Making Chamber Reamers

MAKING CHAMBERING By KEITH

O

FRANCIS

NL Y 3 of the 5 basic tool steel types

available are suitable for making chambering reamers. These are the high-speed steels, the cold work steels, and the carbon and low-alloy steels. High-speed steel is used ordinarily for tools that reach a temperature above 3000 F. Chambering reamers have a large area of cutting edge in contact with the work and are susceptible to chatter and breakage at speeds that generate heat above 300 F. Thus, the most important quality of high-speed steel is utilized only by the toolmaker during the grinding process. It is more forgiving to a careless grinding operation than other steels and will withstand temperatures as high as 11 00 F. with no loss 0

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Note: Drawings giving maximum cartridge and minimum chamber dimensions of U. S.-loaded small arms ammunition are obtainable at $5 per cartridge from Sporting Arms & Ammunition Manufacturer's Institute, 250 East 43rd St., New York 17, . Y. commercial

REA

of hardness. Its heat-treatment is involved and i best left to the professional, but cold work and carbon steel may be hardened in the small shop. The beginner should select a steel that is suitable for reamers and also ea y to grind. One of the most easily ground suitable steels is designated SAE 06. It is classified as a 'non-deforming die steel' and has a grindability alue of 85. This is a steel that the beginner may use while developing grinding skill. Two popular brands of SAE 06 steel are Allegheny Ludlum's 'Oilgraph' and Tirnken's 'Graphmo'. Chambering reamers must cut the full length of the chamber. They cannot be operated as fast as end-cutting reamers; red-hardness and other advantages of some of the other tool steels are not required. Getting a good fini h at reduced speed is the main requirement. Tool steel for reamer i available in

various forms. On rod or round bar form. Drill rod h finish (not preci mon high-speed are available in stock is less expeasrve, has 'bark' on i be removed. It . 1116" waste for diameter. For clla~:;>::;;-:::. .30-'06 size (.4'7-- • 9/16" diameter • to about %". I grinding. Drill l/z" diameter as ties on its surfa available ill 3-ft. lengths often run The illustratio and the followi : steps in _ produ chambering ream

Lathe operations with SAE 06 steel are easy. For a reamer such as the .30-'06 a 1 . rpm's with .~03" feed is used in our shop with a much tougher tool steel, making diameter, Carbide-tipped Grade K-6 tools are used for all turning. When high-speed steel t the rpm's should be kept below 500, condition of the lathe and coolant system being the factors. No sharp corners are left when turning the blank. The turning tool has about an .0 lessen the chance of hardening cracks during the heat-treatment. We leave .010" grinding stock and shank and .025" to .030" grinding stock on the rest of the blank. All blanks are tura to provide .030" stock at the back end and .062" grinding STock at the shoulder of a .30Shoulder angles are plunge-en from the rear tool post

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The milling cutter used to the flutes may be one of several types. The most common' the 60 cutter, shown on the right. The sharp corner of these cutters are radiused to avoid hardening cracks. Minimum radius is about .030". The 60 cutter will produce flutes 0 ear uniform thickness from top to bottom. The cutter on Iefr I one made and used by master toolmaker F. K. Ellio 0 his fine chambering reamers. The flute produced with '- radius is much stronger than the other types. Note the tapered e and thicker crosssection of the reamer section on left. I ilmet 29 is used as a milling machine cutting oil, but m good grade sulfur-based cutting oils will work well ith AE 06 steel

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The reamer flutes should be milled a to reduce chatter. Or the flutes may 3 solid stock if that is more practicable.

irregular intervals ground from the . g formulas may be found in the Machinery's Handbook. For a 6-fluted reamer an index circle with 39 holes is placed 0 the dividing head. The first flute is then milled. The second te j milled 4 holes less than regular spacing, the third is ed 5 holes more, the fourth 7 holes less, the fifth 6 holes more and the last 5 holes less than regular spacing. A produc 'on milling fixture is usually built with the irregular spacing machined into an index plate. The milling fixture shown has a bination center and dog which is more conducive to ra id loading and unloading than a standard center and dog would . This center has been ground with 5 .

JANUARY

1963

the milling machine the reamer goes back to the 4carbide-From lathe for the tang or shank squaring operation. A set of tipped cutters is used to straddle-mill the squares. A cut is taken and the reamer then indexed 90° for milling the second pair of flats. The reamer then goe to the marking machine for stamping of identification and caliber

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The heat-treating or hardening of SAE 06 steel is less difficult than that of water- or brine-quenching steels. With a common furnace like the one hown, it is best to do something about the atmosphere contacting the reamer. Timken recommends painting a saturated solution of boric acid over the surface of the tool to avoid decarburization. There are also good commercial nonscaling compounds available, such as Phoenix Brand, sold by the Parker Stamp Works of Hartford, Conn. This is good for hardening temperatures up to 1650° F. and covers more easily and completely than does boric acid. The flute faces, being a part of the cutting edge, should be protected from decarburization. This is no problem if the flutes are to be ground after heat-treatment, since the scale is easily ground away. SAE 06 tool steel should be heated to about 1450° F. and soaked at this temperature only long enough to insure uniform heating. The reamer should then be removed as quickly as possible and quenched vertically in a light quenching oil. Quench with a gentle up-and-down motion until it has cooled to below 200 F. (very warm to the touch). It is apt to warp if dropped to the bottom of the quenching tank too soon. Also, any swishing motion is likely to cool one side of the reamer more quickly than the other and cause warpage. A quenching oil temperature of about 100° F. is recommended to minimize warping and cracking. After cooling to oil temperature. the reamer is taken out for tempering. Relief of machining and hardening strains demands about 2 hours tempering time at 300 F. A kitchen oven will do nicely. It is best to let the reamer cool in the oven. It may then be tested for hardness. SAE 06 steel should have a working hardness of 63 on the Rockwell C-scale 0

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From the hardness tester the reamer goes to the drill press for checki ng of its concentricity (above). It is then centerlapped with diamond lapping cones (below). During this process the centers may be moved over to minimize any warpage. The centers may be lapped with other types of mounted points, such as aluminum oxide 60° center laps. It is very important to lap tbe centers to remove any scale and out-of-roundness and to provide good cones for the reamer to rotate on during grinding. The lower center is fitted to a precisely aligned bole in tbe drill press table. Tbe lower center also goes through a thin plywood table top

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If the reamer is to bave ground fiutes. this is the next operation. It is important to smooth the entire flute so that chips will flow from its face less chance of bonding or fusing. If the backs of the flutes are also ground smooth, the chips will flow out more easily with less packing. Three wheels are used. Roughing is done with a 60-grit aluminum oxide wheel, followed by a 150grit aluminum oxide wheel. Final polishing is done with a fiber MX wheel and finishing compound. Grinding procedures vary, but we grind tbe shanks first. then the pilot shanks followed by cylindrical grinding of the throat, body, neck, and shoulder. With some grinders it is best to circle grind one section of the reamer and then back it off, or relieve it, before changing the taper for another portion of the reamer. Lengths are scribed on the backs of the fiutes using a height gauge. With a good eye Ioupe, it is fairly easy to work to the layout lines. A micrometer-. stop is a must. There are several ways to grind clearance on a reamer. The simplest method is to circle grind to size, tben back off the land leaving a margin no more tban .002" wide. The margin is that portion of tbe cylindrical width of a flute that remains after the reamer bas been backed off. On straight reamers this margin .' is easy to see, being as wide as .020" or so on a 1/2" reamer. A second and somewhat better method, is to circle grind to size plus .001" or so. Then grind tbe primary and secondary clearances. Primary clearances for reamers such as the .30-'06 may be about r for the tbroat and neck and 3 ° for tbe body. Secondary clearances should be about 16° on the throat and neck and 8° for tbe body. If tbe fiutes are thick it will be necessary to grind a third clearance so that tbe beel will not' be too high. Sharpened staggered-flute reamers will mike out of round, so it is necessary to observe the dial reading when the margin disappears and go by the dial. If ring gauges are used the secondary clearance must be ground on first. or the gauges will not come in contact with the primary clearance and cutting edge. A spring-finger may be u ed on most reamers for tbe short section such as the throat and neck, but a slide finger must be used on the body unless it has been ground straigbt. A tbird type of clearance is obtained by cam grinding tbe entire cutting relief. This may be done as a circle grinding operation and is possibly the faste t method. The cams must be precisely timed to match the spacing of the fiutes. This can be a ratber difficult problem \ ith staggered-flute reamers. Tbe flutes must be uniform and not warped. or tbe cutting edges will not be of tbe same heigbt. One or 2 of tbe flutes will do all the cutting and the chamber is likely to be oversize. A fourth and more sopbi ticated method is used by F. K. Elliott. The reamer is circle ground to size plu a predetermined amount, then the secondary clearance is ground first so that it completely eliminates tbe margin. The reamer is then placed in a stoning fixture and the primary clearance carefully hand-stoned. Fixture adjustments control the learance. The cutting edge produced with tbe stone is much smoother tban that obtained with a grinding wbeel. Stoning must be done uniformly or an out-of-round condition will result. This method is more difficult than tbo e previously de ribed, and it is used only for the finest reamers. Selection of grinding whee for SAE 06 tool steel is not as critical as for some of the other reels. For ir Ie grinding, a wheel such as 38A60-K5VBE may be used. For grinding the learan one might choose a 38AlOO-H7VBE wbeel. Steel manufacturer often recommend wbeel gradings for their brands •

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E A

ERICAN

RIFLEMAN