The Puzzle of Fastening and Attaching

Today in class, my fellow engineers and I explored the attaching options that would be available to us when we began to make out well windlasses. These options included using a heat stake, piano wire and notches made with a laser cutter. As a bonus, we also learned about bushings, and the impact small variances in size can have on out prototypes.

Heat Stake

Heat Stake

The heat stake is a machine used to melt the ends of small bits of plastic into 'bulbs' to permanently attach items together.  It requires that the plastic have a small hole pre-cut into it, and that the inserted piece's prong be longer than the depth of the hole, so it can create an effective bulb. Once the prong it threaded through the hole and the two pieces are carefully secured on the base (usually using a second piece of plastic with a hole in it to stabilize them), the heat stake can be gently lowered onto the prong. At 400 degrees Fahrenheit, the heat stake melts the plastic into a condensed half-sphere, which is wider than the opening of the hole cut in the plastic. 

Bulbs created by heat stake

Once excess plastic begins to leak around the edges, pressurized air is blasted at the tip and the

heat stake is turned off. After fifty seconds of cooling, the newly joined piece is finished. 

The heat stake is very effective at joining pieces together with minimum effort from the user. The two pieces do not have to fit perfectly and can have some wiggle room between them (unlike with a press fit, but that comes later). The heat press, when applied correctly, can solve any looseness between the two individual pieces. In any situation where water or excess moisture might be in contact with a product, this method of joining pieces might be particularly effective, as the low-friction Delrin won't slip (it might with a press fit) and won't rust or become water damaged (as the piano wire might). Things built for the outdoors in temperate and tropical regions are an example of such products. However, it does have some drawbacks. Unless the user breaks the bulb off the tip, there is no way to separate the pieces. 

Therefore, if someone wanted to make any minor adjustments to a piece after testing it, they would either have to find a way to break off the bulb or make two new pieces and use the heat stake again.

Piano Wire

Piano wire requires a bit more work than the heat stake, but it is easier to disassemble if needed. Piano wire is what it sounds like: thick, metal wire.

Piano Wire

Drill Press

Using a drill press, the user first bores a small hole in the desired piece of plastic The drill press is designed with an adjustable set of grabbers to accommodate any drill bit. Because of its versatility, the user can make the hole either a tight or loose fit for their wire. A large hole would make it easy for the wire to slide around in and let the plastic slide with it. This called a hinge. If the hole is snug for the wire, then it is considered pinned in place and cannot move. This kind of attachment could be very useful for materials that need strong hinges. In fact, this method is already used in products like door hinges, as the strong attachment can both swing freely and support the weight of the door.I imagine for our purposes that if a product required a movable joint that could stand up to large forces and constant motion, the piano wire attachment would be a perfect candidate.

Although both pinning and hinges are effective ways of connecting pieces (especially ones that need to be able to move easily), it does have its drawbacks. If the cut wire is not perfectly straight, pushing it through the drilled hole can be extremely difficult and can make the joint cockeyed or even unusable. Additionally, the user has to be careful that the drill bit they are using is the desired thickness (especially if they are looking for the wire to fit snugly). Without careful measurement beforehand, the user could accidentally make all their holes too large and render their pieces unusable. Double checking with a digital caliper is very important with this piece of machinery.

Notches and Press Fits

The third option for attaching pieces of Delrin together is by using notches and press fits. Using the laser printer and careful measurements, the user can cut out individual pieces designed to fit snugly together without any extra help. 

Separate pieces

Connected pieces

A press fit is when the two pieces fit together and (through pressure created by the geometry of the two pieces) remain stuck, like a peg in a hole. They can only be removed when a sufficient amount of force is applied correctly. Usually, this requires pliers, as Delrin has a very low coefficient of friction.

Despite the threat of the pieces coming loose, this method is very effective for a user with limited supplies. The pieces, when cut right, hold together quite well. For an engineer building a three dimensional object without excessive material or access to heat or wire, this method is perfect. Rural villages and other such machinery-lacking places would probably utilize this kind of attachment often. For our purposes, any connections that may or may not be permanent and don't have to stand up to excessive force, this kind of attachment would work very well.

However, one of the major difficulties in using this kind of attachment method is the small margin of error. The difference between a tight fitting and a loose fitting notch was very small. A tight notch was about .124in in width (.315cm) and the loose notch was .131in in width (.333cm), leaving only a .006in  (.015cm)  margin between them. As a result, this method requires extremely accurate measurements to be effective. 

Bushings

Bushings are small, circular cylinders with holes through their centers used to either hold objects in place on a rod, keep them spaced evenly or prevent them from falling off.  For our purposes, when can

Bushings

either have the bushing fit our rods loosely or snugly. Both loose and tight fitting bushings are important. Loose bushings will allow easier movement for other pieces flush against them. They will be able to move freely and have less of a frictional impact. Tight bushings can hold objects firmly in place, even in specific places, on the rod and prevent them from coming loose or falling off. They do not require any other aid in staying put. These would be very useful for keeping tires in place, aligning gears correctly and keeping various pieces spread out on an axle or rod.

However, the bushings suffer from the same drawback as the notch method: the margin of error between a loose and tight bushing is extremely low. A tight bushing measured .252in in diameter (.640cm) while a loose bushing was .261in (.662cm). An error of more than .09in could result in a series of bushings that can't hold objects in place.  

The Laser Cutter

The final thing we reviewed was the discrepancy between the measurements in the computers and the measurements on the laser cutter. The laser itself has a width, which can cause the length of various cuts to be slightly off. In Solidworks, my partner and I had measured our bottle opener to be exactly 4cm across. When we cut it out of 1/8' Delrin, though, it was only 3.9cm in width. With the 3/16' Delrin, it was only 3.95cm. It is possible that for the thinner Delrin the laser melts a little more away more quickly than the thicker Delrin. These differences did not affect the functionality of our bottle opener's handle, but they could have a very big impact when cutting out notches and bushings. Particularly with the thinner Delrin. For example, the difference between a tight and loose notch was .015cm. In this case the discrepancy caused by the laser cutter would render any tight notches unusable as their diameters would be too small. Additionally, loose notches would be too tight.


Today's lesson showed just how vital it is in engineering to measure twice and cut only once.