2-240. Form blocks are usually manufactured from steel, phenolic (mechanical grades), kirksite/zinc cast alloy, and
some types of hard molding plastic with high impact strength. The work is accomplished by setting the form block on the
lower press plate or bed, and the blank is placed on the block. The blank is held in place on the block by locating pins
(holes are drilled through the blank and into the form block for the insertion of the locating pins). These holes are
referred to as "tooling holes" which prevent slippage of blank when pressure is applied. If tooling holes are not allowed,
another method of alignment and holding of blank must be utilized. The sheet metal blank should be cut to size (allow
sufficient material to form flange), deburred, and filed prior to pressing. After the block is prepared and placed on. the
plate, the rubber pad filled press head is lowered or closed over the block, and as the hydraulic pressure (applied by a
ram to the head) increases, the rubber envelopes the form block forcing the blank to conform to the form block contour or
shape. It is recommended that additional rubber be supplemented in the form of sheets (usually 1/2 1 inch, hardness of
70-80 durometers) over the form block :3rd blank to prevent damaging the rubber press pad. The design of form blocks
for hydropress forming requires compensation for springback. The form for forming flanges on ribs, stiffners, etc.,
should be undercut approximately 2-8 degrees depending on the alloy, hardness, and radius. In some cases, it will be
necessary to use a combination of hand forming shrinking/stretching using supplemental machinery and pressing to
complete forming by this method.
2-241. DROP HAMMER FORMING. Dies for drop hammer forming are usually made by casting metals such as
kirksite. These dies can be rapidly produced; are more economical than permanent dies; can be melted and recast; and
can be reinforced at selected points of wear by facing with harder material, such as tool steel for long production runs.
2-242. Normally, drop hammer forming is accomplished without benefit of hold down. The metal is slowly forced in
shape by controlling the impact of blows. In many instances, it is necessary to use drawings, rings, 2 or 3 stage dies,
supplemental equipment, and hard forming such as bumping hammer, wooden mallet to remove wrinkles, etc. To
successfully complete forming operations, another aid that may be necessary is to anneal material between die stages
and intermediately for single stage die forming.
Parts should be cleaned prior to annealing to protect finish. Care should be taken to remove
all traces of zinc that maybe picked up from kirksite forming dies, as failure to remove the zinc
will result in penetration of the steel (stainless) when heated and will cause cracking.
2-243. SPINNING. Those steels that have low yield strengths in the soft/annealed condition, and low rates of work
hardening are the best grades for spinning. To overcome work hardening problems, intermediate annealing and 2-3 or
more stage spinning blocks are used. Annealing of the part at intervals also aids the operator when manual spinning,
because less pressure is required to form metal and springback is lower.
2-244. Form blocks for spinning are usually made of phenolic, hard wood, or carbon steel. Manual spinning is usually
accomplished on a lathe specifically adapted and fitted for that purpose. The main requirements are that required speed
be maintained without vibration; clamping pressure is sufficient to hold part; facilities are provided to apply pressure at a
uniform rate; and tools are of proper design. Normally, spinning tools are the roller or round nose type designed in such
a manner that high pressure can be applied without bending. Where local design of tools are required, raw material for
manufacture is obtainable under QQ-T570, Type D2, hardened to Rockwell C40-50.
2-245. SHEARING AND BLANKING. To prevent damage to shear, and to assure clean, accurate cuts, clearance
between shear blades should be approximately one-twentieth (5%) thickness of material to be cut. Also, blades or knives
must be maintained in sharp condition, clean, and free of' nicks. Where only one shear is available, a clearance of 0.005
to 0.006 could be used for general shearing of sheet stock up to 0.125 inches thick. Excessive blade clearance should
be avoided to prevent work hardening of cut area which increases susceptibility to stress corrosion and burring.
Lubrication such as lightweight engine oil or soap should be applied at regular intervals to prevent galling and to clean
blades for prolonged shear blade life.
2-246. BLANKING AND PUNCHING. Blanking and punching requires close control of die clearance, shearing action of
punch/blanking die. Clearance for blanking and punching should be 5% of thickness and closely controlled for all
gauges. In designing dies and punches, it is important that shear action be incorporated to equalize and reduce load.
Double shear should be used when possible to minimize off balance condition and load. Punches and dies should be
maintained in clean sharp condition and lubricated by swabbing or spraying material to be punched with lightweight lube
oil to prevent galling and to aid in keeping punch/die clean.
2.247. GENERAL FABRICATING CHARACTERISTICS.
2-248. PLAIN CARBON AND ALLOY STEELS.
2-249. Plain Carbon Steel 1006 through 1015. This group of steels is used where cold formability is the main
requirement, and have good drawing qualities. This series is not used where great strength is required. The strength and
hardness of these grades will vary according to carbon content and amount of cold work.