Know-how

Preventing burr formation when drilling: Causes, risks and the right tools

Clean drill holes are essential in metalworking to ensure dimensional accuracy, safety, and smooth further processing. However, drilling can produce burrs that leave sharp edges, making assembly difficult and requiring additional finishing work.

Burr formation is not an issue that arises exclusively during countersinking or deburring. It can occur as early as the drilling stage when using twist drills, step drills, sheet drills, hole saws, and other cutting tools. It is therefore crucial to minimize burr formation as much as possible during the drilling process by selecting the right tool and ensuring appropriate machining conditions, and then to remove any remaining burrs in a targeted manner. In this article, you’ll learn why burrs form during drilling, what risks they pose, and how to achieve clean, burr-free holes.

What is burr formation during drilling?

A burr is the term used to describe unwanted material residues or deformations on the edges of a workpiece that occur during machining. During drilling, they can occur at both the entry and exit points of the hole.

Typical forms include sharp or uneven edges, material residues, frayed hole edges, or raised material.

Why do burrs form during drilling?

A burr forms when the material is not completely and cleanly cut during drilling, but is instead partially displaced, bent, or torn away. The extent of burr formation depends primarily on the condition of the tool, cutting parameters, workpiece clamping, as well as the material type and thickness.

A blunt drill or unsuitable tool

A dull drill does not cut the material cleanly but instead displaces and increasingly rubs against it. This causes the temperature to rise, the cut becomes rough, and burrs can form at the edge of the drill hole.

In our article “What happens when twist drills are not precise?”, you can find out what effects an imprecise twist drill can have on the drilling result.

Choosing an unsuitable drilling tool can also lead to burrs. A twist drill, a step drill, a sheet drill, or a hole saw each create different cutting conditions. Therefore, the tool should always be selected based on the application, the material, and the desired hole size.

Incorrect cutting speed when drilling

A speed that is too high can lead to excessive heat buildup. The material will not be cut cleanly; instead, it may smear, tear, or buckle at the edge of the drill hole.

Conversely, a speed that is too low can result in uneven cutting, increased pressure, and an inaccurate drilling result. The optimal speed depends, among other factors, on the material, the tool diameter, and the tool being used.

Therefore, the speed and feed rate should be matched to the material and the tool. Speed tables or application guidelines serve as suitable reference values.

Excessive feed rate or too much pressure

Excessive pressure causes the tool to displace the material more forcefully. This can result in exit burrs when the tool breaks through the back side.

Inadequate workpiece clamping

If the workpiece vibrates or moves during drilling, the cut quality deteriorates. This causes the tool to run more erratically, the hole may tear out, and burrs may form.

Secure clamping is particularly important for thin sheets, profiles, or smaller workpieces. Stable guidance of the workpiece and tool supports a clean and precise cut.

Influence of material type and thickness

Each material reacts differently during machining. Stainless steel tends to generate heat and undergo work hardening if machined incorrectly. Aluminum can tend to stick if the spindle speed is inappropriate or if lubrication is insufficient. Thin sheet metal can deform or buckle during drilling.

Material thickness also plays an important role. With thin-walled materials, burrs often result from deformation and a lack of support. With thicker materials, however, factors such as heat generation, tool wear, and inappropriate cutting parameters become more significant.

Why do burrs form, particularly in through-holes?

When drilling a through hole, the tool must penetrate the workpiece completely. As long as the cutting edges are embedded in the material, the material around the hole is supported. However, this support decreases just before the tool exits the hole.

As a result, the remaining material can deform outward, bend, or tear away, resulting in the typical exit burr. This effect can be particularly pronounced with thin or easily deformable materials.

Controlled tool guidance, appropriate cutting parameters, and, if necessary, rear support help reduce burr formation during the breakthrough.

What risks are posed by burrs in drill holes?

Burrs are more than just a cosmetic issue. Depending on the application, they can compromise the safety, function, and quality of a component and result in additional machining effort.

Risk of injury from sharp drill hole edges

Sharp or uneven edges on drill holes can cause cuts. Clean edge finishing is therefore particularly important for sheet metal parts, housings, and assembly components.

Functional and assembly problems

Burrs can prevent components from fitting together properly or fasteners from being installed correctly. Fits, screw connections, sealing surfaces, and through-holes can also be compromised if material protrudes from the edge of the drilled hole.

More rework and higher machining costs

The more pronounced the burr, the greater the effort required for subsequent rework. This increases machining time and costs. This additional effort can have a significant impact, particularly in series production and recurring machining steps.

Damage and malfunctions caused by burrs

Sharp edges can damage cables, hoses, seals, or adjacent surfaces. In addition, small burr particles can break off and enter mechanical assemblies, threads, guides, or electrical components. This can lead to malfunctions and consequential damage.

Burr formation with different drilling tools

Sharp edges or material residue can result from the use of various drilling tools. The key factors are which tool is used for which application, how precisely it is guided, and whether the speed, feed rate, and material are properly matched.

Burr formation with twist drills

With twist drills, burrs frequently form at the exit of the hole. Especially with through holes, material can tear away from the back side or be forced outward. The risk increases with dull cutting edges, unsuitable cutting parameters, or a twist drill that is not suited to the material.

For larger hole diameters, particular attention must be paid to stable tool guidance and appropriate cutting parameters.

Burr formation with step drills

Step drills are particularly suitable for thin-walled materials, sheet metal, plastic, and housing applications. Burrs are primarily caused by excessive pressure, an inappropriate speed, or when the material thickness does not match the selected tool design.

Exceeding the desired step can also lead to rough edges. For clean results, the step drill should be guided smoothly and used with a steady feed rate.

With RUKO step drills, you can deburr the drilled hole using the next step.

Burr formation with sheet drils

Sheet drills are frequently used to controllably enlarge existing holes in thin-walled materials. Burrs can form if the hole is enlarged too much, the drill becomes misaligned, or the workpiece is not sufficiently secured.

Unlike a step drill, the hole is not created in defined steps but rather through a conical cutting geometry. This allows the diameter to be increased flexibly. At the same time, controlled tool guidance is important to ensure that the hole does not become too large or uneven.

Whether a step drill or a sheet drill is more suitable depends on the diameter, material thickness, and desired hole geometry. You can find a detailed comparison in the blog post Step drills vs. sheet drills: advantages and application.

Burr formation with hole saws

Hole saws are particularly useful when drilling larger through-holes. Since they do not remove material from the entire surface but only create a ring-shaped cut, the amount of force required is reduced compared to drilling large solid holes.

Burr formation with hole saws can be attributed primarily to an incorrect speed, excessive contact pressure, poor centering, insufficient cooling, or a tool that is unsuitable for the material. Especially with thin-walled materials, smooth guidance is important to ensure that the cut remains clean and the material does not tear.

Finishing drilled holes

Even with careful machining, burrs cannot always be completely avoided. The best way to remove burrs is with countersinks or deburring tools.

Countersinking

Countersinking is suitable when you need to remove light burrs from the edge of a drilled hole while simultaneously creating a defined chamfer. This is useful, for example, when you need to round off sharp edges or prepare holes for countersunk screws.

To learn how to use tapered countersinks correctly and what matters when it comes to speed, feed rate, and tool guidance, check out our article Using countersinks correctly’.

Deburring

Deburring tools are used when sharp or irregular edges need to be specifically removed. They are suitable, for example, for through-holes, sheet metal parts, pipe ends, cutouts, and hard-to-reach areas.

Deburring is particularly important when remaining burrs can compromise the safety, assembly, or function of a component.

Whether a countersink or a deburring tool is the better choice depends, among other things, on the size of the hole diameter and on the technical feasibility of the application.

Practical tips: How to reduce burr formation when drilling

Burrs cannot always be completely prevented. However, they can be reduced by following the correct procedure.

1. Choose the right tool

Select a drilling tool that is suitable for the material, material thickness, and desired hole diameter. Also, make sure the cutting edges are sharp and undamaged, as worn tools may not cut through the material as cleanly and can lead to burr formation.

2. Set the cutting speed and feed rate correctly

The cutting speed and feed rate should be appropriate for the material, tool, and tool diameter. Excessively high spindle speeds, excessive pressure, or an uneven feed rate can cause the material to be displaced or torn out more severely.

Use appropriate guidelines for the specific application, such as those found in cutting speed tables or the tool’s application recommendations.

3. Securely clamp the workpiece

The workpiece should be securely clamped during drilling. Movement or vibration impairs tool guidance and can lead to rough hole edges and increased burr formation.

Secure clamping is especially important for thin sheet metal and smaller workpieces.

4. Cool and lubricate as needed

Cooling and lubrication can reduce friction and heat generation, thereby contributing to a clean machining process. At the same time, this can increase the tool’s service life.

The appropriate cooling or lubrication method depends on the material and the tool being used.

5. Carefully finish any remaining burrs

Even under optimal machining conditions, a slight burr may remain. Depending on the application and the desired edge shape, this should be removed with a suitable countersink or deburring tool.

Countersinks are particularly suitable when a defined chamfer needs to be created at the same time. Deburring tools are useful when sharp or irregular edges need to be removed.

Conclusion

Burr formation during drilling cannot always be completely avoided, but it can be significantly reduced by using the right tool, optimized cutting parameters, and stable workpiece guidance. The tool should be guided with care, especially when drilling through. Any remaining burrs can then be removed precisely with a suitable countersink or deburring tool – ensuring safe, clean, and assembly-ready holes.

FAQs on burr formation during drilling and countersinking

Burrs occur when, during drilling, the material is not cleanly separated but is displaced, torn or thrown up at the edge of the drill hole. Common causes include blunt cutting edges, unsuitable cutting parameters, inadequate workpiece clamping or the wrong choice of tool. In through-holes, burrs frequently occur at the exit of the hole.

Burr formation can be reduced by using a tool suitable for the material, sharp cutting edges, appropriate cutting parameters and stable workpiece clamping. Particular care should be taken to control the feed rate as the tool exits the workpiece. Depending on the application, subsequent finishing with a counterbore or deburrer may be required.

Depending on the application, burrs can be removed using countersinks or deburring tools. Countersinks are suitable when the hole is to be chamfered at the same time or prepared for a countersunk screw. Deburring tools are used in particular when sharp edges need to be removed in a targeted manner.