6-Arm Tensegrity Structure

My partner's Printables page for this project: https://www.printables.com/model/377159-tensegrity-3-arm-table

A tensegrity structure durable enough to hold the weight of anything from a phone to a small laptop. Measures at 15 cm  x 15 cm x 18 cm.

Objective:

Create a tensegrity structure with greater or equal complexity to an example provided.

Parts List: Two (2) copies of Phim_Base_2.0_3MF.3mf and Six (6) copies of Phim_Arm_2.0_3MF.3mf

Additional Purchases:

Small screw hooks

https://www.amazon.com/gp/product/B07JMK9GCC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Any thin string (fishing line is recommended as it is very strong and transparent)
 

After completing some research, my partner and I decided on a somewhat non-traditional tensegrity structure. Rather than having a two arm and 3 or 4 strings supporting the bases, we decided on a design with six arms around the edge of the base and no arms in the center. One of the conceptual designs (shown below) had a circular base with curved arms.

We eventually settled on a triangular base with straight arms.

Initially, only the base had rounded edges. However, this contrasted with the more square arm, so we added an arc to the end of the arm to make them match.

We also thought that the arms looked too flat on their own, so we made a cut into the arm to give it more character.

The arm in the initial sketch had a far less steep angle than the final modeled arm. We had to make it steeper in order for each arm to have room in the base since the base had to fit in a 15 cm * 15 cm square.

We had to move the position of the slots for the arms since they were too close to the edges of the base in the sketch.

We wanted to add some sort of design onto the base to prevent it from looking flat like the arm had previously. The initial ideas thrown out included engravings in some sort of pattern.

The final design ended up being a hole through the base. Thai was much simpler than engraving, and an engraving would have been too complex and taken away from the minimal design of the structure.

We had to make the arms longer since they didn’t overlap enough, so we extended them by 0.5 cm.

The arm was designed to be 2 cm across. However, because of how it was modeled, its width increased at the curve in the arm. This was apparent when you looked at the engraving in the arm which had a differing size at the curve. We redesigned the arm, this time creating one edge of the arm and using the Offset Entities tool in Solidworks to create the rest of the arm around it. This resulted in a far more uniform width across the model. Alongside the redesign, we added fillets to the cuts in the arms to have the arm smoothly transition into the cuts rather then having them be a sudden drop into them.

We also added fillets to the edge of the circular cut in the bases. These fillets are far more subtle than the arms, however.

In our original design, we intended for strings to be tied only between each of the arms. That would have likely been too unstable, so we added strings on the corners of the base.

Two main issues arose when assembling the structure. The first main issue was that we created holes for the hooks by hammering a small Phillip’s head screwdriver into a marked point on the arms. This was risky, however, since it was forcing the layers of the arms apart. One of the arms ended up splitting because too much force was applied.

The second issue was that the arms were too wide and ended up resting on each other.

When designing the 2.0 version of our structure, these two problems were the main things we were focusing on fixing.

In order to fix the first issue of splitting arms, we decided to add holes in the arm’s model  for the hooks to be placed.

To fix the second problem of the arms resting on each other, we reduced the thickness of the arms from 1 cm to 0.75 cm.

To go along with this change, the dimensions of the slots in the base were changed to be 2 cm by 0.75 cm. (Plus 0.4 cm to accommodate for tight fit loose fit.)

We also moved the slots 0.5 cm farther from the edge of the base to give the arms even more wiggle room.

These two changes combined gave the arms far more room to breathe, and in the final assembled structure the arms had plenty of distance between them.

We decided on the aesthetic change of making the cuts in the arms go through them. This way, the arms matched with the through hole cut in the base and allowed for more of the structure to be seen.

We wanted to use knots to secure the strings in the corners of the bases rather than hooks like the arms. We added holes in the bases that the knots could hide in. Eventually, we had the holes go straight through the bases. This was so that we wouldn’t have to drill through the arms. However, if the model is printed in hard plastic and with a high density, drilling may still be required as the holes would be smaller.

The only significant problem that arose with the 2.0 version was that we initially printed the pieces in PEC plastic,which is very hard. We also printed it with a higher density. We underestimated the strength of the plastic when screwing in the arms, and we screwed the screw hooks too deep into the arms, causing them to break.

We learned to be careful screwing in screw hooks when using a harder plastic and printing with a high density.

We reprinted the pieces with another, softer plastic and with a lower density. This time, the holes in the arms were too big to fit the screw hooks. We solved this by adding some hot glue into the hole and then inserting the screw hook into the hole.The hot glue was able to hold the screw hook in the hole firmly.

Assembly Instructions:

Parts: 6 of the Arms, and 2 of the Base. 
Tools Necessary: Small Phillips Head Screwdriver, Small Needle Nose Pliers, Hammer, Vice, Drill with 1.5 and 2 mm bit, and Scissors/Cutters. 

Extra Parts required: 12 Small Hooks (depicted below), Any Fishing Line or Small String. 

Hooks: https://www.amazon.com/Shapenty-Projects-Bottles-Jewelry-Findings/dp/B06X9C8KYH/ref=sr_1_3?crid=1JV0U88IKTBAQ&keywords=shapenty%2Bhooks&qid=1673446059&s=arts-crafts&sprefix=shapenty%2Bhooks%2Carts-crafts%2C77&sr=1-3&th=1

Step 1 - Prepare your arms and bases. 

• Tools and Parts Necessary: Sandpaper of a large grit, a scraping tool is recommended too depending on the quality of your prints. All of your printed parts.  

• After taking your arms off of the print bed, It is time to assemble! Three arms go with one base, but before you can put them together you need to prepare your arms. 

1A) Start with cleaning out any excess plastic and errors.  

Before:  After: 

1B) The printing may not be exact so test the arms by inserting them into the base, and checking the fit. If the fit is too tight, or the arms warped and do not fit. Take a higher grit sandpaper, we used 120 grit paper. Constantly check the fit and make sure that you do not oversand!

Now you're ready to add hooks to all of the arms!

Step 2 - Place Hooks into all 6 Arms

Tools and Parts necessary: Hammer, Small Phillips Head Screwdriver, Small Needle Nose Pliers, 6 small hooks, and 6 Arms. 

NOTE -  First locate the small reference holes on the bottom face of the curve in the arm. This will be where you place your hooks. Depending on the hooks you are using and the plastic, a common issue can be shear forces breaking hooks off into the arm. If this happens you can either reprint the arm, or place a new hook as close to the original as possible. Following these instructions carefully can help to prevent this. 

← What to avoid →

2A) To start, take a small Phillip’s head screwdriver with as small of a head as possible to dig into the hole.The simplest strategy is to get a screwdriver with a top that rotates. Hold onto that top and rotate the screwdriver, using it as a handheld drill bit. You will eventually get a hole deep enough for the hook. This will help prevent breaking of your parts and hooks. 

If you do not have a screwdriver with a rotating top, you should push and twist into the hole until it is deep enough to set a hook in. 

If you decide to use a hammer to try to drive a hole into the arm, be careful so as not to split the layers of the arm, as happened in the example below:

Hit the screwdriver with the hammer lightly, and check often to see if the hole has become deep enough.

NOTE - A good depth for the hole is about half of the length of the screw part of the hook, or where you can comfortably start screwing the hook in. 

2B) Now carefully pick up a hook, then place it into the hole and screw it in until it can stand on its own. It may be easier to hold the hook in place with your fingers and instead rotate the assembly piece to get it screwed in for this step. Once it is solidly set, continue screwing it in with your fingers. 

2C) - Continuing, once you can no longer screw it in using your finger, use small pliers to continue screwing it in (THIS IS WHERE YOU MUST BE VERY CAREFUL TO AVOID A SCREW BREAK). The optimal way to hold the hook with the pliers is shown below. THe optimal depth for the hook is with about half of the screw part of the hook inside the arm. The hook does not need to be all the way in the arm. If you tug on it and it feels firmly set, it is deep enough in the arm. Once your hook is solidly in the arm and rotated to your desired way, you're all set. Repeat on the 5 other arms the same way now. 

Step 3 - Drill Holes into Base 

Tools Necessary: A drill, a 2 mm bit and 1.5 mm bit are necessary, and a vice or extra pair of hands is recommended.

3A) To start, take the drill and place the 2 mm bit into it, then vice clamp the base to the table. Making sure the side with the pilot hole is rightside up, this is very important. 

3B) Now place the larger drill bit into the drill, and drill into the pilot hole until you are about halfway through the base. This will become where the knot will slip into the base later on.

3C) Now repeat on all 3 holes of both bases, pay attention to what side is up on each. 

Step 4 - Tying the String on the Arms and the Base

Tools: Ruler, Fishing line, and a Pair of Scissors and a vice. 

4A) Take two arms and set the hooks parallel to each other. Making sure they are 3 cm apart.

4B) Now place the string through one hook and tie it using an overhand knot, then use the pliers to pull it tight. (https://www.youtube.com/watch?v=SwdJ5op25SM) Then line up the next arm with the correct distance. 

4C) Tie the other arm tightly using the remaining string and another overhand knot, constantly check using the ruler to make sure the distance remains at 3 cm. 

4D) Now repeat on the other 5 arms. 

Step 5 - Put the Tied Arms into the Bases. 

Tools Necessary: N/A

5A) Gather all of the pieces and start to place the arms into the base slots. These should be a snug and solid fit. Make sure to place the side of the base with the smaller holes facing upwards. 

Step 6 - Tie the final strings into the bases. 

Tools necessary: Take a pair of pliers and Cutters and a Level. 

6A) Take out the roll of the fishing line and cut three roughly 25 cm pieces out. Tie an overhand knot near one end. This will make the sting not pull through the base when you lace it through. Pull the sting completely through both bases. 

6B) Tie the top length of the string tight to the top base using an overhand knot, referencing the level to ensure correctness. 

6C) Repeat on the other two base corners. Constantly checking the level. 

Step 7 -  Finishing Touches 

Tools Necessary: Cutters

7A) Take the cutters and snip all excess string. Making sure to not over-cut is important. Enjoy your new structure!