The basics of cut rifling a rifle barrel: using modern machines and tools to create those important grooves.
Early rifling cutter boxes were made of hardwood or brass. They were hand fitted to the bore size of the gun barrels to be rifled and were tapered on each end to permit easy entry into the bore. A small slot was cut into them and a hand-made cutter was inserted into the slot. The cutters were made of hardened iron and usually had multiple cutting edges, somewhat like a saw blade. The cutting edges were formed with a triangular file. Because the angles of the file's corners were 60[degrees], the cutting edges formed in this fashion usually had a negative rake of 30[degrees]. With a such a steep negative rake angle, these cutters were incapable of actually cutting material away from the barrel. The cutters could only scrape away fine shavings from the wrought iron barrels. As with the other early tools used to make rifle barrels, these early rifling tools required quite a bit of time and labor to accomplish the task of rifling a barrel. And the cutter boxes had to be replaced after rifling a few barrels, although the ones made of brass tended to fare better than those made of hardwood.
In order to deepen the grooves in the bore with successive passes of the rifling cutter box, paper shims were placed under the cutters when enough material had been scraped away from the groove bottoms to cause the cutter to become loose. In the pre-industrial days, barrel makers had no pillow-block bearings with which to hold a rotating spindle precisely on the centerline of the rifling bench. The barrel blank was simply clamped into position on the bench with a pair of hardwood clamping blocks. The barrel had to remain absolutely fixed in this position until the rifling operation was complete. If the barrel was allowed to slip either longitudinally or radially during the cutting process, it would be extremely difficult to get the grooves to once again align with the path of the cutter and the barrel would likely have to be reamed to a larger caliber so that the rifling process could be started anew.
The rifling cutter box had to be rotated as it was drawn through the barrel's bore in order to cause the grooves to spiral around the circumference. At the time, there were two primary methods to index the twist rate of the helical grooves. One method was to layout the desired rate of twist onto a hardwood cylinder and then cut grooves into the cylinder. One groove was cut into the cylinder for each groove that would be cut into the barrel. The cylinder was placed into a fixture that could be slid back and forth on the rifling bench and the cutter box's pull-rod was attached to the cylinder. A shoe, or pin, was affixed to the bench and engaged a groove in the cylinder. The shoe caused the cylinder and the attached cutter box to rotate as the fixture was slid along on the bench.
The second common method to rotate the cutter box involved using a square iron rod as the pull-rod for the cutter box. The square rod was twisted to the desired rate of rifling pitch and the cutter box was affixed to its end. The pull-rod would then have a handle attached to the other end which would allow the pull-rod to rotate while the handle did not. As with the rifling cylinder method, a fixture on the bench would engage the pull-rod to impart rotation to the assembly. In this case, a square hole through the fixture interfaced with the square profile of the pull-rod and caused the assembly to rotate as it was slid back and forth atop the rifling bench. As before, this caused the cutter box to rotate as it traversed the barrel's bore.
Since the barrel had to be firmly clamped into position on the early rifling benches, the entire tooling assembly had to be capable of being positioned in discreet radial increments in order to cut multiple grooves into the barrel. With the rifling cylinder method, the shoe or pin would be disengaged from the groove in the cylinder after drawing the cutter box through the bore. The cylinder and cutter box assembly would then be rotated to the next groove in the cylinder and the shoe would be replaced into the fixture to engage the newly aligned groove. When the twisted pull-rod method of indexing twist was used, the square-holed fixture and cutter box assembly would be rotated slightly and a pin or screw would hold the fixture in the new position to cut the next groove in the barrel. This process of indexing the cutter box assembly with consecutive grooves in the barrel, while adding paper shims under the cutter on each trip around, would continue until the barrel's grooves had all finally been scraped to full depth. The entire duration of the rifling operation was measured in days, not hours.
Rifling Cutter Box Geometry
The basic design of the rifling cutter box has changed little over the centuries. Contemporary cutter boxes are still simple cylinders about five or six inches in length and they still hold a centrally mounted cutter. However, cutter boxes are now made of hardened tool steel instead of hardwood or brass and the mechanism used to raise the cutter for successive passes through the bore has become quite a bit more sophisticated than paper shims. There are several styles of mechanisms used in modern rifling cutter boxes to raise the cutter but most use a screw to precisely control the amount of the cutter's rise. Some mechanisms use a screw that impinges directly on the cutter itself and the interplay between the cutter's geometry and the geometry of the cutter box slot determine the amount of cutter rise for each full turn of the screw. Other mechanisms use an adjustable ramp under the cutter to control the cutter's rise. Advancing the screw, or a nut on the screw, will change the ramp's depth of penetration under the cutter, thereby raising the cutter as it incrementally climbs the slope of the ramp.
The most basic design of rifling cutter box to employ an adjustable ramp to raise the cutter does not use any type of screw to precisely control the amount of cutter rise for successive passes through the bore. This type of cutter box uses a section of ordinary drill rod with the ramp milled onto one end. To adjust the cutter, the cutter box is drawn into the barrel just far enough for the cutting edge to touch the leading edge of the bore. The end of the drill rod is then rapped lightly with a small hammer, or other appropriate tool. This forces the ramp tight under the cutter and simultaneously forces the cutting edge into firm contact with the bore so that steel will be cut away as the cutter box is drawn through the barrel. When the cutter becomes loose in the bore, the drill rod is tapped again to bring it back into firm, cutting contact. This mechanism is obviously not very precise as the exact amount of cutter rise cannot be accurately controlled. However, this simple system is used to make first-class barrels by those who employ it. The barrel maker must use a light touch in advancing the ramp during the final cuts to bring the rifling to full depth, but no deeper. A heavy-handed approach with this system during the finishing cuts can lead to a slightly over-sized groove-circle diameter.
There are two different types of cutter boxes that use a screw to control the depth of ramp penetration under the cutter. The first type employs a ramp with an integral threaded stem. In this system, a retaining nut is fabricated to slide onto the ramp stem, without engaging the threads of the stem, and screw into the cutter box. As its name implies, the retaining nut has an inside diameter smaller than the ramp diameter and it retains the ramp within the rifling cutter box. An adjusting nut is screwed onto the stem. The position of this nut on the stem determines the depth of ramp penetration. The ramp can slide under the cutter until the adjusting nut butts against the retaining nut. The adjusting nut is normally graduated with ten index marks. When the adjusting nut is turned one-tenth of a turn to the next indexing mark, the cutter will be raised 0.0001" over the previous adjustment when the ramp is slid into position under the cutter. Because the rise of the cutter at each adjustment can be precisely predicted, this system allows the barrel maker to maintain tight control over groove depth without the need for constant measurements between cuts during the final passes through the bore. However, this system is not suitable to automation of the cutter-raising adjustments. This system requires that the barrel maker manually adjust the depth of cut after one pass has been made through each groove at a particular depth setting.
The other type of mechanism that employs a screw and ramp to control the depth of cut does allow the adjustment to be automated. This system employs a screw and a separate ramp as individual components. The screw threads directly into the rifling cutter box and impinges upon the rear of the ramp. When it is time to raise the cutter, the screw is turned by the increment required to raise the cutter by the desired amount. As the screw is turned, it penetrates more deeply into the cutter box and forces the ramp to drive deeper under the cutter ahead of it. This mechanism can be automated by incorporating a radiused slot in the end of the screw. This slot is engaged by an indexing driver that looks somewhat like the tip of a hollow-ground screwdriver. When the cutter box has made a cutting pass through each groove, the driver turns by the amount required to advance the screw and raise the cutter for the next series of passes. As the cutter box exits the bore on the return stroke after cutting the last groove in the series, the radiused slot in the screw engages the driver in its new position and the screw is forced to turn. This automatic indexing feature is quite useful on machines that cycle automatically under hydraulic power because it allows the rifling operation to flow seamlessly with no stops required to raise the hook cutter.
The final type of rifling cutter box employs a screw which directly impinges on the cutter itself. As with the preceding example, the screw threads directly into the cutter box and the adjustments can be automated. However, the screw used by this system must be screwed out of the cutter box as the rifling operation proceeds, rather than being screwed deeper. As the screw is retracted from the cutter box, it allows the cutter to be driven back by a spring. Both the rear surface of the hook cutter and the interfacing surface of the cutter box must be ground to just the right angle in order for this system to work properly. As the cutter is forced backwards by the spring to the limit allowed by the screw, the interaction of the sloped surfaces on the cutter and cutter box cause the cutter to rise incrementally out of its slot to cut the grooves deeper. Because the geometry of both the cutter box slot and the cutter itself must be just right, this type of rifling cutter box is generally employed only by large manufacturers who have the necessary tool room equipment required to accurately machine these components. Barrel makers who fabricate their own tooling and do not have a well-equipped tool room usually choose one of the other mechanisms simply because the components and the cutters are easier to fabricate.
The machines used for rifling gun barrels are very specialized pieces of equipment. These machines are of no use whatsoever for any task other than cutting grooves into rifle barrels. As with the older style of rifling bench, modern rifling machines move the rifling cutter box back and forth on the longitudinal axis of the machine to accomplish the cut. However, unlike the antique rifling cutters, the hook cutters used to cut rifling on modern machines are ground with a positive rake, usually 5[degrees], and actually cut the barrel steel rather than simply scraping it. This change in cutter geometry, along with several changes made to the machines themselves, have made the act of cut rifling a barrel with a modern rifling machine a much less time-consuming proposition than it used to be.
One major difference between the old rifling benches and modern rifling machines is the use of pillow-block bearings to mount a rotating spindle to the bed of the machine. On many older machines, the spindle is rotated in discreet increments after each groove has been cut at a particular cutter depth setting. An indexing fixture is used to lock the spindle in position for each cut. Some fixtures are designed to engage a series of holes drilled into the spindle, or a collar around the spindle. Others are designed to employ a ratcheting action to engage cut-outs on a spindle-mounted collar. Either of these options locks the spindle by mechanical means and they require that the rifling cutter box be spun as it is drawn through the bore. Modern, CNC-controlled machines typically do things the other way around. The rifling cutter box does not rotate on these machines. Instead, a servo motor is used to rotate the barrel spindle at a rate which matches the desired twist as the cutter box is drawn through the bore.
Rifling machines can be made to use either hydraulic or mechanical systems to impart longitudinal movement to the rifling cutter box. On most modern machines designed for mass production, hydraulics power the linear movement of the tool carriage. As mentioned before, these machines will typically employ an automated system to raise the hook cutter after a series of cuts has been completed. The hydraulics used to power the carriage will also cycle automatically once the rifling process has begun. It is important to realize that the hydraulics are not used to allow a deeper cut to be taken, nor are they used to increase the speed of the cut. In order to keep the hook cutter sharp for as long as possible, the rifling cuts must be made at a relatively slow speed. Because of this, the hydraulically powered machines are usually setup so that the stroke of the piston which drives the cut is slower than the return stroke. Since no steel is being cut away during the cutter box's return, it is beneficial to accomplish the return stroke with all due haste.
The rifling machines that use mechanical power to move the rifling cutter box typically use one of three options. The options for driving the cutter box are chains and sprockets, rack and pinion gearing, and leadscrews. The machines which use chains and sprockets to power the carriage are usually built from scratch. A hand crank is placed on the sprocket axle which lies closest to the barrel spindle and this allows the barrel maker to remain in one position as the carriage moves. The machines which use either rack and pinion or leadscrew systems are typically built on the bed of an old long-bed lathe that has been reconditioned for use as a rifling machine. The lathes original gearing and/or leadscrew setup is used to power the carriage. And if the original lathe had powered feed, the rifling machine can also be made to have power feed. However, advancing the carriage by use of the hand wheel on the apron will prove to be a far faster way to accomplish the cuts on most machines due to the powerfeed rates typically employed in lathe work. This option will require that the barrel maker follow the carriage as it moves back and forth.
There are several different types of twist indexing systems employed on modern rifling machines. One of the most common and well known is the sine bar system. With this system, a steel bar is set at an angle to the centerline of the machine. A sliding rack is made to engage the sine bar. The sliding rack is mounted on the carriage of the machine perpendicular to the longitudinal axis of the machine. As the carriage moves back and forth, the rack's engagement with the sine bar causes it to slide along a path at right angles to the motion of the carriage. A pinion gear at the rear of the rifling cutter box's pull-rod engages the rack. As the rack slides, the pinion gear rotates and this causes the cutter box to rotate along with it. The twist rate cut into the barrel is determined by the circumference of the pinion gear in conjunction with the angle of the sine bar. The smaller the pinion gear and the greater the angle of the sine bar, the faster the twist rate of the rifling will be. These sine bar systems are adjustable to cut all standard twist rates, and any twist rate in between.
The second common twist indexing system seen on modern machines is the rack and pinion system. These machines employ a fixed rack that is mounted to the machine bed parallel to the longitudinal axis. A pinion gear engages the rack and rotates an axle which is mounted on the carriage perpendicular to the machine's axis. A miter gear on the axle interfaces with a miter gear mounted to the rear of the rifling cutter box's pull-rod. As the gears spin, the cutter box is forced to rotate as well. In this case, the twist rate of the rifling is determined by the circumference of the pinion gear alone. In order to change the twist rate of the rifling, the rack must be adjusted higher or lower to accommodate a pinion gear of a different size. Because of this, these systems are not infinitely adjustable for twist rate. Only the twist rates that can be cut with the available, discreet gear sizes are available to the barrel maker.
One additional feature of the automated rifling machines is a cutting oil system. Since these machines cycle automatically, they must be capable of clearing chips with no intervention by the barrel maker. As with the machines used to drill and ream rifle barrels, these machines pump oil through the shank of the rifling cutter box. The oil will typically exit the cutter box at the hook cutter slot to flush the chips way from the cutting edge. The oil then carries the chips away through the barrel to be deposited in the chip tray upon exit. The machines that are operated under manual power and/or with manual cutter feed settings do not usually have an oil pumping system. The chips are cleared away from the cutting edge at the end of each stroke by the barrel maker before proceeding with the return stroke.
The rifling operation is the make-or-break moment in the process of fabricating a rifle barrel. If this delicate job is botched the barrel will have to be scrapped, or at least reamed to a larger caliber to try the rifling operation again. The most troublesome problem that can arise during the rifling operation is rough groove bottoms. A second issue that can occur is incorrect groove depth. The grooves must be cut to within specified tolerances of depth in order for the rifle barrel to be safe and accurate.
One of the most common causes of rough groove bottoms is a too-aggressive cutter feed. Rifling grooves are typically cut a mere 0.0001" deeper on each successive pass of the cutter box through the bore. If the cut is any more aggressive than this, the groove bottoms tend to be very rough. Lapping is mandatory for these rough barrels because they will foul quickly and lose accuracy if the grooves are not lapped smooth. When using a cutter box with the capability for precisely controlling the cutter feed, excessively heavy feed and rough grooves should not be an issue. However, if using the simpler cutter box design in which the ramp is simply tapped to advance it, rather than being positively controlled by a screw, then a too-heavy feed is certainly possible. The condition of the groove bottoms should be monitored to check for this condition when using one of these cutter boxes.
An easily correctable, but sometimes frustrating, problem that will definitely occur at some point in every barrel makers life is a dull hook cutter that refuses to cut. Since the rifling cuts are so very shallow, any slight imperfection of the cutting edge can cause the cutter to skip and scrape rather than cutting the way it should. This condition is easily detectable when using a manually operated machine. The barrel maker will be able to feel and hear the hook cutter sliding through the bore rather than cutting. When using an automated machine, a dull hook cutter can be a little harder to detect simply because the barrel maker is not intricately involved in every step of the process. If the barrel maker has a grinding fixture that can be used to accurately sharpen the cutter, the rifling cutter box can be removed from the machine to sharpen the cutter. However, if there is a barrel in place in the spindle that is not yet completely rifled, the barrel maker had better be very certain that the rifling cutter box can be re-installed on the machine in the exact same orientation after the cutter is sharp. If the cutter box is misaligned with the incomplete grooves by even a tiny amount, the finished results will likely be less than satisfactory. The other option is to sharpen the cutter by hand while leaving the rifling cutter box in position on the machine. This technique requires a bit more manual dexterity, but it does avoid the possibility of misaligning the rifling cutter box during re-installation.
The final issue that must be dealt with is accurately measuring the groove-circle diameter. The time-honored method used to accomplish this is by pushing a slightly oversized, soft lead slug through the bore. The slug emerges with raised ridges that correspond to the grooves in the barrel. The measurement across the ridges on the slug is the same as the measurement of the groove-circle. The cutter is fed out of the cutter box and the rifling operation continues until the desired groove depth is reached. Another alternative for measuring the groove-circle is to fabricate a simple tee-gage that is dimensioned to represent the lower limit of groove-circle diameter. When this gage can be slid into opposing grooves, the barrel is finished. Please do note that either of these procedures will be far easier to employ if an even number of grooves is cut into the barrel because each groove will have another groove directly across the bore from it. If an odd number of grooves is cut into the barrel, each groove will be mated to a land on the opposite side of the barrel. This configuration makes measuring the groove-circle diameter a little more problematic.
More detailed information on rifling barrels is provided in Steel Helix (steelhelixrifle.com).
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|Title Annotation:||SPECIAL REPORT|
|Author:||Moore, Charles J.|
|Date:||Dec 1, 2015|
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