Since carbon fiber doesn’t behave the same as metal or other materials that are often used in car manufacturing, we must find ways of fastening the panels to the chassis without causing damage to the carbon fiber.
Drilling Holes:
As some projects would like to mount onto the panels with screws, the following are some ways I found to drill holes into panels.
The first few methods I found depending on the number of holes we’d actually like to make was adapted from the website Elevated Materials.
The first method is by using a simple electric drill:
Is the simplest way but also least efficient
You can drill the holes using high-speed steel bits
Most Efficient for 5-10 holes
Will dull very quickly after this and will only serve to heat and burn the resin
The second method is by using carbide tipped bits:
Can also be done using an electric drill
Can drill many more holes than an HSS bit
Although these bits are more expensive, they’re still reasonably priced
The last method is using a solid carbide bit:
Beware as these bits are very brittle and can only be used in a rigid setup like a drill press/mill
The bits will easily shatter using a hand drill
These bits just turn the carbon into dust at a high rpm
Cut fibers without fraying/delaminating the material and they eliminate splintering inside the hole
Can withstand hundreds of holes if they are used properly
The following drill bit has been said to be very efficient for drilling through composites:
Carbide Dagger Drill, you can get this drill bit from any retailer, this is just the one I found, pictured below.
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Note that when drilling holes, it is good practice to gradually work up to your final hole size
taking a look at this later: https://www.sciencedirect.com/science/article/pii/S2212827119313174The following are ways to properly fasten/machine into CF panels as some of our projects are looking to do this.
Small Introduction:
Since composites behave much differently than metals, we cannot use the same methods of fastening into metal as we would into composites. Luckily, many companies make specialty fasteners for these needs. A couple of them are threaded fasteners, lock bolts, blind bolts, blind rivets, and specialty fasteners for soft structures, such as honeycomb panels.
Fasteners:
The following are the most common and efficient materials to use for composites fasteners:
Fastener System for Sandwich Honeycomb Structures:
The adjustable sustain preload (ASP) system provides an easy way to fasten composites. Below are some pictures of how they work, these fasteners have a controlled clamping force.
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This is the only place these, specifically are available: (I think)
https://www.howmet.com/global/en/products/product.asp?cat_id=220&prod_id=1607
Hi-Lok® and Huck-Spin® Lockbolt Fasteners
Apparently, in the aircraft industry, these two fasteners are used a lot.
The former is a threaded fastener that incorporates a hex key in the threaded end to react to the torque applied to the collar during installation. The collar has a frangible portion that separates at a predetermined torque value.
The latter has a collar that is swaged (joined/connected??( into annular grooves.
The lockbolt comes in two types, pull and stump.
The pull is the most common, a frangible pintail is used to react to the axial load during the swaging of the collar. When the swaging load reaches a predetermined limit, the pintail breaks away at the breakneck groove.
Hi-Lok’s and the pull-type Lockbolt’s can be performed by person but the stump-type has to have multiple people? “it requires support from the head side of the fastener” I don’t know if this means multiple people but I think it does.
The following are some specifications:
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The following are some pictures detailing these fasteners:
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Places to buy these:
https://www.lisi-aerospace.com/en/product/hi-lok-and-hi-lite-fasteners/
https://www.hfsindustrial.com/us/huck
These are just a couple but there are more
Eddie-Bolt® Fasteners
These are similar to Hi-Lok’s and are apparently a “natural” choice for carbon fiber composite structures.
The pin is designed with flutes in the threaded portion, this allows a positive lock to be made during installation using a specially designed mating nut/collar.
The mating nut has 3 lobes that serve as driving ribs.
During installation, at a predetermined preload, the lobes compress the nut material into the flutes of the pin and form the locking feature.
This is what eddie-bolt’s look like:
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Where to buy them?
https://www.howmet.com/global/en/products/product.asp?bus_id=1&cg_id=88&cat_id=216&prod_id=536
After looking for a bit, this seems like the only place I could find these.
Cherry’s E-Z Buck® (CSR90433) Hollow Rivet
This rivet is meant to be used in a double flush application for fuel tanks.
The main advantage is that this type of rivet is that it takes less than half the force of a solid rivet.
These are installed with automated riveting equipment/ a rivet squeezer.
Special dies ensure that the squeezer is always centred during installation, this avoids structure damage.
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This is the diagram, this is all gibberish to me so…yeah.
Where to buy them?
https://www.usatco.com/item/WEB+839B1-4/Cherry-Hollow-End-E-Z-Buck-Dies/ (these are the dies not the actual rivets)
https://www.skygeek.com/cherry-aerospace-csr90433-4-5-rivet-mfg-c-of-c.html (I think these are the rivets)
https://phil-industries.com/catalogue/esd-products/cherry-hollow-end-e-z-buck/ (Same with this, I think its right)
Precautions
The following precautions in relation to riveting and bolting should be taken into account:
Riveting:
Don’t hit the rivets, this can lead to poor resistance to the impact of the laminates
Pay attention to the risk of possible “bolt lifting” (pictured below) due to the small thickness of the laminates.
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Note the necessity to assure the galvanic compatibility between the rivet and the laminates to be assembled.
Riveting accompanied by the bonding of the surfaces to be assembled provides a gain in the mechanical resistance by 20-30% but, the disassembly of the joint will be impossible, and the weight will obviously increase.
note the following are different types of riveting:
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Bolting:
The following was an example provided in a source:
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Tightening of the bolt will lead to a distribution of contact pressure
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Therefore, tightening of the bolt is necessary
But, since the laminated facings are fragile, they can’t admit high contact pressures and therefore, a metal washer should be inserted as pictured below:
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Bolted joints can also be bonded for higher strength but suffer the same fate as riveted joints where they cannot be taken off.
Bonding:
Bonding is also a method used that has many principal advantages which include:
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Adhesives:
The following adhesives are used:
Epoxies
Polyesters
Polyurethanes
Methacrylates.
When assembling pieces, the following must be taken into account:
Degreasing
Surface Cleaning
Protection of Cleaned Surface
The following schematic shows an exaggerated view of deformation that can occur in a double-bonded joint:
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Geometry
The Geometry of the joints must be able to follow the following specifications:
The joint must work in shear in its plane
Tensile stresses in the joint must be avoided
The following diagram portrays ideal geometries that we should emulate
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