Two sizes of air hammers and some of the many tools they can drive.
Some extremely useful additions have been made to the marble sculptor's tool kit in modern times.
This tool is just a smooth walled socket to hold the chisel, with a piston at the back that strikes the chisel hundreds of times a minute. The chisel does not lock in--if you let go, the hammer will bounce it out. They are noisy, vibrate, and raise clouds of dust, but the stone just spews from the tip. They're fantastic. Both of the hammers shown are equipped with a valve so the air can be turned on and off during use.
Claws and straight chisels work very well in a pneumatic hammer. Punches can a disappointment even on soft stone if your hammer is underpowered, because they require some weight behind them when you are doing bulk removal of stock. The ideal size for working with the claw on marble or limestone may be too small for working with the punch.
The performance boost provided by pneumatic power is probably the greatest for bushing, particularly for hard stone--bushing a substantial mass of stone without a pneumatic hammer is a waste of time. A wide assortment of bushing heads are also available.
Sculptor's pneumatic hammers are small--anywhere from the size of a Mont Blanc fountain pen, up to the size of a can of spray paint. The larger of the two shown here is a general purpose hammer, and is about an inch and a quarter in diameter and eight inches long. It's actually a bit small to drive the punch shown, even with the pressure turned up high, but is more than adequate for all the other tools.
The choice depends on how big you're working and on the type of stone. They are all very expensive compared to commercial pneumatic tools, but the big ones aren't much more expensive than small ones.
Other than the overall size, the big variable is stroke length. For a given cylinder size, the longer the stroke, the harder it hits, and the slower the cycle time. Hammers designed for granite have a longer stroke, which means they consume more air, and the larger hammers consume a lot--up to 8 CFPM, which is greater than a three to five horsepower contractor grade compressor can provide. But that's a very powerful handle. Most marble carvers will be more than happy with a model that consumes a maximum of 4 CFPM. More about this below.
Both steel and carbide tipped chisels are available. Steel tools:
Carbide tipped tools:
For softer stones, many carvers find that for tools with edges, steel tends to work somewhat better than carbide, but this has to be traded off against the frequent maintenance required. Carbide seems to have the edge for claws.
Carbide tooth chisels last a long time without sharpening, but they tend to lose or break their outer teeth very easily if they are used in any situation that results in pressure from the side. Running the chisel into a tight space, approaching an inside corner from an angle, or even grazing against a slight knob of stone when chiseling across a surface can cause this. Steel is much more resilient, and less likely to break in this situation, but wears down more quickly and eventually breaks from fatigue.
Only when the corner is clear can you safely run a tooth chisel parallel to the corner. To clean out inside corners, go into the corner first on low power with a sharp punch to get the solid rock out, or come in perpendicularly from one direction then the other with the tooth chisel.
The down side of pneumatic hammers is the noise, vibration and dust. In addition to the safety glasses and mask, you need ear protectors. Padded gloves are a good idea for extended use, because any vibrating tool can cause a variety of problems, explained in detail in the Safety section.
Pneumatic tools are rated for both air pressure (pounds-per-square-inch, or, PSI) and volume, expressed in cubic-feet-per-minute (CFPM). The CFPM consumed by a tool varies with air pressure, so both a low and a high number are usually given in the product specification. The numbers given for hand tools are the minimum pressure it requires, and the maximum pressure it is designed for, and the amounts of air it will use at those pressures.
Almost all compressors, regardeless of size, produce at least 120 PSI, which is more than enough for almost any air tool. The two places where compressors differ significantly is in the CFPM rate, and in the size of the air tank. Think of it like a truck--they all have more or less the same top speed (PSI) but a semi can haul more (CFPM) than a pickup truck.
The compressor motor has only has one setting--flat out. If the tank pressure falls below a set amount, say 120 PSI, the motor will automatically turn on and run until the tank pressure reaches some slightly higher limit, say, 135 PSI, and then cut off automatically. Should the cutoff mechanism fail for any reason, the pressure that can build up in the tank is limited by a spring-loaded valve which can only hold back slightly more pressure than nominal maximum for the machine, say, 140 PSI. Above this maximum, the air pressure in the tank will lift the valve and bleed off the excess. These valve usually also have a finger ring that can be pulled to manually empty the tank at any time.
If you are taking air out of the tank a rate lower than the CFPM rate of the compressor, the tank pressure should always stay in the range established by the automatic on/off levels, which is more pressure than you usually want. Therefore, every compressor comes with a device called a regulator, which maintains the output air pressure to the air hose at any set pressure, regardless of fluctuating pressure in the tank. Most compressors out of the box have two pressure meters, on for the tank pressure, and one for the regulator, which shows the line pressure. You simply turn the regulator knob until the line pressure is where you want it, and it will maintian that pressure for you automatically. If you turn the regulator knob down, it will automatically bleed air from the line until it reaches the desired setting.
The CFPM rating of a compressor varies with the line pressure. The compressor pictured here puts out 6.6 CFPM at 40 PSI, and 5.8 at 90 CFPM, which easily covers the typical operating range of the pneumatic hammers shown above, but would not be enough to run an angle grinder or other heavy duty tool. In the long run, the CFPM for your compresser must be greater than the combined CFPM's of all the tools that are running, but a big air tank will let your total air usage spike to a much higher rate for a limited amount of time. The more intermittent the tool usage, the more a big tank extends the number of tools that can be supported. This can greatly extend the number of users in, say, a gas station, where multiple mechanics use high-CFPM tools like wrenches, but only for a few seconds at a time. Carvers, however, tend to use tools like chisel handles and grinders, that stay on continuously, the CFPM rate of the compresser, must be more closely aligned with the tool ratings. In practice, it is best to have a wide margin of extra CFPM capacity above what you intend to use, so that the machine does not have to run continuously to keep up. Even a big tank doesn't provide more than a few minutes of heavier use--the tank pictured holds 26 gallons, which is only 3.35 CF. The hammers shown here are rated at 3.0 and 4.0 CFPM, so you could run two of the small ones more or less continuously, but two of the big ones would overdraw the maximum output of the compressor by two CFPM. It is easy to see that draining a 3.35 CF tank at 2.0 CFPM doesn't provided much of a buffer even at a low pressure setting.
Most of the contractor and home grade electric powered compressors you will find at a big-box store for $200 to $300 produce less than 3.5 CFPM, which is just barely enough for the small pneumatic chisel, but not enough for the big one.
At 3.5 to 6.0 CFPM, pneumatic chisel handles are on the low end of air consumption for pneumatic tools. Pneumatic grinders and sanders that can run wet, die grinders, drills, etc., are available, and are often cheaper and much more powerful than their electric counterparts. Pneumatic die grinders are appealing because they are powerful, you can't burn them out, safe to use around water, and cheap--you can get a decent one for under $60--but the smallest of them use about as much air as a chisel handle, and they go up from there. Angle grinders arehogs, and may require up to as much as 40 CFPM. Unfortunately, this level of air usage adds a decimal place to the price range for the required compressor, and the total energy cost goes up too--air tools consume more total power by far than do electric tools.
Compressors need regular oil changes, and you should check the oil level periodically. Consult the manual to find out what grade of oil yours requires. In most machines, the plug for the oil intake hole has a built in dipstick. Dirty oil, low oil, or the wrong kind of oil can will shorten the life of the compressor.
All compressors have an air filter that should be cleaned frquently. If you don't clean it, it quickly chokes up with dust and the machine has to work too hard to suck in air. Take the filter out, blow it clean from front and back with compressed air, and put it back daily, or more often, depending on how fast it gets dirty. No filter is perfect--it's best to locate the compressor away from dust sources to minimize the dust and grit it is exposed to. Ideally, put the compressor in a closet with an outside vent, as you would a water heater. This will keep the noise level down as well.
The compressor tank collects condensed moisture as the compressor runs. At the end of the day, the compressor should be turned off, and the water drained by opening the cock at the bottom of the tank, allowing the remaining air in the tank blow the water out. Leave the cock open until the tank is empty, then tighten it again.
Water also condenses when it leaves the pressure regulator. Most compressors have a built in water catcher after the regulator, but before the air hose connecton. These devices have a bell shaped jar beneath them where the water collects. There is a cock at the bottom to let you drain it periodically.
Most air tools require frequent oiling. Two drops of 3-In-One oil into the air intake every hour or two is ok for hammers and many other air tools. A better way is to install a line oiler near the point of use. These devices spray a little oil into the compressed air just before it enters the tool. Don't use this device on air lines you intend to use to spray paint!
One extremely nice studio accessory is fixed pipe on the wall to supply compressed air to several outlets, each with its own regulator, so the pressure can be regulated independently for different tools. This lets you connect the tools with thin, ligh, plastic air hoses, rather than the usaul heavy rubber hoses.
A second big benefit of this is that water can condense in a long air hose. A fixed pipe lets you put a second water catcher close to where the air will be used eliminating most of the water that would otherwise collect and end up spraying out your tool's exhaust port. The main pipe should be installed with a down-slope and a valve at the lowest point, so you can drain the water periodically.
Almost as handy as a pneumatic hammer are the many variations of rotary grinders. These tools are phenomenaly versatile, and come in many formats and sizes, and can be powered by either electricity or air. They range in size from heavy, two handed die grinders that can drive a heavy grinding wheel, or a ten-inch diamond saw blade, down to a Dremel tool the size of a tube of toothpaste, driving a bit with a diamond grinding wheel the size of a match head.
The physical hazards are much like those of pneumatic hammers: noise, dust and vibration, plus the added danger of severe cuts, abrasions, and electrocution. But again, it's easy to protect against these hazards if you use some sense: never work sitting down, keep your hair tied back, your shirt tucked in, and your sleeves squared away, and wear ear plugs, glasses, and a respirator. And wear padded gloves if there is significant vibration for long periods.
The aesthetic hazards of grinders are harder to protect yourself from. Of all the tools available to the stone sculptor, grinders pack the biggest danger to the artwork. They work a too well, doing what they do so effectively and effortlessly, that it's easy to get led around by the tool. Its probably fair to say that grinders are probably the leading cause of insipid sculpture.
These tools are characterized by small, one-handed size, and may be either pneumatic of have either a built-in electric motor. They work at very high speed (up to 30,000 rpm), and therefore are intended for use with bits and wheels of small diameter, usually under 1.5 inches. Within that limitation, they can be used with a huge variety of stones, steel or tungsten carbide cutters, diamond and fiber saws, drills, cut-off wheels, polishers, and other accessories.
They can have the shaft rotaion inline with the tool, or at a right angle, and may may either have a cylindrical shape or a pistol grip. The collet size is usually 1/4 or 6mm, but may occasionally be smaller or larger. They work very well for general purposes but tend to be somewhat clumsy because the motor is part of the tool. On the left in the picture below is an electric 1/4 inch die grinder, and on the right, a similar air powered tool. Both tools are priced at about sixty dollars. Air-powered grinders tend to be smaller, but they last a long time if they are oiled periodically when in use. Electric tools are much cheaper to run, but the brushes may need to be replaced occasionally.
Dremel , though a proprietary name, has become almost a generic term for small die grinders. Other companies make similar tools but Dremel dominates the market. They are light duty, but extremely versatile and flexible, and are manufactured in a in a range of grades.
These tools have a 1/8" collet, and a huge variety of bits are available for sawing, sanding, grinding, drilling, etc. For stone carving, they are only useful for small details. Buy a good one; they burn out fast under heavy use. A typical Dremel tool and accessories can be seen below. This kit sells for about sixty dollars. Buy the kind with a power cord, not the rechargeable models, which are not adequate for heavy use.
Flexible shaft tools are a heavy duty alternative to die grinders and Dremel tools. They have 1/4 to 1/2 horsepower stationary motors, providing as more power than an ordinary die grinders, with but with a smaller hand piece, because the tools are driven by a flexible shaft connecting the hand piece to the motor. The motors may be either mounted on the floor or hung from the ceiling, and are usually controlled by a foot pedal. Hanging configurations are more convenient.
Flexible shaft tools handle a wide range of bits that are interchangeable with other die grinders, and can usually accommodate a many collet sizes from 1/16" to 3/8". The prices for these tools vary wildly, from $50 to $750 or more. Two flexible shaft tools and accessories are shown.
These heavy-duty tools are useful for shaping, smoothing and polishing big, simple sculpted shapes as well as blocks for bases. They can also be used to drive cut-off wheels for almost any hard material. Be aware that different wheels are required for steel and stone.
Angle grinders may be powered by either electricity or by air. Air powered grinders consume large amounts of compressed air, but unlike electric tools, can be used with and around water. Some pneumatic grinders are equipped to supply a stream of water to the center of the cutting tool to clear the chips and keep the tool cool.
For granite and other hard stones a heavy duty angle grinder is almost an essential, both for carving and for finishing. Shown below. are some typical large angle grinders that can be used for sawing, grinding, and polishing hard or soft stone, or for grinding and cutting metals. Smaller versions that use 4 1/4" wheels are also common.
An angle grinder can be used to remove large amounts of stone quickly by cutting as illustrated
a series of deep, parallel cuts into the block using a diamond wheel. Use a hammer or a hammer and chisel to knock out the resulting leaves. If the cuts are not on a corner, it may be important to free the leaves on their ends with perpendicular cuts to avoid wedging the stone apart when you break them free.
Angle grinders drive a wide variety of cutting, grinding and polishing tools suitable for working with stone. Among these are:
The color of a grinding stone is significant. White, pink, and grey signify aluminum oxide, while green and black indicate silicon carbide. Aluminum oxide stones are suitable for soft stones, but will not cut harder stones like granite. The surface of some aluminum oxide stones may tend to pack with waste, reducing the capability of the stone to cut. Silicon carbide stones, also called "carborundum" will cut practically anything hard except diamonds.
The blades and motors of rotary tools, particularly the larger tools, can carry a lot of momentum. Should an abrasive or metal wheel bind in a cut, the tool can kick back hard. The operator should be in a comfortable, centered stance, feet well planted, knees slightly bent. Arms should be tight to the sides, in such a position that some of the force of a kickback will be transmitted through the back of your upper arm to the side of the chest--you don't want to rely on the strength of your arms alone for safety. Long hair must always be tied back, and your shirt tucked in tightly. Sleeves should be buttoned or rolled up securely. A shirt brushing against a blade can wind instantly around the wheel, yanking the tool toward the operator's body.
Another common accident is plugging in a grinder when the switch lock has been inadvertently left on, causing it to skitter across the table or floor. Trigger locks automatically release when you pull the trigger, so be in the habit of squeezing the trigger once before plugging in any power tool.
For obvious reasons, never operate an angle grinder sitting down, and never operate one in an uncomfortable or awkward position.
Wheels and bits can disintegrate because they are damaged or defective, or because they are rotating at a speed beyond their designed limit. A broken wheel can throw pieces very hard, so keep your face out of the plane of rotation. It's a good idea, to let a wheel run a briefly in a safe position before putting it to the stone.
In the age of product liability, everything comes with a warning label; do not confuse this one with the kind of warnings that come on toothpick boxes; these accidents happen often!
Polishers look like angle grinders, but are lighter duty. These are good for flat surfaces, or large curved surfaces, but they are not of much use for figurative sculpture.
Some polishers accept a water hose input, and are capable of running water into the center of the cutting head for lubrication and cooling of the cut. Usually, water-equipped polishers and grinders are pneumatic, but electric versions also exist which have water resistant bodies and ground-fault protection built in. Needless to say, wet work must not be attempted with an electric grinder that is not specifically designed to be used wet. Check the CFPM requirements of your compressor before spending money on a pneumatic polisher--they are hogs for air. Some typical polishers can be seen below. Neither is an expensive tool-under $150 for the wet polisher, and under $100 for the dry polisher.
A bench grinder is is an essential tool in the studio. The most common are electric powered, with two wheels, one on each end of the shaft, as pictured below. A wire brush wheel on one side, and a silicon carbide stone on the other is a good combination. The wire brush is good for removing rust and corrosion from metal, and the silicon carbide will grind both steel and carbide tools.
The trouble with powered wheels is that they spin fast, and heat tools very quickly, so it's easy to burn steel cutting edges. Keep a pot of water on the bench, as shown, so you can wet the steel frequently. As the steel gets thinner it heats faster and can burn is a split second--as you get closer to a fine edge, use a lighter touch, and only for a second or less, before backing off to let the steel cool.
A good way to practice is on old junk tools. As you grind, look for any discoloration near the tip as you work. The instant you see it, the hardness will have been removed. This will help develop a sense for how fast the metal heats. If you are making tools, this is not an issue, as you will be hardening and tempering them again anyway.
The final grinding should be done either by hand, with a coarse oil stone, or with a water wheel, as shown. The base of the water wheel is filled with water, and it is cranked by hand (the crank is on the far side in the picture.) Even though they are slow moving, wet wheels cut fast, because you can grind continuously, and it is almost impossible to burn a tool on a wet wheel. Powered wet wheels are the best, but they are very expensive and don't add much for stone tools, which do not demand the kind of precision grinding that one would want for woodworking tools. Water should not be left standing in the tools when they are not in use, as it can sometimes soften the stone on the side that remains wet, causing it to go out of true sooner than it otherwise might.
Almost all grinders come with an adjustable tool rest. It is not always necessary, and sometimes it is convenient to work without it. It has been temporarily removed from the powered grinder, but the wet wheel is shown with it mounted. More expensive grinders often have a quick-release catch for the tool rest, so it can be removed and replaced easily. This is a feature worth paying a little more for.
Always inspect wheels carefully when remounting on a powered grinder. Verify that the rotational speed is consistent with your motor speed, and check visually for any damage. Do a tap test as well. A cracked wheel will usually have a distinctive dead sound, like a cracked bowl or cup. If you discover that a wheel is cracked, break it with a hammer to prevent reuse. The pieces can be useful on both steel and stone.
After being used for a while, wheels can cake up with residue of metals (especially if you grind non-ferrous metals) and they can develop grooves, rounded corners, etc. All of this can be fixed with a dressing tool, which come in two varieties. Diamond dressing tools cut a new surface. They can consist of a single diamond on a long handle, or an edge with several diamonds mounted on it. They are rested on the tool rest, and the diamond(s) held to the wheel and moved from side to side to shave a layer from the surface. If the wheel is out of round, a fixture can be used to hold the tool steady enough to cut it back to where it is a perfect circle with the hole exactly in the center. This cannot be done without a fixture of some kind.
The other kind of dressing tool consists of a heavy handle with a cylindrical stack of star shaped wheels on the working end. The handle is held to the tool rest, and the wheels pressed to the spinning stone. Because of the gaps, they impact the stone hundreds of times a second, acting like a precision bush hammer against the point where the cylinder touches the wheel. The wheels on the tool grind down eventually, but they are replaceable.
The picture below shows a range of standard drills for the studio. A one-handed 3/8-inch cordlless drill
is handy in the studio for drilling holes in wood and driving small screws. The use of these drills is limited by their light motors, but even more by their keyless chucks. The chuck is the part that holds the bit. Keyless chucks consist of two rings, which turn in opposite directions, and tighen three jaws around the drill bit to clamp it in place. A keyless chuck is quick and easy to use, but limited in how tightly it can hold the bit, and therfore, how much torque it can apply when drilling with conventional bits. This can be obviated by using bits with hex shafts.
The second drill is a 3/8" corded version, also with a keyless chuck.
The third and fourth are 1/2 drill. These are necessary for heavy work, such as driving large screws, drilling holes in stone, boring holes in timbers, mixing plaster and cement, and similar tasks require a lot of torque. They can look somewhat like a 3/8" drill, or they can have a D handle in the rear and straight handle sticking out the side. The two different formats are pictured.
If a 1/4" or 3/8" drill binds in a hole, you hand will usually easily stop the drill, but 1/2" drills are much more powerful, and are stronger than your wrist at full power. Therefore, even the pistol grip types usually have a detachable handle on the side for applications where a lot of torque is applied.
Good 1/2" drills often have a switch that lets them function as hammer drills. This is a feature worth paying for. The red drill in the picture has this feature. With the hammer feature, these drills have enough power for the majority of stone drilling in the studio, and quickly switch back and forth, which can be very convenient when drilling marble. A drill is often used in conjunction with carving, but with marble, you must not hammer drill close to the finished surface, because the impact can leave deep permanent bruises in the stone. With a switchable drill, you can drill most of the hole with hammering turned on, then switch it off for the last inch or two.
Conventional drills with a switch that lets them also hammer, while they are quite powerful, are the least powerful class of drills for this purpose. True hammer drills are the next step up. Thse drills are intended primarily for masonry. They still have a conventional chuck with three jaws, like regular drill, but are intended primarily for masonry work. Some of them rotate as much as four times faster than ordinary drills in hammer mode, and they are considerably more capable on stone, but are not really indended to be used as drills for wood and metal.
At the top of the heap are true rotary hammers, which are very different from conventional drills or hammer drills. The most obvious differences center around the chuck. The masonry bits used by an ordinary drills and hammer drills have a plain cylindrical shaft which is gripped tightly by the chuck. In contrast, the rotary hammer takes an SDS-style shaft, which fits loosely in the drill, and is slotted, so that it can slide back and forth, but cannot fly free. This allows the hammer mechanism to impact only the drill, without having to also move the chuck and shaft. A rotary hammer and some typical SDS bits are pictured below.
Most rotary hammers can be set to drill-only or drill-and-hammer. Some can also be set to hammer only, without rotation, which is a nice feature. A range of bits such as punches, chisels, and spade--like chisels are available that let the rotary hammer double as a demolition hammer. These tools are too clumsy to be useful as general purpose carving tools, but their large size gives them a lot of impact, and they can be useful for special purposes.
Old fashioned star drills that you tap with a hammer have power-driven cousins that are intended to be used with a pneumatic chisel handle. The drill tip shown below. consists of a plain shaft with a chisel-like carbide blade across the tip. Like a manual star drill, it is twirled with the fingers as it is danced against the stone, and gradually pulverises a round hole into the stone. These drills are not usually used for utility work such as drilling holes for splitting, but are often used more often for holes that are part of the carving. They are convenient because they are driven by the same pneumatic handle that the sculptor is using for the rest of the work.
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