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Cutting Cardboard - Drag Knifes

Re: Cutting Cardboard - Drag Knifes

Postby keith » Mon Sep 08, 2014 8:02 pm

I would order #3.

I think it would be the best option as it would give the owner the OPTION to use both types of blades.

Just my 2cents.....
The Titanic was built by professionals; the Ark was built by Amateurs..........
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Re: Cutting Cardboard - Drag Knifes

Postby Ringo42 » Wed Sep 17, 2014 7:54 pm

I just found this site. Love the idea of a less expensive drag knife. Put me on the list for one.
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Re: Cutting Cardboard - Drag Knifes

Postby Awesomeness » Wed Sep 17, 2014 8:10 pm

I'm still waiting to get my CNC motor mounts for my CNC [metal] mill. As soon as I do, making a prototype of this drag knife will be one of my first projects. I was supposed to have the motor mounts about 5 weeks ago, so that's what the delay is.
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Re: Cutting Cardboard - Drag Knifes

Postby Ringo42 » Wed Sep 17, 2014 9:10 pm

I'm interested in cutting dollar tree foam board. It is almost 1/4" thick. The dontek guy did a demo recently where he was cutting it without issue. I bet lots of RC airplane people would be interested in this.
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Re: Cutting Cardboard - Drag Knifes

Postby Awesomeness » Wed Sep 17, 2014 10:05 pm

There's no real issues with cutting 1/4" foam board. It just requires an increased offset of the blade tip from rotary center. That obviously also limits how tight a turn radius you can turn.
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Re: Cutting Cardboard - Drag Knifes

Postby GRÜNBLAU » Fri Nov 28, 2014 9:18 pm

I just wanted to insert my $.02 since I have been down this road of figuring out my own drag knife. I am glad to see there is a lot of interest in pursuing DIY drag knives as this is exactly what I did. There are only very expensive options out there like the Donek Drag Knife and I did not know how much I would actually use it. I just wanted one in my arsenal just in case the need ever came up. There has been posts about people DIYing drag knives around the internet for years even before there was any software to use them.

A couple notes from experience.

I initially wanted to use a #11 Exacto blade as I have a bulk of them and I know that they can cut everything that I would want. The problem with any blade like this from a drag knife standpoint is that the trailing, unsharp portion of the blade needs to be vertical to help with the 'rudder' effect while it is cutting the material. When I was cutting the watercolor board in my video this became apparent during the first 3D printed prototype. For thicker material you will likely need take multiple passes. A vertical rudder will keep the pivot points interacting with the material consistently at all levels of the cut. Might not matter for very thin vinyl/paper, however.

There is a lot of force being applied by a machine when cutting, so it is necessary that it be made from materials that can withstand the force. I chose aluminum, eventually.

One thing you should be aware of is Donek Tools drag knife is patent pending. Below is the published application. In it he refers to the drag knife as an Apparatus for Manipulating Substrates in the title as there are likely examples of drag knives dating back to the 1800's.

https://www.google.com/patents/US20140091511

There are a variety of old patents and prior art that should be cited that apply. Here is an old patent from 1993 that is very similar in scope using computer numerically controlled machine to cut using a drag knife.

https://www.google.com/patents/US5438896

The only difference being that the blade used here is specific to the application. Happily there are also many DIY examples of using standard blades in the sphere of public forums dating back at least to 2005. This is one that I remembered complete with a drawing that looks almost exactly like the ones in the DT patent application...

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http://www.cnczone.com/forums/printing-scanners-vinyl-cutting-and-plotters/13922-cnc.html

Here is one holding a utility blade...

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http://www.rccrawler.com/forum/tools-procedures/416470-cnc-drag-knife.html

In any case, should his patent be granted, your investment in producing them may be at risk. One can only hope that the examiner does their research and does not simply rubber stamp such a broad application. Resulting in turning good hearted DIYers into outlaws. I have nothing against Donek Tools and I think they have produced a very professional drag knife. They have taken a DIY tool that people have been making for years and made it available to regular people that have no interest in tinkering. So patent or no, they will continue to have a large market share. I almost bought one, but decided to turn it into a fun project instead with an out of pocket cost of about $20 not including a broken endmill. I have always maintained that I have no interest in producing drag knives for others as I simply don't have the time, instead I have been trading the CAD file for a 'beer' so people can cut their own or make their own modifications etc... of course that would change if the patent was granted, obviously.

Here is the video showing my process..

https://www.youtube.com/watch?v=YqvgA1P5hWg

These are the finished parts. Tapped and ready to assemble
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Final assembled knife
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Assembly-

The stem that goes into the collet is simply a 1/2" OD aluminum tube stock that is press fit into the top plate with a bit of epoxy.
The square peg at the top of the knife holder is slightly chamfered (1/32") and press fit into the bearings. The left over 'D' spaces could be filled with epoxy as a forever solution (I didn't). The bearings used were these.. http://www.vxb.com/page/bearings/PROD/1-2inch/Kit7527 just because I had them laying around. There are much lower profile bearings that would change the holder geometry and make it much smaller diameter although this has no real bearing on function.

I look forward to seeing Awesomeness's final design and prototype! I might end up getting one as well! :D

Best,

Brian O.
http://www.grunblau.com
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Re: Cutting Cardboard - Drag Knifes

Postby Awesomeness » Sun Nov 30, 2014 12:56 am

Thanks for the background info and input.

GRÜNBLAU wrote:One can only hope that the examiner does their research and does not simply rubber stamp such a broad application.


You'd hope, but the patent office doesn't have a great track record.
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One thing with patents though, is that if you make meaningful (sometimes even trivial, but describe them as meaningful) improvements to something, you can patent yours or ignore previous patents. So you hold it a different way, make it smaller/taller/lighter/etc., or in some other way do a better job, and you can avoid infringing on a utility patent. For a design patent, you just need it to look different.

In any case, I finally got my motor mounts for my CNC mill about a month ago, and I'm hoping to get everything up and running over the holidays. Then I can make a couple prototypes.
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