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Thread: Drag Knife

  1. #1
    Join Date
    Oct 2011
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    Colorado
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    Default Drag Knife

    I built my first drag knife in 2000. It was developed for die cutting (inlayed) snowboard base graphics. Over the course of about a year and a half, I developed 2 different holders that were very effective in different circumstances. I set myself the goal of creating a new knife holder this summer that could do both jobs.

    My new knife holder uses a standard irwin utility blade rather than an expensive custom blade. Having spent a bit of time looking for a commercial product, it came to my attention that there is a potential market for such a device. Tangential cutters are great, but far more expensive and not always necessary. I put together a second one that can be used for materials up to 1/8in (like cardboard and leather). Both holders can be used to cut a variety of substrates. I have used drag knives for cutting gaskets, snowboard graphics, leather, flexible magnet material and probably others. I have also used them to score ABS plastic to later be broken apart.

    I am curious if others would be interested in such a device and what they might be willing to pay for one.


  2. #2
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    Jun 2010
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    Vancouver Island
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    Looks like a brilliant idea. Does it have a spring? Having the ability to adjust spring tension may alow you adjust for different material.

  3. #3
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    Quote Originally Posted by VanIslanddan View Post
    Looks like a brilliant idea. Does it have a spring? Having the ability to adjust spring tension may alow you adjust for different material.
    A knife like this is not intended for thin materials like sign vinyl. If you surface your vacuum table frequently enough, there is no need for a spring, just accurate zeroing of the knife. You can quite easily set it to just cut through your material without cutting into the table.

  4. #4
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    Sep 2006
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    cnc routing, portland or
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    looks really good.I made one for 1/2" cardboard. I used the spindle to swivel it. but I had to have the knife have a lot of tilt backward about 1/2" or so. if I had it like yours the knife would not turn and would just bend going around a corner. 90 degree cuts I had to manually line up the blade and cut all of the cuts in one direction.

  5. #5
    Join Date
    Jun 2004
    Location
    Springfield Mo
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    809

    Default spring / price

    Seems like the commercial one I saw somewhere was $150.. which would be ok if one used it all the time.

    Does one need a spring ? Seems a cushion or a pad under the material to be cut would provide the nearly same function.
    The decimal point seems to be the most important on the z axis... x & y not so much....
    ShopBot... Where even the scraps and things you mess up and throw away are cool....

  6. #6
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    Quote Originally Posted by knight_toolworks View Post
    looks really good.I made one for 1/2" cardboard. I used the spindle to swivel it. but I had to have the knife have a lot of tilt backward about 1/2" or so. if I had it like yours the knife would not turn and would just bend going around a corner. 90 degree cuts I had to manually line up the blade and cut all of the cuts in one direction.
    That's some thick cardboard. The typical technique for making sharp corners is to raise the cutter to the surface of the material and move the cnc through a circle/arc with a radius equivalent to the offset, then lower the cutter into the material and resume cutting. Minimum cut radius is usually equivalent to the offset of the tip of the blade.

    Quote Originally Posted by curtiss View Post
    Seems like the commercial one I saw somewhere was $150.. which would be ok if one used it all the time.

    Does one need a spring ? Seems a cushion or a pad under the material to be cut would provide the nearly same function.
    The $150 cutter requires a $14 replacement blade and can only cut up to 1/32" thick material. I'd be surprised if it could really do that as the blade is similar to my vinyl plotter blade, and they wouldn't last long in such an application. They are very very small. Most gasket material is thicker than 1/32, as is leather, snowboard base material, and (I think) the flexible magnet I've cut. Perhaps the applications are too specialized to make production affordable. I could never produce them for $150. You could easily spend that on 10 blades for the other device. A pack of 50 blades for my holders costs $15. When you are cutting thicker material, there's really no need for a spring or cushion. Materials like ABS can be cut partial depth and then broken much like you score glass and break it. Some materials are forgiving of an almost complete cut, but as I said earlier, setup is quite simple as long as your table is surfaced periodically. We just skim off .005 to .010in once a month and we're good.

  7. #7
    Join Date
    Apr 2001
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    South Elgin, IL
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    It looks like a well made tool. But if you need to get more than $150.00 for it, I think you will have a limited market and the companies already in that market probably have expensive flat bed plotters that can handle thick materials. I'm just making assumptions here...

    If you want to sell these to the occasional user, I think something between $50.00 and $100.00 would be an affordable price for a tool that saves time manually cutting various materials.

    Have you tried using an Exacto blade with more of an angle on them?
    They might allow cutting of thicker material if you have the cornering issue figured out.

    Is your knife holder is a swivel holder which allows the blade to turn the corner because of the blade off set?

  8. #8
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    Sep 2006
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    Quote Originally Posted by donek View Post
    That's some thick cardboard. The typical technique for making sharp corners is to raise the cutter to the surface of the material and move the cnc through a circle/arc with a radius equivalent to the offset, then lower the cutter into the material and resume cutting. Minimum cut radius is usually equivalent to the offset of the tip of the blade.
    I thought of that too but it was a hassle. I finally just drew everything straight cuts with just single lines and all x cuts and y cuts had the same start points. so I would cut all of the x cuts rotate the knife 90 and cut the y's
    the cutting I did was fairly limited so it was ok and it did not change.
    his is what I tried like yours but this would not work well it needed a lot more trail. I ended up having about 1/2" or so trail. once I had it I could cut these parts with little problem.


  9. #9
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    Quote Originally Posted by knight_toolworks View Post
    I thought of that too but it was a hassle. I finally just drew everything straight cuts with just single lines and all x cuts and y cuts had the same start points. so I would cut all of the x cuts rotate the knife 90 and cut the y's
    the cutting I did was fairly limited so it was ok and it did not change.
    his is what I tried like yours but this would not work well it needed a lot more trail. I ended up having about 1/2" or so trail. once I had it I could cut these parts with little problem.
    I see no bearings in your cutter Steve. You have to have bearings. The spindle or router has way too much mass to be turned by the tip of a knife blade. Minimal inertia and freely spinning bearings are critical.

  10. #10
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    Default

    Just a quick video of the motion on the knife. I'll try to put together some video of the knife in action this week.

    Looks like I can't embed, so here's a link:
    http://youtu.be/_9G2IZlW0gY

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