What is a Burr in Metalwork?
When machining or modifying a metal workpiece, sometimes the process used leaves a small ridge or protrusion of metal known as a burr. For the part to be completed to a high standard, these will need to be removed.
The formation of burrs is a complex process of material deformation affected by many factors, so being able to control burrs from forming is quite difficult.
The machining method involved will determine the kind of burr as they come in many different forms, and they are usually created when the tool enters or exits the workpiece.
Once the part has been machined or laser cut etc., if required, it will be deburred using one of the many methods available to produce a smooth and even part and remove any edges which have the potential to interfere with the fitting or movement of parts. If burrs are left on the parts, they will decrease the dimensional accuracy (how well the finished part matches the specifications on the engineering drawing), reduce the surface quality of the part, are a risk for employees and end-users when handling the parts, and render the parts more susceptible to fatigue as burrs can act as failure initiation sites.
Types of Burr
Five common burrs in metalwork are:
- Poisson burr
- Rollover burr
- Tear burr
- Cut-off burr
- Thermal burr
A poisson burr occurs around the tool due to the materials propensity, when compressed, to bulge to the sides until it results in plastic deformation (permanent distortion which occurs when a material has been subject to compression, bending, tensile or torsion stresses which exceed its yield strength, causing it to twist, bend or elongate etc.).
When the tool is applied to the workpiece under a downward force the material may bulge outwards at this point. It may also occur when the tool removes a layer of the surface laterally, leaving a poisson burr on the edge of the workpiece.
A rollover burr, otherwise known as an exit burr because it is typically formed at the end of the cutting process when the tool exits the material, is a result of burr accumulation along the exit edge of the workpiece.
Burr accumulation is where the burr effectively increases its height after each tool pass, resulting in an often substantially larger burr to other types.
Due to tearing produced by the chip separation from the workpiece. It is a result of material tearing loose from the workpiece rather than shearing clean off. This usually occurs when a material is being drilled or punched.
A cut-off burr is produced when a piece of material falls off the main workpiece tearing it, which leaves behind a burr.
Thermal burrs (otherwise known as dross or slags) are a result of the intense energy generated by a laser cutting machines beam bringing the temperature of the metal past its melting temperature, with the assist gas which is supposed to remove this molten metal from the kerf not doing so. In cases where the molten metal solidifies quicker than the assist gas can remove it, this results in stalactites forming at the bottom of the kerf, with these making up the burrs.
What is Deburring
Burrs are not very aesthetically pleasing, and more importantly they reduce the safety and functionality of the parts if they are not removed.
Deburring is the process of removing these imperfections which can be done manually, mechanically, thermally or electrochemically.
What Methods of Manufacturing Can Cause a Burr
- Manual Deburring
- Punch Deburring
- Electrochemical Deburring
- Thermal Deburring
- Grinding and Rolling
- Hole Deburring
- Brush Deburring
Manual deburring is done by using hand tools which are typically made up of a curved or hook-shaped edge that scrapes the burrs off the workpiece, a deburring cutter which sits on a hexagonal shank where the tip helps to find the centre of the machined hole to assist the cutting edge with removing any burrs, leaving the edge of the machined hole with a chamfer (the small angled edge which is left once deburred), or with grinders, brushes, files and sanders
Punch deburring uses a punch machine to get rid of the burr, which can be used for deburring holes and the corner of parts. This method requires rough blanking dies, fine blanking dies and sizing dies to achieve the punch mould work and is more efficient and productive than manual deburring although it is not a viable option for complex shapes.
Electrochemical deburring uses an anodic metal dissolution with a negatively charged electrode (cathode) being the deburring tool, to dissolve the burrs away in a “reverse plating” action. The cathode acts under DC current in an electrolyte solution of salts dissolved in water (sodium chloride or sodium nitrate) which creates the anodic reaction; this removes the workpiece surface material precisely and with no risk of the parts becoming heat affected in non-targeted areas.
It is typically used on smaller parts for areas which can not be reached with traditional deburring tools and methods and can easily deburr holes and intersections of internal passages, or other hard to reach areas of the workpiece.
Metal tumbling is a mass finishing option, meaning it can be used to debur (among other finishes) large quantities of parts at the same time.
It works by placing the parts in a barrel along with water, a compounding agent and abrasive media before rotating the barrel which causes the parts and media to tumble around inside the barrel creating friction, abrading the parts. This results in a fast, efficient deburring of all the parts at the same time which is why tumbling is one of the more efficient and cost-effective methods of deburring.
Thermal deburring (or Thermal Energy Method, TEM) is achieved with an accurately defined mixture of gases being fed into a deburring chamber before being ignited by a spark. It is a process which can accurately remove all burrs in a very short period, but emphasis must be placed on determining the correct proportion of oxygen and fuel which will be used to generate the heat to not damage the workpiece. It does however require further processing such as pickling as ferric oxide deposits on the workpiece surface following the TEM process.
Grinding and Rolling
Mechanical deburring by grinding or rolling is similar to manual deburring but is a lot faster and subsequently more cost effective as many parts can be done at once, on a continuous run. Mechanical grinding uses a rotating wheel which is made up of abrasive materials, with the wheel revolving around a central axis that removes burrs once contact is made with the surface of the workpiece.
Hole deburring is the method of deburring inner parts of a hole using a cutting tool which is spring loaded and mounted on a spindle, positioned in a way that it can go through the hole smoothly to remove the burrs. This method requires special tooling and is only used on holes but provides a good finish and is a cost-effective method of deburring.
Brush deburring uses brush tools which may contain abrasive substances that rub (or brush) the workpiece to remove burrs. This method is fast and cost effective, can be done manually similarly to using sandpaper although most brushes are designed to be used in machine tools providing the ability to automate this process and is one method which can be used on complex designs.
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