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The basics of gundrilling rifle barrels: a little history on gundrilling and the use of modern machines and tools.

Prior to the Industrial Revolution, the hole through gun barrels was formed by forging red-hot iron skelp around a mandrel, then forge-welding the edges together. As you can imagine, this was a rather time-consuming and labor-intensive process. It was also the type of task which required the skill and experience of a master blacksmith to ensure that the job was done properly. It also limited gun barrels to the low-pressure black powder loads that would be safe in a wrought iron barrel. The Industrial Revolution brought steam, and later electric, motors onto the scene and these inventions allowed the development of ever more sophisticated machining tools. At about the same time, there were also several innovations in the field of small arms development, most notably the self-contained cartridge and nitrocellulose (smokeless) powder. The pressures that could be produced with the combination of smokeless powder and brass cartridge cases was awesome and demanded that gun barrels be made of stouter stuff than wrought iron. Fortunately, the machine tools of the period were fully capable of cutting the strongest steels known.

Although the machinery of the period could be made to serve the purpose of creating gun barrels quite readily, the drills used to cut the holes through the barrel blanks were still in their primitive forms. The twist drills used for most types of drilling were the first adapted to the task. These drills were manufactured with a small hole through the shank through which oil could be pumped. The oil flushed the chips backwards out of the hole effectively, but the drills did not perform all that well. Twist drills have a tendency to wander off-center when employed to drill deep holes due to their geometry. Twist drill geometry incorporates two opposed cutting edges of equal length which are set at equal angles in relation to the shank of the tool. This means that there is no net lateral force produced by a twist drill, at least not if the cutting edges truly are equal and opposite. The problem is that they can seldom be ground to be perfect mirror images of each other, and so the drills tend to wander a bit. And the fact that their central webs prevent them from being truly free-cutting made it obvious that a more effective tool must be developed for the particular job of drilling gun barrels.

The first improvement in gundrill design came in the form of a flat drill that was equipped with a collar behind the cutting edges. If properly started in the correct location, flat drills will cut reasonably straight holes. The collars used on the drill shanks helped to keep the drills going in the right direction by following the portion of the hole that had already been cut. However, if the drill did begin to wander off-center, then the collars would ensure that they continued to deviate from their intended course by forcing them to continue cutting in the direction of the crooked portion of the hole. Also, only very small grooves could be formed in the collars to allow cutting oil and chips to flow out of the hole. If the grooves were made too large the usefulness of the collar as a supporting structure would be compromised. The small grooves, in turn, mandated the incorporation of chip-breaking grooves on the cutting edges to keep the size of the chips small. This arrangement complicated sharpening the drill. There was still a lot of room for improvement in the tools used for gundrilling.

At long last, the final form of the gundrill began to take shape with the D-drill. This drill used a single cutting flute that extended all the way to the center of the tool so that the drill would be truly free-cutting. It also incorporated a single, large groove to allow for the passage of oil and chips out of the barrel blank. This tremendously eased problems with chip packing when cutting at industrial speeds. Later, Pratt & Whitney changed the geometry slightly, primarily by moving the point of the drill slightly to the side. By moving the point of the drill off-center, a teat is created in the center of the barrel blank while drilling and the teat greatly aids in keeping the drill moving steadily down the centerline. This geometry also creates two opposing cutting edges on the same side of the drill's centerline. By altering the angles of these cutting edges, the cutting action can be made to induce a net force on the drill which will further discourage any tendency to wander off center. The modern gundrill design had finally been realized.

Gundrill Geometry

There are several subtle design features of a gundrill that allow it to stay on a straight course during the process of drilling a gun barrel. As explained above, these features have been selected through a continual process of trial and error. New design features that improved the function of the tools were retained, with slight modifications, in subsequent designs. During each iteration of experimentation, new light was shed on what features a gundrill must have in order to drill the straightest possible barrel blank. The geometry of our current gundrills is a testament to the ingenuity and innovation of the men who made the Industrial Revolution happen, for the design of these tools has changed little in the last 100 years.

To those who are familiar only with conventional twist drills, the first and most striking feature of a gundrill is the shape and location of the cutting edge of the tool. Gundrills have a cutting edge on only one side of the drill body itself. This edge lies at the front of a plane that is placed exactly on the centerline of the drill and it is sharpened all the way to the center of the tool. There is no central web on a gundrill as there is on a twist drill. The central web of a twist drill does not do a good job of actually cutting steel. It merely forces material out of its way by displacing the material in a lateral direction. This dramatically increases the thrust force on the drill and makes it far more likely that a twist drill will wander off center. But a gundrill, with its single, centrally-located cutting edge, is absolutely free-cutting in its action and does not create any undue stresses on the tool or the workpiece that could cause the drilled hole to run crooked.

To further reduce the likelihood that the drill will wander, the cutting edge is divided into two equal-length cutting edges that are set at opposing angles to each other. This tends to transmit a portion of the cutting force sideways, and these laterally directed forces cancel each other because of the opposing angles of the cutting edge. A key feature of the angles of the cutting edges is that the outer edge is set at a slightly steeper angle than the inner cutting edge. The principles of physics tell us that all forces can be broken down into component forces which are at right angles to each other. Because the outer cutting edge is closer to parallel with the drill shank than the inner cutting edge, the lateral force induced by the cutting action of the outer edge will be stronger than the lateral force produced by the inner cutting edge. The increase in cutting speed at the outer edge of the drill (cutting speed is a function of the rate of revolution and the radius from centerline) compounds the effect, producing a very definite net lateral force on the drill. Because this force is strongest at the outer periphery of the tool, a gundrill actually pushes itself away from the hole on the cutting half of the drill and towards the hole on the non-cutting side. Since the drill cannot cut on that side, the drill can't possibly wander off-center in that direction. As the guys in the beer commercial would say, "Brilliant!"

There are a few conditions, primarily chip-packing and too-high cutting oil pressure, which can push a gundrill off-center towards the cutting half of the tool. If this occurs, the cutting speed at the very edge of the cutting surface will increase (again, cutting speed is a function of cutting radius) and this will produce an ever-increasing net force pushing the drill towards the non-cutting half of the drill. Also, the cutting edges of a gundrill are sharpened to slightly past the lateral centerline of the tool. This means that, if the drill begins to wander in the direction that it can cut, it will begin to cut a very slightly oversized hole. Because there is a net force pushing the non-cutting side of the drill against the hole, the drill will very quickly self-correct its course as the non-cutting half of the drill will drift back the other way until it is in firm contact with the hole once again. As long as the cutting oil pressure is maintained at a high enough level to quickly and completely flush the chips out of the way, a gundrill will run on auto-pilot down the rotational centerline of the barrel blank.

Gundrilling Machines

The commercial machines used to drill gun barrels are designed to perform only this one task to the highest degree of accuracy possible in the shortest period of time. In order to drill the straightest hole possible, these machines rotate the barrel blank while the drill remains fixed in a stationary position on a moving carriage. As the drill is moved into and through the barrel blank, cut ting oil is pumped inside the shank of the drill and emerges at the tip. The oil then flushes the chips away from the cutting edges and carries them backwards out of the barrel through a V-groove that has been pressed into the drill shank specifically for that purpose. Once clear of the barrel blank, the chip and oil slurry is captured and redirected by a chip box. On most machines, the slurry dumps out of the chip box into a chip pan under the machine. The oil then drains out of the pan and is filtered for recycling, while the chips remain in the pan.

If the object of a machining operation is to drill a very long, very straight hole in a barrel blank, spinning the drill to impart the necessary relative cutting motion between the drill and the barrel blank is not the best way to accomplish this goal. When a spinning drill begins to wander off-center, the portion of the hole that has already been cut tends to guide the shank of the drill and causes it to continue progressing forward in the wrong direction. However, if the barrel blank is being spun instead and the drill begins to wander off-center, then the hole will begin spinning eccentrically as it moves off of the centerline. Obviously, the shank of the drill will also be carried along by the hole. As the drill is carried in this eccentric circle, the shank will continuously flex, first one way and then the other. This flexion causes the shank to load up somewhat like a spring, and the spring tension produced tends to help force the drill back towards the centerline of the barrel. This is why machines designed to drill gun barrels spin the barrel blank, rather spinning the drill.

When drilling a barrel blank, the drills are fed very slowly into the solid steel bar. The drill advances only a few ten-thousandths of an inch per revolution of the barrel. This slow feed rate causes the chips produced by a gundrill to be very thin and light. This is an important consideration because the chips must be very easy to flush out of the hole by way of cutting oil injected at the drill tip. The cutting oil is pumped at high pressure through the hollow shank of the drill and emerges at the tip. After exiting the drill, the oil finds itself at the head of the hole with nowhere to flow except backwards along the drill shank. The shanks of gundrills have a wide V-groove crimped into them to allow the passage of the oil. As the oil reverses direction to flow along the V-groove, it picks up the chips that are being produced and carries them out of the hole. Without this ability to clear away the chips, gundrilling machines would not be able to produce satisfactory barrels. The chips absolutely must be flushed away from the cutting edge as soon as they are produced in order to avoid chip packing. Packed chips will counteract the forces produced by the geometry of the drill that keeps the tool on the centerline, and the drill will actually be pushed off center by the chips.

The pressure and flow required to keep things running smoothly is determined by the diameter of the drill. A small drill will obviously produce a small hole through which to force the chips backwards out of the barrel. This necessitates fairly high cutting oil pressures to ensure that the chips are flushed out effectively. By contrast, large drills produce spacious holes through which chips can easily pass, but the increased volume of the hole requires an increased volume of cutting oil to fill the void. The pump used to supply the demand for oil, whether at high pressure or a high rate of flow, must have quite a bit of power to get the job done. For this reason, the motor used to power the cutting oil pump on a gundrilling machine is quite a bit more powerful than the motor used to drive the spindle that rotates the barrel blank. Because the drills are so free-cutting and the feed used to drill is so fine, the spindle motor does not require much power to rotate the barrel blank at the required speed.

The final considerations for beginning a gundrilling operation are starting the drill precisely on the centerline and keeping the long shank of the drill from bowing during the drilling operation. A hardened steel bushing with an inside diameter no more than a few ten-thousandths of an inch over drill diameter is used to ensure that the drill will begin cutting in the proper location. A solidly mounted fixture must be used to ensure that this bushing is held precisely on the centerline of the barrel blank. Once the hole has been drilled deep enough for the entire periphery of the drill tip to enter the barrel, the self-correcting forces caused by the geometry of the cutting edge and the rotation of the barrel blank will keep the drill on course. Between the barrel blank and the carriage that holds the drill driver, a device called a whip guide is placed about midway along the shank of the drill. The whip guide is another bushing which is just over drill diameter. Because the drills used for gundrilling are very long in relation to their diameter, the drill shanks tend to bend to the side under the cutting load. The whip guide is designed to prevent this and keep the drill shank nice and straight so that it cannot flex, because flexing would encourage the drill itself to run off course. However, the whip guide is most useful on gundrilling machines which spin the drill (these machines generally drill things such as crankshafts, not gun barrels). Gundrills are inherently out-of-balance due to the V-groove pressed into the shanks, and the shanks will definitely flex if the drill is being spun.

Troubleshooting Drill Runout

Despite the self-correcting design of gundrills and the development of machines specifically designed to use them, there are things that can go wrong during a gundrilling operation. If one of the parameters of the operation is not as it should be, the result can be a crooked hole that does not run through the center of the barrel blank. This condition is called runout and if it becomes excessive, the barrel blank will not be worthy of continuing on to the reaming and rifling operations. Many have tried to correct crookedly drilled bores by using long reamers to try to straighten things out, but results are extremely variable and usually not too impressive. If the hole in a barrel blank isn't properly drilled, then that particular barrel blank is a likely candidate for the scrap heap.

The primary cause of drill run-out is chip packing. If the cutting oil pressure and flow aren't high enough to flush the chips out of the hole before they can accumulate, then chip packing will be the inevitable result. As mentioned above, chip packing counteracts the forces which are intended to act upon the drill to keep it on centerline. Once these forces have been neutralized, then the force of the packed chips pushing against the barrel wall will take over, and these forces are definitely not designed to keep the drill on center. Even if the oil pressure is correct at the beginning of a drilling operation, chip packing can still result at some point during the drilling operation. As drilling progresses, the cutting oil will become hot as a result of the continuous friction of the cut. The oil temperature will commonly exceed 100[degrees] F by quite a lot before the hole has been drilled to full depth. As the oil temperature increases, its viscosity will begin to decline. Cutting oil is necessarily thin to begin with. As it heats, it begins to flow like water. Since the oil is now easier to pump than it was before, the pressure on the delivery line will drop. A drop in oil pressure inside the barrel blank, especially when the oil is hot and thin, can cause the oil to flow past the chips instead of picking them up and carrying them out of the barrel. For this reason, the oil pressure should be monitored when drilling a gun barrel and increased to the necessary level if it begins to drop.

The other major cause of drill runout is improper operation of the gundrilling machine, meaning that either the speed, the feed, or both are excessively high for the size of gundrill being used or the type of material being drilled. Excessive rotational speed or drill feed will cause the cutting forces acting upon the drill to be greater than they should be. This can lead to unpredictable results and a decided lack of barrel-to-barrel consistency. One barrel may come out of the machine well within tolerances, while another is a complete mess. Excessive speed and feed can also cause premature cutting edge erosion and an overall shorter working life for the gundrill. Of all of the parameters involved in gundrilling, feed rate is the one that must be followed most closely. The reason for this is that the feed rate has a major impact on the thrust force encountered by the drill. Feed rate also determines the thickness of the chips that are coming off of the cutting edges. If the chips being produced become too thick, they will be harder to flush out of the hole and that can lead to chip packing.

Assuming that all goes well during the drilling operation, a nicely drilled barrel blank will be the result. Bore runout does not have to be, and usually isn't, zero from one end of a thirty inch barrel blank to the other. But it doesn't have to be. As long as the runout is gradual and there are no crooks or dog-legs in the bore, the runout can be effectively eliminated during the barrel profiling operation. As long as the runout of the bore does not exceed 0.015" the drilling operation can be considered a success, however, runout exceeding 0.015" indicates the operating parameters of the machine should be examined carefully to determine the cause of the excessive runout before drilling any more barrel blanks.

The preceding discussion was intended to be a general overview of gundrilling. My book, Steel Helix (, provides a thorough discussion of making rifle barrels from solid bars of steel, including the design and operation of a home-shop built machine for making rifle barrels.
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Title Annotation:SPECIAL REPORT
Author:Moore, Charles J.
Publication:American Gunsmith
Date:Oct 1, 2015
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