Fastening & Attaching

Heat Staking

Heat Staking refers to the method where pegs are inserted into a slot and then melted to join the peg and the slot permanently.

Advantages:

• Heat staking is permanent. This means that the product cannot be altered unless destroyed.
• As a result, it is very strong.

Disadvantages:

• Heat staking creates a round top, which makes it inconvenient to stabilize on a flat surface.
• Since it is permanent, the product cannot be adjusted if any problem occurs.

Possible uses:

Heat staking seems to be useful in many situations. For example, the permanence of the structure would be helpful in children's toys so that little parts that are swallowable would not come off and become potential danger to children. The permanence is also helpful when a product requires a lot of strength as opposed to the ability to be altered.

Piano Wire

Piano wire can be used to join two pieces of Delrin together so that they can open and close like book pages around the joint edge.

Advantages:

• The piano wire makes sure that the two pieces will join firmly together (unless the wire breaks).
• The piano wire gives mobility to the structure.

Disadvantages:

• The piano wire does not provide a still structure, i.e. it cannot hold pieces together at a specific angle, since it will spin.
• The piano wire requires a lot of precision.
• The piano wire cannot be used on pieces that are too thin. The drill hole makes the material thinner and more vulnerable to break.

Possible uses:

The piano wire, as mentioned above, is very good for structures requiring mobility. It can be used with structures like doors.

Pegs and Notches (Press fit)

Pegs and notches is a way to join Delrin together by friction caused by pressure. One cuts out pegs on one piece of Delrin and notches on the other piece. Then the pegs are inserted into the notches. If the size is correct, the two pieces will be held together very well due to the friction caused by the pressure.

Advantages:

• Press fit is a very sturdy structure.
• However, it is still reversible, unlike heat staking.

Disadvantages:

• It takes many tries to find the right size.
• As a result, making multiple test pieces wastes a lot of material.

Possible uses:

• Press fit is perfect for sturdy structures that require smooth surfaces.
• It is also good for structures that may require dissembling, e.g. when adjustments are needed.

Tolerance and measurements:

For a peg that is of size 3.15 mm * 12.63 mm, the size of the slots of different fits are:
Tight fit = 3.16 mm * 12.65 mm
Looser fit = 3.44 mm * 12.93 mm

Clearly, the slot has to be bigger than the peg, but not by too much. The slot has to be almost the same size of the peg, otherwise the structure is very unstable.

Discrepancy:

For a notch with a nominal width of .115 inch, the three measurements we took are .126 inch, .117 inch, and .117 inch.
For a notch with a nominal width of .125 inch, the three measurements we took are .135 inch, .136 inch, and .135 inch.

Clearly, the laser cutter makes the notch bigger than it is intended to be, possibly because it cuts by burning the plastic and thus consume parts of it, i.e., it does not cut through a line with a negligible width.

Bushings

A bushing refers to a circle cut out on a Delrin sheet that fits over a Delrin rod. It is normally used to fasten a rod to another piece of Delrin.

Advantages:

• Makes sure that the rod stays in place, perhaps against a sheet of plastic
• Can be also made loose to separate pieces on a rod.

Disadvantages:

• Just like press fit, it is very hard to find the correct measurement.
• Bushing does not guarantee that the rod would not move at all. There is a limit to the force that it can stand.

Possible Uses:

• A good use of bushing would be our project, as we would like to fix the rods onto a wall or a plate that spins. A tight fit bushing would work very well.
• A loose bushing may be used to separate pieces on the rod.

Tolerance:

For a rod with a diameter of 6.35 mm, the diameter of bushings of different fit are:

Tight fit = 6.40 mm

Medium fit  = 6.52 mm

Loose fit = 6.61 mm

Just like press fit, the bushings must be slightly bigger than the rod, but not too much. While we have no data for discrepancy in bushing, I can imagine that the actual measurement may be slightly bigger than the nominal measurement. Another observation we made was that sometimes the laser cutter does not cut perfectly vertically, making it tighter to push in from one side than the other.