February 6, 2012

Choosing‚ And Using‚ Abrasive Stones

From the hand-held Arkansas stone to the high-dollar ScotchBrite wheel, your options for smoothing metal are wide open.

 

Smoothing metal with abrasive stones is one of the most important operations a gunsmith can perform. It also has ranked right up there with dental surgery on my list of favorite entertainments.

Smoothing Metal with Abrasive Stones

Courtesy, American Gunsmith

Arkansas was known for its abrasives well before the 1992 election, including, far left, a flat Arkansas stone. Other abrasives include, from left, a round India, a square Carborundum and a triangular ceramic alumina stone. Lower right, the author holds a flexible stone suitable for working on curved surfaces or in hard-to-reach places. They are made of aluminum oxide bonded to a flexible spine.

I always considered stoning one of the necessary evils associated with metalworking and went to great lengths to minimize my association with stones. My subconscious might be partially responsible for my attitude, as I really hate to put out my hard-earned money for something that vanishes while I’m expending so much energy. I forced myself to overcome this attitude when stoning saved some lengthy projects from the frustration container I line with garbage bags.

Before we get into their different uses, let’s first look at some of the types, shapes and compositions of abrasive stones, along with some do’s and don’t’s concerning their use. We have to know which stones do what to choose the right one for the job. Learning the correct use of the different types and shapes of stones didn’t come easily, but the experience has repaid me many times over.

Composition And Cost

Stoning is, by definition, the use of abrasive pieces of synthetic or natural stones which have been shaped for particular jobs. The larger the granules, the coarser the cut of the stone. The shape of the granules is also important in determining the smoothness of the finish left by stoning. Also, the hardness of the stone is as important as its cutting ability, for hardness determines the formability of the stone, including influencing how the stone resists forming when you don’t want it to.

Smoothing Metal with Abrasive Stones

Courtesy, American Gunsmith

A Dremel Moto-Tool, shown with an assortment of mounted stones, Cratex, cutoff wheel and sanding drum, is a favorite in the author's 130-degree gunsmithing shop.

There are times when files just cannot do the smoothing required in gunsmithing; yet, seemingly less precise stones can do the job. Even diamond files, the only ones that can cut into hardened steel, are really stones with a steel core. You cannot cut dead-hard steel with common files, yet almost any abrasive stone will do so easily.

Natural stones come from the earth, having undergone great heat and pressure over a long period of time. The grade of the stone is determined by the amount of foreign minerals present. Coarseness is determined by the loose proximity of the granules with respect to each other, while hardness is a matter of chemical makeup and physical structure.

Arkansas stones, as they are known in the trade, are actually novaculite stone. Per their name, they are usually found in and around Arkansas and are the most widely used natural stones. Arkansas stones are fine-cutting and are usually white, resembling white marble. The gunsmith can easily feel an Arkansas stone’s smoothness, and he can predict the finish to be correspondingly smooth. These stones are fairly highly priced, because they require a lot of energy to be expended in their shaping.

In contrast, India and Washita stones are generally dark and coarse and remove larger amounts of material than the Arkansas stone. I have heard of the Washita stones, but I haven’t used one. They have been so overshadowed by the India stones that little is written about them. Most of the softer stones are India. Found in black and a red, sand color, India stones are natural aluminum oxide and are usually subjected to a man-made process called sintering to form them into useful shapes. Available in fine, medium, and coarse grades, they generally cut faster than hard Arkansas stones. I like these stones for smoothing curved surfaces and for forming a stone with which to get into crevices. These, like most natural stones, are also somewhat expensive compared to artificial stones. But the modern processes of sintering them into shapes reduces the cost below that of Arkansas and polycrystalline Ruby stones.

Smoothing Metal with Abrasive Stones

Courtesy, American Gunsmith

When stoning the inside of a flared magazine well with a coarse, soft stone, move the stone linearly and axially and use water liberally to remove fouling.

More expensive than the India stones, superhard polycrystalline Ruby Stones are especially suited for working hard metals like tungsten carbide and hardened tool steels. They come in three grits, all relatively finer than those grades in other stones. With the advent of ceramic alumina stones, I find it hard to pay for high-priced Ruby Stones, but they will give a beautiful finish.

Further along the finish scale is the ceramic alumina material spawned by the U.S. space program. This ceramic material was developed for use in the thrust nozzles of rocket exhausts due to its great heat tolerance. It was also required to withstand the incredibly corrosive effects of the fuel, unsymmetrical dimethylhydrazine, (UDH2). All fuel lines, valves, and other components that come into contact with this fuel need to be gold plated to withstand its corrosive effects. The final finish obtained from these synthetic stones is phenomenal, but compared to what other natural or artificial stones do, they remove a minuscule amount of material.

Carborundum is the trade name of a material and the manufacturing process of abrasives manufactured by Union Carbide Corporation. Used for most grinding wheels and inexpensive sharpening stones, it is easily formed into precise shapes by pressure and heat. Thus, it does not require the expensive carving necessary when shaping natural stones. This material is usually fairly coarse and cannot compete with the finish left by the natural Arkansas stone or the space-age ceramic alumina material. Its main attractions are its relatively low cost, the amount of material it can remove, and its ease of manufacture.

Diamond stones are a cross between natural and artificial stones, in that real diamond particles are embedded into a nickel coating on an unbreakable plastic or metal base. They are used with water, not oil, and should last indefinitely because they don’t get dull. They cut faster than other stones and are generally available in two textures, coarse (325 grit), and fine (600 grit), and in the form of bench stones, laps, or files. Their cost makes me keep them locked away so that they will not be used in place of regular, cheaper stones.

Smoothing Metal with Abrasive Stones

Courtesy, American Gunsmith

Dykem steel dye is often used to identify high areas when stoning with a flat stone. For the gunsmith on a budget, dark permanent-felt markers serve as well.

How To Buy And Use Stones

Because stones come in so many shapes and sizes, the gunsmith can be confused when he starts buying stones for the shop. I would recommend buying the following shapes in this order: a sharp-cornered square of about 1/4 inch per side, a triangular stick of about the same size, and a flat stone 3/4- by 3/16-inch in cross-section. I find sticks about 5 inches long easiest to handle, while those used to level a larger flat surface need be around 6 inches in length.

Starting gunsmithing requires more of the fine-cutting stones than coarse ones, since the type of jobs a starting gunsmith would take on don’t usually require removing any significant amount of metal. I use the coarser stones more than most gunsmiths because I hate filing, then I use moto-tools with Cratex to do quite a lot of my finish work. I often use fine or medium stones that have wide, flat surfaces to level out flat planes such as the sides of 1911 slides. I also have found that it is economical to glue sandpaper to a stick and save wear on the more expensive stones. I would suggest that a starting gunsmith start out with some Arkansas stones of the proper shape for trigger jobs and a couple of Carborundum stones for quick metal-removal chores.

I avoided soft stones when I saw them crumble away so rapidly, but soon found them of as much value as the harder, flat stones. That they crumble away allows them to conform to an irregular surface and be contoured into the proper shape with which to work in corners more readily than harder stones.

Smoothing Metal with Abrasive Stones

Courtesy, American Gunsmith

Stoning a cylindrical piece of metal in a drillpress requires the gunsmith to use light strokes with the wet stone to avoid bending or possibly breaking the rod.

Harder stones hold their shapes better and retain the perfect angle cut into them by the manufacturer. They excel in leveling surfaces in a way that allows the ‘smith to see the surface approaching flatness. For this work, it is handy to have a hard India stone with a sharp 90-degree corner for making sear notches and other critical angles true.

However, the wide, flat stones get most of my attention, because a lot of my work involves leveling surfaces. For this, I use the medium-hard Carborundum stone, or when features allow, stearite sandpaper on a flat surface. The sandpaper is more economical than the stones, and I can follow the sandpaper with a finer stone if necessary. I have also found the white stearite paper cuts faster and finishes smoother than the grade indicated on it. It is also longer lasting than other types of papers.

I recommend this type of sandpaper for applications requiring 100-grit to 320-grit metal removal—and I recommend 3M papers exclusively. Having owned automotive body shops and run manufacturing plants in the past, I have found an appreciable difference in the papers made by 3M and those by Bear-Manning, Norton, and Bay State. My sandpaper bills were reduced by almost 50 percent when I changed to 3M; the workers noted the reduced energy required to do their jobs; and the quality-control examiners lauded a corresponding increase in finish quality when we used 3M papers.

Motor Tools and Stones

There aren’t many tools that come into use around a gunsmithing shop more than the small, hand-held rotary grinder, generically called a motor tool. The thing most of the brands have in common is their 1/8-inch shaft size, which keeps gunsmiths from being limited to only those tools made for particular grinders.

There are many brands of motor tools on the market, including one you can charge on the Sears card, but I have found the Dremel Moto-Tool to be the best. The Moto-Tool, Dremel’s copyrighted name for its motor tool, is the most widely used and for good reason. Dremel has been building hobby handgrinders longer than anyone else on the market. Foredom has been making hand-held flexible-shaft tools for a long time, but they just don’t lend themselves to gunsmith use as well as does the Dremel tool. In my opinion, gunsmiths should celebrate Mr. Dremel’s birthday, because there is no end to the chores a motor tool can perform. For stoning jobs in my shop, the Dremel gets fewer days off than a priest.

The gunsmith may buy Dremel Moto-Tools as stand-alone products or in several types and sizes of kits. There are motors with bushings, ball-bearings, single speeds, two speeds and variable speeds. Dremel also offers a kit with a flexible shaft and handpiece. There are some interesting accessories also offered, such as the router attachment, drill press and other fixtures not really aimed at the gunsmith. The router attachment is well suited for woodworking and hobby use, however.

The Dremel-type rotary stones can be shaped with the rectangular, gray stone found in the kits by spinning the rotary stone at a high speed and holding a corner or edge of the shaping block against the stone. This technique is much like using a wood lathe, in that the workpiece is spinning and the shape of it is influenced by manually holding a cutting implement against it judiciously.

Smoothing Metal with Abrasive Stones

Courtesy, American Gunsmith

A soft stone is used when cleaning up after silversoldering on a custom front sight, so that it will contour into the square joint.

I keep my rotary stones separated into two distinct groups, the hard and the soft. It is apparent at a glance which stones are coarse and which are fine so they can be mixed together without wasting your time searching for the proper stone for a job. More often than not, I gravitate to the one nearest the desired grit and hardness and in the best condition. Nearing the end of the life of my sampler collection, I have quite a few stubs in the bottom of the box, and I try to use them when I can bear the extra strain it places on already tired hands. Handling small pieces of abrasive does little for the skin or the muscles of the fingers and hands. I find myself buying new stones the day after I have made myself use up the bits and pieces of stones.

Of course, all power tools should be used only when wearing safety glasses or a face shield. In hand stoning, this isn’t necessary unless you happen to be incredibly fast.

Using Popular Stones

The most commonly used stone in a pistolsmith’s shop will be the flat, hard, fine stone with a 90-degree corner to be used for sear and sear-notch work. I also use diamond-coated files because they cut harder metal and leave a fairly smooth surface that can be touched up with Cratex wheels. (Note: Take care to use water with diamond files. They are expensive, and the diamond coating will be quickly removed without water to lube the way.) When I must use a stone for sear or sear-notch work, I usually choose an ceramic alumina stone for its hardness and fineness of cut. If the sear or notch is rough enough to require a coarser stone, you should consider case hardening the surface.

Another popular stone is the 3/4-inch-wide, 1/8-inch-thick medium-cut stone used with plenty of water to flatten a surface. I use this whenever I am unable to level a surface with flat-glued sandpaper. There might be some protrusion that prevents the use of a large, flat abrasive surface, so this stone is a must. I wash stones like this with a good handcleaner and a fingernail brush under running warm water. It would be a good idea not to use the spouse’s vegetable or nail brush because the stone tends to work well on bristles, also.

I keep several grades of fine-cutting round stones around to use inside shotgun bores after installing a bead sight near the muzzle. Marks from these are much easier to polish out than those from a Dremel sanding drum. The sanding drums, wrapped with a tissue liberally laced with semi-chrome polish, are good for the final polishing of the shotgun muzzle’s interior.

Another good use for abrasive stones is against rotating parts. This confines their use on round surfaces that are concentric about an axis which can be chucked up. I use the stones to give a final finish to many of the parts which I make with my lathe, and even use the stones with my drillpress projects. As with filing, caution must be exercised when using a handheld object against a rotating part; not only with respect to the workpiece, but with the spinning chuck as well. Letting the end of the file or stone be hit with one of the chuck’s jaws can cause it to effortlessly ventilate one palm or knock a thumb out of joint. Either is bound to leave an impression on even the most unimpressionable gunsmith.

It is imperative to use water or oil as lubricant and anti-filling agent when stoning a rotating part. Use normal filing motions; the stone will load up immediately if you don’t. Also be aware of the surface speed because the rotating part increases the relative speed between the stone and the surface of the workpiece by several magnitudes. The larger the workpiece diameter, the greater the difference of the relative surface speed. I use this technique when dressing the end of a threaded part made in the lathe or in breaking the sharpness of an edge cut on the lathe.

Brownells sells some useful items called Flex-Stones, which can be used in quite a few applications that call for curved surfaces and hard-to-reach places. They are aluminum-oxide particles bonded to a flexible spine that conforms to curved shapes yet is stiff enough to transmit the pressure needed to remove material. They are 1 millimeter thick (.039 inch) and last well if not subjected to sharp edges or solvents.

When fitting parts or trying to achieve a perfectly flat surface, it is useful to dye the surface to be stoned with an minute coating of color to readily show the low spots. The mainstay in the machining industry is a dye named Dykem. It is so well entrenched that the generic name for surface dyes is dikem.

In the abrasive-stones section of the catalog, Brownells offers a Prussian Blue, a paste used mainly to check the fit between parts. It also can be used to locate low areas of a flat surface, but Dykem is better for this purpose. Dykem comes in many hues, but intense blue is the standard of the metalworking industry. It is an alcohol-based lacquer that smells good and gives the ‚Äòsmith a high if he isn’t careful. Being a lacquer, it is waterproof and needs acetone or Dykem Remover to remove it. It comes in either spray can or liquid in poly bottles with a squeeze-bulbed brush applicator/cap.

For those of us who are stingy to a fault, a good felt marker in appropriate color is almost as good as the relatively low-priced Dykem, even though they ultimately cost a lot more. The advantage of felt markers is their small initial cost for those just getting their feet wet in the business.

Coat the part to be stoned flat with whichever dye method you choose and commence stoning. A moment should be taken to think out which direction the abrasive tracks (scratches) should be oriented in order to lessen the chance that they will be visible after stoning. For instance, on semi-auto pistol slides, the stoning should be from end-to-end with the stone parallel to the length of the slide. This is the direction in which any unevenness of the surface will show up. After several strokes with the stone, the bright metal colored areas will give away the low area’s hiding places at a glance.

Stoning, with water, should continue until the high areas are reduced to the level of the last remaining dye spot. You might then follow this with some final stoning with a smoother stone or a trip to the ScotchBrite wheel. This wheel and its use will be described later.

The same technique is used on curved surfaces. Just think out what the stone will do at its worst and orient it in such a way to lessen the effect. Strokes along a curved surface tend to cause me less trouble than trying to rock across the curve. These minute flat planes are easily eliminated, while low spots caused by the edges of the stone while rocking across a curved surface can be murder to eliminate. Again, just close is enough if you have a good buffer equipped with a ScotchBrite wheel.

When using the dye method to find interference between mating parts, you should put the dye on the part that seems to need the metal removed. After examining the interference mark(s), it is more easily determined how one should go about removing the offending metal. A scratch all along the marked part would indicate to me that I had dyed the wrong part. It also indicates a spot or area on the mating part would be the culprit which needed rubbing out. Remember, the dye is so thin that it doesn’t really enter into the picture, fitwise. Remove the dye from the first dyed part, and dye the mating part in the suspected area. Repeat the mating action between the parts and notice the interfering area or spot now. Stoning where there is an interference between parts should be done with the stoning marks diagonal to the action of the reciprocating or rotating part. This enables you to spot any rub marks and allows the marks to carry lubrication.

A Stone In The Hand

There is a natural way almost everybody finds to hold abrasive stones, but this might not always be the best way.

It seems as if the natural way to hold them always results in their being broken into shorter pieces that defy further use. The best way for me to combat finger and hand fatigue is to hold the stone through the palm and little finger with the tips of the next three fingers resting on the stone, and the index finger pushing the stone into the workpiece. With this hold, you should only apply maximum pressure to the stone when the index finger is over the point of contact with the pressure lessening with the reduced proximity to that point. Retaining the same pressure throughout the stroke will result in broken stones.

The preferred motion depends on the type of stoning and the shape of the work surface. When stoning a surface like the inside of a trigger guard, it helps to add a slight rocking motion to the natural back and forth stroke because most trigger guards have cross sections similar that of a football. If the trigger guard has flats across the inside, the rocking motion would not be appropriate.

Instead, this is a good place to use the soft, shape-conforming stones that will crumble until they match the contour of the workpiece before really working. Some soft India stone about a 1/2-inch across and a 1/4-inch thick is particularly well suited to the task. The thickness is for strength and to give the stone some material with which to conform to the trigger guard shape.

On large, flat surfaces, there are spots where stoning marks parallel to the long direction of the flat would suffice. But just as often, you will want to apply the stone in circular motions much smaller than half of the distance across the flat. These circular motions should orbit around the flat area in another, larger circular path with an ever-descending radius.

This ensures that the stone attacks the metal from a different direction each time it comes back to the same place. The path should overlap a different portion of the circle with each reduced circular path.

The arch on the tops of slides makes me want to stone in a criss-cross pattern so I am compromising a linear path and an arched path. As mentioned before, the linear paths would cause small flats to develop along the length of the slide and stoning across the radius of the top might induce some edge marks from the stone. The criss-cross pattern is a good way for the gunsmith to prepare for final surface treatment.

One Other Abrasive Tool

There is no other single tool in my gunsmith shop more appreciated by my students than the buffing motor with its ScotchBrite wheel.

I’m afraid that I concur and might be guilty of overusing it. Since their cost rivals the national debt, I cringe every time I hear the Baldor buffing motor bog down even the slightest. This wheel is an adaptation of the kitchen scouring pad but is firmer and more tightly secured. It does the same thing, just in an accelerated manner.

There is nothing in my experience that will remove irregularities, burrs, blueing and rust, silversolder overruns, and a host of other maladies as well as the ScotchBrite wheel. It also deburrs the ends of threaded devices, blends in faceted stoning, and prepares parts for buffing better than anything else available. The bad news is the wheel’s cost. I was paying $38 for a 6-inch by 3/4-inch wheel in Los Angeles and now pay about $32 at a good industrial hardware store in San Diego. I’ve tried several different wheel brands but have found none that suit me as well as the ScotchBrite. The pads, however, are not much different from the abrasive-impregnated pads of other manufacturers, in my view.

I use them for everything from re-brushing brushed stainless guns to finally smoothing things on a lathe or drillpress prior to buffing. The grit, denoted by the color of the pad, most used around my shop is the maroon ScotchBrite material, approximately equivalent to 320-grit white sandpaper. It gets impregnated with the metal it has contacted, so I cut it into small pieces dedicated to a particular material’s use. The cost of the impregnated pads is not much more than the cost of sandpaper, and it lasts longer. It does not work well with any liquid, as it tends to overcut when wet.

When Not To Use Stones

There are situations in which you should avoid using stones if there is any other way to do the job:

I usually try not to use stones around the muzzle for fear they are too coarse, too hard or too anything which could alter the shape or surface condition of the muzzle.

However, I do use long, thin stones to touch up the inside of the bore at the bottom of the ported holes when using a mill or a drill with which to make gas ports. In this case, it is important to use a hard, fine stone to keep from damaging the rifling. I force a copper-jacketed slug past the ports before stoning to remove what burrs I can.

Stones, unless they are very coarse, aren’t used very often on wood. Not only will the stone develop heat that will burn the wood, the wood will likely load up the stone, make it stop cutting, and start burning anyway. A rotary stone can usually be cleaned of wood by heating it with a propane torch or the low flame of a welding torch. This turns the wood into ash, which can easily be scrubbed out of the grain of the stone.

Aluminum, pewter, magnesium, lead, tin, or zinc alloys will load up a stone permanently.

Aluminum is the worst offender of the non-ferrous metals, bonding with such ferocity that it takes re-dressing the stone to remove the clumps of aluminum embedded in the work surface. These aluminum clumps tend to scratch steel badly and can ruin some otherwise good work in the blink of an eye.

Similarly, magnesium fouls up a grindstone, and it also flashes off or explodes when steel or iron are used on the wheel afterward. In essence, the wheel is just a large flashbulb without a housing. The same goes for buffing magnesium. It causes so many explosions in the buffing industry that few companies making automotive racing equipment can find shops that will risk working with the metal.

Keep Stones Clean

Using water or oil to lubricate the stones and float away the fines, metal particles that are rubbed off, is imperative. The stone will load up and scratch the workpiece without this liquid to clean the pores as you stone away. Dipping the stone into the water bath or oiling it cannot be done too frequently. I prefer water because it’s cleaner and easier to remove after use. It also is easier to wipe from your hands before picking up something else.

I use oil only on my bench stone but water on all hand-held stones. On my motorized diamond lapping wheel, I use a glass cleaner like Windex. This seems to loosen the particles more readily and it is easier to wipe up anything thrown off the wheel during the honing.

Click here for American Gunsmith Website