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CNC milling, or computer numerical control milling, allows you to carve digital 3D models out of blocks of solid material like foam, wood, plastic, and metal. Using RhinoCAM, an add-on to Rhinoceros 3D, you can create your own models for desktop CNC milling machines, and build objects that are stronger than those created by additive means, including 3D printers.
In this short course, author Taylor Hokanson explores RhinoCAM, focusing on converting a 3D model from another graphics program to a "toolpath", or a set of instructions for a CNC mill to follow. From there, you can import the toolpath into the mill of your choice. Note, however, you should already be familiar with model generation in Rhino (a Windows-only program) before attempting this material. We recommend viewing Rhino 5 Essential Training to learn more.
This course was created by Taylor Hokanson. We're honored to host this training in our library.
So when you've been working with CNC for a while, there's a couple of mistakes you see all the time. And you see them, you know, because you make them. So hopefully that can share with you some of the common troubleshooting steps that I take. And this can prevent you from wasting some of the time that I wasted. Okay so here we have a box of stock. It's about three inches deep. And I'd like to cut a hole in it all the way through that's in the shape of a plus sign like this. So let me show you what happens if we just jump in here, make a two axis path, and do profiling. And again I want to cut out the interior.
So I'll select my drive region that's already selected. I've already defined a flat mill tool. And for my cut parameters I want to, or rather levels, I want to cut all the way down through the three inches of my stock. And let's say, just because we're doing this quickly, that we do this in half-inch depth passes. So I'll hit Generate. And you'll see that by default, oops, I got those tool paths on the outside of my material. So, that often happens if you forget, down here in parameters, to set Use Inside and Outside for Closed Curves.
I'll make sure to click that, and click on Inside. So you'll notice that even when it's unchecked, it'll still just make the choice for you, but it not, might not be the one that you want. So hit Generate. And now while the cut is on the inside, we have this problem where we're not fully executing the cut. So the reason for this is that even though these slots are exactly half an inch and my bit is exactly half an inch, those two things don't go together in the real, real world, right? Have you ever tried to put exactly a half inch object into a half inch hole? Doesn't work. And the computer recognizes that. So in this particular case we're going to need to, if we want that hole to be exactly half an inch, pick a bit that's actually smaller than half an inch.
So I'll head in here to my flat mill, and edit this tool. And we can bump this tool diameter down to 0.25. Save Edits to Tool. I'll get a warning because it's an existing tool, but we know what we're doing. And you can also see a picture here of how much bit we have exposed outside the tool holder, and that will become important in a moment. So Save Edits to Tool. Okay. Generate. And then now that bit can fit on the inside of this containment area just fine. So everything looks okay, but here's another example of why simulation is so important.
Let's go ahead and simulate this. And you see that even know we get this cut in the middle, we have this strange red surface. And what the red surface tells us, is that the tool holder is actually making contact with the stock, once the tool gets low enough. And it's actually eating away at some of that material, so that red surface is a warning sign. So let's head back to our flat mill. And then here we can see that we have this much amount of material, this is the sort of cutting surface of the bit, and just a little bit of non cutting shank sticking out with the tool holder right here.
So if the bit allows for it, we should try to pull it out of the collet a little bit, say maybe to 3.125, so three and an eighths inches, again, assuming that our bit is long enough. So I have the same bit, but now that it's exposed a little bit more, we should be able to use it to get all the way down to the bottom of this cut without damaging the top of the surface. So, let's regenerate that path, still looks the same, but now if we simulate it, you'll see that we make that cut with no errors, also got this funky little piece in here; but that's so slim it'll probably just blow away as saw dust.
Okay, so finally, if you've been watching carefully as we've gone along. You'll see here in my simulation that the interior of my sharp corners are actually rounded over. Now in many cases that won't necessarily matter but if you're trying to make something, say, for inlay, or that requires a very careful fit, if your model has sharp corners in it, you want your result to have a sharp corner in it. So it's just plainly impossible to get an absolutely sharp corner if you're working with a cylindrical bit. You can get a really tiny bit so that, that roundover is very small.
But you'll never totally get rid of it. Now if you're trying to fit something in here, and all you care about is making sure that a square object can fit into this void. We'll do what some people call dog boning. Some people call bunny ears, I'll leave that up to you. So let's come in from the top and I'll temporary hide my stock. And now I'll make a circle, using my snaps will be useful in this case. So I'll make a circle right here, and I want this to be slightly larger than the diameter of my bit. We picked a quarter inch bit, so let's just make it something like 0.26.
We're just trying to make something that the bit can tuck into. Okay. I'm going to copy it over. I'm going to center over here. Now I can copy these two at the same time. Copy this fella here, here, here and here. So there's a couple of ways to do this. I can go ahead and get in here, and actually trim this out, so it's all one continuous area. Another way that I could do it, is just to make a separate profiling path. So let's just go ahead and trim it, because I think that looks kind of cool. So here are my cutting objects, the central area, and then I'll come around and cut all of these little interiors.
Enter to finish. And now we'll do it in reverse. So I select everything and then deselect the plus, so these are my cutting objects, and then my objects to cut are the insides of the cross. You can't see this happening because this is currently selected as a region for cutting. But bear with me, you'll see it's there. All right, now if we join this all together, that looks pretty good. You'll see up here it's joined into one closed curve. We might need to refresh our machining features, remove all, drive containment, like so.
And now when I hit generate, what we should get, is a little secondary drill pass that heads into each of these. And this will reveal itself it I refresh the stock and simulate it. Make the stock visual. Let's do that one more time. And there you go. So, if we want something to plug into these corners, now a sharp corner will fit into there. It will leave a little bit of a visual gap, so it just depends on the type of result you're trying to accomplish. Okay, so those are some common mistakes that I've made, so you don't have to.
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