Fastening & Attaching

When working with Delrin, there are many ways to fasten it together. Today we learned three methods that can be employed on our new project of building a windlass; using piano wire, heat staking, and notches/pegs.

Each method is described below.

Piano Wire

Piano Wire is often used to create a hinge between two pieces. While it is a good method to create a point of movement, it is not the best method to keep two objects bound tightly in place.

When using this method, you must first construct two pieces with overlapping pegs in SolidWorks. In practice, a hole will be drilled through these overlapping pegs to create a path through witch to thread the wire. For example, if you were joining two pieces of Delrin, each with two pegs, you would first drill through the top three pegs to create “clearance” holes. These “clearance” holes are to be made with a drill bit that has a diameter slightly large than that of the wire you are using. This is to make sure that the wire can fit through the pieces without bending and still allow for movement. For the last peg, you must switch to using a drill bit that has the same diameter as your wire. This is to create hole that will keep the wire snugly in place. After threading the wire through the first three pegs, it is press fit into this final hole to secure it in place.

When setting up the drill for use, there are several things you must consider. Firstly, you must make sure that the two pieces of Delrin are far enough apart that they do not get in the way of each other’s movement. You must also make sure that the drill bit is perpendicular to the cutting surface when it is put into the drill chick, and then completely secured with the drill chuck. When actually drilling your pieces, it is often wise to employ a “pecking” technique (drilling a little portion and then pulling the drill out in order to clear the charts), to prevent the drill from getting stuck in the material you are working with.

Heat Staking

Heat staking is a method that connects two pieces of Delrin together by melting a peg that protrudes from one component into a notch in the second component. This method of joining is extremely strong, and useful in creating secure structural connections. It is, however, a permanent method of joining, so one must be sure of their design before employing it.

Before heat staking two Delrin pieces, one must first ensure in SolidWorks that the dimensions of the first piece’s peg matched that of the other piece’s notch.  While it does not have to be an exact fit, it can only help the structure’s stability to have no leftover spaces between the two pieces.

Notches/Pegs and Bushings

This last method of joining relies on design, rather than machinery to hold two separate pieces together. In the notch/peg method, the dimensions of one piece’s notch should match those of another piece’s peg closely enough that they can be joined in a snug and secure fashion.

The benefits of this method are that the two pieces can be joined and unjoined as much as needed. This allows for alteration and substitutions to occur in the overall structure of your object.

The major issue with this method, however, is that it can be challenging to get the dimensions of the pieces close enough to create a “press fit” between them. In the trials we did in class regarding this, we found that even less than a millimeter difference in size could mean the difference between fitting, or falling out/not going in.

When working with Delrin rods, rather than sheets, the notch/peg method is a little bit different. In this sub-method, hollow circular pieces of Delrin called bushings are cut from Delrin sheets to fit around the rod. Unlike the notch/peg method described above, bushings can be either loose or tight. Tight bushings are used to keep rods in place, while loose ones allow them to rotate or slide back and forth freely. Thus, this method can be used to create both mobile and stationary objects.

Our in-class trials with this joining method were much like those for the notch/peg method. Using a caliper to get exact measurements, we observed that even the slightest difference in diameter could mean the difference between tight and loose bushings. In our trials we measure the diameter of the Delrin rod to be 6.35mm, the diameter of the tight fitting bushing to be 6.41mm, and the diameter of loose bushing to be 6.53mm diameter. The observed 0.12mm difference between tight and loose fit bushings just shows how exact design dimensions need to be. 

This brings us to the issue of discrepancy between the dimensions of objects drawn in SolidWorks, and their printed counterparts. Even if one is careful in setting dimensions in Solidworks, the printed outcome is not always the same. This is due to several reasons. For one, one must take into account the thickness of the laser. Although set to the thinnest setting (hairline), as seen in our in class trials, even this tiny thickness can throw off the fit of the knotches/pegs. One must also take into account the material being used. While considered the same material, Delrin sheets may vary slightly in temper. For this reason, it is often wise to use the same sheet for one project.

Moving forward this discrepancy means that certain precautions should be taken. Firstly, one should always test the tightness of there notch/peg dimensions using their selected material before printing out their whole design. It would also be wise to employ het staking when possible.