The start of a CNC'd knife

#1
I've been working for the last year or so (very part time) on machining some knives. I thought it would be good to contribute something to this section of the forums. This is a little overview of how I currently make a CAD model of a knife. I've learned this stuff on my own through forums/youtube/help files and just figuring it out as I've gone. So this is probably not the best way to do things from the perspective of someone with actual training in this stuff, but it's worked well for me so far. This does not cover every small step, but hits the highlights.

Step one is to insert an attached canvas, which is just a photo of a knife you want to model. This is a small fixed blade of mine. You have to scale the image, I know the holes are .25", so I scaled to the holes. Next I trace the outline. In this case I used the sketch splines. They work well for curved surfaces. They end up smoother than a bunch of arcs.
Image 2-18-19 at 8.54 AM.jpg
 
#2
Next hide the canvas and extrude the blade. I recommend keeping the origin plane in the center of the knife by extruding to both sides, extrude, dimension, symmetric, then your blade thickness. Image 2-18-19 at 8.56 AM.jpg
 
#3
Next I'll do the bevels. I use a sweep to do it. I make a construction plane along a path at the cutting edge. A construction plane gives you a made up surface to draw a sketch on, in this case. Image 2-18-19 at 9.24 AM.jpg
 
#4
Next a make a new sketch on that construction plane. I'll draw the profile of what I want cut off of the blade to make this side of the bevel. The profile is offset from the center plane of the knife by half of the thickness of the edge. The line with dots is the origin plane viewed from the side, where the knife profile was sketched. Image 2-18-19 at 9.25 AM.jpg
 
#5
Next I'll make another construction plane. This time I'll use the tangent plane, selecting the edge face. Then I'll make another sketch, this time for the plunge. Throughout these steps using the project/include feature for some reference points is key. Image 2-18-19 at 9.27 AM.jpg
 
#6
Next is the satisfying part. Using the sweep cut feature select the triangle for the bevel as the profile and the line along the cutting edge as the path. Same for the plunge, use the sketch for the profile and the line along the plunge/bevel for the path. Image 2-18-19 at 9.28 AM.jpg Image 2-18-19 at 9.29 AM.jpg
 
#8
In order to get the bevels/plunge/swedge onto the other side of the blade I use the mirror feature, selecting the bevel face, plunge face, and swedge face, mirror plane can be the origin plane, since that is along the center of the blade body.

Image 2-18-19 at 9.33 AM.jpg
 
#9
That will get you a blade with bevels modeled in CAD. That is really just the beginning. Next is figuring out how to hold the blade for machining, then figuring tool paths and machining methods in the CAM. This is my blade fixture set up. If anyone is interested I can add some more about how I designed the scale, fixtures and then into CAM.

Image 2-18-19 at 9.36 AM.jpg
 

KenH

Well-Known Member
#12
Looks good - I've played around with Fusion some, but not enough to really get 3D out of it. Good work, and YES I'd be interested in seeing how to make the tool paths and export to gcode. What CNC program are you using? I know about Mach4, and use LinuxCNC for my work.
 

Sean Jones

Well-Known Member
#14
Cool stuff. I've played with AutoCAD LT a bit...but not enough to really do much with it. Plus I can't afford a personal copy. Fusion 360 is a good tool from everything I've seen. Your work with it is very good.
 
#15
For making the scales I made another sketch (maybe not necassary), projected in the circumference of the handle and the holes. Then I drew the front of the scales. Image 2-19-19 at 8.56 PM.jpg
Next I extruded the scales. This time I just did a single side extrude up from the origin plane, the same plane the knife outline sketch is on. I made the holes have an 82 degree countersink. Then I put a fillet on the perimeter. Image 2-19-19 at 8.58 PM.jpg
 
#16
This looks a little too plain and simple, though I did make set of scales like this and they were pretty comfortable. I decided to add a slight curve to the sides of the scales. I did more constructions planes and sweeps. Not a massively contoured scale, but I think it will be a little nicer than without the sweeps.
Image 2-19-19 at 9.21 PM.jpg
 
#17
Next I'll show the designing of the fixture plate for the blade. I sketch out some stock and some holes in the stock for locating pins and 1/4-20 screws to hold it down.
Image 2-19-19 at 9.08 PM.jpg
Then extrude that out, so that it looks like what I want the stock to be going onto the fixture. I'll do the locating holes, screw clearance holes and skeletonize the handle on an "OP 0" where I just put the bar stock in the vise. Image 2-19-19 at 9.10 PM.jpg
 
#18
Next I'll sketch up the fixture plate. I'll use a piece of 1x6x12 aluminum. I project the holes onto the fixture plate sketch, then draw a line down the middle of the fixture plate and mirror all of the holes onto the other side of the plate for flipping the blade over. The hole in the center of the fixture plate sketch will be for locating or zeroing the fixture once on the machine.
Image 2-19-19 at 9.11 PM.jpg

I made a construction line along the same center line of the fixture plate sketch so that I can mirror the blade and the blade stock.

Image 2-19-19 at 9.13 PM.jpg
 
#19
Then extrude that out and make it a new component. I should have mentioned that it seems to be best to make the scales and blades independent components. The blade fixture and scales fixture will be individual components as well.

For this extrude, I used an "offset plane" which allows you to start the extrusion, in this case, lower than the plane the sketch is on. The sketch plane is in the center of the blade. It is a .156 blade, so offset down in Z by half of that, .078. So now the model of the blade will sit right on top of the model of the fixture plate.
Image 2-19-19 at 9.14 PM.jpg
Image 2-19-19 at 9.15 PM.jpg
 
#20
I also made a fixture for the scales, basically the same process. I did set the scales up on a raised boss I guess you'd call it. That way when I machine it, I can use a ball mill to hit the sides of the scales, because I can have the ball portion below the bottom of the scale. That will probably be more clear when I get to the CAM portion.
Image 2-19-19 at 9.18 PM.jpg
 
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