Hook Cutter rifling. The most simple method of cutting the grooves is by use of a 'single hook cutter'. In this, a hardened steel cutter, in the shape of a crochet hook, is set into a recess in a steel rod of slightly smaller diameter than the bore of the barrel being rifled. As the cutter is dragged through the bore, the barrel is rotated at a fixed rate to impart the spiral of the rifling. Each pass of the tool only cuts one groove, and the barrel must be repositioned and the process repeated for the number of grooves required. As each pass of the cutter only removes a few thousandths of an inch of metal, the height of the ' hook' must be raised and the grooves all cut again, with up to 80 passes being made for each groove. This is a very time-consuming and, as the hooks rapidly wear out, an expensive method of rifling (Figure 4.5).
Hook rifling can be identified by (i) the presence of longitudinal striations in the cut grooves and (ii) the similarity, as the same tool is used for every groove, between the micro stria in the grooves.
Scrape cutter rifling. The Scrape cutter i method of rifling uses a bar with curved and hardened steel scrapers set into it. The number of these scrapers corresponds to the number of grooves required. As a result, all grooves are cut with one pass after which the height of the scrapers is increased and further
Figure 4.5 Hook cutter.
passes are made. This method produces extremely fine rifling and is used on some of the best weapons.
Scrape cutter rifling is very similar to hook cutter except that as a different scraper is used for each groove, there will be no underlying similarity between the grooves.
Broach rifling. The most commonly used rifling method is called 'broach rifling'. This, in a very simplified form, can be thought of as a series of 20 or 30 steel discs on a rod, with each disc being slightly larger than the one preceding it. Into each disc is cut the profile and number of grooves required with the last disc possessing the final calibre and dimension required. A broach cutter can thus cut all the grooves and lands to the final dimensions in a single pass.
Broach cutter rifling can be recognized by the longitudinal striations on the lands as well as the grooves on the weapon's bore (Figure 4.6).
Button rifling. 'Button rifling' is a very commonly used method but is generally only used on the cheaper weapons, particularly those of 0.22" calibre.
In this type of rifling, the barrel is bored slightly smaller that the final required diameter. A ' button' on the end of a long rod, containing an exact negative of the rifling required, is then pushed or pulled through the bore forcing the metal to expand into the final shape required (Figure 4.7). This is a single operation and is a very cheap way of rifling a weapon.
Button rifling is, especially in the cheaper weapons, very easy to identify as the circular marks produced during the reaming of the bore are not eliminated during the rifling. These marks are simply pressed into the metal and are visible on both the lands and grooves (Figure 4.8).
Figure 4.6 Broach rifling cutter.
Figure 4.7 Button rifling tool.
Swage or hammer rifling. Another method which is similar to button rifling but produces a very much higher quality is called 'swaging or hammer rifling'. In this method, the bore is reamed slightly larger than the required final diameter. A mandril (an extremely hard steel plug tapered at both ends) containing an exact negative of the rifling profile required is then passed through the bore whilst the outside of the barrel is either hammered or hydraulically squeezed on to it. This method causes the metal not only to work-harden, but also to increase in density. Assuming the mandril is of a good quality, rifled barrels of an exceptional quality are produced.
This type of rifling can, if the outside of the barrel has not been turned down, be recognized by the peculiar spiral indentations on the outside surface due to the hammering or squeezing process. Other than that, the only other identifying characteristic is the mirror-like finish and lack of striations in the rifling.
Other methods of rifling. Other methods of rifling, such as electrolytic and gas cutting, do exist and are used but only to a very limited extent.
A report on the use of electrochemical machining for the production of rifling in barrels appeared in the journal of the Association of Firearms and Toolmark Examiners in 1988 (Pike, 1988). The barrels reviewed in the report were being made by Cation Co., a small company in Rochester, New York, USA, for an arms manufacturing company called Coonan Arms.
Since 1993, Smith & Wesson (S&W) has also been using an electrochemical machining technique to rifle most of their revolver barrels. The only revolver barrels that S&W still broach rifle are their 0.22 calibre barrels and ported barrels.
Electrochemical machining is not exactly a new method of rifling barrels as it was reportedly used by Krupp, the famous German arms-making company, to manufacture their cannon barrels as early as 1920.
In the modern process, a mandril is made slightly smaller than the bore size of the drilled barrel blank. Strips of plastic are glued to the mandril in a spiral pattern corresponding to the desired shape of the rifling. The mandril is then inserted into the barrel blank and an electrolytic fluid is circulated down the gaps left between the plastic strips and the bore.
A direct current is then applied between the barrel blank and the mandril, with the mandril being made the cathode. The current strips away metal from the exposed areas of the barrel between the plastic strips forming the grooves.
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