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using millis() for timing | multi-tasking the arduino - part 1 | adafruit learning system

using millis() for timing | multi-tasking the arduino - part 1 | adafruit learning system

Onesimple technique for implementing timing is to make a schedule and keep an eye on the clock. Instead of a world-stopping delay, you just check the clock regularly so you know when it is timeto act. Meanwhile the processor is still free for other tasks to do their thing. A very simple example of this is the BlinkWithoutDelay example sketch that comes with the IDE.

At first glance, BlinkWithoutDelay does not seem to be a very interesting sketch. It looks like just a more complicated way to blink a LED. However, BinkWithoutDelay illustrates a very important concept known as a State Machine.

Instead of relying on delay() to time the blinking. BlinkWithoutDelay remembers the current state of the LED and the last time it changed. On each pass through the loop, it looks at the millis() clock to see if it is time to change the state of the LED again.

We also have code that looks at the state and decides when and how it needs to change. That is the Machine part. Every time through the loop we run the machine and the machine takes care of updating the state.

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thread milling, simplified | cutting tool engineering

thread milling, simplified | cutting tool engineering

While thread milling is a well-established process, some machine shops are still afraid to try it, fearing it is too complicated. Thread milling is not a big mystery, but some shops are hesitant to do it, said Joe Mazzenga, sales manager for J.M. Sales - USA, Troy, Mich., which offers solid-carbide and indexable-insert thread mills. In reality, we can set up a customer to successfully thread mill fairly easilyas long as they have the right tooling.

Thread milling requires a machining center capable of helical interpolation, which requires three axes of simultaneous movement. Two axes perform circular interpolation while the third moves perpendicular to the circular plane. Most CNC machines built in the last 10 to 15 years have this capability.

Rarely do I run across someone who cant do it, noted Jim Hartford, vice president of Advent Tool & Manufacturing Inc., Antioch, Ill. Advent manufactures single- and multiple-flute inserted thread mills and solid-carbide thread mills.

The inserted type with multiple flutes is typically for thread milling holes " in diameter or larger because there has to be enough room for the tool body and inserts. Smaller inserted thread mills are available but they usually have only one or two flutes.

In a lot of cases, when you get into holes " and smaller you can still get a 4- or 6-flute solid cutter in there to do the job, Hartford said. With the inserted cutter, you could only get possibly one insert in there so your cycle times would be slow. For example, in 316 stainless steel, a single-flute thread mill would take 40 to 45 seconds to thread a hole. With a 4-flute, solid-carbide thread mill, it would take 6 to 8 seconds.

While both solid-carbide and inserted thread mills are suited for larger diameter holes, it can be advantageous to use smaller solid-carbide tools as they can typically run at higher feed rates, said Wolfgang Ruff, vice president of engineering for KOMET of America Inc., Schaumburg, Ill. For instance, if you have an M801.5 thread, you can run a 50mm-dia. inserted thread mill, which would have five cutting edges. You can also choose a 20mm-dia. solid-carbide thread mill, which will also have five cutting edges, but will run at more than twice the feed rate, effectively cutting the production time for each hole by half. This makes a huge difference for large production runs.

Inserted thread mills can be good for smaller job shops with small production runs. One tool body can use multiple replaceable inserts with different thread forms, so it is more versatile and less expensive overall.

They operate the same, but there is the cost difference, Hartford said. The tool body would cost about $300 to $450, but the inserts are only about $30 when replacement is needed. With the solid-carbide thread mill, the entire tool needs to be replaced at a $200 to $300 price tag. Also, it is easier for an operator to change an insert than to change an entire tool and requalify it.

KOMETs Ruff pointed out that inserts must be precisely positioned in the pockets. If they are not, there is a mismatch from one insert to another and you cannot produce the proper thread. KOMET offers solid-carbide thread mills as standard tools and inserted ones as specials.

We use the helical almost exclusively, Mazzenga said. With a helical-flute cutter, you get a much smoother, quieter cut because the engagement of the teeth is spread over a greater range. On a straight-flute cutter, all the teeth on a given flute engage at the same time. This creates a greater amount of radial pressure, causing chatter and deflection.

Most thread mills, solid carbide or inserted, are multitooth tools. The teeth are arranged parallel, rather than helically like a tap. Multitooth thread mills cut the full depth of the thread in a single rotation around the hole.

J.M. Sales offers its Quattro indexable-insert thread mill (left), GFT thread mill with three rings of teeth for small diameters and 3 diameters deep (second from left), BGFS-W drill/thread mill for steels and titanium (second from right) and BGFS-H drill/thread mill for hardened steels (right).

One multitooth thread mill can cut threads of the same pitch in a range of diameters. This is because the diameter is determined by the CNC toolpath instead of the tool (as with a tap). With a fixed pitch, the multiform tool can cut any 20-pitch thread, whether it is -20, -20 or 2"-20, as long as it can fit in the hole, said Stephan Francescone, production manager for Harvey Tool Co. LLC, Rowley, Mass., which makes single- and multiple-form solid-carbide thread mills.

The multiform thread mill is fast because all those peaks and valleys are cutting at the same time to create the thread, said Jeff Davis, vice president of engineering for Harvey Tool. The downside is that youre locked into the pitch because of those peaks and valleys. Anytime you see more than one triangular thread form on a tool, you have a fixed-pitch situation. Each thread pitch requires a different tool.

In addition to being able to cut any diameter, the advantage of a single-form thread mill is it can cut any thread pitch or a range of thread pitches. However, a single-form thread mill can only cut one thread in a single pass and must move around the hole as many times as there are numbers of threads.

Even though they have to buy a thread mill for each pitch, larger production shops lean toward a multiform so they have a nice array of tools, Davis said. The single-form tools lend themselves more to smaller job shops that want a more flexible tool.

Harvey Tools AlTiN-coated thread mills are suited for threading difficult-to-cut materials, ferrous materials, steels and aerospace materials. They are also available with TiB2coating for aluminum workpieces.

J.M. Sales - USAs Mazzenga said his company doesnt subscribe to the belief that thread mills can be used for any diameter as long as the pitch is the same. It is somewhat correct, but we dont define it that loosely, he said. Our thread mills are designed to cut the thread on the minor diameter as well as the flank and the crest. Most of our thread mills are manufactured for a specific pitch and diameter.

We define that thread mill more completely than just 60 included threads (standard thread forms are 60 included), he continued. In reality, if you try to make your thread mill as large as possible for a specific thread and dont compensate that form in the tool itself, you will produce threads that border the thread tolerance boundaries. It is quite possible to make an adjustment on the machine for pitch diameter and then have an oversize minor diameter. We correct the thread form of the tool to allow for this.

KOMET offers both types of thread mills: those with the so-called corrected profile (M16 and under) and those without the corrected profile. When using the latter for larger diameter holes, it is crucial to follow the 23concept where the diameter of the thread mill has to be no larger than23of the hole to maintain the correct pitch for the threads.

You really dont want to take a thread mill any deeper than three times diameter, particularly with a course pitch, Mazzenga said. Anything deeper, and there is a tremendous amount of radial pressure being applied, which causes deflection. The chances of being successful in one shot are greatly reduced. If you have to go deeper, you should take a pass at the top of the thread, maybe go half way down and take a pass, then drop down deeper and take another pass. And, with CNC, your threads will link up.

However, some manufacturers question the need for deep threaded holes. When you get to 2.5 diameters deep, you have reached maximum strength for the thread. There is no benefit to designing a deeper thread, Ruff said. There is no need to overengineer the parts and make them more costly to produce.

Because the thread mill is circular interpolated, the actual feed rate at the cutting edge will be different from what is programmed at the center of the tool. A thread mill is always traveling in a circle, so the centerline of the tool is going to be traveling at a different speed than the OD of the tool, Harvey Tools Francescone said. So you need to compensate for that in your feed rate. In the case of an internal thread, you scale your linear feed rate down, and in the case of an external thread, scale it up.

Many thread milling programs start by going perpendicular into the hole wall, which causes vibration in the tool and tool marks on the thread, according to KOMETs Ruff. Once the tool starts vibrating, it will not stop for the duration of the operation. To eliminate these prolems, he recommends starting the process by gradually approaching the hole wall with a circular motion, then going to the full cutting depth. KOMET offers a program for this that can be downloaded from its Web site. Apple and Android apps are also available.

Advents Hartford gave this example. Say you need to cut a 6"-12 thread and then a 5"-10 thread. Using an inserted thread mill, all you need to do is change inserts from the 12 pitch to the 10 pitch at $30 per insert. A tap of this size would probably cost $2,500, and you would need to buy two.

If a thread mill breaks during operation, the pieces can be removed from the hole and a new tool makes the thread. Because a tap produces a great deal of contact along the cut, creating a lot of force, taps can break and become stuck in the hole, possibly causing scrap.

With only a change in programming, one thread mill can produce a left- or right-hand thread. A tap can only do what is ground into the tool, so the user needs one tool for left-hand threads and another for right-hand threads.

Thread mills can produce a full thread close to the bottom of a blind-hole. When the hole is blind, a tap can only reach so far because it has a tapered point. Your thread form will be within half a pitch from the bottom of the hole, Advents Hartford said. That is as close as you can get.

Thread mills produce a thread in one or more passes. With thread milling, you can probably do it in one pass if it is a softer material, like aluminum, Harvey Tools Davis said. But in a hard material, you could take more passes without breaking the tool. Tapping uses the whole thread form and it jumps to final size in one pass.

Thread mills can start at the top of the hole and go to the bottom or vice versa. Taps must start at the top and go to the bottom. Very often, people like starting at the bottom and working toward the top because, hopefully, gravity will pull the chips down and they wont get recut, Davis said.

And finally, thread mills, unlike taps, can combine various operations into a single tool. For instance, manufacturers offer tools that can drill a hole, chamfer it, and then mill the thread in one step. CTE

ball mill - an overview | sciencedirect topics

ball mill - an overview | sciencedirect topics

The ball mill accepts the SAG or AG mill product. Ball mills give a controlled final grind and produce flotation feed of a uniform size. Ball mills tumble iron or steel balls with the ore. The balls are initially 510 cm diameter but gradually wear away as grinding of the ore proceeds. The feed to ball mills (dry basis) is typically 75 vol.-% ore and 25% steel.

The ball mill is operated in closed circuit with a particle-size measurement device and size-control cyclones. The cyclones send correct-size material on to flotation and direct oversize material back to the ball mill for further grinding.

Grinding elements in ball mills travel at different velocities. Therefore, collision force, direction and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles, as well as collision energy. These forces are derived from the rotational motion of the balls and movement of particles within the mill and contact zones of colliding balls.

By rotation of the mill body, due to friction between mill wall and balls, the latter rise in the direction of rotation till a helix angle does not exceed the angle of repose, whereupon, the balls roll down. Increasing of rotation rate leads to growth of the centrifugal force and the helix angle increases, correspondingly, till the component of weight strength of balls become larger than the centrifugal force. From this moment the balls are beginning to fall down, describing during falling certain parabolic curves (Figure 2.7). With the further increase of rotation rate, the centrifugal force may become so large that balls will turn together with the mill body without falling down. The critical speed n (rpm) when the balls are attached to the wall due to centrifugation:

where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 6580% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.

The degree of filling the mill with balls also influences productivity of the mill and milling efficiency. With excessive filling, the rising balls collide with falling ones. Generally, filling the mill by balls must not exceed 3035% of its volume.

The mill productivity also depends on many other factors: physical-chemical properties of feed material, filling of the mill by balls and their sizes, armor surface shape, speed of rotation, milling fineness and timely moving off of ground product.

where b.ap is the apparent density of the balls; l is the degree of filling of the mill by balls; n is revolutions per minute; 1, and 2 are coefficients of efficiency of electric engine and drive, respectively.

A feature of ball mills is their high specific energy consumption; a mill filled with balls, working idle, consumes approximately as much energy as at full-scale capacity, i.e. during grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.

The ball mill is a tumbling mill that uses steel balls as the grinding media. The length of the cylindrical shell is usually 11.5 times the shell diameter (Figure 8.11). The feed can be dry, with less than 3% moisture to minimize ball coating, or slurry containing 2040% water by weight. Ball mills are employed in either primary or secondary grinding applications. In primary applications, they receive their feed from crushers, and in secondary applications, they receive their feed from rod mills, AG mills, or SAG mills.

Ball mills are filled up to 40% with steel balls (with 3080mm diameter), which effectively grind the ore. The material that is to be ground fills the voids between the balls. The tumbling balls capture the particles in ball/ball or ball/liner events and load them to the point of fracture.

When hard pebbles rather than steel balls are used for the grinding media, the mills are known as pebble mills. As mentioned earlier, pebble mills are widely used in the North American taconite iron ore operations. Since the weight of pebbles per unit volume is 3555% of that of steel balls, and as the power input is directly proportional to the volume weight of the grinding medium, the power input and capacity of pebble mills are correspondingly lower. Thus, in a given grinding circuit, for a certain feed rate, a pebble mill would be much larger than a ball mill, with correspondingly a higher capital cost. However, the increase in capital cost is justified economically by a reduction in operating cost attributed to the elimination of steel grinding media.

In general, ball mills can be operated either wet or dry and are capable of producing products in the order of 100m. This represents reduction ratios of as great as 100. Very large tonnages can be ground with these ball mills because they are very effective material handling devices. Ball mills are rated by power rather than capacity. Today, the largest ball mill in operation is 8.53m diameter and 13.41m long with a corresponding motor power of 22MW (Toromocho, private communications).

Planetary ball mills. A planetary ball mill consists of at least one grinding jar, which is arranged eccentrically on a so-called sun wheel. The direction of movement of the sun wheel is opposite to that of the grinding jars according to a fixed ratio. The grinding balls in the grinding jars are subjected to superimposed rotational movements. The jars are moved around their own axis and, in the opposite direction, around the axis of the sun wheel at uniform speed and uniform rotation ratios. The result is that the superimposition of the centrifugal forces changes constantly (Coriolis motion). The grinding balls describe a semicircular movement, separate from the inside wall, and collide with the opposite surface at high impact energy. The difference in speeds produces an interaction between frictional and impact forces, which releases high dynamic energies. The interplay between these forces produces the high and very effective degree of size reduction of the planetary ball mill. Planetary ball mills are smaller than common ball mills, and are mainly used in laboratories for grinding sample material down to very small sizes.

Vibration mill. Twin- and three-tube vibrating mills are driven by an unbalanced drive. The entire filling of the grinding cylinders, which comprises the grinding media and the feed material, constantly receives impulses from the circular vibrations in the body of the mill. The grinding action itself is produced by the rotation of the grinding media in the opposite direction to the driving rotation and by continuous head-on collisions of the grinding media. The residence time of the material contained in the grinding cylinders is determined by the quantity of the flowing material. The residence time can also be influenced by using damming devices. The sample passes through the grinding cylinders in a helical curve and slides down from the inflow to the outflow. The high degree of fineness achieved is the result of this long grinding procedure. Continuous feeding is carried out by vibrating feeders, rotary valves, or conveyor screws. The product is subsequently conveyed either pneumatically or mechanically. They are basically used to homogenize food and feed.

CryoGrinder. As small samples (100 mg or <20 ml) are difficult to recover from a standard mortar and pestle, the CryoGrinder serves as an alternative. The CryoGrinder is a miniature mortar shaped as a small well and a tightly fitting pestle. The CryoGrinder is prechilled, then samples are added to the well and ground by a handheld cordless screwdriver. The homogenization and collection of the sample is highly efficient. In environmental analysis, this system is used when very small samples are available, such as small organisms or organs (brains, hepatopancreas, etc.).

The vibratory ball mill is another kind of high-energy ball mill that is used mainly for preparing amorphous alloys. The vials capacities in the vibratory mills are smaller (about 10 ml in volume) compared to the previous types of mills. In this mill, the charge of the powder and milling tools are agitated in three perpendicular directions (Fig. 1.6) at very high speed, as high as 1200 rpm.

Another type of the vibratory ball mill, which is used at the van der Waals-Zeeman Laboratory, consists of a stainless steel vial with a hardened steel bottom, and a single hardened steel ball of 6 cm in diameter (Fig. 1.7).

The mill is evacuated during milling to a pressure of 106 Torr, in order to avoid reactions with a gas atmosphere.[44] Subsequently, this mill is suitable for mechanical alloying of some special systems that are highly reactive with the surrounding atmosphere, such as rare earth elements.

A ball mill is a relatively simple apparatus in which the motion of the reactor, or of a part of it, induces a series of collisions of balls with each other and with the reactor walls (Suryanarayana, 2001). At each collision, a fraction of the powder inside the reactor is trapped between the colliding surfaces of the milling tools and submitted to a mechanical load at relatively high strain rates (Suryanarayana, 2001). This load generates a local nonhydrostatic mechanical stress at every point of contact between any pair of powder particles. The specific features of the deformation processes induced by these stresses depend on the intensity of the mechanical stresses themselves, on the details of the powder particle arrangement, that is on the topology of the contact network, and on the physical and chemical properties of powders (Martin et al., 2003; Delogu, 2008a). At the end of any given collision event, the powder that has been trapped is remixed with the powder that has not undergone this process. Correspondingly, at any instant in the mechanical processing, the whole powder charge includes fractions of powder that have undergone a different number of collisions.

The individual reactive processes at the perturbed interface between metallic elements are expected to occur on timescales that are, at most, comparable with the collision duration (Hammerberg et al., 1998; Urakaev and Boldyrev, 2000; Lund and Schuh, 2003; Delogu and Cocco, 2005a,b). Therefore, unless the ball mill is characterized by unusually high rates of powder mixing and frequency of collisions, reactive events initiated by local deformation processes at a given collision are not affected by a successive collision. Indeed, the time interval between successive collisions is significantly longer than the time period required by local structural perturbations for full relaxation (Hammerberg et al., 1998; Urakaev and Boldyrev, 2000; Lund and Schuh, 2003; Delogu and Cocco, 2005a,b).

These few considerations suffice to point out the two fundamental features of powder processing by ball milling, which in turn govern the MA processes in ball mills. First, mechanical processing by ball milling is a discrete processing method. Second, it has statistical character. All of this has important consequences for the study of the kinetics of MA processes. The fact that local deformation events are connected to individual collisions suggests that absolute time is not an appropriate reference quantity to describe mechanically induced phase transformations. Such a description should rather be made as a function of the number of collisions (Delogu et al., 2004). A satisfactory description of the MA kinetics must also account for the intrinsic statistical character of powder processing by ball milling. The amount of powder trapped in any given collision, at the end of collision is indeed substantially remixed with the other powder in the reactor. It follows that the same amount, or a fraction of it, could at least in principle be trapped again in the successive collision.

This is undoubtedly a difficult aspect to take into account in a mathematical description of MA kinetics. There are at least two extreme cases to consider. On the one hand, it could be assumed that the powder trapped in a given collision cannot be trapped in the successive one. On the other, it could be assumed that powder mixing is ideal and that the amount of powder trapped at a given collision has the same probability of being processed in the successive collision. Both these cases allow the development of a mathematical model able to describe the relationship between apparent kinetics and individual collision events. However, the latter assumption seems to be more reliable than the former one, at least for commercial mills characterized by relatively complex displacement in the reactor (Manai et al., 2001, 2004).

A further obvious condition for the successful development of a mathematical description of MA processes is the one related to the uniformity of collision regimes. More specifically, it is highly desirable that the powders trapped at impact always experience the same conditions. This requires the control of the ball dynamics inside the reactor, which can be approximately obtained by using a single milling ball and an amount of powder large enough to assure inelastic impact conditions (Manai et al., 2001, 2004; Delogu et al., 2004). In fact, the use of a single milling ball avoids impacts between balls, which have a remarkable disordering effect on the ball dynamics, whereas inelastic impact conditions permit the establishment of regular and periodic ball dynamics (Manai et al., 2001, 2004; Delogu et al., 2004).

All of the above assumptions and observations represent the basis and guidelines for the development of the mathematical model briefly outlined in the following. It has been successfully applied to the case of a Spex Mixer/ Mill mod. 8000, but the same approach can, in principle, be used for other ball mills.

The Planetary ball mills are the most popular mills used in MM, MA, and MD scientific researches for synthesizing almost all of the materials presented in Figure 1.1. In this type of mill, the milling media have considerably high energy, because milling stock and balls come off the inner wall of the vial (milling bowl or vial) and the effective centrifugal force reaches up to 20 times gravitational acceleration.

The centrifugal forces caused by the rotation of the supporting disc and autonomous turning of the vial act on the milling charge (balls and powders). Since the turning directions of the supporting disc and the vial are opposite, the centrifugal forces alternately are synchronized and opposite. Therefore, the milling media and the charged powders alternatively roll on the inner wall of the vial, and are lifted and thrown off across the bowl at high speed, as schematically presented in Figure 2.17.

However, there are some companies in the world who manufacture and sell number of planetary-type ball mills; Fritsch GmbH (www.fritsch-milling.com) and Retsch (http://www.retsch.com) are considered to be the oldest and principal companies in this area.

Fritsch produces different types of planetary ball mills with different capacities and rotation speeds. Perhaps, Fritsch Pulverisette P5 (Figure 2.18(a)) and Fritsch Pulverisette P6 (Figure 2.18(b)) are the most popular models of Fritsch planetary ball mills. A variety of vials and balls made of different materials with different capacities, starting from 80ml up to 500ml, are available for the Fritsch Pulverisette planetary ball mills; these include tempered steel, stainless steel, tungsten carbide, agate, sintered corundum, silicon nitride, and zirconium oxide. Figure 2.19 presents 80ml-tempered steel vial (a) and 500ml-agate vials (b) together with their milling media that are made of the same materials.

Figure 2.18. Photographs of Fritsch planetary-type high-energy ball mill of (a) Pulverisette P5 and (b) Pulverisette P6. The equipment is housed in the Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR).

Figure 2.19. Photographs of the vials used for Fritsch planetary ball mills with capacity of (a) 80ml and (b) 500ml. The vials and the balls shown in (a) and (b) are made of tempered steel agate materials, respectively (Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR)).

More recently and in year 2011, Fritsch GmbH (http://www.fritsch-milling.com) introduced a new high-speed and versatile planetary ball mill called Planetary Micro Mill PULVERISETTE 7 (Figure 2.20). The company claims this new ball mill will be helpful to enable extreme high-energy ball milling at rotational speed reaching to 1,100rpm. This allows the new mill to achieve sensational centrifugal accelerations up to 95 times Earth gravity. They also mentioned that the energy application resulted from this new machine is about 150% greater than the classic planetary mills. Accordingly, it is expected that this new milling machine will enable the researchers to get their milled powders in short ball-milling time with fine powder particle sizes that can reach to be less than 1m in diameter. The vials available for this new type of mill have sizes of 20, 45, and 80ml. Both the vials and balls can be made of the same materials, which are used in the manufacture of large vials used for the classic Fritsch planetary ball mills, as shown in the previous text.

Retsch has also produced a number of capable high-energy planetary ball mills with different capacities (http://www.retsch.com/products/milling/planetary-ball-mills/); namely Planetary Ball Mill PM 100 (Figure 2.21(a)), Planetary Ball Mill PM 100 CM, Planetary Ball Mill PM 200, and Planetary Ball Mill PM 400 (Figure 2.21(b)). Like Fritsch, Retsch offers high-quality ball-milling vials with different capacities (12, 25, 50, 50, 125, 250, and 500ml) and balls of different diameters (540mm), as exemplified in Figure 2.22. These milling tools can be made of hardened steel as well as other different materials such as carbides, nitrides, and oxides.

Figure 2.21. Photographs of Retsch planetary-type high-energy ball mill of (a) PM 100 and (b) PM 400. The equipment is housed in the Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR).

Figure 2.22. Photographs of the vials used for Retsch planetary ball mills with capacity of (a) 80ml, (b) 250ml, and (c) 500ml. The vials and the balls shown are made of tempered steel (Nanotechnology Laboratory, Energy and Building Research Center (EBRC), Kuwait Institute for Scientific Research (KISR)).

Both Fritsch and Retsch companies have offered special types of vials that allow monitoring and measure the gas pressure and temperature inside the vial during the high-energy planetary ball-milling process. Moreover, these vials allow milling the powders under inert (e.g., argon or helium) or reactive gas (e.g., hydrogen or nitrogen) with a maximum gas pressure of 500kPa (5bar). It is worth mentioning here that such a development made on the vials design allows the users and researchers to monitor the progress tackled during the MA and MD processes by following up the phase transformations and heat realizing upon RBM, where the interaction of the gas used with the freshly created surfaces of the powders during milling (adsorption, absorption, desorption, and decomposition) can be monitored. Furthermore, the data of the temperature and pressure driven upon using this system is very helpful when the ball mills are used for the formation of stable (e.g., intermetallic compounds) and metastable (e.g., amorphous and nanocrystalline materials) phases. In addition, measuring the vial temperature during blank (without samples) high-energy ball mill can be used as an indication to realize the effects of friction, impact, and conversion processes.

More recently, Evico-magnetics (www.evico-magnetics.de) has manufactured an extraordinary high-pressure milling vial with gas-temperature-monitoring (GTM) system. Likewise both system produced by Fritsch and Retsch, the developed system produced by Evico-magnetics, allowing RBM but at very high gas pressure that can reach to 15,000kPa (150bar). In addition, it allows in situ monitoring of temperature and of pressure by incorporating GTM. The vials, which can be used with any planetary mills, are made of hardened steel with capacity up to 220ml. The manufacturer offers also two-channel system for simultaneous use of two milling vials.

Using different ball mills as examples, it has been shown that, on the basis of the theory of glancing collision of rigid bodies, the theoretical calculation of tPT conditions and the kinetics of mechanochemical processes are possible for the reactors that are intended to perform different physicochemical processes during mechanical treatment of solids. According to the calculations, the physicochemical effect of mechanochemical reactors is due to short-time impulses of pressure (P = ~ 10101011 dyn cm2) with shift, and temperature T(x, t). The highest temperature impulse T ~ 103 K are caused by the dry friction phenomenon.

Typical spatial and time parameters of the impactfriction interaction of the particles with a size R ~ 104 cm are as follows: localization region, x ~ 106 cm; time, t ~ 108 s. On the basis of the obtained theoretical results, the effect of short-time contact fusion of particles treated in various comminuting devices can play a key role in the mechanism of activation and chemical reactions for wide range of mechanochemical processes. This role involves several aspects, that is, the very fact of contact fusion transforms the solid phase process onto another qualitative level, judging from the mass transfer coefficients. The spatial and time characteristics of the fused zone are such that quenching of non-equilibrium defects and intermediate products of chemical reactions occurs; solidification of the fused zone near the contact point results in the formation of a nanocrystal or nanoamor- phous state. The calculation models considered above and the kinetic equations obtained using them allow quantitative ab initio estimates of rate constants to be performed for any specific processes of mechanical activation and chemical transformation of the substances in ball mills.

There are two classes of ball mills: planetary and mixer (also called swing) mill. The terms high-speed vibration milling (HSVM), high-speed ball milling (HSBM), and planetary ball mill (PBM) are often used. The commercial apparatus are PBMs Fritsch P-5 and Fritsch Pulverisettes 6 and 7 classic line, the Retsch shaker (or mixer) mills ZM1, MM200, MM400, AS200, the Spex 8000, 6750 freezer/mill SPEX CertiPrep, and the SWH-0.4 vibrational ball mill. In some instances temperature controlled apparatus were used (58MI1); freezer/mills were used in some rare cases (13MOP1824).

The balls are made of stainless steel, agate (SiO2), zirconium oxide (ZrO2), or silicon nitride (Si3N). The use of stainless steel will contaminate the samples with steel particles and this is a problem both for solid-state NMR and for drug purity.

However, there are many types of ball mills (see Chapter 2 for more details), such as drum ball mills, jet ball mills, bead-mills, roller ball mills, vibration ball mills, and planetary ball mills, they can be grouped or classified into two types according to their rotation speed, as follows: (i) high-energy ball mills and (ii) low-energy ball mills. Table 3.1 presents characteristics and comparison between three types of ball mills (attritors, vibratory mills, planetary ball mills and roller mills) that are intensively used on MA, MD, and MM techniques.

In fact, choosing the right ball mill depends on the objectives of the process and the sort of materials (hard, brittle, ductile, etc.) that will be subjecting to the ball-milling process. For example, the characteristics and properties of those ball mills used for reduction in the particle size of the starting materials via top-down approach, or so-called mechanical milling (MM process), or for mechanically induced solid-state mixing for fabrications of composite and nanocomposite powders may differ widely from those mills used for achieving mechanically induced solid-state reaction (MISSR) between the starting reactant materials of elemental powders (MA process), or for tackling dramatic phase transformation changes on the structure of the starting materials (MD). Most of the ball mills in the market can be employed for different purposes and for preparing of wide range of new materials.

Martinez-Sanchez et al. [4] have pointed out that employing of high-energy ball mills not only contaminates the milled amorphous powders with significant volume fractions of impurities that come from milling media that move at high velocity, but it also affects the stability and crystallization properties of the formed amorphous phase. They have proved that the properties of the formed amorphous phase (Mo53Ni47) powder depends on the type of the ball-mill equipment (SPEX 8000D Mixer/Mill and Zoz Simoloter mill) used in their important investigations. This was indicated by the high contamination content of oxygen on the amorphous powders prepared by SPEX 8000D Mixer/Mill, when compared with the corresponding amorphous powders prepared by Zoz Simoloter mill. Accordingly, they have attributed the poor stabilities, indexed by the crystallization temperature of the amorphous phase formed by SPEX 8000D Mixer/Mill to the presence of foreign matter (impurities).

mills college(usa) courses & fees structure 2021 - duration & eligibility for uae students

mills college(usa) courses & fees structure 2021 - duration & eligibility for uae students

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