Defining the quality of hole machining is actually quite difficult, and the reasons are...

If a hole has strict tolerance or surface finish requirements, secondary machining such as boring or reaming is usually used to bring the hole to its final size. In these cases, the primary value of the drill bit may be to create as many holes as quickly as possible, and what the user sees is whether the positioning is accurate.

But that’s not always the case. In some applications, spending a little more time and effort can help the drill bit achieve quality standards in a single operation. Or, it can be determined that the quality of the drilled hole affects its ability to accept high-quality secondary machining. For example, drilling at too high a speed may generate heat, causing work hardening of the material, which can greatly reduce the lifespan of a tap or even make the material too hard to thread.

Whether a carbide drill bit drills 2 holes or 200 holes may differ; if it’s 200, the focus may mainly be on speed (efficiency). If it’s just 2 holes, spending a little more time and effort during drilling, or using a specially designed tool to drill and ream in a single operation, can produce holes that meet quality specifications without additional processes.

Here, I can think of three key issues:

1. Does it meet the hole tolerance?

2. Does it meet the hole machining requirements?

3. Is the concentricity good?

Carbide drill bits are used in many fields, but many technical details are often overlooked. The design of the helix angle is also important. For example, low-helix or straight-flute drill bits are very suitable for short-chip materials such as cast iron or ductile iron. Helix angles of 20–30° are generally good for general drilling in various hard materials because they aid in chip evacuation.

Aluminum and copper, however, tend to favor high helix angles, which help with chip evacuation. Choosing drill bits with the right characteristics for specific materials and applications will extend tool life and achieve a good surface finish.

Coatings also make a big difference. Many drill bit brands use composite coatings that combine titanium, chromium, and titanium-silicon layers. Silicon provides excellent lubricity, allowing chips to slide off and preventing built-up edge formation. Preventing built-up edge is key to maintaining cutting ability and avoiding marks on the hole wall.

Some new coatings are designed to remove material at higher speeds while producing a high-quality finish. These coatings need to withstand the heat generated at high cutting speeds.

Controlling Drill Bit Details

Choosing suitable blank material: hole quality starts with process design. Excessive runout will compromise hole accuracy, surface finish, and concentricity. The correct web thickness at the drill tip is important to keep the drill stable when engaging the material, preventing enlargement or deviation that affects hole straightness.

When quality requirements include improved tolerance and surface finish, switching from a single margin to a double margin drill can help. These margins provide four contact points in the hole to stabilize the drill and give a polished effect, leaving a very smooth surface. Double-margin drills also guide the drill straight, especially in deep holes, preventing wobble and providing relatively round holes.

Although double-margin drills produce a good surface in short-chip materials, for long-chip materials, single-margin drills are recommended. For materials like aluminum or stainless steel, single-margin drills are preferred. Using double-margin drills in stainless steel can trap chips between the drill and the material.

Controlling runout is another key to hole quality. Excessive runout enlarges the hole diameter, and as drill speed increases, it may lead to progressively larger holes.

Long drill bits may have insufficient rigidity and vibration. These vibrations, especially in small-diameter drills, can cause breakage and leave marks inside the hole.

Cutting Fluid Management

Proper cooling fluid management—including maintaining optimal concentration, filtration, and pressure—is crucial in drilling applications.

Proper coolant concentration increases lubrication while removing heat from the cutting edge. Filtration removes metal contaminants and other debris, improving drilling performance and preventing clogging in small-diameter drills.

Preventing chips from getting between the drill and the hole wall is essential for hole quality. The shape and color of the chips can help operators assess the hole quality.

Drill flutes should produce nicely conical chips. Two or three curled or braided chips may tangle in the flutes and rub against the hole wall, causing scratches and rough surfaces. The back of the chips should be silvery and shiny, which indicates good heat management. Blue chips indicate excessive heat at the cutting edge, which accelerates tool wear.

Many times, the stability of a tool is a core indicator of quality—this is what distinguishes large manufacturers from small ones. Therefore, it is recommended that factory owners do not be swayed by temporarily low prices, as the time cost is significant. It’s best to choose a professional manufacturer for the most suitable tools.