Manage costs by drilling efficiently

After labor, cutting tools are the largest expense at a machine shop, and drilling is the most common machining operation. Therefore, creating efficient drilling operations is important to managing machining costs.

Three elements — cutting speed, stability and chip evacuation — result in a stable, robust drilling operation regardless of the material, machine or drill.

Obtaining optimum cutting speeds and feed rates is the foundation of efficient drilling. Cutting speed is also the most significant factor in drill life. As cutting speeds increase, so does the volume of heat produced in a drilling operation, and elevated temperatures cause faster tool wear.

Cutting speed is also the most influential factor in cycle times, which is why machinists and engineers usually look to increase the rotational speed and feed rate when productivity improvements are needed. One of the basic machining calculations is rpm × chip load = feed rate. So increasing the rpm or chip load yields a faster feed rate and shorter cycle time.

Effective chip evacuation improves tool life and productivity. Cutting Tool Engineering image

Stability is the second factor in efficient hole drilling. The most common stability problem is vibration, or chatter, which can occur for a few reasons. When a drill chatters, the cutting edge, or lip, can chip. A chipped drill creates more heat, which reduces life. In the worst situations, this causes a workpiece material to weld to the cutting edge. Chip welding prevents a drill from forming a chip. Instead of cutting, the drill rubs, and the axial loads eventually cause the drill to break. While all drills can chip, advanced drill materials, such as polycrystalline diamond and carbide, are more prone to this type of failure than high-speed steel and powder metals.

A drill point vibrating at the bottom of a hole is the most frequent chatter issue and usually results from incorrect cutting parameters or having the wrong edge and point geometry for a workpiece material. Other times, this can be caused by a toolholder and drill combination that lacks rigidity or when a drill protrudes too far from the toolholder.

Spot drilling is another way to induce chatter that damages high-performance drills. When the point angle of a spot drill does not match the angle of the drill, the point of the drill is left unsupported, resulting in chatter. Chatter can fracture the corners of the drill, preventing proper chip formation at the corners, which reduces drill life or causes catastrophic failure.

When everything works right, a drill forms small chips, and it is important to evacuate them from the hole or else chip packing and re-cutting can occur. Chip packing is when chips do not clear a hole and become packed in the flutes of the drill. The result is a poor-quality hole or a broken drill.

Re-cutting happens when a chip becomes trapped between the drill lip and the bottom of the hole. Ideally, the drill cuts the chip, and the pieces are evacuated. Worst case, the chip stays trapped. The drill then is unable to form new chips, stops advancing in the hole and breaks.