MAKINGCHAMBERING REABy KEITH FRANCIS

ONL Y 3 of the 5 basic tool steel typesavailable are suitable for making

chambering reamers. These are thehigh-speed steels, the cold work steels,and the carbon and low-alloy steels.

High-speed steel is used ordinarilyfor tools that reach a temperature above3000 F. Chambering reamers have alarge area of cutting edge in contactwith the work and are susceptible tochatter and breakage at speeds that gen-erate heat above 3000 F. Thus, the mostimportant quality of high-speed steel isutilized only by the toolmaker duringthe grinding process. It is more forgiv-ing to a careless grinding operation thanother steels and will withstand tempera-tures as high as 11 000 F. with no loss

Note: Drawings giving maximum cartridge andminimum chamber dimensions of U. S.-loadedcommercial small arms ammunition are obtain-able at $5 per cartridge from Sporting Arms &Ammunition Manufacturer's Institute, 250 East43rd St., New York 17, . Y.

of hardness. Its heat-treatment is in-volved and i best left to the profes-sional, but cold work and carbon steelmay be hardened in the small shop.

The beginner should select a steelthat is suitable for reamers and alsoea y to grind. One of the most easilyground suitable steels is designatedSAE 06. It is classified as a 'non-de-forming die steel' and has a grindabilityalue of 85. This is a steel that the

beginner may use while developinggrinding skill. Two popular brands ofSAE 06 steel are Allegheny Ludlum's'Oilgraph' and Tirnken's 'Graphmo'.

Chambering reamers must cut the fulllength of the chamber. They cannot beoperated as fast as end-cutting reamers;red-hardness and other advantages ofsome of the other tool steels are notrequired. Getting a good fini h at re-duced speed is the main requirement.

Tool steel for reamer i available in

svarious forms. Onrod or round bar -form. Drill rod hfinish (not precimon high-speedare available instock is less expeasrve,has 'bark' on ibe removed. It .1116" waste fordiameter. For clla~:;>::;;-:::..30-'06 size (.4'7-- •9/16" diameter •to about %". Igrinding. Drilll/z" diameter asties on its surfaavailable ill 3-ft.lengths often run

The illustratioand the followi :steps in _ produchambering ream

1 Lathe operations with SAE 06 steel are easy. For a reamer such as the .30-'06 a. 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 tthe rpm's should be kept below 500, condition of the lathe and coolant system being thefactors. No sharp corners are left when turning the blank. The turning tool has about an .0lessen the chance of hardening cracks during the heat-treatment. We leave .010" grinding stockand shank and .025" to .030" grinding stock on the rest of the blank. All blanks are turato provide .030" stock at the back end and .062" grinding STock at the shoulder of a .30-

Shoulder angles are plunge-en from the rear tool post

2 The milling cutter used to the flutes may be one ofseveral types. The most common' the 600 cutter, shown

on the right. The sharp corner of these cutters are radiusedto avoid hardening cracks. Minimum radius is about .030".The 600 cutter will produce flutes 0 ear uniform thicknessfrom top to bottom. The cutter on Iefr I one made and usedby master toolmaker F. K. Ellio 0 his fine chamberingreamers. The flute produced with '- radius is much strongerthan the other types. Note the tapered e and thicker cross-section of the reamer section on left. I ilmet 29 is used as amilling machine cutting oil, but m good grade sulfur-based

cutting oils will work well ith AE 06 steel

The reamer flutes should be milled a irregular intervalsto reduce chatter. Or the flutes may ground from the

solid stock if that is more practicable. . g formulas maybe found in the Machinery's Handbook. For a 6-fluted reameran index circle with 39 holes is placed 0 the dividing head.The first flute is then milled. The second te j milled 4 holesless than regular spacing, the third is ed 5 holes more,the fourth 7 holes less, the fifth 6 holes more and the last 5holes less than regular spacing. A produc 'on milling fixtureis usually built with the irregular spacing machined into anindex plate. The milling fixture shown has a bination centerand dog which is more conducive to ra id loading and unload-ing than a standard center and dog would . This center has

been ground with 5 .

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JANUARY 1963

4 From the milling machine the reamer goes back to thelathe for the tang or shank squaring operation. A set of

carbide- tipped cutters is used to straddle-mill the squares. A cutis taken and the reamer then indexed 90° for milling the secondpair of flats. The reamer then goe to the marking machine for

stamping of identification and caliber

5 The heat-treating or hardening of SAE 06 steel is less diffi-cult than that of water- or brine-quenching steels. With a

common furnace like the one hown, it is best to do somethingabout the atmosphere contacting the reamer. Timken recommendspainting a saturated solution of boric acid over the surface of thetool to avoid decarburization. There are also good commercial non-scaling compounds available, such as Phoenix Brand, sold bythe Parker Stamp Works of Hartford, Conn. This is good forhardening temperatures up to 1650° F. and covers more easilyand completely than does boric acid. The flute faces, being a partof the cutting edge, should be protected from decarburization.This is no problem if the flutes are to be ground after heat-treat-ment, since the scale is easily ground away. SAE 06 tool steelshould be heated to about 1450° F. and soaked at this temperatureonly long enough to insure uniform heating. The reamer shouldthen be removed as quickly as possible and quenched vertically ina light quenching oil. Quench with a gentle up-and-down motionuntil it has cooled to below 2000 F. (very warm to the touch).It is apt to warp if dropped to the bottom of the quenching tanktoo soon. Also, any swishing motion is likely to cool one side ofthe reamer more quickly than the other and cause warpage. Aquenching oil temperature of about 100° F. is recommended tominimize warping and cracking. After cooling to oil temperature.the reamer is taken out for tempering. Relief of machining andhardening strains demands about 2 hours tempering time at3000 F. A kitchen oven will do nicely. It is best to let the reamercool in the oven. It may then be tested for hardness. SAE 06 steelshould have a working hardness of 63 on the Rockwell C-scale

From the hardness tester the reamergoes to the drill press for checki ng of

its concentricity (above). It is then center-lapped with diamond lapping cones (below).During this process the centers may be movedover to minimize any warpage. The centersmay be lapped with other types of mountedpoints, such as aluminum oxide 60° centerlaps. It is very important to lap tbe centers toremove any scale and out-of-roundness and toprovide good cones for the reamer to rotateon during grinding. The lower center is fittedto a precisely aligned bole in tbe drill presstable. 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 isimportant to smooth the entire flute so that chips will flow from its face

with less chance of bonding or fusing. If the backs of the flutes are also groundsmooth, the chips will flow out more easily with less packing. Three wheels areused. Roughing is done with a 60-grit aluminum oxide wheel, followed by a 150-grit aluminum oxide wheel. Final polishing is done with a fiber MX wheel andfinishing compound. Grinding procedures vary, but we grind tbe shanks first. thenthe pilot shanks followed by cylindrical grinding of the throat, body, neck, andshoulder. With some grinders it is best to circle grind one section of the reamerand then back it off, or relieve it, before changing the taper for another portionof 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 simplestmethod is to circle grind to size, tben back off the land leaving a margin no moretban .002" wide. The margin is that portion of tbe cylindrical width of a flutethat 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 some-what better method, is to circle grind to size plus .001" or so. Then grind tbeprimary 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. Secondaryclearances should be about 16° on the throat and neck and 8° for tbe body. Iftbe fiutes are thick it will be necessary to grind a third clearance so that tbe beelwill 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 andgo by the dial. If ring gauges are used the secondary clearance must be groundon first. or the gauges will not come in contact with the primary clearanceand cutting edge. A spring-finger may be u ed on most reamers for tbe shortsection such as the throat and neck, but a slide finger must be used on the bodyunless it has been ground straigbt. A tbird type of clearance is obtained by camgrinding tbe entire cutting relief. This may be done as a circle grinding operationand is possibly the faste t method. The cams must be precisely timed to match thespacing of the fiutes. This can be a ratber difficult problem \ ith staggered-flutereamers. Tbe flutes must be uniform and not warped. or tbe cutting edges will notbe of tbe same heigbt. One or 2 of tbe flutes will do all the cutting and thechamber is likely to be oversize. A fourth and more sopbi ticated method is usedby 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 tbemargin. The reamer is then placed in a stoning fixture and the primary clearancecarefully hand-stoned. Fixture adjustments control the learance. The cuttingedge produced with tbe stone is much smoother tban that obtained with a grind-ing wbeel. Stoning must be done uniformly or an out-of-round condition willresult. This method is more difficult than tbo e previously de ribed, and it isused only for the finest reamers. Selection of grinding whee for SAE 06 tool steelis not as critical as for some of the other reels. For ir Ie grinding, a wheel suchas 38A60-K5VBE may be used. For grinding the learan one might choose a38AlOO-H7VBE wbeel. Steel manufacturer often recommend wbeel gradingsfor their brands •

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E A ERICAN RIFLEMAN