How to Make a Percussion Drill
In this section we will discuss the parts that make up a percussion drill and how to build them.
While we will be describing designs that have been used successfully for years, remember that there have been many other designs used throughout the 3,000 years that humans have been drilling wells using the percussion method. We will try to describe the reasons behind the designs so that you can design tools that work best for you.
We encourage you to experiment and use whatever you have to make your tools. Please write to us and let us know what you come up with!
Designing the Bit
The bit is a heavy drill tool which is lowered into the well hole on a rope and lifted and dropped to chop up the earth.
The bit's primary job is to make mud.
Because the bit needs to drop with enough force to cut earth and crush stone, it must be very heavy. The bits shape needs to be such that it can move up and down quickly in a hole full of mud. The teeth of the bit need to be made of very hard steel so that they can cut the earth and stone for a long time without becoming rounded and dull.
The weight of the bit depends on the type of drill being used. If the drill is to be powered by people pulling on a rope, then the bit should weigh 65 to 125 pounds. If the drill will be powered by an engine, experiment with heavier designs. In some cases, we have welded strips of rebar (steel concrete reinforcement) to the side of our bits to make them heavier.
The size of the bit depends on the size of the hole to be drilled. Common bits used by modern drillers vary from two inches to twelve inches wide and can range from two feet to sixteen feet in length. For the typical hand pump installation, where a four inch casing will be installed, a four inch bit will work fine. Because the bit will slop around inside the hole as it drills, the hole may end up being more than six inches wide when drilling in loose soils. The drill bits we used most in Liberia and Nigeria were four feet long, four inches wide at the teeth, and weighed 85 pounds.
The bit should have a shaft which is thinner than its cutting edge, or head, so that there is enough room for the mud to move quickly around the bit as it lifts and drops inside the hole. The head itself should be designed so that it can move through the mud quickly. In the drawings on this page, you can see that one bit has four slim teeth that allows mud to move between them while the other bit is wider in one direction so that the mud can move past it on the sides.
The shaft of the bit can be made of any sort of scrap steel which is not likely to break or shatter. Pieces of railroad ties work nicely, as do lead filled pipes, truck axels, or the solid steel of hydraulic lifts. Scrap pieces of smaller lengths of steel can be welded together to create a stronger and heavier final product. Make sure the welding is done well, because this piece is going to take a beating!
The head of the drill bit needs to be carefully designed. It must be the strongest piece because it will be the first to hit the rock. The heads of forged steel bits can be hardened by the blacksmith or they can have hardened steel imbedded into them. Heads that are welded together should be made with hardened steel -- like that of an automobile or truck spring leaf. If you have access to a metal shop, it is worth the small investment to buy the best hardened steel available. The teeth are small and not much steel is needed to make them, but they will last much longer and they will not need to be sharpened as much if you start with the hardest possible materials.
The head should be welded on with the greatest care. If the head should break loose at the bottom of a 90 foot hole, not only would the bit become useless, but it would be almost impossible to drill past the broken pieces meaning that the well hole would have to be abandoned.
The bits head must also be shaped so that it will cut a straight hole and not tire the driller and wear out the tools. It should have many edges to cut the earth without being pointed. This is because one wants to make small cuts when drilling. It is always better to chop up a small amount of earth at a time and have patience. If the drill bit is too pointed and too sharp, it can stick into the ground like a knife, making it difficult to pull it back out and straining the rope and those pulling the rope. If the bit is too light and thin it will be easily pushed aside by the small rocks at the bottom of the well hole, leaving you with a crooked hole. If the bit is sharp on one side only, it will scrape the side and drill a crooked hole. If the drill is dull and rounded, then it will simply pack the earth down at the bottom of the hole.
In these drawings of the drill bits you can see two designs that work well. The welded star bit and the forged rock bit.
The Welded Star Bit
This bit with four teeth, called a "star bit", has the teeth tapered into the center of the bit. This causes small rocks to be moved to the middle of the hole so that they receive the hardest blow from the falling bit. But the teeth are only two inches longer on the outside than in the center, which means that the bit will take small bites and not get stuck.
The Forged Rock Bit
The forged bit, like the one in the drawing below, has been formed by the blacksmith to have a broad end that tapers gently and has numerous edges. The many edges do the cutting while the broadness keeps the bit from sinking into the earth too far or "biting off more than it can chew." This design is especially good for chipping away at larger rocks.
Designing a Bailer
The bailer is a hollow steel tube with a valve at its base. It fills with mud from the bottom of the well hole and traps it so the driller can carry it to the surface. It literally "bails" out the cuttings made by the drill bit. The bailer is another tool that needs special attention paid to its construction since it will, like the bit, do a lot of pounding on earth and rock.
The bailer may be the same width as the bit (and it must be when drilling in hard rock), but we have found that when drilling in looser earth using a larger bailer is easier and it helps to keep the hole straight. We used a four inch bit with a six inch bailer in most of our tests and found the results to be excellent. The bailer needs to be large enough to carry a sizable amount of cuttings to the surface, but not so large that it is too heavy for the power source to lift. On our hand powered drill we used a bailer that could carry five gallons when filled.
The bailer has two parts: the tube and the bailer bottom.
The tube can be any steel pipe of good strength. In looking for the tube, be aware that it must take a good pounding and will need to be strong, but also understand that if the walls of the tube are too thick the bailer will be heavier and more difficult to use. Look for a strong pipe with 1/4 or 3/16th-inch thick walls.
The top of the tube is tapered on both sides of the loop to prevent the bailer from catching on the side of the hole when it is being pulled upwards.
The bailer bottom is the part that actually comes in contact through earth and contains the flap valve which traps the drill cuttings. The flap valve is designed so that, as the bailer is being lowered into the hole, the flap will open upwards to allow the cuttings from the drill to enter the tube. When the bailer is then lifted upwards, the flap falls back to its flat position, sealing the bottom of the tube and trapping the cuttings inside. The bailer is then lifted to the top of the hole and emptied.
The bailer bottom can be reinforced with extra steel so that it can withstand its impact with the earth inside the hole. To do this, the inside of the bottom of the tube is lined with a 2-inch long sleeve of steel 1/4 to 1/2 inch thick. The sleeve and the wall of the tube are tapered inward to provide a cutting edge. The hinge of the flap valve is attached to the top of the sleeve and the flap valve rests on the top edge of the sleeve when closed. The hinge can be a steel mechanical hinge, or made from leather or rubber from a car tire.
The bit and bailer have a sturdy loop welded to the top to attach the rope or cable. This loop should be welded on very carefully so that it can not possibly break loose. A 1/2" steel bar is sufficient for this task. The loop should be big enough so that it can be hooked by the fishing tools should the rope or fastener break and tools get stuck inside the hole.
Instead of using the simple tool loops described above, you can improve your drill tools by making fancy loops that will provide you with both an easy and fast way to connect the tools to the rope, as well as the ability to "knock" the tools upwards should they get stuck in the hole.
Jars get their name from the "jarring" action they provide when the hook slides up within the loop to pound against the top of the loop.
The jars are made of two parts: the jar hook and the the jar loops.
The jar loops can be made entirely out of 1/2" steel rod, but you'll want the steel to be strong and smooth since they will rub against each other a lot and take a lot of pounding. Note that there's a extra wide part of the loop above the tool to make it easier to "fish" or hook the tools when they get lost in the hole.
Because they will take a lot of pounding, it is very important to make the jar hook as strong as possible.
The tip of the hook is the most important, since if this breaks inside the hole it will cause a lot of problems and you could lose your tools. It would be a good idea to invest in the best metals and the best welding for this part. Inspect this part often and re-weld the pieces whenever you see cracks. This one piece receives the greatest force when loosening tools stuck at the bottom of the well hole. The thicker the rod, the stronger this piece will be. One inch rod has proven very sturdy in field tests.
Choosing a Pulley
A pulley is a wheel that allows the rope to turn a corner without binding or causing friction.
There are many designs for pulleys. Most hardware stores will have pulleys in stock but pulleys can also be easily made from scrap.
Store bought pulleys usually have a hook attached at the top and come in many sizes. Look for a pulley which is wide enough to hold the rope you will be using. Most drillers will use a 1/2 inch rope, so the pulley should be at least 1/2 inch wide. Look for a pulley which is made from strong steel and has bearings around the axle for smooth operation.
There are many ways to make your own pulley. You can get a scrap pulley from the engine of a broken car or truck. You can use a wheel (with the tire removed) from a motorcycle, bicycle, trailer, or other vehicle. You can make a pulley from wood. Make sure that the parts are strong and that all the rough edges are smoothed down so that they will not cut the rope.
Designing a Tripod
The tripod, the largest part of the drill, is a simple frame that holds the pulley in place over the hole as it is drilled. The design of the tripod is to allow the tools to be lifted in and out of the hole with ease and to allow the driller plenty of room to operate around the hole.
The simplest tripod to build involves three poles tied together at the top with a chain, strong rope, or vine. More portable and re-usable tripods can be built with steel poles or angle iron bolted together at the top. Four-legged structures, called derricks, can also be built with bamboo or wood planks.
The tripod is built tall enough to allow the tallest tool to be lifted out of the hole with room to spare. For most drilling that does not require casing, each leg of the tripod can be 12 feet long. When they are strapped together with the poles overlapping at the top and the bottom ends placed six feet apart in an triangle on the ground, there will be a height of 9 1/2 feet from spot on the ground where the well hole will be drilled to point where the tripod is tied together at the top. You may want to experiment with different heights. In one test we used 18 foot poles and placed them 14 feet apart on the ground. The distance from the ground to the top of the tripod was 12 feet. We had plenty of room to work under the tripod. In another test our poles were thinner than 3 inches and they wobbled from the weight of the full bailer.
The strength of the tripod should be much greater than that which would be needed simply to lift and drop the drilling tools. If the tools get stuck at any time during the drilling, you may need to apply many times the usual pull to get them loose. Use good strong new poles that are not dry and brittle. Look for poles that are at least six inches thick at the base and four inches at the top. In many of our tests we used eight inch poles very successfully. If the poles are much larger they may be difficult to tie together without first cutting notches at the top.
If steel is being used to build the tripod or derrick, then look for pipes that will not bend easily. Also attach small plates to the bottom of each leg to keep them from sinking into the ground.
Erecting the tripod can be done simply by standing the three poles and having many persons hold them in place while one person climbs up and ties them together. But a safer and easier way to do it is to lay two poles together on the ground with the tops crossed and the feet spread apart and tie them together temporarily. Then, while two or more persons stand them up together, another person places the third pole into place. Using a ladder or standing on someones shoulders, one person ties the three poles together in a loose but more permanent fashion. The pulley is then attached to the center of the unionmaking sure that the pulleys wheel is pointing towards the place where the rope is to be pulled.
Before tightening the chain or rope at the top of the tripod make all the adjustments that are needed: make sure that the center of the tripod is directly above the spot where you will be drilling; place the feet of the poles equal distances from each other; make sure that the angle of each face of the tripod is equal; and then bury the end of each pole in a one foot deep hole for stability.
With the tripod positioned just the way you want it, tighten up the rope or chain that ties the top of the tripod so that it cannot move while the drill is working. The easiest way to do this is to drive wooden wedges into the loops of rope or chain until there is no looseness left. After the drilling has begun you will want to check these wedges often to make sure they stay tight.
One small addition to the tripod can save the driller a lot of time and energy. Tying a pole from one leg to another, positioned about three feet above the ground, creates a shelf. The drilling tools can be leaned against the shelf so that the driller does not have to bend over to lift them up and the smaller tools can be hung on the shelf so that they do not get buried in the mud around the well hole.
The rope is used to lift and lower the bit and bailer inside the well hole. It is the one part of the drill that can wear out the fastest. We used 1/2 inch rope in all of our drilling and found it to be strong enough. Thicker rope would work as well, but thinner rope would probably stretch and break too easily.
Polypropylene rope is the easiest to come by and is a very sturdy rope. Polypropylene rope will stretch, however, making it easier for the rope to tangle and harder to leverage tools that are stuck at the bottom of the hole.
Manila rope is also easy to find and can sometimes be less expensive than polypropylene. When drilling with manila rope one must wear gloves to protect the hands from splinters and must pay close attention to cleaning and drying the rope after every use so that it will last longer. A wet manila rope will spoil quickly and lose its strength -- creating both an expensive and potentially dangerous situation -- so be sure to dry your rope in the sun and hang it in a dry place with lots of air circulation.
Any rope that is twisted together will start to untwist during heavy use. One way to fix a untwisting rope is to relax the rope and re-twist it by hand until it has returned to its original shape.
The best possible rope to use, even though it is more expensive and difficult to find in many developing countries, is braided nylon. Nylon rope will last longer than manila rope since it is not affected by water. Nylon rope is stronger and does not stretch nearly as much as polypropylene rope. Since nylon rope is softer, there is less need to wear gloves. Most importantly, nylon rope is wound together by braiding, not twisting, so that it cannot become untwisted.
The Rope Eye
The rope eye is a rounded metal insert that makes a strong loop at the end of the rope so that it does not wear or break during drilling.
Rope eyes are very common and can be found in most hardware stores. They are inexpensive and come in sizes to match all rope widths. They can also be made from scrap.
The shape of the rope eye is rounded on one end and pointed on the other, just like a human eye, to act like a loop of rope being pulled. This allows the rope to pull on the drill tool without rubbing or stretching. The rope eye is smooth on all edges to prevent the rope from cutting.
The rope eye is attached by weaving the rope together just above the eye, knotting the rope with a slip knot, or by lacing the rope together with small, strong cord.
The Safety Hook
To change quickly between the bit and the bailer with little fuss, a safety hook is attached to the eye at the end of the rope. The safety hook is a hook of any design that will not become unhooked while the tool is down in the hole. The hook must be smaller in width than the smallest drilling tool to be used and it must be strong enough to withstand both the upward and downward stroke of the drill.
The hook makes the change from bit to bailer happen faster and prevents the drillers having to untie and re-tie the rope each time a tool is changed.
A common problem that can bring the drilling to a halt is when a tool becomes unhooked from the rope during the drilling. At this time the fishing tools are used.
The fishing tools are long hooks that attach to the rope and are used to catch the loop at the top of the drill bit or bailer. In shallow holes, poles can be used to help hook the loose tool, but if the hole is too deep the driller will have to rely on the weight of the fishing tool and the shape of the hook to grab the loop. The fishing tool needs to be heavy enough to sink into mud if the tool is lost in mud at the bottom of the hole.
Miscellaneous Parts and Pieces
To measure the depth of the well hole and keep track of the drills progress, it is necessary to have a measuring tool. A 50-foot tape measure works perfectly, but other measures will work as well.
The depth gauge is a simple string that measures the depth of the hole. The depth gauge is usually made of cheap material because it will get very dirty inside the well. Using the depth gauge to measure the well will keep the tape measure from getting dirty and spoiling.
The depth gauge has a small but heavy weight on a string which is lowered into the hole to measure the depth. The driller can feel when the weight has hit the bottom of the hole and make a mark on the string at the top of the hole. When the depth gauge is removed, the driller can tell the depth of the hole by measuring the distance between the mark on the string and the end of the weight.
The second use of the depth gauge is to detect where the water stands inside the well when the water table has been reached. If the bottom face of the weight is cup shaped, then it will make a small "pop" noise when it hits the flat surface of the water in the well hole. In one design we add to the depth gauge a piece of bamboo that was one inch wide. We cut the bottom edge one inch from a joint to make the small bottom cup and then cut the top edge just below the next joint. We put some small stones in the upper part of the bamboo for weight, and then tied a string thru the top. This design makes an especially loud and cheerful "pop."
When the well has been dug and the casing installed, the part of the well hole left open around the casing needs to be re-filled. The well will work much better if the bottom of the casing is surrounded by large grained sands that will allow water to pass through but keep out gritty, smaller dirt. The sand screen is used to sort sand and collect the larger sized sands.
The sand screen is simply a frame that has wire mosquito screen stretched over it. The frame can be made of wood, bamboo, metal, plastic, or whatever is available. We have even used a two foot square piece of mosquito screen without a frame.
The sand screen can also be used to wash and sort samples of the mud that are brought out of the well hole by the bailer. By looking at the washed samples the driller can tell what kind of earth is being drilled at the bottom of the hole.