Holemaking accounts for nearly 40% of all machining time. Selecting the wrong drill doesn't just waste time—it leads to poor accuracy, broken tools, and scrapped parts. From general-purpose jobbers to high-speed indexable U-drills, understanding your options is critical.

1. Standard Twist Drills

The most common tools in any shop, defined by their length-to-diameter ratio and flute geometry.

Jobber Drills

The "standard" length drill bit. The length of the flutes is typically $9$ to $12$ times the diameter. These are versatile but can flex or "walk" if not spotted correctly.

Best for: General manual & CNC work

Stub Drills (Screw Machine Drills)

Significantly shorter and more rigid than a jobber. Because they are shorter, they offer much higher positional accuracy and can often skip a spotting operation in shallow holes. Ideal for tight spaces in CNC lathes.

Best for: High-precision CNC, shallow holes

2. High-Performance Carbide Drills

Solid carbide drills are designed for speed. They are categorized by their L/D Ratio (Length to Diameter).

  • 3xD: Very rigid, high speed, usually used for shallow holes.
  • 5xD The industry standard for most production drilling.
  • 8xD to 30xD Deep hole drills, almost always requiring Through-Spindle Coolant (TSC) to evacuate chips effectively.

Warning: When using carbide drills with a ratio higher than $5 \times D$, it is mandatory to use a pilot drill of the same diameter (but shorter) to guide the long tool into the work.

3. Heavy-Duty Holemaking

When you need to move a lot of material quickly or drill very large diameters, twist drills become inefficient.

Spade Drills

A spade drill consists of a reusable holder and a replaceable carbide or HSS blade. They are incredibly cost-effective for large diameters—typically from 12mm 1/2inch up to 100mm 4.000inch

  • Pros: Low tool cost per hole; easy to change size.
  • Cons: Lower feed rates than solid carbide.

U-Drills (Indexable Insert Drills)

These tools use indexable carbide inserts (usually an inboard and outboard insert). They are the kings of production speed. Unlike twist drills, they don't have a point—one insert cuts the center, and the other cuts the periphery.

  • Capabilities: Can drill off-center, can be used as a boring bar, and produces a near-flat bottom.
  • Requirement: Needs high pressure coolant to work safely.

4. Specialized Drill Types

Spotting & Center Drills

Used to create a precise starting point. Spotting drills are preferred for CNC because their angle (usually $120^\circ$ or $140^\circ$) can be matched to the drill point angle to prevent the corners of the main drill from chipping.

Step Drills

Custom or standard tools that drill multiple diameters (and often a chamfer) in a single shot. Essential for reducing tool changes in high-volume production.

Calculate Your Parameters

Whether you're using a tiny stub drill or a massive U-drill, getting the speeds and feeds right is the only way to avoid tool failure.

Open Tooling Calculator

Selection Summary

Tool Type Rigidity Max Depth Speed Level
Jobber Low Medium Low (HSS)
Stub High Low Medium
Solid Carbide Very High Variable Very High
U-Drill Extreme $4 \times D$ typical Extreme

Conclusion

Don't just reach for the closest jobber drill in the drawer. Assess your depth, required accuracy, and material. Switching to a stub drill for shallow holes or a U-drill for large diameters can cut your cycle times in half and drastically improve part quality.