Fastening & Attaching

This week, we learned four different ways of fastening and attaching Delrin pieces to each other. These methods, listed below, will help us construct our well windlasses.

1. Heat staking
2. Piano wire
3. Tabs and notches
4. Bushings

Heat staking involves inserting one piece of Delrine into another, and heating the piece so that the Delrin melts and the two pieces are securely attached. This method is quite effective for making sure that the two pieces do not come apart, but it is also a very permanent method. There is no way to undo heat staking, unlike the other methods. Heat staking should be used with careful planning so that you do not attach something only to find out something should have been done differently. It is best for pieces that will undergo a lot of pressure or force and need to be very strongly secured.

Using piano wire to connect pieces of Delrin is done by lining up the two pieces, drilling a hole into both, and inserting the piano wire into the hole. Piano wire is mostly useful for creating hinges out of two pieces of Delrin. It can also be used to simply attach two pieces if piano wire is inserted in multiple spots on the pieces. This method can be tricky, as when we tried to create a hinge with piano wire with the drill, we found that the pathway of the drill often deviates and therefore does not create a perfectly vertical hole.

Tabs and notches are another way of attaching two pieces of Delrin together. Like the name suggests, this method has one piece of Delrin with a small tab sticking out, and the other with a hole for the tab to fit into. The two pieces are attached perpendicularly. Tabs and notches are a less permanent way of securing two pieces together than the heat staking, but also less secure. When using this method, it is important to determine what kind of fit is appropriate for the project. When the notch is wider than the tab, a loose fit is obtained. When the notch is the same size, or barely winder, as the tab, the fit is tight. Using calipers, we measured the width of the notches for tight and loose fits. The difference in tolerances were mere tenths or hundredths of a millimeter, showing that the importance of exact measurements. A loose fit or a tight fit determines whether or not a piece of whatever is being constructed will work properly. If a piece is supposed to be loosely attached but is too tight, the lack of movement will cause the device to not function properly. And, vice versa, if a piece is too loose but needs to be a tight fit, the piece could easily fall out or be in the wrong position. 

Bushings are used to secure pieces of Delrin on a rod. A bushing is slid onto a rod to prevent pieces from sliding side to side on the rod. Like the tabs and notches method, there are also loose and tight fits with bushings.  The inner diameter of the bushing and the diameter of the rod determine the tightness of the fit. The loose and tight fits can differ by only 0.01mm, as we measured with the calipers. If a piece needs to be secured to avoid sliding around, it is important to have tight fit. If the rod is meant to rotate in the bushing, the fit must not be too tight, or else the rotation will be hindered.

In the tabs and notches, we found that the dimensions specified on Solidworks did not match the dimensions actually cut on the laser cutter. On a plate of notches that displayed the widths of the notches used in Solidworks, the actual width was found to be approximately 0.01in. larger than indicated on Solidworks. This is very important knowledge to have when working with the laser cutter. The implication of this is that we must test the size of tabs and notches before printing out our complete project pieces. Like mentioned earlier, a proper or improper fit of bushings or tabs and notches may be the difference between a project being successful or not. Also, when using Solidworks, we may want the width of the notches, or the diameter of the bushings, to be smaller than the desired actual dimensions. 

I believe that the reason for the discrepancy in the Solidworks to laser cutter dimensions is the fact that when the laser cutter cuts out a piece of Delrin, the laser goes over the design several times. If the material is thick and requires several passes, or the velocity of the laser is low, the laser will melt more Delrin in the process. Therefore, the holes in the notches or the bushings are enlarged. The thickness of the material does matter, as the thicker the material, the more intensely the laser goes over the Delrin. If the laser goes over the Delrin so many times or for such a long time, the Delrin will melt more and the cut will thus be less precise. 

Ultimately, precision is key when attaching pieces of Delrin cut from the laser cutter -- especially with tabs and bushings. A few hundredths of a millimeter could have a dramatic impact on the result. On the other hand, tabs and bushings are less permanent and have more room for adjustment than heat staking and piano wiring. Regardless of which method is used, the use of test pieces and careful measurements are extremely important.









 
x