How to put a knife edge on a piece of 7075 aluminum?
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    Default How to put a knife edge on a piece of 7075 aluminum?

    I have a piece of 7075 .100" thick aluminum approx 5" x 3", one end is curved and along the curve I need to have a bevel cut like a knife. I have tried to grind it down with my handheld grinder and smoothing it with a file. It does not look very good...looking for a better way to put this kind of edge on it...any ideas?

    My tools are mostly for woodworking but not limited. I don't have a mill or something to cut the edge..here is a picture of the curve I am talking about and also if you could give me some ideas on how to bend the piece to a smooth curve after I add the edge it would be helpful too

    turnfin.jpg

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    use a sharpening stone. submerged in kerosene or even soapy water.

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    Carbide router bits or saw blades run at woodworking speed will cut aluminum and leave a smooth surface. You need to be very cautious in how you hold the metal and take very small cuts. Aluminum takes more power to cut than wood, hence the small cuts.

    Never use aluminum oxide abrasive to try to grind aluminum. Zirconia or silicon carbide will work OK.

    Larry

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    Quote Originally Posted by L Vanice View Post
    <snip>
    Never use aluminum oxide abrasive to try to grind aluminum. Zirconia or silicon carbide will work OK.

    Larry
    Yeah, but everyone does. They grab the angle grinder and hit the aluminium like it is steel. If you are doing this, make sure you keep the wheel clean and not not choked up. Drag an old masonry drill across the spinning wheel to clear the clogging regularly.

    The danger is the wheel can clog up, quickly go out of balance, then fly apart after vibrating like hell.

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    I should think a large file should do it nicely.

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    Vixen files are made for stuff like that.

    1" Vixen File Single Cut

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    That looks like a Turn Fin from an RC boat.

    I use a belt linisher to sharpen the leading edge. Only sharpen the outside edge.

    I see the initials TI at the top, pretty sure you have a Titanium fin there, Not 7075T6.

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    Quote Originally Posted by Kenre View Post
    That looks like a Turn Fin from an RC boat.
    title of image is "turnfin".

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    Yes it is a turn fin for an rc boat. You'd be surprised how little people want to share about how they make these.
    The one in the picture is Titanium and one I currently have, but I want to make my own out of 7075. I need one with a slighly different shape/length

    I am leaning towards using my router (and table), not sure how I can hold it safely though.

    If I can make a nice edge on it, I will bring it to a shop so they can roll slip it to curve, but maybe I can do that myself too?

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    I would file it if it didn't take 3 hours to make perfect and kill my arms...

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    I use brown grinding wheels for roughing aluminum, on a 4 1/2" grinder.
    Then, for finish, I use alumina zirconia flap discs- probably 100 grit or finer.

    brown wheels- from Rexcut- Rex-Cut Abrasives - Home
    most decent welding supply stores will stock these, or you can order direct, or, most likely, from MSC or McMasters.

    Lee Valley has em for six bucks
    4-1/2" x 7/8" Rex-Cut Max Flex Wheel | RC-4-1/2MF - Lehigh Valley Abrasives

    the flap discs I use are klingspor, but norton or pferd or sait all make good flap discs- blue, alumina zirconia is what you want, 100 or maybe 120 grit.

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    Very nice- I guess the right disc on my 4-1/2 will work great too
    I say that because I can get a pretty good edge with a black grinding disc, it's just a matter of cleaning it up with the right disc. And If I use a better disc for the first rough cuts, that will help later on to finish.

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    It's not rolled I think. It's bent in a press with a die. You can make that sort of simple die yourself. you can use a belt sander to sharpen them pretty darn quickly.

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    Quote Originally Posted by aaronhl View Post
    Yes it is a turn fin for an rc boat. You'd be surprised how little people want to share about how they make these.
    The one in the picture is Titanium and one I currently have, but I want to make my own out of 7075. I need one with a slighly different shape/length

    I am leaning towards using my router (and table), not sure how I can hold it safely though.

    If I can make a nice edge on it, I will bring it to a shop so they can roll slip it to curve, but maybe I can do that myself too?
    You will need to anneal the 7075 if you want to bend it like the sample.

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    Interesting... How do you know it's bent in a press and not rolled?

    How can I make one of these or what is the name of it so I can search for it

    If I torch it and bend it, will the 7075 be the same strength again once cooled?

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    You have a table saw? Router? Use the saw to make an angle jig. Use the router to carve the bevel. Make a jig to fit in a bench vise or shop press to make the bend. Practice on some scrap and expect to make more than one version of the bend jig to get the curve the way you want it.

    If it takes you three hours to file it, you have the wrong files, Get a coarse double cut file for roughing out the bevel, A vixen file is probably overkill, but would work and with practice, would leave a pretty good finish too, a couple single cut files for finishing, like as not a Bastard and a Fine cut. Should take you all of about ten minutes to put a good bevel or bevels on the fin, then bend it. Secure your stock on the edge of the bench, and put your back into it. Take care of your files. Pick them clean if they get pinned (bunged up with filings and metal), and keep them from rattling against each other. Replace as required. The are cutting tools, they wear, get blunt, and generally deteriorate in use.

    Look into learning about draw filing.

    Wet or Dry paper wrapped around something solid and flat to finish. Use water with a drop or two of dish soap in it. I liked using offcuts of acrylic (Plexiglas) for flat sanding stuff. Start coarse, work your way up, it goes really quick.

    You can spot anneal the 7075 by drawing a line on it with a Sharpie Marker, and heating it with a torch until the line vanishes. Drop it in water to stop the process instantly. It's aluminum. Quenching it like that simply stops the anneal process, does not cause it to harden like steel does.
    By spot annealing, you still keep the temper up at the point it is connected to the boat. The area you want to bend will be soft.

    Cheers
    Trev

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    I've only seen fins for full-sized hydros, and they're always straight, adjustable angle mount. Seems like the curve would cause extra drag without doing any work. Is this common on RC boats?

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    With 7075 you should start by lapping it against strong sunlight and get the final edge by honing it on waning moonlight.

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    Quote Originally Posted by aaronhl View Post
    I would file it if it didn't take 3 hours to make perfect and kill my arms...
    Practice makes perfect. Also, it sounds like you're using the wrong file for the job. Get a coarse toothed file for roughing, only switch to the fine ones for the final clean-up. I have a big old curved tooth file that happily hogs aluminum and leaves a smooth finish.

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    No need to anneal it for that radius bend, it will spring back somewhat, so a little over bending will be required to get the correct shape.

    you can make a radiused bend with a couple of dies, one being a piece of round bar, the other something concave to press into. A press or large vice may even be enough. Hint, press the shape first, then cut to shape.
    Several bends close together will achieve the same thing as a constant curve.


    RC turnfins have the hook on the bottom, or an angle to stop them coming unhooked or riding up out of the water.
    Heres an example, Twin Engine Hydro - YouTube

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