By Rob Brown (Brewing Techniques)
A good crush is a critical first step toward optimum extraction and efficient lautering in all-grain brewing. Do-it-yourselfers can build their own mills for home use, with excellent results.
At one time or another, serious home brewers travel down the road to all-grain brewing. A critical step in this process is crushing the grain. Some brewers purchase their grain precrushed, while others use a mill at a local homebrew shop. But for many inventive home brewers, rigging their own version of a mill is part and parcel of the art and science of home brewing.
The simple design of two-roller hobby mills makes home fabrication a reasonable possibility. Those willing to make the effort can construct an effective mill superior to, or less expensive than store-bought models. Whether you mill your own grain for precise control of the ctush, for the aesthetics of having your own mill, or simply because the beer tastes better that way, the convenience and personal thrill of using your own mill is equal only to the cost savings gained through bulk grain purchases.
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The extent to which the grain is crushed will affect your brewing session and the end result — the beer you will proudly call your own. It’s a critical step in the brewing process, and one that deserves some attention.
Before you begin building or milling, it’s important to understand and keep in mind what you are trying to accomplish. The primary components of the grain to consider are the husk, which forms the filter bed when lautering, and the endosperm, which is the starch and enzyme source. Milling the endosperm exposes the starch and the enzymes contained in the grain.
During milling, the husk is best left as intact as possible while releasing the endosperm within; a largely intact hull will form an ideal filter bed during sparging. A milling that has ground the husk to a flour, or otherwise broken it into too many small pieces, will form a poor filter bed, leach more tannins, and increase the likelihood of creating the ever-dreaded stuck mash. Although crushing the endosperm to a flour would technically provide maximum conversion, this is only really useful as a theoretical baseline for percent extraction calculations.
A commercial mill with a multiroller design is capable of removing the hull from the endosperm and separating the hulls and different-sized starch particles using sizing screens. The hard ends of the endosperm are further separated from the husk, and the larger starch particles are crushed in subsequent rollers.
Many two-roller mills could reduce much of the grain to flour, but with fully modified malts the milling of the endosperm into small granular pieces (grits) will provide for a complete and speedy conversion while maintaining hull integrity.
A coarser crush of well-modified malts will result in negligible loss of extraction compared with the multistep crush required by poorly modified malts to achieve similar results. Poorly modified malts will adhere to the husk as hard ends, whereas fully modified malts will more readily separate from the husk with a two-roller mill.
If you wish to construct or purchase a two-roller mill, keep in mind that its design must yield a high-quality crush within a reasonable amount of time and effort. Although the rollers are undoubtedly the focus of a mill’s productivity, a mill must also effectively combine a hopper, a frame, an adjustment mechanism, and a drive source for the rollers to properly perform their function. Before delving into the all-important rollers, let’s examine the other components.
Hopper: A hopper made of metal, plastic, or wood is essentially an elongated funnel positioned above the rollers. It should distribute grain along the length of the rollers in an even and continuous fashion. Ideally, a gravity-fed hopper will hold the entire grain bill, or at least be large enough to avoid excessive refilling. Whether it’s a separate piece or part of the mill’s frame, the base of the hopper also acts as a guide to hold the grain in place. It should (if applicable) be made with roller-shaped V-grooves at either end, and fit snugly with the rollers to control the grain’s flow or escape.
Frame: A frame, made of wood or metal, must hold all the other components solidly and securely. Its construction can be as simple as a four-sided frame with room to install the other components. A flat, even surface to mount the rollers is essential, as the rollers must meet evenly to function properly. The frame should also incorporate a stand or some means of attachment to a table or collection bucket.
Adjustment mechanism: A roller adjustment mechanism controls the gap between two rollers. Although a fixed-gap mill is workable, it is not as versatile as one with an adjustable gap and requires precise construction techniques. For optimal milling, a homemade mill is best equipped with an adjustment device to enable you to cope with varying grain sizes. A simple screw mechanism that adjusts the gap between rollers can be easily constructed. The accompanying figure shows some of the possible configurations for adjustment mechanisms. In all cases, of course, the bolts that hold down the bearings would have to be loosened before making an adjustment.
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Reverse Belt Drive Terence Tegner used a pair of smooth 6-in.-diameter rollers made of mild steel. They were 10 in. long and grind 10 kg/minute – a lot of grain! The motor-driven mechanism (¾ HP, running at 350 rpm) used a round section belt to drive both rollers – one roller with the “front” of the belt and the second with the “back” of the belt. In addition, he used varying-sized pulleys on the roller shafts to create a speed difference between rollers, aiding the crush. A drive design provides an easy and effective way to drive both rollers. The differential speed of the rollers helps create a tangential sheering force similar to that used in commercial mills, helping to remove the husk from the endosperm. |
Drive mechanism: The means for turning the rollers can vary considerably. For the small amounts of grain a homebrew mill must crush, the simplest solution is to drive a mill using an ordinary crank handle. The small diameter of rollers on homebrew mills, however, may require a bit more patience during cranking sessions. Many people, especially those brewing large batches, attach a motor to speed up the process, giving them some extra time to savor the fruits of past labors. Some simply use a variable-speed drill, but many attach a small electric motor with belts and pulleys. Generally a ½- or ¾-HP motor is geared down with different-sized pulleys to run at rates between 200 and 300 rpm.
When power is supplied directly to only one roller, a few different methods are available for effectively transferring the power to the second roller. In the grain-transfer method, the grain, when forced between the rollers, transfers the energy from the drive to the follower roller. This is the simplest method of power transfer and is very effective. One of two more-complex methods uses O-rings (belts) or gears. One or more grain-sized O-rings are fitted around the rollers so that the spinning energy of one roller is directly transferred to the other. Gears can be added to the ends of the rollers to transfer the drive power in a similar fashion. Terence Tegner (Walkerville, South Africa) used a third and very effective method, a reverse-belt transfer, to drive both rollers with a single belt (see box below).
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Adjustment Mechanisms Here are some simple adjustment mechanisms to consider; they use common bolts. A simple adjustment mechanism is best from a functional and construction standpoint for the home brewery. |
As part of your drive system, you can use either external bearings or bushings, depending on the speed and frequency of the mill’s use. If, however, you find suitable rollers with internal bearings, you may be tempted to seize the bearings and drive the shaft using external bushings or bearings. Instead, I recommend attaching a cog/pulley (around the shaft) to the end of the roller. A chain or belt sitting between the frame and roller can then be used to drive one or both of the rollers. Using internal bearings is cheaper and will also ensure that the roller(s) spin freely and truly, as designed.
The rollers are the most important and, unfortunately, expensive or elusive component of a mill. It’s a good idea to wait to finalize your mill plans until you have obtained rollers. Consider yourself fortunate if you have the money to have a pair of rollers machined, or access to the right equipment to make them yourself. A pair of first-run rollers from a machine shop can be prohibitively expensive. Most mills are, and will continue to be, made with minimal funds and maximum ingenuity.
To counter the limitation of rollers with small diameters, home brewers have turned to texturing their rollers to better grip the grain. This texturing enables home brewers to achieve a good crush.
Rollers refers to the traditional roller (length greater than diameter) that is the mainstay of hobby and commercial mills. Rollers are available used, but often are too small in diameter (no grip), thin walled (can’t be knurled), or are generally unsuitable. The best sources of rollers appropriate for use on a grain mill are from a conveyor belt (the end or drive rollers), an old printing press (check for toxic ink residues), or an agricultural mill. Obviously, these items can be obtained new from a conveyor/press manufacturer or a farm-supply shop. One producer of roller mills, Jack Schmidling Productions (Marengo, Illinois) shipped his first 40 mills with used photocopy machine rollers. Some conveyor manufacturers (check your Yellow Pages) even specialize in stainless steel. Buying new rollers can be expensive, so for reasonable prices check for used or discarded rollers from local manufacturers or at mills, scrap yards, or farm auctions.
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Wooden Rollers |
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Tom Clifton (St. Louis, Missouri) turned 4-in.-diameter, hard maple rollers. He used a router to add axial grooves (fluting) for extra grip. He hand-cranks this mill at low revolutions and is happy with the grind. He based his mill on designs from Chris Barnhart. Clifton’s project was relatively expensive, but he says he enjoyed doing the work himself. He reports that the bearings that he bought show some slack when under load. He would consider using bronze bushings, considering the low rpms and its infrequent use. |
If you want to have a roller machined, you have a choice of three basic approaches. For a large-diameter roller, you can have a tube’s ends capped, center-drilled, a through-shaft added, and then turned true. For small-diameter rollers, a single piece of stock can be trued, the ends reduced for shafts, and then knurled. Chris Barnhart (Geneseo, Illinois) took the most economical approach. In addition to making his own mill, he drafted plans that have aided others with construction of theirs (see box below).
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Concrete Rollers |
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The basic roller construction method of C.D. Pritchard (another Home Brew Digest contributor) starts with plastic tubing or buckets as a basic mold. He pours (store-bought, stoneless) quickrete around a ⅝-in. steel rod as a shaft with a So far he has built two (one 4 in. and another 7 in. in diameter) single rollers against tangential plate mills and is considering a double-roller mill. He reports that glazing over with malt components on the grinding surface and tearing of the husks have been the only concern of otherwise successful crushing sessions. If attempting a concrete mill, I would try a two-roller mill with smooth, large-diameter rollers to avoid glazing on a roughened surface. At this point, he is still working on his design. A copy and description of his trials is available through alpha.rollanet.org, in the technical library under malt mills. |
Roller materials: Metal. The ideal material for a roller is stainless steel; it is hard, durable, and virtually maintenance-free. Unfortunately, it is much more expensive than mild steel and harder to work with. Stainless steel can cost up to four times as much as other materials, and the labor will be 50% more expensive at any machine shop. Mild steel can rust and must be maintained. Although not strictly food-grade, cast-iron rollers are common on older commercial two-roller mills. Softer metals, like aluminum and brass, are easier to work with and should be considered, if available, though they may wear more if heavily used.
Wood. If you have access to a lathe, you can consider using a suitable hard-grained wood (see box, previous page).
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An Economical Approach to Machining Rollers The rollers in Chris Barnhart’s mill are made from 2-in.-diameter, cold-rolled steel, which was medium-knurled, center-drilled, and tapped to fit a bolt as a shaft. It is driven by an internally geared motor. Although his knurled, 2-in. rollers are close to those found on hobby mills, he suggests that a 4–8 in. diameter would provide a sufficient grip on the grain with little or no texturing. These rollers cost him only $ 20, but plan on paying at least twice this amount for the materials and labor. You could also omit the tapping and just ream shafts in. Ideally, you would remachine the rollers true after assembling the unit, although this is not strictly necessary for a well-centered economy job. |
Plastic. Plastic can be easier to work with, food-grade, and suitably hard.
Stone. Traditionally, grain was crushed on stone flour mills, though with more damage to the husk. I asked a local memorial shop about turning granite rollers around a metal shaft. It can be done, but the price is high. Anyone considering this approach should ensure that only nontoxic methods are used to shape the stone. Suitably hard stone or concrete should work if the equipment, materials, and skill are available.
Concrete. I originally planned to make a cement roller mill but this investigation is being tackled by others (see box on previous page).
Roller dimensions: The question of roller dimension and texturing is very important to the design and success of a mill. We know a 1½-in.-diameter roller with a medium knurl works well on a homebrew mill. A quick search through various mill designs shows that a 9¾-in.-diameter (250 mm) smooth roller is considered the practical minimum for a multiroller commercial mill, based on desired throughput and the need for sufficient grip. Depending on your roller’s diameter, you will need to a decide on a smooth, machine-knurled, or roughed (using a file or a grinder) surface.
Adding texture to hobby mills allows the use of a small-diameter roller that would otherwise be useless. With smooth rollers (traditionally cast-iron), the angle of the rollers where the grain is initially gripped must be such that the frictional force between grain and roller is greater than the resistive force required by the compression and cracking of the grain. Rollers forming an obtuse angle (small-diameter rollers) will leave the grain riding on them. Only an acute angle (large-diameter rollers) will form a sufficiently “parallel” surface to draw and crush the grain. In addition, a wider gap setting (coarser crush) will place the grain, or the gripping point, closer to this “parallel” crushing surface.
6-in.-diameter rollers seem to be the home brewer’s safe minimum for smooth rollers (based on Terence Tegner’s mill — see box, page 27). However, the use of smaller rollers with a roughened surface (for those without access to a machine shop) will also work. Although untested, a pair of 4- or 5-inch rollers, where both rollers are driven to improve grip, should be more than capable of providing a suitable crush.
“Wheeler” designs: A wheeler (in which diameter is greater than width) can be thought of as a slice of a commercial mill. Instead of miniaturizing the whole roller and adding knurling, a broad wheel will more closely mimic one section of a full-size mill.
For a hobby mill, the wheels from older agricultural- or industrial-material-handling machines could be used. If they have a rubber coating around the cast-iron hub, remove the rubber to ensure a true surface beneath.
Although much thinner, the greater diameter will make up for the lack of length with each crank of the wheel. The benefits of this design are the availability of these wheels, and their large diameter negates the need for knurling or texturing. A suitable used wheel 6 or 8 in. in diameter and 2–3 in. wide should be easily found. Multiple wheels could even be placed together to make a greater length of the wheeler/roller. New wheels or casters can be found at a caster center, farm, or a material-handling supply center. A thinner frame and a hopper grain-guard would be needed, but this would otherwise follow designs similar to those of a roller mill.
Noncentered designs: Will Self (Billings, Montana) offered a good example of noncentered roller-mill design (see box, right).
Off-centered designs: Off-centered design involves positioning a readily available set of wheels on a shaft (from a lawnmower, for example) inside a large-diameter cylinder. The wheels would hold and drive the cylinder and could be adjusted for a proper grinding gap in a manner similar to that used for typical rollers. Though the shaft is internal, it is offset, transferring the motion to the grinding surface by the intermediary wheel. The wheels should be rubber or otherwise provide suitable traction on the inside of the cylinder — the closer the wheel diameter to that of the cylinder, the better.
The appeal of this design is that it makes use of materials that are easily found.
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Noncentered Roller Design Will Self’s design consists of two 4-in., 40-schedule PVC tubes (rollers), jockeyed into place by casters in an adjustable frame. The rollers are held in place by eight furniture casters attached to a 2 X 4 in. frame, adjusted with attached hinges and tightening wires. The plastic rollers are textured with a utility knife and have a crank handle on one roller, which drives the follower using a series of screw heads and holes placed in the rollers at either end to form a gear-type drive transfer. Although this mill crushes at a slow speed, it’s a good roller-type mill. Its real strength is the fact that its design consists of readily available materials and requires minimal tools. |
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Hand Driven Roller Design I salvaged rollers for my mill from a conveyor-belt system. The rollers and attached frame cost only $ 10, and all of the other materials were already on hand. The drive roller is knurled aluminum, 16 in. long and 4 in. in diameter; the follower is smooth and 2 in. in diameter. The frame is a squared “C” shape, used along with two pieces of angle iron to form slots along which the square bearing housings of the follower roller can run for adjustment. The follower roller had to be adjusted at an angle to form one of two 8-in. grinding faces. I attached a simple crank handle to the drive-roller shaft and added a 1-lb formed hopper, with a feeder bucket above it. The first batch took only a few minutes to grind – about 15 cranks of the handle per pound – producing a medium to coarse grind. This bulky mill is somewhat of an ugly brute, but it gets the job done. The unknurled follower roller does not allow for an ideal two-roller grind because of its poor grip on the grain. I am looking for a pair of dream rollers for a future upgrade – not out of necessity, but just because I like making beer gadgets. An additional consideration for a driven-roller mill is the addition of a spring-loaded adjustment mechanism. A sufficiently hard spring can be added to the screw mechanism in between the fixed adjusting plate and the interior nut. This would provide enough pressure to crack the grain while allowing the gap to open for a passing rock or any other foreign object that may jam or damage the driven rollers. A sheer pin or slip belt could also be used to protect the rollers and the motor from damage, if deemed necessary. |
The designs shown in this article demonstrate the vast possibilities for building homemade mills. If you are considering fashioning your own, take note of how these homebrewing mills work and copy where you can, but don’t let these examples restrict your efforts. Like your beer, your own mill can be the best in the world.
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-in. rod attached to provide torque. He then trues the roller with a masonry attachment for his grinder while spinning it on its bearings.
