Finishing and Polishing Machines
Finishing machines are machines used for metal finishing, which is the last stage of the metal fabrication process; the stage encompasses the use of machines in finishing the metal surface...
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This article gives you a complete guide to the deburring process and deburring machines. You will learn about:
Deburring is a secondary operation of a machining process that enhances the final quality of the product by removing raised edges and unwanted pieces of material, known as burrs, left by the initial machining processes. Burrs are created from shearing, bending, cutting, piercing, and compressing materials. These are mostly seen on soft and ductile materials. Deburring machines are used to remove burrs by mechanical, electrochemical, and thermal methods.
When applying shearing or bending forces up to failure, the material is subjected to plastic deformation. Regions along the edges become bent, elongated, and extruded. These elongations and extrusions appear on the cutting edges particularly on the entrance, sides, and exit of the tool. The formation and size of burrs are difficult to predict and quantify. Thus, deburring efficiency is highly empirical and is based on several production trials before being perfected by manufacturers.
As mentioned earlier, deburring can be done through various methods. Most of these processes are "global", which affects the entire product. The point of application for global or general deburring cannot be controlled. Deburring processes can affect the final quality of the product by altering the final dimensions and contaminating the surface from chemical and abrasive residues. Thus, deburring processes must be carefully selected and controlled to prevent any alteration to the desired characteristics of the product.
The cost associated with deburring processes is about 30% of the manufacturing cost for high precision parts used in aerospace applications. In automotive applications, the deburring costs about 15 to 20% of the manufacturing expense. This increment in manufacturing cost is significant considering that deburring does not add intrinsic value to the product.
Knowing the difficulties and the associated costs, deburring is still an integral part of the production process because of the following reasons:
Edge quality has a significant impact on the form, performance, and life of a product. Burrs and raised edges take a toll on the correct fit and assembly of machine parts. These are critical for precision components used in the aerospace, automotive, and electronics industry. Aside from the correct fit, mating parts such as gears, rollers, and other sliding surfaces, burrs can impinge and damage the parts.
Products with burrs have significantly reduced fatigue life compared to parts with no burrs. Machining processes create residual stresses due to work hardening along the sheared and bent edges. These cause changes in the mechanical properties in these regions. Holes, slots, and notches are features where stress becomes concentrated. Since burrs have a more irregular profile and are usually located at the outermost edges of these features, burrs can act as crack initiation sites.
Thermal and roll-over burrs create crevices that promote localized corrosion. Water, electrolytes, and other corrosive material can accumulate and stagnate in crevices. Also, corrosion can occur when the coating is not applied properly. Raised areas on the surface of the part can have thinner coatings compared to flushed surfaces.
Burrs are also safety hazards since sharp edges can pierce through pressurized lines and electrical cables. Personnel handling burred edges can be harmed as well. Sharp edges are usually chamfered or radiused to prevent unwanted cuts on personnel and equipment.
The best way to prevent burr formation is to change the workpiece with less ductile material. Using less ductile material causes unwanted parts to chip or separate from the main part. However, most of the time, this is not an option especially for applications with strict material requirements. In machining ductile materials, burr formation is almost inevitable. Because of this, non-conventional methods such as chemical etching and laser machining become desirable options. However, these processes have their constraints on limited workpiece thickness and high investment costs.
Finishing, deburring, and tumbling are terms used in secondary machining operations. Finishing is a broad term that consists of operations such as deburring, blasting, polishing, grinding, coating, plating, and so on. Deburring is a type of finishing for removing burrs, irregular edges, and flashes. One method of deburring is through mechanical equipment. Mechanical deburring machines create abrasion imparted by an abrasive media placed together with the part inside a chamber. The chamber is agitated by vibration or tumbling. Thus, tumbling is a deburring process that uses abrasive media and the rotation of the chamber, known as a barrel, to scrape part burrs.
Aside from the classification of burrs according to the cutting direction, burrs can also be classified according to their mechanism of formation. There are four types, namely: Poisson, roll-over, tear, and cut-off burrs.
"Poisson" came from the term Poisson effect which means expansion of directions perpendicular to the application of stress. Exerting compressive forces onto the material causes the edges of the area in contact to plastically deform and elongate creating burrs. During cutting, as the tip of the cutting tool strikes the workpiece, the edges of the cut become deformed due to the compressive and shearing forces. These deformations are seen as entrance burrs which are formed at the entry point of the cutting tool.
These are chips that are bent rather than sheared from the cutter‘s path. As the cutting tool exits the cut, some material rolls and goes along with the tool. The material folds toward the feed and along the cut edge. If the material is ductile enough, the chip does not easily separate from the part. The depth of cut also contributes to the formation of roll-over burr since the chip or roll becomes thicker as the depth increases.
Tear burrs are side-burrs that occur when the cut part is plastically deformed rather than completely sheared. This is observed in punching processes where a sharp, jagged edge is left along the contour of the punched hole. This is material tearing loose from the workpiece.
Cut-off burr is a result of the leftover material as the cut part separates or falls off from the main part. This can be a positive or negative burr. Cut-off burrs are mostly observed on saw cuts and automatic screw machine parts. These types of burrs are prevented by supporting both sides properly until the cut is finished.
These types of burrs are usually referred to as slags, spatters, or dross. Slags are a result of hardened molten metal from welding, plasma, and laser cutting. Slags have different mechanical properties than the base metal due to the residual stresses brought about by heating and uncontrolled cooling. Slags can usually chip-off through manual power brushing, but in some cases, grinding is necessary.
Deburring is done in a variety of methods that depend on the material, part geometry, size and location of burrs, product volume, and cost. Manual and mechanical deburring operations are conventional methods for deburring. Electrochemical, thermal energy, and cryogenic are non-conventional methods for specific deburring applications. Below are common deburring processes and the advantages and applications of each.
This method is utilized when there is a large quantity of parts/components that need to be finished, which can be run as batch systems or as continuous systems and can be performed as dry or wet processes (depending on the material of the products being finished). Mass finishing requires an initial investment of time and materials to determine the exact mass finishing recipe required for your needs, but will save time, money, and manual labor in the long run. Some examples of mass finishing equipment include rotary vibrators, continuous flow installations, drag finishing machines, high-energy disk systems, and tub vibrators.
This method refers to deburring operations that use hand-held or mechanized tools which use tools such as deburrers, grinders, brushes, files, sanders, and so forth. This process is "localized"--meaning it does not affect the entire part. This is employed in locations where there is a high tolerance for dimensional variations since the parameters for manual deburring cannot be defined perfectly. In-house time standards have been developed by most fabricators and manufacturers. However, these do not solve the problem regarding the consistency of the process. This process is slow and is generally done at the end of the production line making any mistake costly for the manufacturer. Below are some of the types of manual deburring methods.
Brushes made of metal filaments or thin wires attached to a rotating disc are used to scrape off burrs along the edges of the cut. This is a fast and relatively low-cost method but is limited by the consistency of the deburring action. Intensity depends on filament diameter, free length configuration, texture, density, material type, disc width, angular velocity, and contact.
This method uses abrasives such as aluminum oxide, silicon carbide, and zirconia compounds bonded into sheets, belts, pads, wheels, and discs. Mechanized reciprocating or rotating action of the abrasives removes materials that are raised from the surface of the workpiece. Abrasives can vary grades from coarse to exceptionally fine depending on the dimensions of the material to be removed, desired surface finish, and application.
Sheet metal edging machines have small grinding wheels or pinch rolls that smoothen edges of sheet metal with various thicknesses ranging from 0.025 to 0.25 inches. Sheet metal edging machines are stationary where sheet metal is fed manually or automatically. Some machines can deburr the top and bottom surfaces and can also create chamfers or fillets. For a set of multiple rollers, the pressure exerted at each roller pair progressively forces the burrs and raised edges over, under, or into the sheet metal. However, compressive forces must be carefully controlled especially on soft and malleable materials since these can warp or buckle under roller pressure.
This involves a chamfering, grinding, or deburring tool mounted on a robotic arm. Since the main disadvantage of manual deburring is inconsistency, slow turnover, and labor-intensiveness, robotic deburring solves these problems by eliminating the human factor. Robots can perform repeatable movements consistently and rapidly. CNC programming allows operators to input predefined movements and other parameters such as force and tool speed. Despite the bigger initial cost, robotic deburring is beneficial in the long term because of the reduced operating costs. Moreover, robotic systems are a much safer process than manual.
Mechanical deburring employs machines to perform a general deburring on the workpiece. The operator has less control over the aggressiveness and localization of the deburring action, as compared to manual deburring. Manual, robotic, and waterjet deburring are also considered as mechanical due to the nature of abrasion application. Examples of mechanical deburring machines are as follows.
Barrel tumblers are one of the most economical deburring machines in regards to operating costs. This equipment not only removes burrs, but it also polishes the surfaces of the part. The machine works by loading a part or several parts into a chamber or "barrel" along with the abrasive media. Special compounds are also added depending on the material and surface finish. This is usually a batch operation, but in-line batch processing and single-pass processing are also available. Barrel tumbling can be divided into wet and dry tumbling.
In wet tumbling, water is loaded which acts as a lubricant and helps wash out residues to improve deburring efficiency. The level of water affects the speed of processing and fineness of the surface finish. Compounds designed to work with ceramic or plastic media are also added to the water. These compounds impart corrosion resistance, cleanliness, cosmetic finish, shine, and other additional surface qualities. Compounds also improve the deburring operation by removing oils, lengthening the life of the abrasive media, and eliminating foaming from plastic and synthetic media.
The use of water and liquid compounds proves to have a lot of benefits. However, there are also downsides such as wastewater contamination and uncontrolled reactions with fluids used in upstream operations.
As the name suggests, this process only uses dry media such as sand and dried organic materials. Using sand does not only improve polishing but also acts the same way like water in wet tumbling. Sand carries the residues and prevents them from embedding onto the surface of the part. Organic material, on the other hand, is more absorbent than sand which can remove dirt and oils. Organic materials used are corn cob grits, walnut shell grits, and wood pegs.
Using dry media does not have the disadvantages of wet tumbling compounds; however, the process is relatively slow. Heavier abrasive composition tends to have a shorter processing time. Dry abrasives are lighter than their liquid counterparts and generally take about two times longer to produce the same deburring effect. Because of this, dry tumbling is not usually applicable to large-volume production.
A special configuration of a barrel tumbler is a centrifugal tumbler. This consists of an array of two or four tumblers mounted on a turret. The turret rotates in one direction which causes the barrels to rotate in the opposite direction. Each complete rotation of the turret represents one rotation of the barrels. Turning the turret fast enough causes centrifugal forces greater than gravity to be applied on the barrels. This creates greater abrasive forces resulting in faster deburring times.
Vibratory deburring machines are similar to deburring barrel tumblers where the parts are loaded into a chamber along with the abrasive media and other additional compounds. Their main difference is the movement of the chamber. While tumblers rotate to generate agitation inside the chamber, this type of machine vibrates to generate motion. The chamber is mounted on springs or dampers which isolates its movement from the foundation. An off-center revolving weight is attached which shakes the contents of the chamber. Various configurations are available such as tubs, circular bowls, or trough machines. Choosing the configuration depends on the geometry of the part and its application.
This process utilizes the impact force of high-velocity water jets to erode burrs and debris from the workpiece. Water jets are CNC controlled, similar to tooled robotic systems. Waterjet deburring uses lower pressures compared to waterjet cutting to prevent damaging the part. Thus, it only removes thin and loosely attached burrs. Larger burrs cannot be easily removed without damaging the edges. The main advantage of using water jets is that it can reach features that are inaccessible to ordinary deburring systems. Also, the resulting product is free of oils and debris.
This is a deburring process that utilizes the principles of electrolysis. Electrolysis is accelerated in areas with small interelectrode gaps. Meanwhile, it is prevented in areas with insulation between electrodes. The cathodic tool is shaped as a negative of the workpiece. This is used to focus electrolysis on regions where burrs are located. The workpiece is attached to the circuit and acts as an anode. To complete the circuit, an electrolyte is added which transfers charge between the tool and workpiece. Portions of the tools are insulated to prevent dissolving other surfaces. This method is suited for deburring difficult to machine geometries and poorly machinable but conductive materials. Also, there is no tool wear. A disadvantage for this process, however, is the difficult wastewater treatment since it uses environmentally harmful chemical compounds.
This process is also known as thermal energy method. In this process, the workpiece is exposed to hot corrosive gases for a very short period. A thermal shockwave is generated which quickly vaporizes the burr. The rest of the workpiece is unaffected because of its low surface-to-mass ratio and short exposure time. Small amounts of metals such as burrs and raised edges sublimate since they are unable to dissipate the intense heat to the surrounding parts. Thermal deburring is effective on materials with low thermal conductivity that can easily oxidize.
Cryogenic deburring is mostly done on precision plastic parts which possess inherent impact toughness. In this process, liquid nitrogen is flashed into a chamber containing the parts to be deburred. The flashing process cools the chamber near the glass transition temperature of the part material. This embrittles the burrs and flashes but is not enough to change the properties of the rest of the parts. The part, along with the abrasive media, are tumbled in the chamber.
This process involves impacting the surface with abrasive media to remove any surface irregularities. Abrasive blasting is used to remove larger burrs and to create a required texture and surface roughness usually for equipment used in fluid shearing applications. Micro-abrasive blasting is more precise in creating smoother surfaces without damaging or changing the dimensional accuracy of the part. This process uses very fine abrasive media such as aluminum oxide, glass beads, and plastic media and a miniature nozzle to produce a controllable abrasive jet that can target and remove microns of material. Micro-abrasive blasting is used for high-value precision parts.
There are different types of abrasive media available in the market. Common abrasive media are ceramic, steel, plastics, and organic compounds. These materials are available in a variety of shapes and sizes depending on the geometry of the part. The deburring media not only scratches and cuts, but it also acts as a cushion preventing different parts from impinging one another.
Ceramics can deburr different types of metals and plastics. Depending on the composition, density, and geometry, it can provide different surface finishes at varying cutting speeds. Also, since most ceramics have an inherent hardness, they are extremely durable when deburring hard metals.
Steel media are used for light deburring and burnishing. They have a high initial cost but are widely used because of their minimal attrition rate and extreme cleanliness.
Synthetics are composed of 50 to 70% abrasives by weight. Abrasives can be alumina, emery, and silicon carbide. The abrasive is embedded within a softer material. As the softer material erodes, the abrasive is exposed which then deburrs the part.
Plastics can be formulated to serve specific applications. They are available as low density that is used for general-purpose deburring, or high density which is for both ferrous and non-ferrous metals.
Examples of these are walnut and corn cob. Organic media are used for drying purposes since they can easily absorb water and oil.
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