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AFD Nightstalker

21st century vampire hunting weapon: repeating electromagnetoelastic compound crossbow with night vision

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Living in Romania poses some unique challenges, and the one this project aims to address is particularly peculiar: the local strigoi population, semi-sentient resurrected bodies that pester the countryside. These ghouls have inspired the far more romantic myth of the vampire. In reality they are about as hazardous as the occasional wildlife that wonders into settlements and are treated similarly.
Since their activity is mostly restricted to rural areas, protection from and disposal of these pests is traditionally handled by a village volunteer, though some coordination with local law enforcement and veterinary sanitation authorities exists. The most pressing issue these tradespeople currently face is their woefully outdated equipment, which consists of farming and woodworking implements, various clubs and stakes and in rare cases firearms.
AFD herein aims to describe the design, production, testing and optimization of a weapon suited to the specific needs of Romanian strigoi hunters.

Main development objectives:

  • simplified construction: avoid the need for specialized tools and procedures;
  • accessible materials: components should not be more exotic than generic hardware store stock; use previously sourced material whenever possible;
  • specific features for use in residential areas at night: low noise and (active) night vision;
  • the Rule of Cool must factor into every decision.

Surely the most thematically fitting type of weapon for such a purpose is a crossbow, and that was indeed the starting point for development: a modern compound crossbow enhanced with a Gauss gun module and of course equipped with IR-sensitive electronic scope and IR illuminator. This choice was based on, apart from the thematic evocation, one particular advantage of the crossbow-Gauss gun integration: the string provides a means to activate a mechanical contact that energizes the coil when the position of the projectile is known (right in front of the string).

A second choice had to be made when it became obvious that the proper material for the limbs would have to be a glass or carbon fiber composite, which was deemed to exotic and expensive. So the switch to a sling-bow configuration was made, where power is provided by the elasticity stored in rubber bands or coiled steel spring as an alternative. This change also allowed for a more compact design, free of protruding limbs.

In order to optimize the amount of power transferred from the elastic material, a lever system like the one used by compound (cross)bows was needed. A simple 2:1 pulley system was considered, based on tests published by user [rolynd] on The Arbalist Guild forum (link to thread). His results showed that pulley weight was critical, which could potentially lead to the same issue with exotic materials.

The solution was to use the geometry of a compound pulley/cam system, with three individual elements rigidly connected by a shaft: Two bowstring cams and a central driving cable cam, with its track built as a spiral, thus providing the necessary mechanical advantage in a more compact configuration. The string also winds twice around its tracks, thus halving their required diameter. This dimension also sets the maximum cam ratio, since the corresponding cable cam minimum diameter can be no less than that of the connecting shaft (8 mm). So any further reduction would probably be counterproductive, leaving us with 83 mm cams for a ~500 mm power stroke, which can reasonably be integrated into an ergonomic design. Having the string cams coupled to a single driving cam (and thus to each other) also eliminates the issue of cam synchronization, provided they are built to the same precise diameter.

Locking in these choices allows more design elements to come together: the elastic bands shall be situated centrally, almost parallel to the bowstring. A drum-style magazine fits around them, forward to the end of the bowstring/string locking mechanism in order to clear for a loading ramp. This ramp sets the projectile in position once extracted from the magazine.

The next design focus was on the cocking system. One option was to engage the cam shaft directly, with a motor and worm gear reduction, such as a wind shield wiper motor assembly. This would have required a separate system to move the bowstring back in place and a way to decouple the reloading mechanism from the shaft for firing. In addition, it would not take advantage of the cams' levering action. Also it would have meant purchasing the assembly while the hand drill motors I already had would go unused, in lack of a suitable reduction. Another idea involved a bicycle chain that can engage the string and the shaft optionally, and could provide continuous cocking motion. The bike chain was impulsively bought and much time was sunk in sourcing a worm gear reduction and appropriate sprockets to no avail, and this solution had to be scrapped.

The ceaseless utility of the inclined plane...

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LTC 0455 Dinion.pdf

Bosch security camera

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hx711_english.pdf

Strain gauge sensor module

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panasonic_nv-r10_nv-r100.pdf

Panasonic camera

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View all 7 files

  • 1 × M8 stainless steel threaded rod, 1 m
  • 1 × 10 x 1 x 100 mm aluminium square tubing
  • 1 × handheld digital scale
  • 1 × Bosch Dinion LTC0455/11 security camera
  • 1 × Panasonic NV-R100 camera

View all 13 components

  • Log 7

    FiveseveN09/08/2019 at 17:52 0 comments

    I had promised (mostly to myself) to test the mechanics this week but I kept getting distracted with minor parts. However, that means I am very nearly done with fabrication.

    To finalize the cam assembly the spiral was trued up and some spacers made: first I tried MDF impregnated with CA but they were not strong enough, so I remade them from Dibond scraps.

    That allowed me to align everything and glue the forward assembly in place. I hadn't properly planned this part out, which lead to me having to cut the hardwood transverse element no less than three times.

    While on the subject on doing things thrice, it turned out I had cut the slot almost a centimeter too long, making it ever so slightly weaker than it needed to be, so I replaced it, again. After finding a pretty part in my box of greebles I also decided to redo the trigger. The same box provided a brilliant end piece for the coil frame: a brushless motor stator. The reloading shuttle was then completed and the reloading ramp mounted.

  • Log 6

    FiveseveN08/25/2019 at 12:56 0 comments

    Good news, everyone! Documentation has taken a back seat to getting things done, I hope understandably so. So I’m going to stick to a briefer play-by-play for now. Though things are coming together:

    Was rather stuck on how to motorize the magazine, dreading the thought of having to file matching teeth for a gear, then I got inspired by the classics, channeling a bit of the old Sam Colt. So a ratcheting system is to both move and index the magazine, and I got away with filing 14 teeth instead of dozens.

    Took this time to drill out and paint the security camera. Who needs an oven when it’s August?

    Turned out I had to redo the pusher as I had made it too short. Took this time to grind a bolt for the interfacing bit, thus finishing this part of the mechanism.

    On the theme of remaking parts, the foam recoil pad slipped on the grinder just as I was finishing it, so I had to make another.

    Next came a crucial component that I had initially planned to make from rather thin sheet stock. Luckly I had part of a hinge that was already mostly the right shape. Here it’s pictured with the initial nut I tried to fit but I later found a better fit.

    I added proper springs and did some fitting on the trigger group. The disconnector is yet to come since I’ll have to finish the ramp assembly first.

    After a lot of filling with hot glue and cutting it back and gluing and deciding I wasn’t satisfied, and having to do the whole thing again including a new track, the “barrel” was finally in place.

    The beast in its full glory, resting upon my own Empire of Dirt:

  • Log 5

    FiveseveN08/18/2019 at 21:08 1 comment

    More progress was made after sourcing a strain gauge from a handheld digital scale. This will allow to compensate for variations in elasticity due to temperature and the rubber hysteresis effect by measuring tension and factoring it into the ballistic calculation. Slots for it and the required linkage were cut, followed by some more work on the magazine well, stoppers, and the butt plate insert.

    The aluminium parts for the projectile ramp were cut and ground to shape. A captive nut assembly was constructed out of aluminium pipe, then the rest of the main motor assembly. 

     And then I had a little accident where my Dremel saw blade slipped into my left middle finger, requiring a visit to the ER for some stitches. I wasn’t sure I would be able to include all the features I wanted before the deadline but now I guess I’ll just have to focus on getting it to any working condition with less time and without the full use of my limbs. Wish me luck!

  • Log 4

    FiveseveN08/09/2019 at 00:13 0 comments

    Parts for the magazine were cut consistently using another quick jig, then sanded/deburred.

    Ample use of the available tube stock when constructing the trigger group, with the rest of the parts being fabricated out of sheet metal, rod and bolts.

  • Log 3

    FiveseveN08/09/2019 at 00:11 0 comments

    A long session of scroll saw use left my hand bruised and sore as the 4 sheets of MDF flooring were cut using a printed template on one pair and transferring the cut parts to the other. Two registration holes were drilled all the way through for long M4 screws. Preliminary sanding of the top and handle area followed and we are left with a frame that can be clamped together and sanded to final dimensions.

    First the elastic bands were fitted to their slot, with some more room made at the anchoring point.

    Then the slot for the main motor was sanded to a tight fit.

    The slot for the rechargeable battery followed, with provisions for locking into place and a tensioning rubber slug.

    Finally a cylindrical slot was cut for the keyed main power switch.

    The large space for the magazine proved much harder to finalize, with an entire day dedicated to it and a custom sanding drum constructed, to mediocre results. The technique of using a hacksaw blade in a rasping motion was more effective, but it still involved much back-and-forth between registration, clamping, checking, de-clamping and actual work.

  • Log 2

    FiveseveN08/08/2019 at 23:58 0 comments

    The following component was the reloading threaded rod assembly, which includes bearings, a gear and means to attach them in a reversible fashion. In addition to the motors, the cheap hand drills also provided their (cca. 10:1) planetary gear reduction, and one of the metal gears contained was used in this assembly, while another serves as an intermediate gear that couples it to the main motor. The gear was hand filed to a square interior slot, and the threaded rod ground to the corresponding mating shape. Other diameters and threads were also created for bearings and nuts.

    The rolling nut was constructed from a hardwood cylinder sandwiched between two M8 washers and rolls on an 8 mm pipe. Slots were cut using a hacksaw and then filed.

  • Log 1

    FiveseveN08/08/2019 at 23:53 0 comments

    Fabrication started with the cams, specifically the circular, bowstring ones. Each is constructed from a disc of 4 mm MDF sandwiched between two 2 mm Dibond type aluminium-plastic laminate sheet discs. These were cut with sheet metal scissors and ground using a small jig to ensure circularity. Space for the string tracks was first scraped using a wood screw and then filed to shape. Weight relief holes were drilled, then pop rivets and contact adhesive were used for final assembly.

     Next the central spiral cam was cut from a piece of hardwood, then drilled and cut using a combination of sawing, bench grinding and hand filing. The cams are then connected through three long screws, after the cable is secured and some paint is applied.

View all 7 project logs

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Tom Nardi wrote 08/25/2019 at 19:02 point

Have to admit, when this project first popped up I thought it was just a joke. Glad to be proven wrong with all the work you're putting in, and can't wait to see the final result.

  Are you sure? yes | no

FiveseveN wrote 08/26/2019 at 14:26 point

Thanks, man! I'm finishing up the mechanical part, so there will be some footage of it in action soon.

  Are you sure? yes | no

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