How to put a knife edge on a piece of 7075 aluminum? - Page 2
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  1. #21
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    Thanks for the replies, very useful information from everyone. Excited to try some new ways

    Here is a picture of the type of boat it is going on, this one I made out of wood
    Lavigne Marine Hydroplane Outrigger - YouTube

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    Quote Originally Posted by Kenre View Post
    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
    I NEED TO MAKE ONE OF THOSE CARRIERS!!! such a great idea. Any pictures of it?

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    An Aluminum Knife???????? ok
    how about using one of those sharpeners sold on late nite TV ?
    unless a chunk of concrete would do?

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    Rough it out with whatever you have; file, router, ??

    Then:

    Belt sander with 1" belt. Start with 150 grit, then 220, 440, 600, 800, etc.

    High speed, don't slow it down.

    Use a light oil on the belts as you progress to the finer grits. You can get a mirror finish.

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    Quote Originally Posted by aaronhl View Post
    Thanks for the replies, very useful information from everyone. Excited to try some new ways

    Here is a picture of the type of boat it is going on, this one I made out of wood
    Lavigne Marine Hydroplane Outrigger - YouTube
    Sounds just like the real thing:
    35cc Hydro Test Run - YouTube

    They meant to say 350cc.

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    Quote Originally Posted by Kenre View Post
    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
    That one doesn't have a hook, it's angled. It's just not adjustable and the angle is bent into the fin. A hook like the OP showed wouldn't do quite the same thing as the angle. They way I see it, a hook like that not perfectly aligned with the angle of attack of the boat, would cause either lift or down force in the straightaway. An angle fin properly set won't do that. The nose of a hydro does lift at times, and so you would have to set a hook like that to never create lift when the nose lifts, thereby creating undo drag on the inside sponson on a straightaway.

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    Quote Originally Posted by Forestgnome View Post
    That one doesn't have a hook, it's angled. It's just not adjustable and the angle is bent into the fin. A hook like the OP showed wouldn't do quite the same thing as the angle. They way I see it, a hook like that not perfectly aligned with the angle of attack of the boat, would cause either lift or down force in the straightaway. An angle fin properly set won't do that. The nose of a hydro does lift at times, and so you would have to set a hook like that to never create lift when the nose lifts, thereby creating undo drag on the inside sponson on a straightaway.
    That's right, so when you look at the fin the curve should be pretty much parallel with the water line when the boat is at speed. Maybe a little pointed down to keep the sponson on the water

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    Quote Originally Posted by Kenre View Post
    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.

    [/url]
    You'll never bend 7075 like that without annealing, If I was the OP I would make sure the grain was 90deg to the bend to avoid cracking as well.
    Last edited by triumph406; 02-28-2016 at 10:54 PM.

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    The bend cannot be a 90 degree, it's a slight curve or radius...I have to measure exactly how it's bent but I know it's not 90 degree. I will post more pictures later

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    Grain not bend

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    Gotcha that makes sense, I ripped the aluminum on my table saw going along the grain and the bend will be going across the grain

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    Quote Originally Posted by aaronhl View Post
    The bend cannot be a 90 degree, it's a slight curve or radius...I have to measure exactly how it's bent but I know it's not 90 degree. I will post more pictures later
    Sorry, I meant to grain not bend

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    Quote Originally Posted by Forestgnome View Post
    That one doesn't have a hook, it's angled. It's just not adjustable and the angle is bent into the fin. A hook like the OP showed wouldn't do quite the same thing as the angle. They way I see it, a hook like that not perfectly aligned with the angle of attack of the boat, would cause either lift or down force in the straightaway. An angle fin properly set won't do that. The nose of a hydro does lift at times, and so you would have to set a hook like that to never create lift when the nose lifts, thereby creating undo drag on the inside sponson on a straightaway.

    Forestgnome,

    No hook on that one is correct! 18 degree bend to be exact. Only made about 50 of them so far lol, Never had one break yet.
    You are right in that the ones with a hook do need to be aligned with the running angle of the boat.
    When set up correct the boats will corner like there on rails! The curved fin is great on the smaller lighter boats, helps stop them from flipping over in the corners when they hit rougher water.

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    You have any pictures of your turn fins?

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    OK guys I finished the fin I was working on before this thread. I roughed up the knife edge using my 4" grinder and then filed and sanded to smooth. Not too pretty but usable

    I also bent the fin slightly- sandwiching the fin in a vice between 2 steel angles resting on the vice jaws and pulling the fin down with a heavy pipe wrench...The bends are ok except for the areas on the knife edge that pressed wrong in the vise. The rear of the fin has a smooth curve BUT the front knife edge is crooked

    I found a router bit on ebay (similiar to the one posted, might get a 12 degree instead) I am going to get and install a larger diameter bearing on it to machine the knife edge. I will have to double sided tape a thick template on top of the 7075 sheet to run the bit.
    Magnate 383 Raised Panel Router Bit, Horizontal - 5 degree Face Cut Profile; 1-1/16" Reveal - Edge Treatment And Grooving Router Bits - Amazon.com

    Then maybe I will bring it to a shop to have them bend it...Or I can figure out how to make mold I can install in my vise to form the 7075 sheet.

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    Quote Originally Posted by aaronhl View Post

    I found a router bit on ebay (similiar to the one posted, might get a 12 degree instead) I am going to get and install a larger diameter bearing on it to machine the knife edge. I will have to double sided tape a thick template on top of the 7075 sheet to run the bit.
    Instead of pissing about chasing angled bits that are only good for one angle, you ought to put some more thought into an adjustable angle holder for using a regular cutting bit. I had something like this in mind. Easy enough to make adjustable for the angle. Screws to hold the stock onto the angled table.untitled.jpg Use the router you have, to cut away the material, flip it for the other side, then bend.

    Just saying. Cheaper.

    Got a band saw? Access to one? Hand held jig saw? Grab a block of hardwood and draw the shape you want to end up with, a bit tighter radius than you want, to allow for springback. Squeeze your part in between the two halves.
    Gonna take some cut and try, but you should be able to afford the 2 or 3 blanks that it will take to get there. I have found old pallets to be a pretty good source of hardwood chunks. YMMV.

    Cheers
    Trev
    Attached Thumbnails Attached Thumbnails untitled.jpg  
    Last edited by trevj; 03-01-2016 at 11:52 AM. Reason: trying to get picture uploaded...and big enough to see....

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    Is it Plate or extrusion?

    Only rolled plate will work. The rolling work hardens the outside edges, by compressing the grain structure. The sharp blade you want should be using one of the outside edges where it's HARD *smile.

    Stan-

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    Quote Originally Posted by trevj View Post
    Instead of pissing about chasing angled bits that are only good for one angle, you ought to put some more thought into an adjustable angle holder for using a regular cutting bit. I had something like this in mind. Easy enough to make adjustable for the angle. Screws to hold the stock onto the angled table.untitled.jpg Use the router you have, to cut away the material, flip it for the other side, then bend.

    Just saying. Cheaper.

    Got a band saw? Access to one? Hand held jig saw? Grab a block of hardwood and draw the shape you want to end up with, a bit tighter radius than you want, to allow for springback. Squeeze your part in between the two halves.
    Gonna take some cut and try, but you should be able to afford the 2 or 3 blanks that it will take to get there. I have found old pallets to be a pretty good source of hardwood chunks. YMMV.

    Cheers
    Trev
    Gotcha, more than helpful... I have everything to do all of that...only concern is the angle of the edge I might need to build an extreme slope for that router jig

  19. #39
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    Quote Originally Posted by aaronhl View Post
    Gotcha, more than helpful... I have everything to do all of that...only concern is the angle of the edge I might need to build an extreme slope for that router jig
    Not getting you. Were you thinking of cutting with the stock up on edge? Don't. Screw the stock down to the angled section between the two side rails. That way, if you wanted to, you could bevel the fin stock across a width far wider than the length of the router bit.

    The jig that I proposed above is to be used with a regular square ended router bit or end mill in a router, with the table of the router on top of the two side rails.

    A table saw or some care with a jig saw or even a hand saw and protractor, and you can make the bottom block at any angle that you wish. As I see it, a fella would be able to make the jig angle adjustable by adding a couple blocks of wood across the ends and pivoting the bottom block that the stock is screwed to.


    Set it to half the angle if you want a centered edge, or at the full angle if you want it at the edge. Adjust your depth of cut until you reach the depth you need it to be.

    I have cut a few hundreds of feet of various sheet aluminum aircraft parts out with a cheap Dewalt router. It works pretty well, though you REALLY want good eye protection and a zipped up coverall on.

    Clear? Or not?

    If not, I'll really (really!) crudely mock up what I mean, and take a picture of it. Tomorrow, though.

    Cheers
    Trev

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    something tells me your forearms could handle it...


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