Bat House Project

[reality_boy]

This is a 3D bat house I have been working on. I did not want to post the plans until I have had time to validate the design with real bats. However I need a place to talk about the design, so I'm posting it here.

Background

My wife is a 2nd grade teacher and she does a whole segment on bats (she teaches them for about a month). I have been looking around for a while for a way to give her kids more hands on experiences with bats, beyond videos and books. One idea was to build a bat house, so I sent off for a copy of BCI's guide to building bat houses (http://amzn.com/0974237914/). I quickly realized that while the guide was good, the plans were well beyond what a grade school club could put together. They needed power tools to cut the wood up, and a very serious pole to mount the house on. Since I was already using a 3D printer as a hobby I wondered if a 3D printed bat house could be made for a low cost and with tools kids could use.

Arial]As I was researching this subject I cam across the plans for the Bat Bunker (https://www.thingiverse.com/thing:2057552). This was an impressive project by a junior high school robotics club, but looking at the plans it seemed clear to me that this design would not work very well for a family of bats. Since I could not find anything else, I set out to design and build my own house with the hope that the design could be shared freely and inspire others to put up a bat house of there own.

3D printers are great, but they tend to have a relatively small build volume. After doing some research on it I decided that targeting an entry level printer with a 150x150x150 mm build volume would allow most users to print a small 2 chamber bat house, and going up to a 200x200x200 mm build volume would let many users with full sized machines print a more substantial 3 chamber house. Having a small square build chamber lends itself to a rocket box style house (http://www.batcon.org/files/RocketBoxPlans.pdf), and a rocket style house can be mounted on a single pole more easily. By my calculations the 2 chamber house will weigh less than 1.5 kg (about 3 lb) and the 3 chamber house will weigh less than 2.5 kg (about 5.5 lb) so it does not need a substantial pole. After searching around and trying several pole ideas I settled on using a 1 3/8" top rail for a chain link fence since it is readily available in 10 foot lengths and is strong enough to hold the house and cost only $13 per segment.

Design

The basic design is either 2 or 3 19 mm wide chambers with 1.5 mm deep horizontal groves in the wall spaced 9 mm apart. The houses are each split up into 5 segments, each either 140 or 190 mm tall for a total height of 660 mm or 860 mm for the 2 and 3 chamber houses respectively. You could choose to add or remove segments to create a larger or smaller house. There are three different designs for the center segments, one with a row of 13 mm vent holes in the outside of the box, one with 38 mm passages between the chambers, and one with no holes at all. These can be mixed and matched as needed for the environment.



Each segment of the house slots together with the segment above. This interface is designed to create a solid seal to keep the elements out and to help keep the structure strong even if heat causes minor warping of the plastic (although I don't expect any warping, it is good to be prepared). In addition I use #6 1/2 in sheet metal screws to secure each segment together. And have a box around the end of the screw that prevents it from penetrating into the bat chamber. In my tests this provides a very solid structure that could withstand a large impact and hopefully will keep the house sealed up tight for years to come. Finally the pole is run up through the entire length of the house to add rigidity to the design so that the screws are not the main structural support. The base of the structure clamps to the pole with 3 screws as well to further keep the structure together. And it also forms a 3" landing strip for bats to land on.

The 2 chamber house made with 5 segments has an outer dimension of 142x142x660 mm (around 23" tall), uses 1.4 kg of plastic (a spool and a half) and has a living space of around 6,860 cm^3. By my estimates it would cost $26 to print and take about 6 days to print all 5 pieces. That puts it close to the 6,500 cm^3 space of BCI's single chamber bat house plans (http://www.batcon.org/files/BCI_Sing...se_Plans-3.pdf). Each additional segment adds around 1,700 cm^3 of living space and 130 cm of vertical height.

The 3 chamber house made with 5 segments has an outer dimension of 189x189x862 mm (around 34" tall), uses around 3 kg of plastic (3 spools) and has a living space of around 16,800 cm^3. By my estimations it would cost around $60 dollars and take about 16 days to print all 5 pieces. That puts it close to the 15,000 cm^3 space of BCI's triple chamber bat house plans (http://www.batcon.org/files/FourCham...usePlans-3.pdf). You can save a small amount of money and time by printing only 4 segments, and of course you can add additional segments as well. Each segment adds around 180 mm of height and around 3,700 cm^3 of living space.

To complete the project you need a box of #6 1/2" sheet metal screws and a 10' length of 1 3/8" galvanized top rail, for an additional cost of around $15. Once inserted in the house the remaining pole will only be about 7' tall, you will need to attach it to an existing pole already in the ground to get the house 12' off the ground. You can use any pole that is 5' off the ground or higher, or use a second 10' top rail and a $5 panel clamp set to attach the poles together. Having a split in the pole is actually a benefit, the top pole weights around 10 pounds with the house on it and can easily be lifted and installed by one person, with a little pre-planning.

There are still several unknowns about this design:

• I have tried to research how well 3D prints hold up in the outdoors and there are several posts on line with anecdotal evidence that indicates they hold up well for several years, but only time will tell if this is true or not. The exterior of the house could be painted with an exterior latex paint to ensure a longer life.
• The house could be printed with any material but PLA is the most common. I have a test piece outside here in Tucson, AZ to see how well it holds up to heat. Again my research indicates that PLA should hold up even in the desert heat, but if it does not we could switch to a stronger plastic like PETG. I verified that PLA looses its integrity at around 130 F, but I don't know if that is enough of a margin to allow the house to survive the summer heat.
• I'm not sure if the texture I added to the walls of the house is rough enough for bats to grip. I could go with a different pattern if needed.
• I'm not sure if it is important to use a dark filament to print the house out of. The walls are thick so most light would be blocked with any non translucent filament and exterior paint would help darken up any plastic.
• I'm not sure how well the plastic will hold up to bat guano, or if it is bat friendly. However it is a solid plastic that is corn based and should be both stable and non toxic.
• The design of the walls could be reworked to reduce the plastic used and increase the print time. By my calculations it may be possible to reduce the plastic by around 30%, however that may reduce the strength of the walls and for now I figured it was better to error on the side of caution. The shape of the walls has a big impact on print time, it is possible that using a different texture on the walls could speed up the prints by a large factor.

I left the outside of the house undercoated on purpose so that others can decorate the houses as they see fit. Here are a few ideas off the top of my head for possible designs. You could use a tool like MeshMixer to quickly decorate the walls without a lot of cad experience.



Here are some renders of the final design. In the first cutaway you can see part of the passage between the chambers, along with some of the vent holes on the lower chamber. And you can clearly see how the segments attach to each other and where the screw blocks are located.





This is a render of the 3 chamber bat house.



Below is a rough outline of the instructions I hope to provide with the plans. For 3D printers you don't give cad drawings, but rather 3D models in STL format that describe how to print the segments out. These would be stored on the Thingiverse website under a creative commons licence that lets anyone modify and print the plans without compensating me in any way.

Print Settings

Printer Brand: Creality
Printer: Ender 3
Rafts: No
Supports: No
Resolution: 0.2 mm
Infill: 20%
Walls: 3
Filament:AmazonBasics PLA Neon Green
Notes:
This was designed to print on most any printer with a 150x150x150 mm build volume or greater. You should not need any supports at all. I managed to print it fine without a brim, but if you have first layer issues a brim may be helpful.
The models are undersized just a bit, enough so that you can use MeshMixer to decorate the outside however you want. Just be sure to not hurt the integrity of the chamber itself.
By my calculations the house will take around $25 worth of filament to print (1.4kg) and take about six days to print.
It should be OK to print this using PLA, but if you live in a hot climate going with PETG may give you more dimensional stability. I have a test model located at my home in Tucson, Arizona and so far have not had any warping or issues with my PLA print. I will update this if that changes.

Post-Printing

Finishing it up

The box needs to be mounted between 12 and 20 feet off the ground. This was designed to be mounted on a 1-3/8" chain link fence top rail. You can pick a 10 foot length up at Home Depot or Lowe's for around $13. You will need to either connect two together, or mount the post on an existing pole in the ground.
When selecting a site, make sure the box has good exposure to the sun and that the area around the box is clear so that bats flying in and out will not get entangled in anything. Try to locate the box near a source of water. And if mounting it near a park or school, locate it in an isolated area where kids will not be able to throw rocks at the house, or accidentally come into contact with the bats.

All parts below can be easily found at your local Lowes or Home depot, if you live in the United States.

• $13.00 1-3/8 in x 10 ft. 6 in. 17-Gauge Galvanized Top Rail
• $3.50 #6 x 1/2 in. Stainless Steel Pan-Head Phillips Sheet Metal Screw (50 per Pack)

If you don't have an existing post to mount this to you can use the following to install a post into the ground and use the panel clamp set to connect the two pols together.

• $13.00 1-3/8 in x 10 ft. 6 in. 17-Gauge Galvanized Top Rail
• $5.50 1-3/8 in. Galvanized Panel Clamp Set
• $3.25 Quikrete 60 lb. Concrete Mix
• optionally 3 x #6 x 5/8 screws for making the pole clamp stronger

Bats wings are thin and easily damaged. Be careful when assembling this that the screws don't penetrate into the structure and watch for any sharp object near the house or wires that the bat could get entangled in.

You may want to add a layer of paint to the outside of the box to help it last longer in the outdoors. And if you expect a lot of rain you may want to calk the segments when assembling to ensure a more perfect seal from the elements.