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Some extremely useful additions have been made to the marble sculptor's tool kit in modern times.

The Pneumatic Hammer

Probably the most useful new addition is the pneumatic hammer, invented by Willam Holden, of Barre Vermont, in 1888. Two modern Trow and Holden pneumatic carving hammers, medium and small, are pictured below.

Two sizes of air hammers and some of the many tools they can drive.

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 relative 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.

Bushing heads for pneumatic hammer: bush and cup (left), star (right), granite tip.

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:

  • Useless for hard stone.
  • Cheaper to buy, but many have much shorter working life, and all require much more maintenance.
  • More acute edge angle means better penetration for some tools, and more stone removed for a given amount of impact.
  • Better working feel for delicate carving.

Carbide tipped tools:

  • Work for both hard and soft stone.
  • Last many times longer if used correctly, and require much less maintenance.
  • Some tools can be more fragile if used carelessly, e.g., claws.
  • Can be used with higher power settings.

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.


Most compressors that you're likely to run into, regardeless of size or horsepower, produce air pressure of between 100 and 150 pounds per square inch (PSI), which is plenty for almost any pneumatic hand tool. Compressors for workshops and garages usually supply up to about 140 PSI, so inadequate pressure is rarely an issue.

What you do have to beware of is that compressors differ wildly in the volume of air they supply. Air volume is measured in cubic feet per minute (CFPM) which is a somewhat confusing measurement. The important thing to remember about CFPM and PSI is that compressors are like trucks--they all have similar top speeds (PSI) but a tractor-trailor can haul a lot more cargo (CFPM) than a Toyota pickup truck.

The input air, i.e., the air in the room, is by definition at one atmosphere of pressure, which is about 14.7 PSI. A compressor rated at 6 CFPM will suck in six cubic feet of air (about 45 gallons) every minute and squeeze it into the tank at the rated pressure.

If the compressor is set to compress up to 120PSI, the air is being compressed to 120/14.7=8.16 atmospheres of pressure. The volume a given mass of air will fill is inversely proportional to pressure, so the tank will hold about 8 times as much air under 120 PSI of pressure as it would if the tank were open to the ambient one atmosphere.

The meaning of the CFPM rating of a tool is often misunderstood. If a tool is said to consume x CFPM at a given pressure, it does not mean that the tool will consume x cubic feet of compressed air every minute. What it means is that in each minute it will consume a mass of compressed air equal to x cubic feet of air at one atmosphere of pressure. This is the same thing we are saying when we talk about the CFPM of the compressor--it is really a measurement of mass masquerading as a measurement of volume, because it is the same even if higher pressure reduces the volume of the air. The smaller volume of compressed air going into the tool expands to x CFPM of air as it is exhausted from the tool.

Note that the CFPM for a tool changes with the pressure, so values for the minimum and maximum pressures are usually given. CFPM for tools goes up as the input pressure increases because more air is crammed through the tool. Conversely, CFPM for compressers goes down as the tank pressure goes up because it takes more power to pack in a given volume of input air against the pressure in the tank.

Horsepower refers to the power output of the motor that drives the compressor. In theory horsepower should correspond closely to CFPM, i.e., a compressor with double the horsepower should give double the CFPM, but in practice, both the nominal CFPM and horsepower ratings should be taken with a grain of salt (they are notoriously exaggerated.) In practice, if you're shopping for a compressor you should expect to get about 3-4 CFPM per HP at 80 PSI.

Compressors also differ in tank size. Most compressors don't supply air directly out of the pump, but instead put the air into a large intermediate tank, which serves as a buffer between the pump and the tool. The tank serves several purposes, but the most obvious to the user is that it allows you to consume air for a short time faster than the compressor can supply it. If you have a good size tank, you can have lots of intermittent users hooked up and even if once in a while they all happen to hit the trigger at once, the air presssure will remain adequate even if their aggregate use exceeds the capacity of the pump for a few minutes.

Another factor that affects what you get out of your compressor is "duty-cycle," which is the maximum percentage of time that a machine is rated to run. If the duty cycle is 50%, then the compressor should be off at least half of each hour, or you risk wearing it out. For this reason, simply comparing the air consumption of the tools with the CFPM rating of the compressor isn't enough if the compressor will be heavily used. The phrase "continuous duty" means that the machine's duty cycle is 100%, i.e., it is designed to be able to run indefinitely and does not need to rest.

A typical five-horsepower shop compressor capable of running air tools.

The compressor motor has only has one setting--flat out. If the tank pressure falls below a set amount, say 90 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. The differnce between the upper and lower pressures means that unless the air is being used at a high rate, the compressor automatically gets a lot of resting time, as it fills the tank and then rests for a while while it drains.

Should the cutoff mechanism fail for any reason, the maximum 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, 150 PSI. Above this maximum, the air pressure in the tank will automatically lift the valve and bleed off the excess. These valves usually also have a ring that can be pulled to manually empty the tank.

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 most tools require. 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 pressure. The compressor pictured here puts out 6.6 CFPM at 40 PSI, and 5.8 at 90 CFPM (note that at higher pressures the CFPM goes down.) This range easily covers the typical operating of the pneumatic hammers shown above, but would barely cut it for a large hammer, and wouldn't come close to being 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, except for occasional brief spikes in usage.

The more intermittent is the tool usage, the more a big tank extends the number of tools that can be supported, but carvers tend to use tools like chisel handles and grinders, that stay on continuously, so the CFPM rate of the compresser must be more closely aligned with the tool ratings than it would in a gas station or wood shop.

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 or a big one and a small one more or less continuously, but two of the big ones would overdraw the maximum output of the compressor.

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.

Pneumatic angle grinders, sanders that can run wet, die grinders, drills, and a host of other tools are available and are often cheaper and more powerful than their electric equivalents. Pneumatic die grinders are very appealing because they are powerful, won't burn them out, are safe around water, and are cheap--you can get a decent one for under $60.

The catch is that at 3.5 to 6.0 CFPM, pneumatic chisel handles are on the low end of air consumption for pneumatic tools. Small drills and grinders typically use even more air than a chisel handle and the bigger tools, such as angle grinders, are utter hogs, requiring as much as 40 CFPM. Supporting this level of air usage adds a decimal place to the price range for an adequate compressor.

Even if you have a big enough compressor, the energy cost to run pneumatic tools is relatively high. It takes about seven or eight times more electricity to compress the air to generate a given number of horsepower than it than it does to generate the same horsepower directly using an electric motor in the tool. The price of air tools is appealing, but if an air tool is heavily used, the cost the power to run it quickly obviates any advantage.

Bottom line: you will definitely want to the pneumatic hammer as a workhorse, but it's more economical and environmentally friendly to favor direct electric power for most other uses except in special cases, such as working around water, or for occasional use when the compressor is already spun up.


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 will shorten the life of the compressor.

All compressors have an air filter that should be cleaned frquently. If you don't clean the filter, it 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 at least daily, 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 that 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 in mny other places, expecially 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.

Tool Operation

Most air tools require frequent oiling. A drop or two of 3-In-One oil into the air intake every hour or so 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 that 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 tends to 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 slight down-slope and a valve at the lowest point, so you can drain the water periodically.

Rotary Grinders

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 ten-inch grinding wheel or diamond saw blade, down to a Dremel tool the size of a tube of toothpaste, driving a 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. 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. Grinders are one of the leading causes of insipid sculpture.

Die Grinders

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.

Electric die grinder (left), pneumatic die grinder (right).

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.

Some Dremel tools, with accessories.

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.


Two popular flexible shaft tools and accessories.

Angle Grinders

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 air. Air powered grinders consume large amounts of compressed air, but unlike most electric tools, can be used with water. There are also electric grinders that are designed to be used wet. Whether electric or pneumatic, grinders that are specifically designed to run wet will almost always have a hose coupling and valve to supply a stream of water to the center of the cutting tool.

Beware that wet grinding requires wheels specifically designed to run with water.

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.

Some typical seven to nine inch angle grinders.

An angle grinder can be used to remove large amounts of stone quickly by cutting 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 is important to free the leaves on their ends with perpendicular cuts, in order to avoid wedging the stone apart when you break the leaves off.

Some angle grinders can be equpped with a dust shroud that can be hooked to a shop vac.

Battery powered angle grinders also exist, but are not suitable for the extended use that is typical in the studio. They are also relatively expensive.

Angle grinders drive a wide variety of cutting, grinding and polishing tools suitable for working with stone. Among these are:

  • Solid Tungsten Carbide Burrs: These are rotary files and rasps cut from solid tungsten carbide. They are extremely hard and capable of shaping soft and hard stone very quickly.
  • Diamond Burrs: Come is a range of sizes and shapes similar to those available for carbide burrs, but have an abrasive surface composed of diamond particles. They shape any kind of stone and are extremely durable, but cut more slowly than carbide.
  • Abrasive Stones: Also come in range of shapes and sizes similar to the those available for carbide burrs, but are solid, rock-like concretions of silicon carbide, aluminum oxide or other abrasive. They are slower cutting and change shape as they wear. Beware the differences in hardness--silicon carbide, also known as carborundum is necessary for granite-like stones. Silicon carbide, and aluminum oxide will both work on marble and softer stones.
  • Cut-Off Wheels: These function like circular saws, but usually cut by abrasion. They may be metal, with diamond or carbide grit bonded to the edge, or fiber impregnated with resin and abrasive particles. They are capable of cutting into any kind of stone. Diamond wheels are most effective on hard stones. The metal wheels remain the same size throughout their useful life. Fiber-resin wheels wear down quickly until they will no longer reach the work-piece.
  • Flexible Abrasive Disks These are made of heavy plasticized cardboard coated on one side with abrasive. They run mounted over on a rubber backing disk, may be used to work the surface of very soft stones. In general are not very good for stone work.
  • Cup wheels: These rigid metal cups have the abrasive on the rim and are useful for smoothing and flattening the sides of stone blocks and slabs. They are also used for smoothing compoundly curved surfaces. The cup configuration is available in most grinding media.
  • Kutzall wheels: This is a brand-name for a line of structured carbide cutting heads available in many shapes and sizes. They have a very rough texture composed of carbide bonded with softer metal, and are great at ripping through large quantities of stone (or almost anything else.) They come in numerous shapes and degrees of coarseness for both angle grinders and die grinders.

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.

Operating an Angle Grinder

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 tack-hammers or boxes of toothpicks; 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 typical rotary polisher (left) and an electric wet polisher (right).

Bench Grinder

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 this kind of dressing tool wear down, but they are replaceable.

A powered bench grinder with wire wheel and silicon carbide stone (left) and a hand-cranked aluminum oxide wet-wheel.


The picture below shows a range of standard drills for the studio.

Drills: 1/4" cordless, 3/8" standard, 1/2 with switch for hammering, heavy duty 1/2" with D handle.

The first is 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 drills. 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.

Typical rotary hammers.

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.

Non-rotary Drills

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.

Percussion drill manufactured by Trow and Holden.