Modular SlingXBow Family

CC BY-NC-SA license:  https://creativecommons.org/licenses/by-nc-sa/4.0/

These designs, and devices built from such, are experimental.  While some minimal testing has been performed, they are provided as-is with no guarantees whatsoever.

Neither the creator, nor anyone else involved with these files, assumes ANY responsibility or liability for their use in any way whatsoever.  By using these files in any way, the user agrees to assume full responsibility in this regard.

At this time, these designs are currently very much a work in progress.  

While many individual parts have been finalized, there isn't much in the way of documentation, meaning how to set up your printer to make them, how to assemble them, or how to use them.

Working Document (Print orientation, assembly, etc.):

https://docs.google.com/document/d/1QLMOzGKLO2RXOps3jN210q0NSNLkTSxxrtQNkC6O7K4/edit?usp=sharing

All these designs were printed using PETG on a Prusa Mk3s with a powder-coated steel sheet as the print bed.  The standard 0.4mm nozzle and no other upgrades to the printer were used. I am not an expert at 3d printers in general or how to set up other printers.  At a minimum, I recommend using Prusa-slicer as the slicer so you can follow the profiles that are provided.  In general, all parts should be printed with at least 3 perimeters and at least 10% infill, increased to 25% infill in critical areas using modifiers.  For critical parts such as the front that holds the limbs, perimeters can be increased to 4 or 5.  For critical mechanical parts like trigger pieces, use as many perimeters as possible, typically 8, and 100% infill.  

All these designs should be printable on a FDM printer without additional support of any kind (some have support towers built into the files, these support towers are intended to be broken after printing).

All these designs can be printed within the print volume of 250x210x200mm, though some may need to be rotated 50-52 degrees in order to fit on the bed.  Print orientation is often important and that will be covered in the video series (TODO).

There are a several low cost manufactured parts you will need to acquire to build these devices:

If you plan on using elastic bands as a power source, you will need zipties to secure them to the rope.

Zipties:  https://www.amazon.com/gp/product/B0777LWBD9

Zipties of width 0.16" or greater are recommended for draw weights above 25 lbs.

Spring kit:  https://www.amazon.com/gp/product/B000K7M36W

The 23/64" x 1-3/8" compression spring from this kit is used.  This spring is included in many compression spring kits that can be ordered from the site.  Alternatively, some equivalents can be found on McMaster.  P/N 1986K382 (light trigger pull) or P/N 1986K215 (heavy trigger pull).

m3 screws:  These can often be found in a m3 screw kit, available from many manufacturers on amazon.  Get the hex socket head type.

search link for amazon:  https://www.amazon.com/s?k=m3+screw+kit+hex+socket+head

Make sure to get some longer m3 screws.  The vital trigger axle uses a 40mm m3 screw to reinforce the plastic part, the trigger lever and trigger bar use 20mm, 30mm, or 40mm m3 screws for reinforcement.

For the lever spring, a 20mm "Ender 3" bed spring from the following kit is used:

https://www.amazon.com/gp/product/B07V6YCYP5

This spring must be matched to the draw weight and to some extent the string angle, a spring that is too weak will fail to lift the string out of the notch.  Start with the weakest spring in
the kit and move to stronger springs as necessary.  Typically I find myself using the beige spring for draw weights less than 20 pounds, the blue spring for draw weights between 20-35 pounds, and the red spring for draw weights greater than 35 pounds.  

For rope:  At a minimum, take the expected draw weight, and double it.    The rope used should also be thin enough to not jump over the bolt or projectile as it is being fired.  For the inexpensive and small ~6cm long ~6mm diameter pistol crossbow darts you can find on amazon and a variety of sites, use 3/32" solid braid nylon rope and a draw weight no more than 25 lbs., for ammunition that uses 10mm dowels, use at least 1/8" solid braid nylon rope and a draw weight no more than 40 lbs.  

rope:  https://www.amazon.com/gp/product/B076WV4CJP

If you are planning on making a slingbow, the tubular exercise band kits that can be found on amazon do a good job of this.  

https://www.amazon.com/gp/product/B002WK85LC

https://www.amazon.com/gp/product/B001RQ2626

For noise/vibration damper, these adhesive pads work well in smaller devices.  For larger devices, they can be used in conjunction with closed-cell foam from a sleeping pad.

https://www.amazon.com/gp/product/B000SL0KJC

https://www.amazon.com/gp/product/B01LW6JPAB

For projectiles, 10mm or 3/8" dowels:

https://www.amazon.com/gp/product/B07V6YCYP5

and 3/8" slingshot BB's are used:

https://www.amazon.com/dp/B00JZ18K20

These designs can be built with the supplied 3D printed mounting pins, if desired, the mounting pins can be swapped out for M5 screws of equivalent length.

Setup files for prusaslicer are included.  These are a good starting point and may work fine, but it's likely they may need some fine-tuning depending on the actual filament used and environmental conditions.  To use these files, use the help menu from prusaslicer, select “show configuration folder” and then drop these files into their respective directories.  

Printing the solid arms or limbs:  At a minimum, the limbs should be printed with nothing but perimeters (the print should not contain any actual infill).  Beyond that, it really depends on your draw weight.  For draw weights above around 20 lbs., special attention must be paid if limbs are to be reliable and not break.  My best technique to printing these is to intentionally induce a slight amount of over-extrusion by printing wider than the nozzle and increasing the extrusion multiplier.  You may also want to experiment with lowering the fan speed or turning the fan off completely.  When using these profiles, watch the print and printer very carefully to make sure it is not globing up below the nozzle or failing in any other way.  If anything looks like it is going wrong, stop the print, fine-tune the parameters, and try again.  Don't try to continue a print that is going badly, you can end up damaging and having to replace the hot end, or worse.

Possible additions (coming soon):  Currently I am experimenting with arms made from heat-shaped PVC pipe, there are plenty of people making bows using these techniques in videos posted on youtube.  While I have found it's possible to get good draw weights using these techniques (easily 40 pounds or more), the results have been somewhat underwhelming to date due to poor velocity.  If I can ultimately get decent velocities that are comparable to the elastic bands I currently use (150 ft/sec or more) I'll design a front around it.