T. O. 1-1-1/A-9
e.
Temper-drawing should be started with annealed/normalized material and intermediate annealing accomplished
as required. The requirement for annealing (intermediate) usually is needed after reduction exceeds 30-35% for
stainless/20-25% carbon steel on the initial draw, and when reduction exceeds 8-15% on each following draw. Parts
should be cleaned removing all lubrication and other contaminate prior to annealing and desealed after anne2liug. In
instances where draws exceed 22-25%annealingis recommended after completion of the drawing operation followed by
descaling and passivation (stainless). Restriking on final stage die to remove distortion after final anneal is permissible
without further heat treatment.
f.
Drawing Speed - Generally a speed of 20-55 feed per minute is satisfactory. Drawing using a hydraulic powered
press in lieu of a cam operated or toggle type press is usually the most satisfactory.
g.
Lubricant Compounds - used should be of heavy consistency capable of withst2nding high temperature and
resisting pressure necessary to form material. One heavy bodied lubricant used is lard oil, sulfur (one pound of sulfur to
1 gallon of oil) to which lithopone is added in equal parts until consistency equals 600W engine grease, or as desired.
Other compounds such as tallow, mixture of mineral oil and soft soap, powdered graphite mixed to thin paste with
lightweight oil can be used.
h.
Blank size and preparation - A good practice is to use minimum sine required to meet dimensional size of parts
and for hold down. When trimming, consideration must be given to the fact that on rectangular parts, the majority of
drawings will occur on wider portions of the rectangle away from the corners. To overcome this problem, the radius of
the vertical corner should be approximately 10% of the width. Trial, using a very ductile material to determine blank size
and stress areas prior to starting the forming operation is recommended. After size is determined by trial, etc., the blank
should be filed/polished to prevent cracking in wrinkle/stress areas, handling hazard and surface friction which hinders
flow of metal into die.
2-233. The surface condition of the blank also has an effect on drawing. A slightly roughened surface, such as obtained
by pickling (dull surface) improves control of metal under hold down pads and the holding lubricants. On the other hand,
the roughened surface may be less desirable because of greater friction, especially where free flowing drawing methods
are used (without hold down).
2-234. Where facilities are av2ilahle, cold forming of some steels (primarily straight chromium stainless such as 410,
416,430, 442, 446) can be improved by preheating dies and blanks. The preheating tends to reduce work hardening and
the requirement for intermediate annealing during the drawing operation.
2-235. When forming involves more than one draw, the first operation should be a moderate draw with punch diameter
equal to 60% of blanks diameter and reduction of 15-25%. The second and subsequent draws should be made with
punches about 20%. It is recommended that part be cleaned and annealed following each draw. Excessive distortion
may result from final annealing after last draw. This problem can be overcome in most instances, by reducing the
severity of the last draw or restriking after final annealing on last stage die for the purpose of removing distortion.
CAUTION
Parts shall be cleaned of all contaminates, lubrication, filing, other foreign material, etc.,
before heating or annealing and upon completion of forming or drawing operation. Failure to
clean the parts will result in pitting and carburization, which will damage the surface.
2-236. STRETCH FORUING. Stretch forming is a process where material, sheet or strip, is stretched beyond the elastic
limit until permanent set will take with a minimum amount of springback.
2-237. The stretch forming is usually accomplished by gripping ends of material (b1nk) and applying force by a separate
ram carrying the forming die. The ram pressure sufficient to cause the material to stretch and wrap to contour of the die
form blank is applied perpendicular to the blank (see Figure 2-4). This method of forming is usually limited to parts with
large radii of curvature and shallow depth, such as shallow dishing, reverse curves, and curved pan shaped parts
containing flat areas.
2-238. The trimming of edges and removal of nicks and scratches is important to prevent starting points for concentrated
stress, which, under tension loads, would tear. The direction of major tension (stretch) and direction of grain is also
important. It is recommended in forming that the major tension be transverse to the direction of grain. Lubrication aids
in uniform distribution of stress and the lubricant shall be applied uniformly to work piece to avoid distortion which could
result from unequal friction when material is sliding across the forming die during stretching.
2-239. Forming dies/blocks for general production are made from kirksite/zinc, alloy; for piece production from phenolic
and hardwood. Some types and kinds of plastic with good hardness and high impact strengths are also used. The
rubber pad hydraulic press is used to form relatively flat parts having flanges, beads, lightening holes, and for very light
drawing of pan shaped parts having large radii.
2-116
