When it comes to size reduction of principal feed ingredients, recent practice has been to use hammermills as the main grinding machines.
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This wasnt always the case. In the early days of compound feed milling, when raw materials were homegrown and power sources were either wind or water, the effort needed to grind or flake cereals into a form where animal uptake was optimized dictated that roller milling was a more economic and popular means of size reduction.
Thus, roller milling was the traditional method of preparing cereals and fodder for on-farm consumption by livestock. Today, millers have the option of using either method, or both, and there are many factors that impact their choice.
First, and in some peoples eyes the most important consideration, is power consumption per tonne of grinded product. In this case, I am referring to general processing of cereals and proteins, and my comments do not relate to the specialist grinding of micro-ingredients and high-fat raw materials, which both need careful and specific attention when being ground.
In recent years, hammermill diameters have gradually increased, which obviously allows for greater peripheral beater tip speed at lower revolutions. This has meant the impact effect on cereals at the outer extremities of the grinding chamber is increasingly severe. Consequently, power consumption levels in such hammermills have been reduced to a minimum. This is partly due to a combination of increased screen-hole diameter that complements the increased peripheral beater tip speed by accelerating the impact of individual particles between beater and screen.
This increased diameter of hammermill grinding chambers has led to the adoption of machines with greater throughput capacity, and there has been a progressive shift toward the adoption of post-grinding techniques in most mills built today. In this case, post grinding refers to the positioning of the hammermill after the blending stage as opposed to pre-grinding positioning, when hammermills are placed at the early stage in the mill flow, before ingredients are combined together.
ADVANTAGES AND DISADVANTAGES
There are distinct advantages to adopting the post-grind position for hammermills. Building layout is simplified, overall bulk ingredient storage capacity is reduced, and capital costs are thus minimized in new installations. There are some disadvantages, however, as millers are aware, particularly those who have been called out in the early hours of the morning when a hammermill has broken down and there has been no reserve of ground product for manufacture through the pellet mills while the hammermill is being repaired.
Essentially, hammermills rely on the impact of screens and beaters on the product being ground to reduce it to the desired granularity for incorporation into a balanced ration. Roller mills simply roll or crush product between two revolving cylinders. This latter process has the distinct advantage of requiring considerably less power, although it is not possible to achieve the fineness of final grind through a roller mill that can be achieved through a hammermill. In a hammermill, the screen-hole diameter controls the maximum finished particle size of any ground product. When using roller mills, there is no screen being used, and unless the product is sifted and the coarse fraction reprocessed, the resultant particle size is purely reliant on the millers skill in setting the roller mill effectively.
Roller mills, particularly single pass installations, require more care and attention than hammermills in order to achieve a consistent and accurate grind. Ensuring the feed is spread thinly across the face of the roller mills can present some problems as mechanical feed gates can easily become obstructed, impairing the smooth and regular flow of product into the nip of the roller mill. Variability of raw material also needs regular adjustments as opposed to the all-encompassing grinding nature of the hammermill.
The available capacity is also a major consideration when using roller mills as there is a need for machines of considerable size or number to achieve the similar capacity as that of hammermills in the same circumstances. There are other general considerations that may affect capacity such as the cleanliness of the grain and the presence of foreign objects that may restrict flow through the roller mill feed mechanisms.
However, there are some circumstances when roller mills have the edge and it is not completely desirable to reduce ingredients down to a very fine particle size. Ruminant animals prefer to consume flaked cereals, as do horses and outdoor pigs. In such instances, the roller mill comes very much to the fore, particularly where coarse or open rations are being produced and fed. In the case of beef lots, where the finished feed is not required to be pelleted for purposes of cost-effective transportation, the roller mill can be used quite effectively and can be a key part of reducing power consumption at the mill.
One advantage of using flaked cereals is that the ability to incorporate liquid ingredients into a ration is enhanced. The greater surface area presented by a flake allows for greater absorption of liquids. At the very least, it allows for coating of a greater surface area if absorption is not fully achievable with such ingredients as molasses and some fats and oils. In the brewing industry, a standard grist is required that has been proven to allow optimum application and absorption of enzymes into the mash stage of the process. This can only be achieved by the use of roller mills, often triple roller mills where product is ground twice to achieve the desired grist spectrum.
It should be stressed that the roller mill, when equipped with fluted or corrugated roll chills, can achieve a relatively fine grind, particularly when moisture content of cereals is optimized. The use of differential roll drive arrangements, which create a sheer effect between the chills, not only allows for a finer particle size output, but the sheering effect the roller mill has upon starch granules in cereals is advantageous to the nutritionist when compiling rations. This is especially true for young stock, such as baby piglets and veal calves, where the digestive tract is undeveloped and its sensitivity needs to be respected and treated kindly in early stage diets. The use of HTD belt drives to achieve differential roll speeds of up to 2.5:1 is now well proven, and as a result of such engineering technology there is little need for lubrication of the modern roller mill.
One of the biggest disadvantages of using roller mills is that when the roll chills become worn, replacing them with new chills and subsequently recorrugating the old chills is a major endeavor in terms of time and expense. The good news, however, is there are no screens that can burst or become damaged.
Another positive aspect of using roller mills is that they require little or no air flow to operate effectively due to the fact that, with the rollers being mounted horizontally, product passes through by gravity. Hammermills require a steady and balanced airflow in order to operate efficiently and to keep screens clear and unimpeded. The cost of moving that air, the capital cost of filters and fans, and the space requirement must all be borne in mind when drawing comparisons between grinding techniques.
COMBINING TECHNOLOGIES
Recent hammermill designs have been quite innovative, and we have seen the combination of roller mill and hammermill technologies begin to emerge. By using a roller mill, or adopting roller grinding principles as part of the feed mechanism on entry to the hammermill, the raw material is partially ground at that point, which then allows the hammers and screens in the grinding chamber of the hammermill to be fully effective, with often excellent grinding efficiency results.
Not only is a finer grind achievable with far less power consumption, but the control the miller has on the resultant particle size of the grinded ingredient is enhanced tremendously. By partial preparation of the product between the rollers in transit to the hammermill grinding chamber in such an arrangement, the best of both worlds is achieved.
As power consumption becomes increasingly important, you will likely see greater use of roller milling technology as part of overall grinding techniques. Rolls of up to eight inches in diameter are being adopted as feed mechanisms with differential drives and variable gap settings. Compared to conventional, straight forward hammermilling, these new hybrid arrangements can reduce power consumption by around 15%, which cannot be ignored in these stringent times.
The key to successful size reduction, however, is diligence and, as with all aspects of mill management, attention to detail is paramount. The daily walk around the mill, keenly observing minor daily changes in operations, will always prove to be the best defense against rising costs.
Jonathan Bradshaw is a consultant to the agribusiness and food processing industries, specializing in project management and bespoke training programs through his company, J.B. Bradshaw Ltd. He has extensive experience in flour and feed milling in Africa, the Americas, Europe and the Caribbean. He may be contacted at: [ protected]?.
Milling is a generic description for machining processes used in manufacturing to remove material from a workpiece, using rotary cutters. It's a versatile technique that can produce a wide range of shapes, features, and surface finishes in a broad spectrum of materials and via various classes of equipment that loosely fit within the description.
Milling serves in mass production, small runs/one-offs, and repair. Milling is a fundamental machining process employed in various industries, including: manufacturing, aerospace, automotive, and mold-making. It offers versatility, precision, and the ability to produce parts with complex and highly repeatable shapes and dimensions.
This article will discuss what milling is, how it works, its purpose, and the process.
Milling in machining is a much older term than the current usage in metalworking. It originates in the processing of grain to flour using quern stones. A set of quern stones is made up of one static flat stone and one rotating stone with a hole at its center, grain being fed into the hole and flour being expelled at the circumference as the upper quern is rotated. This extremely primitive machine was part of the origin of all rotating machinery, and the term mill broadened in usage to describe all factories early in the Industrial Revolution, later narrowing to primarily describe rotating-tool cutting machines for wood, metal, plastics, and stone.
The origin of metal milling can be traced back to the development of metalworking processes in ancient civilizations. Early metalworkers used hand tools such as chisels and files to shape and manipulate metals. The concept of rotary cutting was crystalized during the Industrial Revolution in the 18th and 19th centuries when mechanical milling machines were invented. These early machines used rotating cutters to remove metal from workpieces, marking the advent of metal milling as a distinct machining process.
Over time, milling technology advanced, leading to the development of highly precise and automated milling machines, including computer numerical control (CNC) machines, which play a vital role in modern metalworking and manufacturing industries.
Another term often used interchangeably with milling in machining is machining. Machining is a broader term that encompasses various processes for shaping and removing material from a workpiece, and milling is one of those specific processes. Other machining processes include: turning, drilling, grinding, and electrical discharge machining (EDM).
As the family of metal processing technologies has broadened, milling has become one of many processes encompassed by machining and more recently CNC machining.
A hardened-steel or exotic-material cutter is rotated on a fixed X-Y position and its position is able to be adjusted in the Z direction. A workpiece is clamped to a table by various means, to be presented to the cutter. The table is equipped to traverse in the X and Y directions, by winding position control screws. The workpiece and the spindle axis (the quill) are adjusted to pass the workpiece across the face of the cutter for material removal. This involves the material being removed by side cutting, as the circumferential faces of the cutter bite into the workpiece and are traversed in X and Y planes. Either the quill or the table Z axis is adjusted for plunge cutting, in which the cutter acts as a drill and the face cuts a cylindrical hole. This can be the preliminary stage in presenting the cutter at the necessary depth to then traverse in X and Y to cut slots and pockets.
More advanced applications of milling can combine the motion of the 3 axes to cut compound and simple curvature, generally under CNC path and speed control. This usually involves automated cutter exchange and some degree of self-calibration to maintain accuracy.
Additional advancements include the ability to tilt the quill to non-vertical positions, to rotate the workpiece on the table, and increasingly to manipulate cutter and workpiece positions in complex and esoteric ways in 6, 9, 12, and more axes.
Milling is primarily employed in the manufacturing industry to remove material from a workpiece. It allows manufacturers to shape raw materials into specific forms, creating components with desired dimensions and geometries. Milling machines can also achieve extremely tight tolerances and high precision. This is vital in industries like aerospace and medical device manufacturing, in which accuracy is paramount.
Furthermore, milling can provide a range of surface finishes, from rough to highly polished, depending on the cutter and machining parameters. It enables the creation of complex 3D shapes, contours, and features that may be difficult or impossible to achieve with other machining methods. CNC milling machines, in particular, can produce large quantities of identical parts with high repeatability. Milling can be applied to a wide range of materials, including: metals, plastics, and composites.
Milling is used in various industries and plays a central and pivotal role in the manufacturing of virtually all product classes. For example, in the aerospace industry, the milling of high-value, lightweight, and complex parts for any aspect of flight-engineered products is undertaken widely. From airframe components to controls, from flight-surface actuator parts to jet and rocket-engine components, multi-axis CNC milling is the normal production method for most parts.
Milling is also extensively used in the manufacture of surgical tools and medical implants. The ability to process a wide range of biocompatible and easily sterilized materials with high precision and repeatability is greatly valued in these high-value applications. Other industries that use milling are: manufacturing, oil & gas, and infrastructure.
Milling machines come in various types, each designed for specific machining tasks and workpiece sizes. The main types of milling machines include:
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Milling setup and operation is a sequence of skilled stages that must all be performed well, for the result to be useful. Listed and discussed below is the step-by-step process of milling in machining:
Depending on the nature of the parts and the volume, the loading of workpieces to be machined can range from being quite low-skilled to highly skilled. Where parts are clamped in a generic restraint such as a vise, or bridge clamped into place using T bolts, the task requires extreme care and precision. Misplacement or poor clamping will result in offset cuts, parts drifting during processing, or parts breaking out during machine operations.
Whenever higher volume dictates the need for specialist nests, these can make setup easy and repeatable, although great skill in such custom nest design and part clamping is required. In extreme cases, for which the volume justifies the investment, loading can be performed by a general-purpose robotic setup or a dedicated loader built into the machine.
The selection of milling tools can be considered a skilled operation. In a CNC machine with a tool changer, the cutters must be depth referenced and have the correct offsets applied, so the machine has correct awareness of tip and cylindrical positions. In CNC operations, the selection of tools will either occur as part of the automated toolpath generation or as a manual selection by the programmer. Errors in tool selection can have catastrophic results.
The setup of the machine involves clamping and tool selection as the first stages. After this, a range of critical operations must be performed. For a one-off part, the machine must be taught the workpiece position. A range of spatial calibration input methods and devices are used, but the central need is to reference the material on three sides (for X-Y-Z positioning) and if necessary clock the part as flat onto the table.
With material loaded and calibrated to the spatial reference of the machine, cutters set up for length and diametral offset, coolant supply assured and the guards closed, cutting can commence.
In general, the great majority of milling is performed on CNC machines of varied capability: from 3 to 6+ axes (degrees of freedom). Whether the process is manual or programmed, a similar level of care and attention is required in the motion of the cutter.
The operator will either a) monitor the CNC operation as the machine executes the cutting process or b) operate the machine axes manually to direct the cutter according to the plan. A manual machine requires full operator engagement with all parts at every stage. This includes the constant need to stop operations and check dimensions to affirm correct operation and plan the next cut.
Roughing involves deep and fast traverse cuts that take off extensive material without much regard to precisionas long as the cut is completed well before reaching the finished surfaces to be extracted from the workpiece.
Semi-finishing requires shallower cuts and lower feed speeds, to start to impose higher accuracy.
Finishing cuts are generally fine and more slowly fed, to deliver the required precision. Some areas will only require roughing, whereas others will require best-possible tolerances, achieved by slow-moving and fine cuts. Finishing is slower and can often consume the majority of the machine's time per part.
Unloading simply requires the release of the clamping that engages the workpiece and its removal either manually or by a robot. Generally, machine cleaning is performed at this stage, to leave the machine prepared for its next use.
Various measurements and surface quality evaluations will be undertaken, either manually, or using advanced inspection equipment. On occasion, fully automated part checking can be used, for particularly high-volume parts.
Post-processing can involve simple deburring and packing for storage or later use. It can also involve various types of heat treatment, surface coatings, chemical surface treatments, or plating.
Milling is a versatile machining process that can be applied to a wide range of materials, including: metals, plastics, and composites. The suitability of a material for milling depends on factors such as its machinability, hardness, and the specific form of milling operation. Listed below are the materials suitable for milling:
The two main milling methods are listed below:
Climb milling (or down milling) rotates the tool against the direction of the workpiece's motion, so the tool moves in the same direction as the material feed. This positions the cutter to climb over the workpiece and it is best suited to high-quality parts and thin sections. This results in reduced tool wear, less heat, better chip management, improved surface finish, and lower workpiece deflection.
The choice to use climb milling depends on the workpiece material, machine capabilities, tooling type, and the required part quality.
Conventional milling (or up milling) is a milling process in which the cutting tool rotates in the same direction as the workpiece's motion, so the tool moves in the opposite direction to the material feed. It is more applicable to higher ductility materials and allows faster machining. This results in progressive cutter engagement, increased tool wear, rougher finish, greater work deflection, and reduced precision.
Conventional milling is a valuable technique in specific machining scenarios in which it provides better results or is more suitable for the given conditions.
It depends. There are too many varied applications of milling to make a general definition of timing possible. However, for the majority of one-off and low-volume components, few alternative processes can compete for speed of outcome, except by sacrificing either material qualities (3D printing for example) or precision (fabrication).
The accuracy of a milling machine can vary widely depending on the machine design, component (particularly slide) quality, and the operator.
For many standard milling machines, positional accuracy can range from a few thou (0.001 or 0. mm) to a few ten-thousandths of an inch (0. or 0. mm). Repeatability is the ability of the machine to return to a specific position consistently. Repeatability varies between machines but is generally considered to be 0.001 in well-maintained equipment. Tolerances within a few thousandths of an inch are generally expected, but better machines and user techniques can achieve higher tolerances. All screw feeds (manual or CNC) suffer from a degree of hysteresis (or slack motion), so the direction of motion must be considered, to take up any slack before cutting commences.
Yes. Milling machines are high-value assets that are generally manufactured to precise tolerances and with low hysteresis in their linear and rotary positioning systems. On that basis, the best milling machines are among the most precise instruments made. However, their operation and maintenance can have a marked effect both on their precision and repeatability in a given task and their ability to maintain these criteria long-term. A badly operated or badly maintained machine will deliver poor accuracy and repeatability in the parts it produces.
Milling is a near-universal machining process in manufacturing due to its advantages. Such as:
Although milling is a versatile and broad-spectrum machining-process family, it carries some disadvantages and limitations. These are:
It depends. The relative costs of milling and alternative manufacturing processes are highly varied and dependent on a range of factors. At one-off component manufacture, milling is generally the low-cost production method. For simple geometries, hand production by a skilled craftsman is feasible, but considerably slower and more costly. It can, for larger parts with low general tolerances, be cost-effective to sand cast a part and only use milling when higher precision is required as a post-casting treatment. At serial production levels, in which tens to a few hundred parts are required, milling can still be cost-effective when compared with molded plastics or die-cast metal parts. At high-volume production levels (thousands of parts), other methods become highly competitive, against full milling of parts. This can include lower-precision first-step part production such as sand or investment casting, which must often be followed by milling for precision finishing of areas of higher tolerance. This is the norm in large steel and aluminum parts in the automotive sector, for example. For the highest-volume parts, die casting and plastic molding are primary methods for cost reduction, and milling is generally uncompetitive.
Yes. Milling is among the most centrally important methods used across all manufacturing sectors. It finds application at every level in every sector, from medical to consumer goods, from one-off to mass production, and in the manufacture of alternative tooling that handles the volumes above which milling loses cost-effectiveness. For volume, precision, repeatability, prototyping, toolmaking, and customization/repairs, milling is a go-to process in all production environments.
The advised precautions for using a milling machine are listed below:
All milling, in a general sense, involves cutting as rotating tools are passed over a part. It precisely removes chips of material as the teeth cut through the billet to extract the required part from a blank. Cutting as a standalone process usually involves a broaching type action, during which teeth pass over the surface of a part in a linear motion to remove an elongated kerf the entire length of the part, resulting in a slot being cut that separates the material into two parts. This motion can be reciprocating (i.e., a saw blade that works in a cut and return sequence) or continuous. The continuous cut can be a bandsaw blade that passes the material in one direction, or it can be a slitting wheel/saw that is circular but traverses the part in a linear motion as it rotates.
The line between milling and cutting is quite blurred in many cases, as milling cutters can often be used for simple cutting tasks. However, linear cutters are not generally used in milling operations.
Milling has developed into a multi-axis methodology that blurs the differentials with other methods/equipment. A 5+ axis CNC mill can perform turning operations as part of its normal programming, with workpieces held in a rotating chuck and presented to rotating or stationary tools to complete full circumferential cuts. Equally, a 3.5-axis CNC machine can perform a tapered (assumed cylindrical) cut-by-side cutting of a circular profile while adjusting the tilt of the arbor/quill to maintain the cylindrical form of the cut.
Taper turning is generally performed on a lathe, with the part spun in a chuck and a non-rotating tool traversed at an angle to the primary axis of the machine, to cut a tapered cylindrical surface. This generally involves setting the compound slide of the lathe to traverse the tool at the required angle, but it can equally be achieved on a CNC lathe by cooperative motion of the longitudinal and transverse axes of the tool motion.
Swiss machines are, in reality, early versions of multi-axis machining centers, developed for clock/watch mass production in the 19th Century. The addition of cutter rotation that can operate alone or in combination with the part rotation in the chuck allowed the production of flat and square features on otherwise cylindrical workpieces, held stationary in the chuck and cut by traverses of rotating cutters.
It was not until the s that lathes began to be able to encompass these types of combined features, and it has taken the development of 6-axis CNC machines to truly compete with the functionality of Swiss machines.
To learn more, see our article on Swiss Machining.
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