T.O. 1-1A-9
c.
Form material across the grain when possible using correct or specified bend radii. Also provide bend relief in
corner when required.
d.
Observe load capacity of equipment such as brakes, presses, rolls, drills, lathes, shears, mills, etc.
CAUTION
Machines rated for carbon steel shall not be used over 60% of rated capacity when cutting,
forming or machining stainless steel unless approved by responsible engineering activity.
When in doubt inquire.
e.
Tool and equipment shall be maintained smooth, free of nicks, rust, burrs and foreign material. In addition to
above, dies, ways, etc., shall be checked for alignment tolerances, etc., periodically/ each set-up.
f.
Surfaces of material, especially finished sheet, shall be protected from scratching, foreign particles, etc. These
surfaces can be protected using non-corrosive paper, tape, other approved material and good cleaning procedures.
Polished sheet material should be protected when forming to prevent die tool marking.
g.
After forming/machining is completed, remove all cutting lubrication, etc., by cleaning, degreasing, pickling, prior
to any heat treat, plating or painting process.
CAUTION
Avoid handling parts, especially corrosion resistant steel, with bare hands after cleaning and
subsequent to heat treating/ passivation because finger prints will cause carburization and
pitting of surface, when heated.
2-228. BENDING (SINGLE CURVATURE). The bending of most steel sheet and thin bar stock can be readily
accomplished provided that equipment with adequate bending and cutting capacity is available and if the materials are
formed in the soft condition/lower temper range. The heat treatable alloys are usually formed in the annealed or
normalized condition and heat treated if required/specified after forming. Some difficulty will be encountered from
warping due to heat treating and precautions must be taken when forming the material to prevent sporadic or uneven
stress in the work piece. Also, parts will require jigs or close control during the heating and cooling phase of heat
treatment. The use of heat treated formed sheet metal parts on aerospace craft are usually an exception in part due to
above and most materials are used in the normalized or annealed condition.
2-229. Springback allowance will vary according to the type and temper of material being formed. The use of sharp
bend radii on parts for aeronautical application shall be avoided and other application where the parts will be subjected to
flexing (cycle) or concentrated stresses, due to possible fatigue or stress corrosion failure. For recommended General
Bend Radii for use on Aerospace weapon/equipment (see Table 2-25 for Low Carbon/low alloy steel and Table 2-26 for
Corrosion Resistant Steel.)
2-230. In utilizing Table 2-25 and Table 2-26 it is recommended that in practice bend area be checked for strain, grain,
or bend cracking. If parts show presence of above, increase radius by one thickness or more until difficulty does not
exist. Other details, inspection requirements, etc., shall be used when specified.
2-231. DRAW FORMING. Control of die design, and material from which dies are made, are essential to successfully
draw form steel. For long production runs, high carbon, high chromium steel is recommended to manufacture drawing
dies because of wear resistance and hardness. For medium and short production runs, Kirksite/case zinc alloy can be
used with drop hammer hydraulic press if the draw is not severe. Hardwood and phenolic can be used in some cases for
piece production where draws are shallow.
2-232. Successful drawing of steel will depend on:
a.
Radii used for forming or bending Use moderate radii, usually equal to 3-6 times thickness of material depending
on specific requirements, and the severity of draw.
b.
Finish of die-all scratches and surface roughness should be removed.
c.
Blank hold down pressure and drawing rings Hold down pressure should be sufficient to prevent wrinkling of
material, but not to the extent that would prevent flow of the metal into the female portion of the die. Drawing rings radii
should be 4-8 times metal thickness and smoothly polished.
d.
Clearance between punch and die - Generally punch clearance should be about 1 1/4 to 1 1/2 times thickness for
the initial draws, and about 1 1/8 to 1 1/4 times for the following draws. If parts show signs of galling, clearance (drawing)
should be increased when clearance is increased, size requirement must be considered.
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