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BYTONYBINGELIS, 8509GREENFLINT LANE, AUSTIN, TEXAS78759

HOW TO BEND TUBINGSUCCESSFULLY

F

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make it almost impossible for you to form a bend neareither end of a %" diameter steel (o r a l u m i n u m ) tube.Always start with a long length of tubing when bendingit. The larger the diameter of the tubing, the greater itsexcess length should be. Allow an extra 12" on each endfor tubing up to %" in diameter. Allow even more fo r largerdiameters . . . you will need th e added leverage it affords.If yo u find yo u need more leverage, you can always slipin a steel rod or slide a larger diameter tube over the endto serve as an extended handle of sorts.

Equipment NeededYo u don't need much in the way of equip men t or mate-rials to make good uniform bends. These items are defi-nitely needed.1. You will need a large rugged vise. The effectivenessof almost an y type bending device will be greatly improvedby clamping it in a heavy duty vise mounted securely ona solid bench. In addition to freeing both hands (and insome cases, feet), it will enable you to more precisely applythe bending pressure to the tubing. A substitute for thevise would be a good solid immobilized bench to which yo ucould bolt a bending device horizontally.2. Some sort of bending device is essential for any bendexceeding, say 15 to 20 especially when that bend isconcentrated around a small radius.3. A filler materia l (sand, bending alloy or salt) forthose hard to make bends.4. Templates cu t from plywood. How else can you checkthe bend you are making?

Tube Bending DevicesYo u ca n bend tubing successfully with just about an ysimple homemade tube bending device if it is properlymade. You have a wide variety of types from which tochoose.Locally you might have access to some commercial tube

bending machine. If so, great . . . just remember to bringyour templates, too.Others of you might be able to locate someone who hasan Electrician's Conduit Bender and arrange for its use.

It should allow you to make simple bends with a highdegree of success provided it can accommodate the diame-ter of the tubing you need to bend. Most of the tu bing w euse in homebuilts is either '/V' or W in diameter (some-t imes VK", too) . If the tube bender yo u have access to ismade fo r larger diameter tubing you probably shouldn'tuse it as it migh t cause your bends to flatten excessively.The design of an electrician's tube bender is simple enoughthat yo u could duplicate it for the size tubing you need tobend.A simple plywood bending form nailed to a work benchis a good basic bending device fo r large bends in smalldiameter tubin g. However, when cu tting the form to shapeyo u must ma ke it s curve sharper as the tubing will have atendency to spring back. This sort of bendin g device is easyto use for uniform curves because you can fasten one endof th e tub ing an d pull th e free end around the form causingit to bend smoothly in one easy sweep.An improved variat ion of the plywood bending jig(form) is one with its edges grooved to the diameter of thetube fo r which it is intended. The groove is important. Itreduces the tendency for tubing to flatten and somewhatsharper curves or bends in the tubing are possible. Aneven greater improvement would be to make the routedgroove somewhat deeper so that the tubing will nestle init beyond it s half diameter. This added depth permits th ewalls of the groove to exert a restraining effect againstth e tube's tendency to flatten. It is equally important,when making an y grooved pulley or bending jig, for thegrooved edges of the form to be strong enough to resist theflattening and w idening of the tube. A wood form, particu-larly a plywood form, is rather weak in this respect so itshould have considerable edge distance between the grooveand the edge of the pulley, form, jig or whatever. A groovedaluminum "pulley" would be much stronger but is harderto make if you don't have a large hunk of a luminum plateor a metal working lathe. Figure 5 shows some options formaking grooved tube bend ing devices, be they pulley typesor the simple wood jig types. Figures 1 through 5 shouldgive yo u enough ideas to help yo u devise a bending deviceof your own.

EXCESS TUBE LENGTH

A SIMPLE RIG FOR SIMPLE BENDSFIGURE I.

BENCH MOUNTED TUBING BENDERFIGURE 2.

SPORT AVIATION 49

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2" SQUARESHOLE FOR CLIP ATTACHMENT

(5/16" TYP)

PLYWOOD-FOR 1/4" TUBEUSE 3/8" PLY.FOR 1/2" TUBEUSE 5/8" PLY.

RESTRAININGI CLIPJ . MLOOSE FIT O.K.)

^WORK BENCHTUBE BENDING JIG

TUBE BENDING FORM IN USE (FOR SMALL DIAMETER TUBING)F I G U R E 3.

GROOVE TO TUBE D IA.WITH A ROUTER

STAY BLOCK(SELF ALIGNING)"PULLEY"-L A M I N A T E D W O O DO R A L U M I N U M( G R O O V E T O S U I TTUBE D I A M E T E R ) PLATE - MAKE 21/8 ALUM. OR STL. HANDLE -STEEL STRAP 1/8

PRESS DOWN ON TUBEWHILE PULLING ON HANDLE

3/4 STEELTUBE TO BEBENT CLAMP BENDERIN V ISE

I,. J 7*Y ^fS \1 TUBE BENDER DETAILS SIDE FRONTFIGURE 4.

Bending ProceduresTubing in aircraft work is usually bent cold. Contraryto what m ay seem logical and contrary to what you mayhave heard, attempting to bend tubing by heating it canturn out to be a lousy adventure . Most of us don't havethe skill and patience to play with a hot bend and will

usually get unhandsome results. The problem lies withlocalized uneven heating and poorly coordinated bendingpressures . . . to say nothing of impatience. Pressing a hottube against a bending form will surely cause it to flattenon the inside of the bend. Hand bending it without the aid50 AUGUST 1983

of a form is likewise very difficult to do successfully. Inshort, bend it cold.Heating tubing to a red hot condition does have a placein bending . Bending causes the metal to harden somewhat.So, by heating the tubing you can anneal it and continuethe bending carefully after the tube cools. In other words,it is possible to make bends over a smaller radius if youanneal the tube once or twice as the bend progresses. Thisis a slow way but it can make an otherwise severe bendpossible. There is another way to reduce the risk ofmakinga poor bend. Use a filler material.

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BEST

A NY BENDING DEVICE

ROUTEDRADIUSMETAL REINFORCEDSIDES

. PLYWOOD

-PULLEY" N O T G R O O V E DM E T A L S I D E S A D D E D. PLYWOOD

PRETTY GOOD/ W O O D " P U L L E Y "/ ( R E I N F O R C E D E D G E S )

PLYWOOD -' XJ IG OR FORM-NO RADIUS

POOR

FIGURE 5.

The Use Of Filler MaterialsIt is easy to see that packing a tube you w a n t to bendwith a solid material l ike sand or a mol ten bending alloywill greatly inhibit the flattening tendency.Before you attempt to bend any large diameter tube(3/t" or large r), you should fill it with densely packed sand.

(I understand that salt works as well al though I have nevertried it.) The sand has to be dry and well sifted to removeall foreign matter and the larger grains of sand. Plug th ebottom end of the tube with a wood plug and pour in thesand. Tap the bottom end of the tube repeatedly against

a solid surface (concrete floor). After the tube is f u l l ofsand , th e continued tapping will cause it to settle and packmore densely. Add addi t ional sand. After th e sand showsno further s ign of settling, drive a wood plug into the topen d of the tube being sure it bottoms solidly against th esand. Your tube is now ready for bending. The risk of af la t tened bend wi l l have been considerably reduced.A more effective fil ler, of course, is any of the commer-cial bending alloys, C E R R O B E N D , for example. Thesealloys have a very lo w melt ing po in t , some as low as 165F . W h e n t h i s bending alloy is heated to its melting pointit can be poured into th e tube you wa n t b e n t . . . sure, plugth e bottom first. Cooling th e tube in water will solidifyth e alloy and you are ready for the bending effort. Aftercompleting th e bend, th e tube has to be reheated to mel tou t th e alloy . The mater ia l can be reused any number oftimes. However, since it is un l i ke ly that most of us couldfind an economical source for a bending alloy, th e sandt r ea t men t is and will continue to be the one most used byhomebuilders .

More Abou t Th e Bending ProcessY ou cannot hurry your tube bend i ng chore. It wil lordinarily take mu ch longer than you would expect . Al lowyourself plenty of t ime fo r bends that do not have a uniformradius.Most any bending device yo u use, with the exceptionof a template-l ike bending jig, will have a rather smalldiameter grooved pulley, roller or disc over which th e tubewill be bent . Obviously, you cannot bend th e tube verymuch in any one place if your bend requires a radius man ytimes larger than th e bending device pulley . That meansth e bending process ma y become quite long as you have tobend a bit, check th e bend in the tube against a template,etc. A s an aid to checking th e accuracy of your bend asyou proceed, you should mark a center l ine around th etube to use as a reference mark keyed to your t empla te .(Use a black laundry mark e r on a l u m i n u m tubes and a

silver lead pencil on steel tubes .)Do not forget to check th e tubing after the bending iscompleted to be sure that it is not twisted, as viewed fromthe ends. Lay it on a flat surface for a quick check. Ifwarpage is present, a twist ing pressure in the properdirection will take out the w arp. Recheck th e tube's bendagainst the template again before cong ratulat ing yourselfon a job well done.

The component parts for this canopy frame w e r e bent a roun da 4" ho mem ade aluminu m pul ley. The tubing was n o t filled wi the i ther sand o r a bend ing al loy , ho wever , the ben d s are prob ab lyt he m in imum rad i us poss i b le w i t h o u t s u c h a provision.SPORT AV IATION 51