What happens when twist drills are not precise?

At first glance, a twist drill looks very simple: spiral/flute, tip, shank, and that's it. But in fact, a drill is a high-precision tool.

A hole drilled with a precise drill has clean edges, is straight, and has exactly the specified diameter. Tight manufacturing tolerances play a key role here—this means that deviations (e.g., in diameter or in the geometry of the cutting edges) must be minimal.

Industry standards such as DIN 338 or DIN define specific tolerance classes, e.g. H7 or H8, which specify the permissible range for dimensional deviations. Maximum precision is essential, especially in metal cutting and in tool and machine construction, as even small inaccuracies can lead to fitting problems or unstable connections.

Important characteristics

• Concentricity

  This describes how precisely the drill moves when rotating around its own axis. Poor concentricity causes the drill to wobble, resulting in inaccurate holes and increased wear.

• Point angle
  The point angle (usually 118° or 135°) influences penetration into the material. If the angle deviates from the target dimension or is unevenly ground, the drill may not penetrate the material properly or may run off course.

• Diameter
  For work that requires high precision, even small deviations in diameter can be critical. A diameter that is too large or too small can significantly impair the accuracy of fit of fasteners or components.

A precise drill...

1. centers itself without wandering
2. drills evenly without pressure
3. produces clean chips and
4. is gentle on the material

1. Inaccurate drill holes

One of the most common signs: the drill wanders. It does not start in the desired center, but deviates. This is particularly noticeable on smooth surfaces or hard materials such as metal.

The result
A hole that is crooked, oval, or offset and is neither visually nor functionally usable. And if you are working on a metal plate or a fitting, this often means you have to start again.

2. The drill does not cut properly

A precisely ground twist drill cuts into the material almost by itself, without much pressure. If the drill is blunt or poorly ground, the drill has to work hard – and so do you.

The result
You have to apply more pressure, and the drill rattles or squeaks. In the worst case, it overheats and is then unusable.

3. The material suffers

Imprecise drills can tear the material – especially wood and plastic, but also steel. Instead of a clean hole, you end up with a large burr, tears, cracks, or splinters. This is obviously not ideal, especially in visible areas. Or it means rework for you.

4. Higher wear and tear – higher costs

A low-quality drill not only has a shorter service life, but also causes greater wear and tear on your machine. Motor parts, bearings, and chucks are subjected to greater stress. There is also an increased risk of damaging the material, meaning you will have to buy replacements.

Manufacturing defects

Faulty manufacturing processes such as inaccurate grinding machines, inadequate quality controls, or unsuitable materials can result in imprecise drills even during production. Such defects are often difficult to detect at first glance, but can significantly impair performance.

Wear and tear from frequent use

Over time, twist drills lose their sharpness due to friction and heat. The cutting edges become blunt, the tip angle changes, and the drill can become distorted during drilling. The wear process accelerates significantly, especially with hard materials.

Incorrect storage or handling

Incorrect storage can lead to damage or corrosion. Even the slightest deformation at the tip or in the shank can impair precision. In addition, incorrect handling, for example due to incorrect clamping, can lead to imbalances.

Cheap products that have not undergone thorough quality assurance

Inexpensive twist drills made from cheap materials or from non-certified production often have larger manufacturing tolerances. The necessary precision is lacking when grinding the cutting edges, maintaining the point angle, or hardening. As a result, these tools wear out more quickly or are inaccurate from the outset.

• Focus on quality: Good twist drills are characterized by excellent workmanship, the use of high-quality steel, and longer durability.
• Make sure your drills stay sharp: If you drill a lot, it is worth investing in a drill sharpener or having them reground.
• Choose the right drill for the material.
• Choose the right chuck and correct speed: Even high-quality drills will run unevenly with an inaccurate chuck. The speed should be adjusted to the material and diameter.
• Use centering aids or pilot drills: A guide is particularly helpful for precise work.
• Do not use force: Do not apply too much pressure when drilling.
• Use cooling: Especially with structural steel or stainless steel, it is important to use coolant to prevent the drills from overheating and the cutting edges from becoming blunt.

Yes, twist drills can be reground as long as the cutting edge(s) do not have any deep cracks or chips. Regrinding involves slightly removing material from the cutting edge to restore the original geometry and sharpness. It is important that both cutting edges are symmetrical and of equal length – otherwise the drill will run out of true later on.

There are several options:

• Manual grinding: Using a grinding block or grinding machine. This requires practice to achieve the correct angles.
• Drill grinding machines: The angles are guided automatically. This ensures consistent results.
• Professional regrinding: This can be carried out in industry or by specialist retailers and is particularly worthwhile for high-quality drills (e.g., HSS-Co, solid carbide).

Important: After grinding, the cutting angles (usually 118°–135°) and the spiral feed edge must be exactly correct. Otherwise, the drill loses its centering ability and drilling precision suffers.

Tip: Store reground drills with a light film of oil to prevent rust and increase service life.

Are imprecise twist drills dangerous?

Yes, they can be dangerous, especially when working with metal, but also with wood and plastic. The danger depends on how imprecise they are and how they are used.

1. Jamming & kickback

If the drill wobbles (poor concentricity or bent tip), it can suddenly dig into the material.

→ The machine jerks, and wrist and arm injuries are common.

2. Drill breakage

Poorly ground or crooked tips put uneven stress on the drill.

→ Broken tips can fly through the air like small projectiles.

3. Uncontrolled heat generation

Imprecise drills cut poorly, which generates increased friction instead of a clean cut.

→ Risk of burns and rapid material or drill wear.

The following combinations have different risk levels

Hand drill + metal: high risk

Drill press + poor tension: medium risk

Wood + low speed: low risk, but not harmless

Our clear recommendation is not to use drills if it visibly wobbles, runs smoothly without load but vibrates when applied or grips jerkily.

A precise twist drill is the basis for clean, safe, and economical work.

• Precision means better results and longer tool life.
• Imprecise drills mean more time, higher costs, and safety risks.

Our clear recommendation is: invest in high-quality drills and check them regularly instead of working with blunt or cheap tools. You will thank yourself for it, as you will enjoy your work more with good tools. This does not only apply to drills!