Summary:
This part will be used in a Remington Model 870 Super Magnum 12 guage shotgun. The main purpose of the part is to retain a compressed spring in the magazine tube. This spring is what forces an unspent round into the shell elevator each time the action of the firearm is cycled. A secondary purpose of the part is to correctly locate a magazine tube plug. This plug limits the number of shells that can be placed in the magazine tube to two. The location of the stop for the magazine tube plug must be correct in order for it to be functional with 3.5, 3.0, and 2.75 inch ammunition. Another secondary purpose of the part is to keep the barrel retaining nut that threads onto the end of the magazine tube from loosening.
The Design:
The part is of a cylindrical shape with a singular O.D., various I.D.'s, 24 notches, 4 milled slots on the part O.D., and 1 milled slot in the notched face. The O.D. of the part is such that it will fit, without interference, on the inside of the magazine tube. The various I.D.'s allow for correct mating and alignment of the notches in the part with the notches in the barrel retaining nut, a place to locate the magazine tube plug, and the removal of excess material to decrease the overall weight of the part. The notches provide a kind of locking mechanism to keep the barrel retaining nut from loosening. The notches mate with corresponding notches inside of the nut. The milled slots perform two functions. One is to keep the spring from extending past the end of the magazine tube if the barrel retaining nut is removed. The other is to keep the part from rotating inside the magazine tube as the barrel retaining nut is screwed into place. The slot milled in the notched face is used as a place to insert the blade of a screwdriver to remove the part from the magazine tube. Below is a link to the part drawing.
Fabrication:
Lathe Work:
- Basic turning of O.D. was done with a regular lathe tool mounted
in the tool holder
-I.D.'s were made using a fluted drill bit mounted in the lathe
tailstock and a boring bar mounted in the toolholer
-The notches were cut by mounting a tool with a 45 degree face
on it into the tool holder sideways. The tool holder was adjusted
so that the tool's centerline height was equal to the centerline height
of the lathe chuck. This ensured that as the cross-slide was moved
in and out, the motion of the tool was perpendicular to the surface of
the part. (See link to "Notching Setup" below.) Then,
the circumference of the chuck was measured and divided by 24 (the number
of notches). A strip of paper of length equal to the chuck circumference
was marked with 24 equal divisions. Then the paper strip was taped
to the chuck so that the marked divisions could be referenced to a fixed
point on the lathe headstock. (See link to "Degree Wheel" below.)
It is a sort of " poor man's degree wheel". The tool was then advanced
by the lathe's cross-slide across the face of the part 10 times taking
.005" off in each pass. Once one notch was finished, the chuck could
be indexed by hand to the next division on the paper strip and the process
was repeated for 24 notches.
-Parting off was done by mounting a parting tool in the tool holder
Mill Work:
-All milling was done using standard end mills of the appropriate size
Process Plan:
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*Note: Radiused corners at slot ends are permitted |
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*Note: Drill or boring bar may be used as long as max hole depth does not exceed .995" (This I.D. serves to decrease the part weight and has no critical function) |
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The total machining time for this part came to be approximately 280 minutes. This time includes setup for the various operations as well as the actual time required to machine the part. Since the nature of the notch making operation isn't considered to be "normal" practice, extra time and care had to be taken when making the setup. If the "degree wheel" wasn't laid out right, the part would have been ruined.