, and outside.
  2. Center height must be calibrated within a control range of generally 0.1mm depending on diameter. Smaller diameter U drills have higher center height requirements. Improper center height can cause uneven insert wear, oversized holes, and reduced insert life.
  3. U drills have high coolant requirements – coolant must reach the center and pressure is better with higher pressure. Excess coolant ports can be blocked to ensure pressure.
  4. Follow manufacturer cutting parameters considerations but also consider different insert brands and machine power. Reference machine load during machining and make suitable adjustments, generally using high speeds and low feeds.
  5. Inspect and timely replace inserts. Do not reverse inserts.
  6. Adjust the depth of the cut based on workpiece hardness and insert overhang length. Harder workpieces use greater overhang lengths and smaller depths of cut.
  7. Do not use excessively worn inserts. Record insert wear vs. machinable workpiece quantities and timely replace inserts.
  8. Use adequate, correctly pressurized internal coolant. Its primary functions are chip removal and cooling.
  9. U drills are not suitable for softer materials like purple bronze or soft aluminum.

What Are The Two Most Important Performance Evaluation Indicators For U Drills?

Rigidity and chip removal ability. Rigidity is increased through helical oil groove design, and chip removal is improved by enlarging the inner cutting-edge chip groove and other technologies to enhance U drill performance.

How Can U Drill Rigidity Be Improved?

Manufacturers increase rigidity through designs like helical oil grooves and thicker shanks. The Kyocera DRX series enhances rigidity by 33% using a thicker shank.

How Is Chip Removal Addressed?

Features like varied chip grooves, and the DRV’s focus on micro-cutting outer chips and lightweight inner chips help promote effective chip evacuation during machining.

What Problems Can Occur With U Drills?

Potential issues include bit wear/breakage, heat/friction problems, difficulty with large cuts/clogging, and low accuracy due to lack of rigidity.


U drills combine inner and outer cutting edges via interchangeable inserts. This modularity enables multi-material and size machining. The main advantages are high efficiency, low cost, and ease of use without re-sharpening. U drills provide versatility compared to solid drills through their modular design. When operated correctly, U drills enable efficient and cost-effective homemaking as a CNC tooling option.

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