The basics of reaming rifle barrels: details on barrel reaming with modern machines and tools.
The first scraper that was typically applied to the bore was designed to remove scale from the interior of the barrel blank. Because early gun barrels were hot forged over a mandrel and then forge welded at the seams, all while exposed to the oxygen-rich atmosphere, heavy scale was formed inside the bore during the process. The same type of scale can be readily seen and felt on the exterior of a piece of hot rolled steel today. Scale contains carbides and other types of very hard compounds. These hard substances are quite difficult to cut and tend to dull cutting tools rather quickly. The scrapers produced to remove the scale were very simple in design and not intended to produce a smooth, uniform finished bore. The form of scraper commonly used for this application was an iron bar that had been hammered square and then twisted until the measurement across the helical corners was just over the size of the bore when it came off of the mandrel. The corners of the twisted scraper would break the scale free of the bore's surface through shear abrasive action and brute force. Once the scale was removed and the raw iron exposed, the finish scraper could be brought into play.
There were two types of finishing scrapers employed to bring the bore to finished dimensions. One type used a round head made of hardwood, such as maple or ash. A mortise was cut in the head into which a small block of hardened iron could be placed. The upper corners of the block would be stoned to ensure they were sharp. Then paper shims would be placed in the bottom of the mortise until the corners were high enough to scrape the bore when the head was run through the barrel. Many passes of the scraper were required to remove a sufficient amount of iron to produce a smooth bore. Paper shims were added under the scraper bit each time the bore had been scraped large enough to make it a loose fit. In this fashion, the bore would eventually be brought to finished dimensions. Since this was a scraping tool rather than a cutting tool, the scraper could be rotated in either direction (clockwise or counter-clockwise) without affecting the action of the tool. Alternating the direction of the tool's spin would also bring both corners into play, which would double the number of passes that could be taken before the scraper needed to be sharpened again.
The second type of scraper commonly used for finishing the bore was a hardened, square iron bit that was slightly under the size of the bore. To use this tool, a half-round sliver of wood was placed under one side of the tool to push the corners of the opposite side into contact with the bore. As material was removed from the bore and the fit of the tool became loose, paper shims were added between the bit and the wood filler to take up the slack. Once again, this type of scraper could be rotated in either direction and it took repeated passes of the tool through the bore to bring the internal dimensions to specifications. This type of scraper was still being used for finishing the bores of gun barrels after the Industrial Revolution but the need for more efficient tools was clearly evident.
As society became increasingly more mechanized and mass production was born, a quick, easy way to produce holes with good surface finishes and tight tolerances in all types of steel components (not just gun barrels) was needed. A tool that could accurately cut, rather than scrape, steel was ideal. Reamer design improved to meet this need. Contemporary machine reamers have a chamfer angle on the nose of the tool. Most, if not all of the cutting action takes place at the chamfer while the remaining length of the cutting flutes provide support to the tool as well as scraping and burnishing the hole. Some reamers have a starting angle that will also cut material away from the radius of the hole as the tool is advanced. The starting angle of these reamers is usually 1[degrees] to 5[degrees]. Since these tools are designed to cut steel rather than simply scraping the surface, they also have a rake angle on the cutting edge, usually 5[degrees]. The rake angle produces less pressure during the cut and provides a smoother surface finish.
The reamers designed for reaming gun barrels usually have a rather long tapered section that leads to a shorter parallel-sided section. All of the cutting takes place on the tapered portion of the reamer flutes. In addition to the rake angle at the face of the cutting edge, the flutes are also equipped with a relief angle behind the cutting edge. The relief angle ensures that the trailing edge of the flute cannot come into contact with the bore. Contact of this sort would prevent the cutting edge from biting into the steel and removing material from the barrel blank. If an unrelieved margin is left on the actual cutting portion of the reamer flutes, this margin must be very thin. Anything more than the thinnest imaginable margin on the cutting edges will prevent them from cutting. As noted above, the taper of the flutes can be set at any angle between about 1[degrees] and 5[degrees]. The actual angle used on an individual reamer will be determined by the length of the cutting flutes and the amount of material (depth of cut) that the reamer will be required to remove from the bore.
Behind the tapered, cutting section of the reamers used for gun barrels lies a short parallel section. This section of the reamer is not designed to cut steel at all. The purpose of this section is to scrape and burnish the finished bore to remove any slight imperfections left behind by the cutting section. A secondary function of the parallel section of a gun barrel reamer is to provide additional support and stability to the back of the tool. This support discourages chatter during the cut and provides for a smoother surface finish. Because this section is for stability rather than cutting ability, the margin is left considerable wider on this section of the flutes. A margin measuring several thousandths of an inch wide is common here. Not only does the additional surface area provide more stability, it also reduces the rate of erosion on this surface and this tends to extend the life of the reamer. Once the tops of the parallel flute sections are worn below the lower tolerance limit for hole size, the reamer can no longer be used to ream barrels of the caliber it was designed for.
At the trailing edge of the reamer flutes is a chamfer. Unlike machine reamers, which have chamfers that are designed to cut, the sole purpose of the chamfer on a gun barrel reamer is to ensure that the last contact between the tool and the barrel blank will not be made with a sharp, scratchy corner. Such a corner could, and probably would, ruin the surface finish of the bore by scratching the dickens out of it. The chamfer is usually made in the form of a straight angle of about 45[degrees], but a simple rounding of the trailing edge will also work quite nicely. The final surface finish of the hole is also improved by the use of cutting oil during the reaming operation. As with the gundrill that created the initial hole, the oil is pumped through the long shank of the reamer. At the end of the shank, the oil emerges between the cutting flutes and continuously flushes the powder-fine chips away from the cutting edges and out the other end of the barrel. However, unlike gundrilling, the reaming operation can be accomplished successfully with low oil pressure and flow. The only requirement is that the chips be flushed away quickly enough to prevent them from interfering with the surface finish produced by the cutting action.
The machines used to ream gun barrels come in two basic types: those which spin the barrel blank and those which spin the reamer. Unlike the gundrilling operation, there is no real advantage that can be assigned to either type of machine. This is because, while a spinning drill can adversely affect the straightness of the finished hole, a spinning reamer can't affect the concentricity of the bore for better or worse. Reamers are designed to enlarge existing holes, not create a hole where there was none before. Reamers, especially those attached to the end of long, flexible shanks, will tend to "float" in the center of the hole due to a natural tendency to balance the cutting forces on the opposed cutting flutes.
This tendency causes reamers to faithfully follow the course of the original drilled hole whether the reamer is spinning or stationary. This is good news for the barrel maker, because it means that if the bore of a gun barrel is drilled straight, it is very likely to remain straight during the reaming operation. The downside is that if there are slight imperfections in the straightness of a freshly drilled bore, the defects will not be improved by reaming. So it's probably best not to bother moving on to the reaming operation at all if there are imperfections in the drilled bore.
Gun barrel reamers are typically pulled through the barrel. This means that to setup the machine for reaming, the shank of the reamer is slid through the barrel before being anchored to the carriage that will draw it through the bore. Since the reamer is pulled through the bore, there is no tendency for the shank of the reamer to bend or flex during machining because the shank is kept under tension by the cutting forces. However, the rotary motion of either the barrel blank or the reamer during the cutting operation will cause torsion forces on the shank of the reamer. The torsion of the reamer shank can lead to issues with the surface finish of the bore if chatter is induced by the shank loading up under the torsional forces and then releasing them in the form of a quick slip periodically. And despite the fact that pulling the reamer tends to preclude flexing of the shank, whip guides are still normally employed on reaming machines. The most useful function of the whip guide on a reaming machine is to hold the shank of the reamer on the machine centerline as the reamer exits the bore. This prevents the shank from bowing or bouncing, which could allow the reamer's flutes to be chipped if they were to come into contact with some component of the machine after exiting the barrel.
The cutting speeds used for reaming are considerably lower than those used for gundrilling, especially if the reamer being used is made of high-speed steel rather than carbide. The slower cutting speed allows the cutting flutes of the reamer to remain sharp for a longer period of time. Cutting speeds that are too high could easily lead to a reamer that is dull by the time it makes it to the end of the barrel blank. This situation must be avoided at all costs, because a dull reamer will not produce a good surface finish. Reamers must be razor sharp to produce a top quality finished surface inside of the bore. Excessively high cutting speeds will also induce chatter during the cut, causing a rough, rather uneven surface finish. This type of defect in a gun barrel is not only unsightly, but it can also cause rapid bore fouling and inaccuracy. So cutting speed is a very important consideration when reaming.
In sharp contrast to the relative cutting speeds used, the feed rate used during reaming is normally quite a bit higher than the feed rate used for gundrilling. The first, and most obvious, reason for the increase in feed rate is the fact that most reamers will have between four and six cutting edges, whereas a gundrill only has one. The feed rate per flute is multiplied by the number of flutes to determine the total rate of feed per minute. The other major reason that the feed rate can be increased is the fact that, as stated previously, the reamer will follow the existing hole. Gundrill feed rates are intentionally very low so that the drill will have the best possible chance of drilling down the centerline of the barrel blank. This is not a consideration when reaming. The feed rates used by various barrel makers is quite variable. Feed rates as low as a few thousandths of an inch per flute per revolution to as high as a few hundredths of an inch have been used successfully. The reason for this leeway in choosing a feed rate is that increasing feed rate does not cause cutting edge wear and erosion to increase nearly as quickly as increasing cutting speed. As long as the cutting speed chosen is properly balanced with the feed rate used, all will be well.
The oil pressure and flow needed to perform a reaming operation are quite a bit lower than the pressure and flow required for the gundrilling operation. When reaming, the chips produced are very fine and are of about the same consistency as grinding dust. Also, the chips do not have to be squeezed out through a narrow V-groove in the shank of the tool. They are carried away from the cutting flutes into the open bore of the barrel where they pass freely out the other end. As long as the oil pressure is high enough to produce sufficient flow to clear the chips away from the cutting flutes as soon as they are produced, the reaming operation will proceed satisfactorily and produce a good surface finish. Another factor that lowers the requirement for oil pressure and flow during reaming is that fact that very little friction-induced heat is produced during reaming. Drilling produces a lot of friction and heat, and raises the temperature of the oil substantially. Reaming does not place the same demand for heat removal on the cutting oil, so the rate of flow can be much lower. All of this means that reaming machines do not require nearly as much power at the oil pump as gundrilling machines.
As mentioned previously, reaming cannot influence the straightness or concentricity of the bore for better or worse. So reaming defects come in the form of a rough surface finish. There are several variables that can, and will, affect the surface finish of the gun barrel's bore. One of the most insidious enemies of a good reamed finish is reamer chatter. There is a phenomenon commonly called roping that is the result of repeated, harmonic chatter. Other potential causes of a rough bore surface are scratching and glazing. If a particular reamer has produced good bores in the past, then the cause of the bore defect was likely due to some oversight in the initial preparation of the reamer prior to reaming the faulty bore.
If the reamer was properly sharpened to razor keenness prior to the reaming operation, then a good first place to check for the cause of the defective bore is the speed and/or feed settings used to make the cut. If the speed or feed of the cut are not set properly for the materials involved (the chemical composition and hardness of both the reamer and the barrel blank must be considered) it is likely that chatter will be induced or the reamer will become dull before the reaming operation is complete. When the speed of the cut is too fast, it is likely that excessively fast cutting edge erosion will cause the reamer to become dull before the reaming operation is complete. Dull cutting edges can cause the reamer to chatter. The chatter produces exactly the type of rough, uneven finished surface that it sounds like it would produce. Chatter can also be induced in a sharp reamer with a feed rate that is too high. In this case, the direct cause is likely to be a chip load that the reamer shank is not stiff enough to cope with. If the chip load is too high, it will cause torsion in the shank. When the force of the torsion finally becomes high enough to resist additional torque, the tension can be released all at once causing the reamer to skip and chatter.
There is, however, a potential pitfall associated with a feed rate that is too slow. An excessively slow feed rate can make it more difficult for the reamer to bite into the material being cut. When this occurs, a condition called glazing is produced. Glazing is basically an excessively burnished finish that causes an excessive rate of cutting edge erosion and heat production. The feed rate must be high enough to allow the cutting flutes to get a good purchase in the barrel walls so that they can cut rather than simply rubbing and basically trying to cold swage the bore to the finished diameter. And, of course, if the cutting edges become dull too quickly through glazing, this can also cause the reamer to chatter.
Sometimes, a peculiar surface finish called roping is seen in a freshly reamed barrel. This condition is caused by repeated, harmonic chatter of the reamer and it leaves chatter marks in the bore in the shape of a spiral. Dull reamers can cause the chatter to begin. The holes produced tend to be polygonal in shape with the number of corners produced being directly related to the number of flutes on the reamer. If the speed, feed, and stiffness of the reamer shank are all just right (or just wrong, depending upon how you look at it) the harmonics of the induced vibration causes the regular repetition of the pattern at ever so slightly different times in the cycle. This produces a spiral pattern as the reamer progresses down the bore. The reamer should definitely be sharpened if roping occurs but the speed and rate of feed may also need to be adjusted to prevent the harmonic vibrations from being produced. It may also be necessary to attach the reamer to a stiffer shank but this would be a last resort as it requires more time and effort to accomplish.
The final defect that may be observed in a reamed bore is a surface finish that is just plain rough. In this case, there will be no corners produced in the hole which would indicate that chatter has occurred. The bore will look nice and round but with a rough and lackluster finish rather than be smooth and shiny as it should be. The first thing to check in this instance is the condition of the chamfer on the trailing edges of the cutting flutes. If there is a sharp corner or a burr on the trailing edge of one or more of the flutes, the bore's surface will show the passage of this imperfection in the form of a definite roughness. If the chamfers are all in good shape the roughness may have been caused by inadequate oil flow. If the cutting oil does not flow with sufficient urgency, then chips can accumulate at the interface between the cutting edges and the barrel's bore. An accumulation of chips at this interface will prevent the flutes from making a smooth, clean cut and the cutting oil pressure and flow will need to be increased to correct the problem. For more information, refer to Steel Helix (steelhelixrifle.com).