Fastening & Attaching

Partner(s): Jiaming Cui

Goal: Learn how to connect two pieces of Delrin (acetal) together using heat-staking or piano-wire. Additionally, learn about loose and tight fitted bushings on Delrin rods.

Station A (drill press, arbor press, piano-wire fastening):

• benefits: 
• a relatively simple way to make a hinge-type attachment of two pieces of Delrin
• easily modified (piano wire is removable, can shave down corners to allow for more movement)
• not time intensive
• drawbacks:
• you must remember to use to different size drill bits to make an effective connection
• must use a file to adjust for more movement in the hinge (but filing down is irreversible) (another way to allow for more movement is to have the holes further away from the end, closer to the center)
• two pieces must fit together relatively well (measurements vary with thickness of Delrin and number of times required to cut through it)
• the hole drilled is irreversible and must be in the right place the first time (note: it may be difficult to get the wire through both holes if they are not in the same place)
• This method of fastening is likely best to use when fastening together two pieces of Delrin into a hinge or moveable joint. In addition, this may be better for prototyping or initial models of a product because the way pieces fit together need not be as exact as say with a peg and slot.

Station B (thermal press/heat stake):

• benefits:
• fastest way to join two pieces of Delrin together
• provides a strong joint between the two pieces involved
• drawbacks:
• permanent
• requires the piece of Delrin sticking through the hole to be sufficient to melt and make the seal but not excessive
• little flexibility
• alters the shape of the pieces involved
• This method of attaching two pieces of Delrin together is a strong and permanent, to be used at times when you want a steadfast, reliable, unmovable connection (in the foundation of a model).

Station C(Calipers, bushing measurements and tolerances, peg and slot measurements and tolerances):

• benefits:
• requires least material (and connection effort) to join together two pieces of Delrin
• flexible/changeable connections between objects
• drawbacks:
• require high level of precision so that the two pieces fit together almost perfectly
• non-permanent
• This method of connecting may be preferable when you want the least permanent yet stable connection between pieces. In addition, in putting together a puzzle like structure, there is an elegance (and aesthetic aspect) in pieces that fit together almost perfectly without the use of additional material. 

Exploration of loose and tight fit bushings:

1. Measure the outer diameter of a Delrin rod (6.35mm)
• Inner diameter of bushing:
• press fit: 6.25mm
• tight fit: 6.45mm
• snug fit: 6.56mm
• loose fit: 6.67mm
• Observations:
• The bushing with an inner diameter less than the outer diameter of the Delrin rod did not fit.
• The bushing approx. 0.1 mm greater than the Delrin rod diameter fit tightly but was not moveable.
• The bushing approx. 0.2 mm greater than the Delin rod diameter fit snugly but still would not slide.
• The bushing with an inner diameter > 0.3 mm than the Delrin rod diameter easily slid on and off.
• A tight bushing may be required for when you want the rod inside to rotate to at a 1:1 ratio as the bushing outside of it, say in a car axel to turn the wheel. 
• A loose bushing may be required on a bicycle's front wheel -- to hold a rod in place, but allow it to turn freely.
2. Measure the relevant dimensions for the peg and slot:
• rod: 
• dimension: 6.35mm
• tight fit: 6.42mm
• difference: 0.07mm
• loose fit: 6.67mm
• difference: 0.33mm
• peg 1:
• dimension: 3.23mm
• tight fit: 3.38mm
• difference: 0.17mm
• loose fit: 3.54mm
• difference: 0.31mm
• peg 2:
• dimensions:6.84x4.92x4.93 mm^3
• tight fit: 7.04x5.13x4.98 mm^3
• difference (in each respective dimension): 0.2x0.21x0.05
• The differences in the tight fit are lower than that of the loose fit, but that is to be expected. Loose fits tend to be about 0.3mm larger than the dimension we are measuring.
3. Measure the thickness of the peg plate and compare to the SolidWorks dimensions:
• Line 1:
• SolidWorks Dimension: 0.135 in
• Actual Dimension: 0.140 in
• difference: 0.005 in
• Line 2:
• SolidWorks Dimension: -0.125 in
• Actual Dimension: 0.133 in
• difference: 0.008
• Line 3:
• SolidWorks Dimension: 0.115 in
• Actual Dimension: 0.125 in
• difference: 0.010 in
• Although the differences between the SolidWorks dimensions and the actual dimensions are not very large, we have not taken into account the thickness of the peg plate and how many times it was cut in the laser cutter. Laser cutters, by nature, do not cut straight down -- meaning that the beam weakens in strength as the distance the beam has to travel increases -- so cuts in the Delrin are actually at an angle. Therefore, measurements using the Caliper tend to favor one side of the cut. Possible improvements to a laser cutter (although I do not know much about the science of it) would be to have the beam closer to the material in an attempt to lessen the impact of distance on the strength of the beam. Another possible improvement to the process is to flip the Delrin sheet after one cut and cut from the other side (this would require a flipped version of the 2D model cutout).
• Note: perhaps using a smaller unit would have been better for this exercise, using mm rather than in. to ascertain more precise measurements.

Thanks for reading! Don't get stuck in the snow!

ps: this is my favorite gif at the moment