Bending 4130 Chrome-moly



Kent,
Well, its time to start bending tubing on the airplane project.... The initial need is to bend a 2ft long piece of 3/4"X0.035" round 4130 tubing on a 27" radius. Later on the plans call out bending 3/4"X0.095" into a catenary arch for the roll bar. Some people are using thinner wall for this but after what happened to Charlie Hilliard (got smushed when his plane flipped over while taxi-ing) I'm not taking any chances. JM

The Tin Man Respondeth:

Gentle bends may be made with empty tube by spanning it over two shot bags spaced a few inches apart. Thumping the tube with a rubber-faced dead blow hammer will then arch the tube. The bend limit depends on tube dimensions,temper, and alloy, and the radius needed.

Filling the tube with some material helps a great deal. Cold-poured materials like sand, shot, glass or plastic beads (all packed tightly), will help support the wall of the tubing while thumping, or even machine-bending. Hot-poured materials like solder, lead, serrobend, or Kirksite will enable tighter bends, yet. Mercury was used in the Texas oilfields years ago to bend many lengths of copper tubing into multiple tight-radius bends, which upon installation had accuracy and appearance like a printed-circuit board!

Ultimately, the ball-pulling mandrel bender (Pines, etc.) make the uniform tight bends we buy for making headers etc. In Europe, there is a method of using a lathe to rotate the tube onto a curved mandrel for making perfect circles. I made up a ball-pulling mandrel bender years ago for polished 3/8 copper tubing. One high-end restoration needed a double "J" on a 12" length which was in plain view on the firewall. It turned out just like the remains of the orig.

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