Step 5: Machining the Housing

Picture of Machining the Housing
This is yet another step where I assume some knowledge of milling machine techniques.

I used a 1/8 inch end mill to machine 70 mil deep channels across the length of the housing. This provides heat sinking and more than doubles the thermally emissive surface area of the housing. The channels were cut front to back with respect to direction of travel of the bike. This allows the airflow from riding to dissipate the heat better. Cross cutting in the other direction resulting in a checker board pattern would increase turbulence and potentially increase cooling effectiveness. I have found though that after my 15 minute commute on cool evenings that the housing is just barely warm to the touch.

There are some excellent comments below regarding the coating of the housing. It turn out that for an application where convective airflow is the dominant cooling mechanism (as is the case here) that coating the surface of the aluminum will not measureably improve its heatsinking performance. In any case I decided to powder coat mine for durability and aesthetics. As stated above, the cooling performance of the housing is sufficient, so overall it seems to be working fine.
aburton5 years ago
Thermal emissivity only applies to radiant heat. In this situation, radiant heat is going to be negligible compared to forced convection. The properties you want to be looking at are surface area and thermal *conductivity*, not thermal emissivity. You're right that it is preferable to increase the turbulence across the surface of the housing, but it is HIGHLY unlikely that you'll be able to coat the surface of the housing with something that can increase radiant heat and keep the convective heat constant. Anodizing won't help either. When you anodize, you're coating the surface of the aluminum with a (relatively) thick layer or aluminum oxide. This also doesn't conduct heat as well as pure aluminum (or 6061 aluminum, which is probably what you're using.) The result will be that you're actually insulating the aluminum from convective heat transfer. The important thing to keep in mind is that you're not radiating much heat. You're going to get several orders of magnitude more heat transfer from forced convection. If you want to increase the heat transfer, try to maximize your surface area.
aburton5 years ago
I'm sorry, but I'm not quite sure what you mean when you say "Bare aluminum is not as good a conductor as one might think." Do you think you could explain this a little further? As it is currently worded, this is a deeply misguided statement.