CAD files for knife blanks

[deker]

I've got a follow-on question on this topic. I've been familiarizing myself with 2d/3d modeling a bit and have a new project I'm working on where I'll have a friend doing the CNC plasma cutting for me at TechShop in Durham. It's work that's being donated to my blacksmith guild and so I can't afford having a shop do the cutting for me. He's taken the TechShop course on how to run the machinery, etc. But neither of us has actually modeled a repeated design out for cutting.

So, assuming I want to cut several shapes out of a single sheet of stock, are there any "gotchas" I should be on the look out for in my designing? My gut instinct is to design the piece that needs to be cut and then simply replicate it across the size of the sheet to be cut. The start points of the cut won't be an issue for me (it's all getting forged down into damascus, so the extra little "hole" will get squished out) so I'm not too terribly worried about lead-in to a cut, I just want to make certain that I give him a design that he can easily work with.

It's a learning experience for both of us

-d

 

[deker]

Back to the top...Anybody?

-d

 

[Nathan the Machinist]

Yeah, I don't know.

Issues with plasma are it generates quite a bit of heat, so thin areas can melt and blow out. The HAZ is fairly large compared to laser and there will be burnt steel and scale around the outside that I wouldn't want forged in.

It doesn't pierce particularly well, so holes aren't going to be that great (I would avoid them)

I'm not sure what you're going after, but if you're planning to forge the bits you're cutting, the accuracy and edge finish of plasma shouldn't be a problem.

I'd make sure to leave at least 1/4" around the shapes so the plasma stream doesn't blow out the side of a cut.

The higher the amperage and the faster the feed, the smaller the HAZ, compared to low amperage and low feed. You should feed fast enough so that the slag will be blowing out of the bottom at about a 45 deg angle. The finished edge won't be perpendicular to the surface if that is an issue.

 

[cjohnson7]

I am a architecture student and one of the engineering proffs. asked if I would do some CAD plans for a water cutter. He is asking for nesting knives, and I can more than design and do the typical plans (hand drafted and such), but I honestly don't have a clue as to how much room i need to leave, or what file type I should export, or how I should set up the drawings.

Could you do a tutorial video, or send me an email, on how you take your knife designs from the drawing board to the exported plans?

Thanks!

It is my opinion that things designed in CAD frequently look like they were designed in CAD. I think it is all the straight lines and constant radius curves, they don't look natural to me. I have seen some prominent knife makers with amazing artistic talent make some mediocre designs because they used CAD and the CAD tool ended up driving the design.

I personally have a lot of hours using CAD, probably more than most anyone on this board, but I still have to design on paper first. I have to use CAD in order to machine my designs. So I draw them in paper first and scan them in and trace them in the computer using NURB splines. This is the only way I've found to prevent the knife from looking too "CAD".

I'm not saying that is a bad look (especially on a tactical), but unless that is the look you're going for, I think one must be very carful using CAD in their process.

Here is an example of a knife designed in CAD that I feel escaped the "cad design" look fairly well:

http://i566.photobucket.com/albums/ss107/Nathan_the_Machinist/wip_thread/cadknife.jpg

But I set out with a design on paper and a plan in my mind to achieve the look I wanted, without letting the CAD tool just do whatever.

 

[Nathan the Machinist]

I am a architecture student and one of the engineering proffs. asked if I would do some CAD plans for a water cutter. He is asking for nesting knives, and I can more than design and do the typical plans (hand drafted and such), but I honestly don't have a clue as to how much room i need to leave, or what file type I should export, or how I should set up the drawings.

Could you do a tutorial video, or send me an email, on how you take your knife designs from the drawing board to the exported plans?

Thanks!

To start, you normally need to know the sheet size that is going to be used. My first step is to create geometry that represents the sheet size leaving a bit of meat around the edges, 1/2" is fine. You want to nest your knives to leave .100" between the profiles. I assemble the profiles in an assembly then take them into a drawing for export. You export it as a DXF. For export settings I recommend polylines rather than splines unless you've talked to your waterjeter and know they can use spline data. Set your accuracy settings appropriately fine to reduce faceting.

Things that cause problems include lines that don't connect (import it and confirm you had a clean output) and double lines due to wireframe display of solid objects.

Be aware that with most grades of steel, there is a grain direction in the sheet. Generally speaking your layout should run parallel to the grain direction.

 

[parbajtor]

Bob,
Thanks for verifying what I suspected, but wasn't sure about. I have encountered one knifemaker's website who chooses not to use cut out blanks and cited material possibly being ill-affected as his reason for not cutting them out in the manner I was asking about. I appreciate you taking the time.

By the time you've ground the bevels in and heat treated the blade, the bits affected by the heat of the laser/plasma cutter have pretty much been lost to scale/carburization anyway. the post heat treat work (which I do by hand, because I believe that power tools affect the heat treat more, especially where the steel is only a few molecules thick) putting the final edge or a secondary bevels takes off those affected bits of metal anyway. Waterjetting would be the least risky method of cutting, but there are other things in the process that can affect the final result far more drastically than whatever method was used to cut the blank out with. Accurate tempering is just one example.