What is deburring?

A burr is a material deformation that can be created through different production or machining processes. It is firmly attached to the actual component without, according to the component drawing, actually supposed to be a part of it.

During casting, for example, the excess material which is pressed out between the two halves of the mould is referred to as separating burr.

When removing a surface at the discharge edge of a mould, a machining centre creates a so-called discharge burr.

Burrs can also be created through drilling, punching or turning.

The range of burrs reaches from microscopically small to those with a thickness of multiple millimetres. Their shape depends on different factors. During a casting process, for example, the shape of the burr is affected by the age and material of the mould but also by the properties of the casting material.

In case of mechanical processing, a burr can be more or less pronounced depending on the parameters of the process, the service life of the tool or the material properties of the workpiece.

Burrs generally have a sharp edge after follow-up processes, such as subsequent production steps, or the assembling process. They are unwanted and have to be removed.

The „deburring“ process describes the removal of a burr using different processes, which can roughly be divided into mechanical, chemical and thermal deburring. Neither the input nor the output variables are specified in greater detail.

The users of deburring applications have to define their own objectives. In most cases, deburring specifications are created for this purpose. They clearly define which target values (and corresponding tolerance ranges) are permissible for the interiors and exteriors of workpieces. In addition, the corresponding dimensions are recorded on drawings.

In order to determine the correct deburring process that can achieve the desired result, the type of burr first has to be defined.

The basic conditions for the creation of a burr are as follows:

• The used of a plastically mouldable material
• A force that is applied to the material. Generally, this is a cutting force or feed force as part of a burr-forming production process

The burr-forming production processes are:

• Casting (casting, pressing, sintering)
• Forming (shaping, forming, pressing)
• Separating (turning, milling, cutting, sawing, clearing, …)
• Joining (welding, casting, …)

When looking at the different processes, one can tell that they form different types of burrs. This formation is the result of different forces which, depending on the production process, are applied to the component.

Let’s examine more closely the formation of a burr that is the result of drilling. Drilling creates 2 different types of burrs – one in the area where the drill enters the material and the other where it exits.

The burr created where the drill enters the material depends on the type of drilling operation. At the beginning of the process that generates the borehole, the drill is pressed into the workpiece and therefore displaces the material at the surface of the workpiece.

Spiral drills, whose cutting speed in the centre of the tool is often almost 0, are very often used for such drilling processes. As a result, in the area of the borehole centre, the drill pushes the material in front of it rather than cutting it out of the full material. Prior to exiting the borehole, the excess material first bulges against the borehole base in the interior of the component. Once the drill breaks through the base, the remaining material is bent over the edge and remains there as a burr at the edge of the borehole.

In addition to the force applied through the production process, the material also influences the formation of the burr. To better understand this, let’s first define the different terminology:

Grid / crystal:
When viewing a metallic material on an atomic level, one can see that the atoms are evenly distributed in the space. There are large binding forces between the atoms. These atomic structures are also referred to as crystals. Depending on the packing density and the number of atoms within a unit cell, we refer to them as different grid types. Overall, there are 7 different main grid types (see table 1), whereby the metals are crystallised cubically or hexagonally. In addition, some metals can form different grid types in different temperature ranges.

Grid type	Form of the unit cell (illustrative)
Triclinic	„Brick“ that is skewed on all sides
Monoclinic	„Brick“ that skews in one direction
Orthorhombic	Regular „brick“
Tetragonal	Cube stretched into one direction
Rhombohedral	Cube that is skewed on all sides
Hexagonal	A piece of hexagonal material cut evenly
Cubic	Cube

Unit cell:
The term „unit cell“ describes the smallest volume unit of a space lattice in which all symmetrical features of a crystal system are represented. By periodically shifting the edges, the creation of a space lattice can be simulated.

An ideal crystal is described with the definitions listed above. In addition to the fact that atoms do not have a spherical shape in reality and are not in a resting position, the real crystal also takes into account:

• A finite limit (this refers to the surface of the metal)
• The existence of disrupted areas (gaps, foreign atoms, displacements)

Displacements:
Displacements are line defects that occur in a grid with a high density. They affect the material properties to a high degree and are characterised by the following properties:

• They have a sense of direction, which means they either attract or repel each other.
• They are able to move, which means they cause a deformation of the material by moving within the grid in large numbers
• They are the cause of internal stresses and solidifications

Plastic deformation:
A plastic deformation also refers to the „flow“ of the material. Thanks to an external force of a defined size, the limit stress within the material is exceeded. This leads to a migration of the displacements and therefore a deformation of the respective material. Corresponding to the geometry of the grid, this takes place in preferred levels and directions.

Consequently, the physical and technical properties of a material are determined by the basic grid of the crystal as well as by the type, quantity and arrangement of grid errors and building blocks that are foreign to the grid.

Barrel finishing
Barrel finishing is one of the most common deburring processes. Like so many other things, humans copied this method from nature, where sand and water grind jagged rocks into smooth pebbles.

In modern production technology, the right combination of machine, grinding tools, compound and water allows the machining of nearly any surface. In this process, the workpieces are put into a container as a bulk good together with the grinding medium. By rotating and oscillating the container, a relative movement between workpiece and grinding medium is created, which leads to the removal of the material.

Depending on the requirements, workpieces can be deburred, polished, barrel finished, descaled, cleaned, smoothed or have their edges rounded.

+ Advantages

• Components can be delivered as bulk goods
• Less space required compared to other processes
• No devices required
• Large output quantity
• Edges can be rounded

– Disadvantages

• Complex component geometries or depressions cannot be machined completely.
• Burrs can enter into the borehole
• Not suitable for impact sensitive or highly polished components
• No defined material removal possible
• Not suitable for linking to a production line for piece goods
• Not suitable for large components

Electrochemical deburring:
Using the process of electrochemical deburring (ECM), all conductive materials can be deburred contact free without thermal, chemical or mechanical influences. In this process, the workpiece is polarised as anode (positive) and the tool is the cathode. An electrically conductive liquid (electrolyte solution) closes the electrical circuit.

Explained in a simplified manner, ECM deburring works as follows:
The workpiece is clamped into a device and, via a generator, it is given a positive charge. The tool electrode/cathode is now moved to a distance of between 0.5 – 2mm of the area of the workpiece that is to be deburred. Between the workpiece and the tool, a so-called „working gap“ is created. The electrolyte solution is run through that gap. If a DC voltage is now applied to the workpiece and the electrode, an electrical current runs across the working gap and results in a charge exchange and/or a process of disintegration. In this process, the intensity of the deburring effect can therefore be controlled via the voltage applied or the amount of time it is applied.

+ Advantages

• Targeted deburring of selected areas is possible
• Mechanical properties do not impact the machinability
• Contact-free process
• With regard to the tool, this process is wear-free
• The component is not subject to thermal stress
• No secondary burr
• Partially short deburring times

– Disadvantages

• Component has to be metallic
• Conventional materials and design principles cannot be used
• Components must be chip-free and grease-free
• Following ECM deburring, components have to be cleaned with clear water
• Components must potentially be preserved
• Depending on the shape of the component, additional contours cannot be implemented easily

Thermal deburring:
Thermal deburring (or, officially, thermal-chemical deburring) is part of the non-targeted machining processes. Only the removal of the burr is guaranteed. Nearly all oxidising materials can be deburred in this way.

During the thermal deburring process, the workpiece is in a deburring chamber. It is filled with an oxygen-combustible gas mix and ignited via a spark or an annealing line. Depending on the mixture and the gas volume, temperatures of up to 3,000°C are reached. The workpiece itself, however, only heats up insignificantly (approx. 100 – 190°C depending on its heat capacity).

Due to the abrupt temperature increase, all those areas of the workpiece are overheated whose surface to volume ratio is very large. Generally, these are burrs that, as a result of the heat build-up that is generated in this process, are first ignited and then burned off.

+ Advantages

• Removes burrs in inaccessible locations
• Burrs are removed completely
• Universal process without workpiece bond
• Very short cycle times

– Disadvantages

• No defined rounding of edges
• Only suitable for oxidising materials
• Not suitable for hardened components
• Workpieces with large volumes can only be deburred conditionally
• Must be free from loose chips and greases
• Dipping into a highly diluted acid mixture is possibly required as an aftertreatment

High-pressure water jet deburring:
During the high-pressure water jet deburring process, the burr is broken off at the thinnest available spot. An abrasive medium is often added to the water in order to improve the removal effect. This deburring process is particularly well suited for light-metal workpieces. A lance guides the jet to the area that is to be deburred. Via nozzles, it is applied to the workpiece in that spot. The water generally has a pressure of between 600 and 1,000 bar. Advantage: targeted removal of burrs, suitable for large series, deburring, chip removal and cleaning in one step. Disadvantage: A lot of programming required for the CNC-controlled machine, long throughput times, extensive handling required and only programmed areas are deburred.

+ Advantages

• Even complicated workpieces can be machined
• Targeted burr removal
• Very well suited for large series
• Deburring, chip removal and cleaning in one step

– Disadvantages

• A lot of programming required for the CNC-controlled machine
• Relatively long throughput times
• Extensive handling required
• Deburring only at programmed areas
• Abrasive medium alters the surface structure

Mechanical deburring (or Robotic deburring):
Mechanical deburring includes any material removal or any chipping work that is performed via driven tools and whose goal is the removal of the burr at the component. This also includes all manual deburring operations. The material is removed with a blade, which can be geometrically defined (e.g. cutter, file) or not defined (e.g. grinding belt, brush).

While manual deburring workstations were used almost exclusively for this process in the past, nowadays robots increasingly perform the task of mechanical deburring. This is necessitated by the demands on the component listed below and a modern industrial production process:

• A deburring result not dependent on the diligence and capacity of the employee
• Process reliability
• Reduction of production costs due to economisation
• Employees are less exposed to dirt, noise, heavy weights, etc.
• Increase of productivity
• Less space required

Mechanical deburring via robots is suited for many different deburring tasks and can be integrated effortlessly in existing production lines as an automation solution. A direct connection to a machining centre for loading and unloading is standard practice.

There are two types of robot-supported deburring. In the first variant, a robot moves the component to the different machining tools. This process is well suited for components with small to medium weights and dimensions.

Illustration: Workpiece guided deburring

The second variant is often used for heavy workpieces. The workpiece is fixed in a defined position and the robot guides the tools to the contours that are to be deburred.

 

Illustration: Tool-guided deburring

In both cases, taking into account the cutting geometry, angle of attack, feed speed, length of stay and material properties, a defined phase for many contours can be created.

+ Advantages

• Flexible machining
• Different tools can be used
• Can be expanded with preparatory and subsequent processes (fettling, polishing, …)
• Removal of large burrs
• Well suited for mass-produced parts
• Suitable for different component dimensions and weights
• All accessible contours that are to be deburred are included
• Can be connected directly with the machining centre without additions

– Disadvantages

• A lot of programming required
• Partially large investment costs
• Drilled holes leave behind root burrs
• Due to accessibility issues, each contour cannot be machined unrestrictedly

 

In 1994, an independent company was founded that took over the customers, technology and patents of SIG. Since that time, WMS-engineering Werkzeuge – Maschinen – Systeme GmbH has designed, built and programmed dependable and process reliable deburring applications in the field of robot-guided mechanical deburring.