Hi guys and gals. I'm making a new head for an Streamlight SL-20X. I'm at the point where I have to decide how thick the fins should be and how wide the grooves should be. The diameter of the head is just shy of 3 inches. I started with 3 inch stock and machined just enough to clean the outside surface up.
I'll be cutting with a parting tool that is .095" wide, so that's pretty much what my groove width is going to be I think. Figure on .100" to account for a little tool wander. Taking partial a thickness cut with it is a sketchy proposition, so getting wider grooves is tough.
I should have thought of this earlier and cut a test piece or two to see what I liked, but I have the head in the 3 jaw chuck now and don't want to take it out. 007, I know. I should be using the 4 jaw. I may end up mounting the 4 jaw and re-chucking it after I do come cuts on a test piece.
So, I guess my question is how wide to make the fins since the grooves will be about .100"? I've been thinking .125 or .150". I know thinner is better for cooling, but I don't want them to be fragile. What say you?
A lot depends upon the depth of the fins. Thinner fins can mean more fins and more surface area for heat transfer. I have made fins about .100” thick but probably not as deep as you can go with a 3” diameter. With that size I am thinking .125” fin thickness.
I personally think they look best with the same width as groove. They look proportional that way to me. The parting tool may cut a little wider than it’s measured width, so I would just use a .1”
Take a cut and move .2” and cut again, .1” for the spacing and .1” for the parting tool.
Just my 2 cents.
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Maglite’s are not that true from the factory. Just make sure that where you are taking the cuts, that this section is running somewhat true before you start, especially if you are only going to be removing a few thousandths, or you will end up with egg shaped groove’s.
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You really need to use the 4-jaw more often. I know the self centering 3-jaw is easier, but the independent 4-jaw will always be the most accurate. I use nothing but the 4-jaw. My 3-jaw has been setting in a box for over 2 years now.
Alan, I didn't consider finning when I started this build. I made the pill already and it's giant, so I don't have a lot of depth to fin. If I ever make a second one, I'll make a smaller pill so that I can make deeper fins. For this one the fins will be about a quarter inch deep.
I have finned several mags with .1” fins and have never had one to break. At the outer shell of the head, they might get beat up from heavy use or a drop, but never broke. I have also finned several 1 AA mini mags. I ground down one end of parting tool to make smaller groove’s and keep the same proportional spacing to groove pattern.
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Not a very good pic, its the only one I have uplinked.
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In this pic the 2AA has the same cut to spacing ratio, but the parting tool chips at the anodizing when it first starts to cut leaving the black anodizing looking a little smaller than the groove. For this reason you might pull a little further than the correct ratio. The fins will be wider than the grooves but the remaining black anodizing should look proportional.
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I’m looking at my favorite headlamp, and the fins are about 1.5mm with about 2.5mm grooves. The depth is about 10mm on a head with a diameter of about 35mm. They are plenty sturdy at that size, and 4 of them are even tapped to hold the lens and mounting bracket. (Sorry about all the approximate numbers… I’m away from any measuring tools.)
If the fins will not be that deep, I would make them the same as the groove width. As 007 and others have said, I think that they will look the best if fin and groove are about the same.
I have to agree with 007 in that aesthetics of a finished light play a large part in the use of that light. When it looks good, you want to use it. When it works as good as it looks, you forget you have other lights. Being only around .250” deep, keeping them .100” shouldn’t be a problem. This is 6061 is it not?
I’d want to anodize it, then re-cut the groove to clean it out back to silver. For me it’s a bling bling thing, the silver stripes look awesome in the anodized head, and my mindset tells me that the raw aluminum disperses heat to the air better than anodized aluminum, whether that’s a fact or not.
Edit: For the record 007, Buck is using a new piece of 3” aluminum not a mag head, or at least that’s how I read it.
A new machined head for a streamlight. I was only referring to what I had done and I figured it will not be long before he has one of those aluminum chunks of a Maglite chucked up in the lathe.
For transfer between surfaces (such as between pill and LED star) polished is better but for external surfaces a dark anodize is better for transfer to air.
if the anodize surface stops electrical continuity, how can it be better for transferring heat to air? All that dark business is a matter of collecting heat, I would think, not dispersing it. Obviously, a dark piece of aluminum will absorb more heat from the sun than a silver one, but when transferring by contact or by air cooling, I wouldn’t think the color would matter. If so, why aren’t computer cpu’s cooling fins anodized black? Big business there, I would think they’d be all over it if black did a better job.
I may be wrong about the colour of anodize although I remember reading about it affecting it (specifically, a black material has a higher emissivity coefficient than silver). This article says the colour doesn’t matter:
This article puts anodized higher than polished aluminium but doesn’t specify the kind of anodizing:
Light colors have low emissivity and dark colors, high. All that color non-sense only matters for radiant heat. Finned heatsinks work by conduction so color doesn’t matter.
I think what you are trying to say is that for moving air, the color does not matter, and that may be true. But usually a flashlight is not moving that fast and color can matter quite a bit. Silvery heat sinks need airflow, black ones may be able to get by without it.
All air-transfer heat sinks ultimately work by radiation of the heat… and that depends upon the surface area (i.e. fins) and emissivity. Heat flows via radiation at the FOURTH power of the temperature difference… this can far outweigh the heat flow due to conduction. Black/dull/textured surfaces have higher emissivity than light/shiny/smooth surfaces.
Conduction heat transfer occurrs where two solids are in contact with each other. Heat flows from higher temp body to lower temp body. Like emitter star to heat sink.
Convection occurs where heat is transferring between one fluid and one solid body (for the most part. It could be two or more fluids too). Convection relies on the motion of the fluid to carry heat away. Like air over a finned flashlight head.
Radiation heat transfer is strictly that, electromagnetic radiation. A body that is above absolute zero emits energy in electromagnetic waves. It is mainly dependent on the temperature of the body radiating heat and it's area. The vast majority of radiation heat transfer is in the infrared spectrum for the temperatures we are addressing, so color doesn't matter for us. Not even a little. What does matter is that shininess is bad for radiation heat transfer. For instance, a brushed aluminum surface radiates much better than a polished one. Even the regular aluminum oxidation layer that forms is much better than shiny polished Al. So called "Bling" is detrimental to heat transfer.
Anodizing is not detrimental to heat transfer for a number of reasons...It is dense, so it conducts heat well. It is extremely thin, so even if it was an insulator it is so thin that it wouldn't matter. Lastly, there is no interface between aluminum and the anodized layer the anodized layer grows from the aluminum and even grows down into it as well. They are one and the same body.
My thoughts on all of this:
-At the temperatures we are dealing with, radiation is the minor player. Convection is numero uno. What the split between the two is...maybe 80% Convection 20% Radiation. That's just a guess and I'm not digging my old text book out to re-learn this stuff so I can do the math.
-So for our purposes, to transfer the most heat away from the exterior of a flashlight we need as much surface area as we can get. Bigger heads, fins and even pins sticking out form the head would work great.
