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dimension used for one side in the cavity cut formulation. (Flat
thickness plus tolerance) and add 1.210" to center of the stock
mounting hole. Once again, it may be necessary to compromise
the thickness allowance for forming more than one flat with
varying thicknesses. .085" should be sufficient for all flats
produced by Cobray (or sold by RPB). Drill and ream this hole
for a slip fit to the dowel pin.
The forming core block is placed in the cavity cut, locating the
two holes of the stock mounting hole location (cavity and core)
together by the guide pin. Thus held, use two pieces of metal
shims (or, drill bits) the size of the metal thickness plus the
tolerance space at the front, one on each side to center the
block in the cavity cut. Clamp the block in place, transfer, drill
and ream the sear stud guide pin holes, both M11/9 and M12/380
to a slip fit.
This is the best part, because it is the last part of the die! It is the
gage block for forming the first bend in the extending tail tab.
The drawing number 5 details this block.
Begin with a piece of cold rolled flat stock at least 1/4" thick. It
must be 1 1/2" wide .If you have chosen a piece of metal wider
than four inches for the cavity half of the die, you may wish to
cut a piece as long as the width you have chosen. Room for
variances has purposely been left in this, as this is a 'survival
level' construction report, and the reader may not have available
anything more than scrap to chose from.
Assuming the cavity is four inches wide, the finished, squared
length of the gage block is also four inches. Just as the cavity
cut was made as closely to center as possible, so will the
clearance cut in the bottom of the gage block be made so close
to center. The clearance cut however, will only be .015" wider
than the tab width, so, 1.150 + .015" = 1.165". The clearance cut
may be .110" to .125" deep. However, for the commercially
available flats, the clearance cut need only be flat thickness plus
.005" to .010". Drill and ream the dowel holes and drill the
clearance holes at the dimensions shown. It is not necessary to
counterbore the screws.
To locate the gage block, use the prepared flat, either factory or
homemade, and place it centered on the cavity plate REVERSED
to the direction of the final form position, so that the tab hangs
over into the cavity forming area. At the same time, the length of
tab protruding into the cavity forming area is measured and the
flat is kept centered on the cavity plate and squared to it. The
measurement must be .650" to 660" from the forming face to the
end of the protruding tab.
Carefully clamp the flat in place solidly, and place the gage
block over the tab, and firmly against the side 'wings' sticking
out to either side. These 'wings' will be the gaging surface to
form the first bend of the tab. This is why the builder was urged
to recognize the importance of squareness in the section
covering the manufacture of the flats.
Thus located, clamp the gage block in place and transfer the
screw holes. Drill and tap them into the cavity plate. Then return
the gage block to it's position and lightly tighten it down to the
cavity plate. Insert the flat again, remeasure the tab distance and
double check the square of the gage block. Tighten the
mounting screws, double check square after tightening, and drill
and ream the dowel holes.
The best doweling fit for these holes is to ream the cavity side of
the holes as a press or even a tap fit. The holes in the cavity side
may be blind drilled, not coming through the plate completely. It
is best that the dowels only protrude the height of the gage
block from the cavity plate. The dowel holes through the gage
block should be a slip fit, or even a carefully drilled hole, using a
lot of lube, and pecking as you go til you pop through the other
side. This plate will have to be removed after each flat formed,
and left off for the second and final form stage of the receiver
channel. Location of the flat for final form will be determined by
the guide pins. How well it forms is determined largely by the
radius hand cut into the top guiding edges of the cavity cut on
both sides, and the radius on the tab forming block at the rear.
These should be about 1/8th radii and polished slick. The radii
on the matching surfaces of the forming core should be very
small, actually just breaking the corners. The better all forming
surfaces are polished, the better the die will work.
In both stages of forming, the liberal use of lubricating grease is
advised. Lube everything well, including the guide pins and both
sides of the flats to be formed at least in the central areas, and
the cavity of the die. Too much grease is not a problem here. No
lubrication however, is quaranteed to be rough on the die. This
is especially true of soft mild steel. Since the die was not
designed or built for massive production it surely doesn't have [ Pobierz całość w formacie PDF ]