2/10/15: Lesson - Fastening & Attaching

Connecting Delrin!

In class we rotated through stations with our partners to learn about four different ways to attach two pieces of Delrin together. 

1) Heat Staking 

To heat stake Delrin pieces together, we started with a t-shaped piece already placed loosely into a peg. It was able to melt the T-stub right on top of the peg. Heat staking requires one piece inserted into the other with a stub sticking out on the other end to melt the stub onto the other. This is a permanent method for attaching Delrin pieces together and would be an excellent way to fully join two pieces together, especially pieces that are foundational and perpendicular to each other, because that is where the most pressure will be applied. However, heat staking should be used with caution because it cannot be undone, and if there are mistakes made afterwards with building the structure, all the pieces might have to be created over again. In addition, the material of the pieces matters when melting them together, because two pieces of different materials might not actually stick.


2) Piano Wiring

We practiced with piano wiring through joining two pieces of Delrin together to create a hinge. Lining up two teethed pieces that fit loosely into the other, we used a clamp on the drill press to secure the overall piece in place while bringing the drill bit downwards. After creating a hole into both of the pieces, we pushed the piano wire into the hole, connecting the two pieces and allowing the hinge to swing freely. Piano wiring is excellent for creating movable and immovable hinges, and the piano wire can be pulled out if the process needed to be undone. The process itself can be difficult to master though. Since the piano wire is very thin and the thickness of the Delrin (3/16'' or less) would often drilled through to create the hinge, the drill bit could move to the side as it drills. This piano wire would consequently not go through the desired path, which we encountered when making our hinge. The wire could bend or not move through the entire length of the desired path as well. The multitude of steps, from the necessary precision of the drill bit's path through the Delrin to the measurement of the matching piano wire to drill bit, makes piano wiring inefficient. 


3) Notches & Pegs 

The method allows for one piece of Delrin to be inserted into a peg shaped piece perpendicularly, loosely or tightly. This is non-permanent attachment of two pieces of Delrin and is not entirely secure if the two pieces are pulled apart with pressure. The tolerances between loose and tight fit can be adjusted as needed for the structure's objective, making this a flexible method for joining two pieces together. This method would be an excellent, impermanent way to allow a single sheet to hold pieces up vertically. To connect the pieces using notches and pegs for a tight fit or loose would be take time to reach though, because there are many tolerances and measurements to take into account with each sheet of Delrin, and from SolidWorks to print. On the plate of Delrin with different sized notches according to SolidWorks, we measured that the tolerance differences for loose or tight fit were around .2 to .3 of a millimeter, and the differences between the tab and a tight fit were around .01 - .02 of a millimeter.  


4) Bushings

As we measured with the Calipers, the tolerances for the notches and pegs are the same for the bushings, where approximately .02 of a millimeter is needed for a tight fit and the tolerances for the difference between loose and tight fit is approximately .2 of a millimeter. Bushings are important to hold pieces of Delrin on a rod, and are flexible when trying to secure them tightly or loosely. Tight bushings would cause the Delrin piece stuck between to be immovable, which is especially important when the rod is rotating, because immovable Delrin would with the rotation of the rod. Loose bushings would still hold the Delrin piece in place, but it would permit the piece to remain stable as the rod rotates. Structures with foundational pieces held together by a rod will need loose bushings in order for the pieces to stay in place, but not rotated along with the rod.


Discrepancy Between Model and Product with the Laser Cutter

The discrepancy that we documented between the dimensions specified in SolidWorks and measuring with a Caliper is approximately 0.01 inch, where the actual size (measured by the Caliper) is larger than the dimensions specified. To use bushings and notches and pegs, multiple trials with the laser cutter on the same sheet on Delrin would be necessary to find the right fit (loose or tight). The tolerances differ from real part and the model because of the Delrin thickness and the laser cutter. The thicker the sheet of Delrin, the more over lapses the laser cutter would have to go through on the design. The velocity of the laser cutter also determines whether or not more or less of the Delrin is melted: the slower the velocity, the more melted Delrin, but the Delrin is cut through more thoroughly, and the faster the velocity, the increased risk of the Delrin rapidly melting back together,  but the cut would be thin.

To print out sheets of Delrin, or any material in the future, we will want to think about trial runs and making measurements on SolidWorks millimeters less than the its theoretical. We will need to be careful with our precision and accuracy when measuring parts for our structure.