Mortars Mortar Plans

8/2/2019 Mortars Mortar Plans

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By William B. ParkPhotosyAuthor

small radius point. The front and sideclearance is kept at a minimum toprevent digging.Lock the cross slide. The compoundrest is now backed off until the tool is1-1/8" from the center of the vertical axisof the compound rest. Do not fret aboutthis; you can adjust as the work proceeds.Now it gets tricky. Loosen the screwsthat control the swivel of the compoundrest until the swivel turns smoothlythrough an arc as you move it by hand.Swing the swivel to the opposite sides ofthe bar and adjust by means of thecompound screw so that it just clears bothcorners of the cylindrical workpiece.Return the swivel to the mid-point,start the lathe, ad vance the carriagetoward the headstock a small amount, andswing the tool by hand to cut the nearcorner of the work. The far corner willtake care of itself. Return the swivel tomid-point. If the lathe is loose in the bear-ings or the swivel, it might be wise to stopthe lathe when returning to take anothercut. Do not turn the compound screwduring cutting. All cuts are made by mov-ing the carriage toward the work in smallincrements (Photo 4). It should be lockedduring each pass.

As Photo 5 shows, the overhang islarge -in fact, in this instance, larger thanneed be because I did not want to lose aninch of the bar, so I used it full length offthe shelf. However, I suggest you keep heoverhang to a minimum. The minimum,unfortunately, is large, since the swing ofthe compound rest must clear the chuckjaws as they whirl past. Check his beforeyou start.Some lathes, such as the Logan, onwhich this mortar was built thirty yearsago, have small dials and cranks on thecompound rest. This enables the crankend to be swung under the workpiece,and with a little devising, the standardtool post can be used. Since the progressof the tool along the arc is controlledentirely by the pressure of the hand, itmay be advantageous to rig some kind ofhandle for increased leverage. Light cutsand a spindle speedof 500 rpm are the onlyway to go.Drill a center in the hemisphereand support the work with the tailstockdead center as you machine the ODbetween the bands. Cut to length. Leave asixteenth for finish.Reverse the piece in the chuck.Protect finished surfaces with masking

M Y neighbor to the south accostedme one day."Bill," says he, exhibiting a 1" steelball, slightly rusty, "look what I found inmy hand-split shakes."I knew it was a 1" ball because hadmeasured it recently, but I said, creatively,"It must have fallen from an airplane."If it had fallen from a plane, it mighthave been in his cellar, not in his roof. Ihave no idea of the terminal velocity of asteel ball falling from 40,000 feet. Ineglected to mention this.He looked at me closely, grumbleda little, and, pocketing my steel ball,stalked away.I mention this incident in case any ofyou cannon aficionados are planning totest fire this mortar.The mortar is a model of a gun I sawon the north bank of the Thames, hard bythe Tower of London, where the Englishdisplay some of their trophies of war. Itook photographs and measurements.This guRis not in any scale since I startedfrom the 1" ball, but the proportionsare right (Photo 1). Mortars of this typewere mounted on the fored~ck of a bombketch which, under cover of darkness,crept in close under the walls of a harborfortification. From this vantage point, safefrom the guns of the fort which could notbe depressed to bear on the little ship, itwould lob"bombshells up and over thebattlements to the confusion of the enemygun crews. The forestay of the mainmastwas a chain, to withstand the heat andblast of the mortar.Our mortar is all brass, for ease ofworking, finishing and brazing (Photo 2).Mine was largely ma~e from the contentsof the scrap box.Do not hesitate to change details aslong as the proportions are preserved. .Ifthe brass bar you have for the barrelhappens to be 2-1/2" in diameter, youcould choose to use it and increase alldimensions 10%. Without any change inother dimensions, the thickness of theplates for the carriage could 'be varied bya sixteenth thicker but no thinner.A characteristic of this mortar is thehemispherical shape of the breech, orbutt. Since this is an unusual operation, itis shown as it is set up. A special tool postis mounted on the cQmpound rest. Thetool bit is set on center of the workpiece,horizontally a{\d vertically, with thecompound set at 0'?,or at right angles tothe cross slide and parallel to the ways(Photo 3). The bit -in this case, he shankof a broken 1/4" high-speed tap -isground with 0back and side rake with a20 LIVE STEAM November1989

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---4 5Dolphins earned the name from theearly ornate design of cannon. The cast-inrings or loops to be used in swaying theguns in,to place were shaped with eyes,mouth and tails. They were covered withscales,and cast n lively attitudes.The ~act location of the dolphins isnot critical, but the holes to receive themmust be drilled on a diameter of thebarrel and be syMmetrical with respect tothe tnfnbion and the axis of the gun. Thedolphins are made of 3/16" diameter x 2"bra...od, heated to redness and plungedin a pickle of 10% sulfuric acid. This isdangerous, Protect your face and clothes. Iknow about inexplicable moth holes incotton jeans, but Virginia doesn't. Thevirtue of sulfuric acid is that it does notrust the machinery, and machinery ismore important than clothing. Muriati


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Easy-Flosilver solder is like glue in that itdoes not make fillets at th~ joints. Thetrunnion is already securely fasteneJ;the gussets are tied in with blackstovepipe wire wound around them; thedolphins are a snug fit in the holes; andthe touchhole cup can be fastened withtwo or three silversmith's stitches.A stitch is made with a graver. Underthe piece to be fastened, a stitch is curledup to the location of the edge of the piecepreviously marked. After two or threechipS have been curled, the piece isplaced in position and the stitchespressed against ~he workpiece. Somebrass is brittle. If this is the case, a small

of solder between the joints to ensurethat enough will be available to float,but you must have a coil of wire solderin hand to touch up all joints. Flux alljoints thoroughly.Place the assembly face down on afirebrick covered with a sheet of asbestos(a dirty word, but I have been using thisstuff for many years) or some approvedrefractory. With a large propane torch,slowly bring the workpiece up to heat(Photo 6).

You will see the preplaced pieces ofsolder start to melt. Move in deftly withthe wire solder to touch up the manyjoints. Remove the flame and, as soon as

cape chisel and a light hammer can beused to raise a chip of sorts. After brazing,the stitches are filed off. You will find thistrick handy in making nameplates and formany other problems of holding smallpieces n place for brazing.When all the pieces are clean of all oiland dirt by careful washing in solventand bound together in final form, theentire assembly is painted with a thinpaste of fluoride brazing flux to which afew drops of dishwasher detergent hasbeen added to make it wet the pieceuniformly. This is to hold down oxidationduring heating.It is wise to preplace thin sheets

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CARRIAGE SIDESmachine as a pair~ ~I- ., 11/4 [

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BOLLARD4 required PIN4 required TRUNNION CAP STRAP4 required22 LIVE STEAM / November 1989


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while brazing as above. The brass pinscan be filed flush on cooling, and theywill disappear. Two 3/16" holes shouldbe drilled in the baseplate to vent thepressure of brazing.Do not mill the trunnion seat untilafter brazing, since the trunnion mustbear evenly all the way across.The trunnionseat must be relieved to permit the barrelto swing forward (Photos 7 and 8).The holes for the pins of thehold-down straps are also drilled laterwith the trunnion caps in place. Eachstrap should be numbered to show position,and the trunnion caps and carriage, also.The trunnion caps give a distinctive

the solder has solidified, grasp the workwith tongs and lower it slowly into alarge vessel of acid as above, being ~iceas careful as before. The brown scale willrub off easily after rinsing well. Do not tryto remove remaining flux mechanically -it won't come off. Boiling water in a panwill remove it at once.Any slight etching by the acid can bepolished off; I chose to leave it on becauseit lends a fine flavor of antiquity.Now is the time to drill the touch-hole. Use number 44 drill- .086".The carriage, in the main, shouldfollow the drawings. The several piecescan be pinned to hold them in position

~ relieve to permit/ barrel to swing forward

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7air to the mortar (Photo 9). They are hin place by straps and pins, which suggthe age of the gun. More recent mortaas used in the Civil War, used threadbolts and nuts to hold down the caCommon use of screw threads iscomparatively recent development in long history of cannons.The caps are made as a pair. Apieof brass 3/8 x 1/2 x 2-1/2" is soft soldeedge to edge at one end of a similar b6" long. This extension serves as a hanfor the milling operation. Blue and out the two caps face to face. Drill aream the common hole 5/8". Mount piece on a 5/8" peg on the table the vertical mill (Photo 10), and with1/4" four-flute mill, mill the outside of cap by rotating it around the peg by ha(Photos 11 and 12). This is a trap, becathe milling cutter will pull the work oof your hand if you attempt to climb~i

November1989/ LIVE STEAM

Top

TRUNNION CAP2 required


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3/16" drillV8" deep fordolphins

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24 LIVE STEAM / November 1989


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14 15The finished mortar is shown inPhotos 14 and 15.Finally, this mortar can be fired, withor without ball, as hinted at the beginningof this article (Photo 16). Photographing acannon at the moment of discharge takes

some doing. The small wire that you seeabove the touchhole leads to the outputterminal of a high voltage neon signtransformer, controlled by a foot switch.When the photographer steps on theswitch and sees the flash in the pan, hepresses the cable release. I tested thesetup by firing a small charge insidethe shop and the smoke detectors wentout of their collective minds. Outside, Igot two acceptable pictures out of six.Mortars of this type were used in thecivil War as coast defense weapons. Theycould drop their explosive shells on thelightly armored decks of ironclads such asthe Monitor, and would be deadly againstthe bomb ketch mentioned earlier. It wasimportant, when loading the sphericalbombshells, to point the fuse forward. If itwas pointed to the rear, the gun wouldblow up. They were sometimes mountedon short eight-wheeled flatcars, and whenset up on a curved track, would haveconsiderable traverse. This excerpt fromArtillery and Ammunition of the Civil Warby Warren Ripley gives us an idea of theeffectiveness of mortar fire:"The mortar, fired with 14 pounds ofpowder, recoiled less than two feet on thecar, which moved 10 or 12 feet on thetrack. The effect of the charge was takenup without damage to the axles...Thismortar, whose shell would crush andexplode any ordinary field magazine,excited dread among the Confederategunners, and was effective in inducingtheir enfilading batteries...to discontinuefire on the right of our line."

CC??/ 16The pins are as shown. I brazed thesquare brass heads on stainless rods,because brass on brass does not slide sosmoothly cthd sometimes galls and seizes,particularly if the pins are taken out andreinserted from time to time whenshowing off the a\odel. The pins are insheer ittfiey are properly installed. The

straps should be padded if necessary tohold * trunnions firmly.The carriage is finished a dark brownto contrast with the polished barrel andtrunnion caps. Use a chemical dip, theformulation of which is too lengthy forthis already too long dissertation. I will beglad to furnish it on request, or if there isenough interest, maybe Joe Rice wouldprint it.

the work. Keep the RPMs up, workagainst the cut, and take small bites; if indoubt, turn off the machine for the non-cutting return stroke. The radius ends of thestraps can be made in the same way. Theselittle jobs hardly warrant a rotary table.The flats to receive the straps aremilled. All that is left is filing the raisedrounded ends, after the pieces are meltedapart, and the excess solder wiped offwhile stillliq\iid. A filing machine is amodelmaker's dream (Photo 13)...howsweet it is! -The trunnion straps are made in thesame way as the dolphins. Another bend-ing jig is made, with a 1/4" slot to holdthe flat. The punch is now 3/8" thick andthe die has a 9/16" opening 3/4" deep.

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Mortars Mortar Plans