Getting Started with 3D Printed Minis, Part 4: Cutting Basics
One of my coworkers asked me to print a Naruto styled Kunai for a shadow box for his son. We hit up Thingiverse and found that model, which he liked, so I told him that it would be no problem to print. And it wasn't, because I have the tools to cut the model to make it no problem to print.
First, let's think about what I'd get if I decided to print that kunai as-is. It's got that nice rounded handle and a tapered blade, so there's barely any contact with the print bed. If I just tried to print it, I'd get a few mm2 of well adhered plastic on the bed, then a fair amount of spaghetti to either side of it as the printer tries to print in thin air.
Obviously, that won't work, but we can set up supports and even rafting! The raft will cause the printer to print the kunai's silhouette on the bed, then print up from there. That'll give me great bed adhesion, and then it'll print support material as needed to print the curved and tapered parts of the knife.
That'll work fairly well, but one side of the knife is going to look pretty good (the top side) and the other side is going to need some touchup work. At best, there'll only be some marks from where the raft and support material were removed. There'll also probably be some drooping between the supports, where the should-be flat surface of the blade is obviously malformed slightly by gravity during the printing process. As a rule, I don't like to have an outer perimeter include the first layer of a bridge if I can help it, especially if it's a large face that should obviously be very flat.
This model is an excellent candidate for cutting before slicing. So, how should we cut it? Well, that depends on how we want the finished print to look. The obvious way to cut it is parallel to the flat of the blade, right at the edge. Then, we can flip the bottom and print it with the cut face downwards on both halves.
That's what I ended up doing, because my coworker said that he liked the print lines and wanted to make it obviously 3D printed. In this case, the "flats" of the blade had the least detail, peaking at the .4 mm flat line that my nozzle can extrude. The tip of the knife, on the other hand, was only .1 mm thick, since it was laying flat on the printer bed (we cut it ever so slightly below the edge so that the edge would be 1 layer thick). This cutting style also ended up with distinct markings on the handle, where the part near the cut was detailed at .1 mm resolution, but as it circled up to the top of the print, it eventually ended with a .4 mm flat line. The curve looked ever so slightly off as it was represented by the .1 mm layer heights on the sides, but by the .4 mm nozzle diameter on the tops.
How else could we print it? How about standing upright? In that case, the only "top" layers that we'd have would be the the tip of the knife (and the layers that would eventually get glued to other pieces to form the single kunai). The tip and edges would be more blunt (due to the .4 mm nozzle width), but the layer lines would be consistent throughout the print. The round handle would actually be a bunch of round layers stacked on top of each other, instead of parallel lines that slowly and then quickly grow closer together.
While this sounds great, bed adhesion becomes a concern. We definitely don't want to print that entire kunai standing upright, as it's pretty much guaranteed to get knocked over. So, we're going to have to cut it into several pieces (I'd guess 3 would be about right, but more might be needed), so that no one piece is tall enough to lose its bed adhesion. We could use a raft (or at least a brim) to help alleviate this problem, but then we're back trying to cut off excess plastic and then hide the fact that we cut off a bunch of excess plastic.
So, how do we do all of this cutting? Depending on the complexity of the cuts, I find myself using anything from Slic3r to Meshmixer to Blender. Slic3r gives me a perfectly horizontal cutting plane that is perfect for cutting a rock column in half, so that it doesn't lose bed adhesion 90% of the way through the print. Meshmixer gives me more control, as I can draw arbitrary cutting planes and even exclude geometry from the cut (which is great for cutting arms off of minis!). Blender is my preferred tool if I am going to make a key to help me glue everything back together in perfect alignment (it can be done in Meshmixer, but I've run into situations where it didn't create sharp enough edges and then it was very difficult to get the key into the slot).
Slic3r is easy to use (and is how I generated these screenshots). Just select the piece that you want to cut and press Cut.... That'll open another window with a preview and a height adjuster. Just move that plane up until you're at a good cutting point, and press Perform Cut. You probably want to keep the defaults, Keep: Upper Part, Lower Part, Rotate Lower Part Upwards.
MeshMixer is also pretty easy. After you've imported your shape, go to Edit and select Plane Cut. You'll see a plane that you can move around, including pivots. You'll want to change the Cut Type to Slice (Keep Both) and you'll probably want to disable snapping (the green circle with an S in it, around the axis in the main display) so that you can get the plane exactly where you want it. Once you accept the cut, you can grab one half or the other, then do Edit and Transform to move it away from the other half. At this point, you can export the STL and move it into a slicing program to get it ready to print (or print directly from Meshmixer, if that's your preference).
Cutting with Blender is a totally manual process. So why do it? I've had better luck with making keys in Blender than with anything else. A key, in this context, is just an oddly shaped piece of plastic that fits into a recess in the two halves of the thing that you're cutting, so that when you put them back together, you know that they'll line up. Here's how I cut with Blender:
1) Import my STL, then go to Add > Mesh > Plane. That will stick a plane in my workspace, but it's probably too small to see. So, I press S and drag my mouse around a bunch to scale it up until the plane is large enough to bisect my model. Then, I grab the axial arrows and move the plane to a position that I like.
2) I make my Key object. I go to Add > Mesh > Cube and then scale it up so that I can see it. Next, I press Tab to enter Edit mode so that I can change the shape of my key around a bit. When making a key, I like to use a trapezoid, so I press Ctrl-Tab to change to Face Select Mode and select one of the faces. Then, I scale it down along the "wide" axis (the X axis, in this case). I do that by pressing S to enter scale mode, then X to limit my scaling to the X axis. Once I like the shape of it, I press Tab to get back into Object mode.
3) Once my key is built, I move it into the middle of my model, somewhere large enough that it can leave a void without causing too much trouble. It helps to turn on Wireframe mode for this process, so press Z to do so. I also like to use Quad View so that I can get it exactly where I want it. Go to View > Toggle Quad View to turn it on. This will give you 4 windows: Top, Front, Right, and Perspective. I use the Top and Front views to get my objects lined up, then turn back off Quad View and Wireframe so that I don't go crazy.
4) Now you get to add a Boolean Modifier. Right click on the Kunai to select it, then from the Modifiers section on the right, click Add Modifier and find Boolean. Under Object, select the Plane and change your Solver to Carve. Flip back and forth between the Difference and Intersect Operations, to get an idea about what's going on here. Don't actually apply the Boolean action yet, though.
5) Now, we duplicate the whole mess. Back in the main window, press the A key to select everything (it might take a couple of presses, depending on what's already selected), then press Shift-D to duplicate it and drag the duplicates off to the side. Make sure that the Boolean operations on one of your duplicates is set to Difference and the other to Intersect, then apply them both. Now, delete those planes since they're no longer needed.
6) And we're going to repeat the process, this time cutting the trapezoids from each of the halved kunais. Once again, add a Boolean modifier to each Kunai half and select the trapezoid as your object (which is probably still named Cube). Do a Difference operation on both of them this time and ensure that you use the Carve solver. Now, you're safe to delete one of the trapezoids, but slide the other one out to where you can work with it.
7) We want to save that trapezoid, since it's the piece that we'll insert into the two halves to guide them together for gluing. If everything in the world were perfect, we could just export these pieces and we'd be good to go... but the world isn't precise enough for that. We need to give some wiggle room to our key; I find that .2 mm works pretty well, but you may need to increase that or decrease that based on your hardware. I select the trapezoid and press Tab to go back into Edit mode. I then Right Click the top face and press G to grab it, press Z to constrain my movement to the Z-Axis, then type -.2 to move that face down .2 mm. I then grab the Left most face, constrain it to the X-axis, and move it .2 to the right (so a positive move this time). Finally, I grab the front face, constrain it to the Y-Axis, and move it .2 backwards (once again, a positive move). In the end, you probably won't be able to tell that you've done anything, but now you've got some wiggle room for your key.
8) Once all of that is done, I go to File > Export > STL and save my modified STL file. Then, I can import it into Slic3r to get it ready for printing... except there's one more thing. You need to break the STL file up into multiple pieces so that each one can lay flat on the printing bed. Within Slic3r, press the Split button to split the STL into its component pieces (you'll probably see its alignment shift as everything gets moved down to the printer bed). One half of the kunai is still upside-down though, so Right Click on that guy, go to Rotate > Around X Axis and enter 180. That should flip it upright, and now, you're actually ready to slice the thing and then print it.
So, how do you slice this thing? Give me a break! No, really, I'm tired after putting this tutorial together, and I'm planning on going in-depth into slicing in a later installment!
Articles in this series (to be updated with links as the posts are written):
Getting Started with 3D Printed Minis, Part 1: Modeling Software
Getting Started with 3D Printed Minis, Part 2: Printing Software
Getting Started with 3D Printed Minis, Part 3: Downloading Models
Getting Started with 3D Printed Minis, Part 4: Cutting Basics
Getting Started with 3D Printed Minis, Part 5: Cutting and Supporting a Mini
Getting Started with 3D Printed Minis, Part 6: Slicing a Mini
Getting Started with 3D Printed Minis, Part 7: Finishing and Painting a 3D Print
First, let's think about what I'd get if I decided to print that kunai as-is. It's got that nice rounded handle and a tapered blade, so there's barely any contact with the print bed. If I just tried to print it, I'd get a few mm2 of well adhered plastic on the bed, then a fair amount of spaghetti to either side of it as the printer tries to print in thin air.
Obviously, that won't work, but we can set up supports and even rafting! The raft will cause the printer to print the kunai's silhouette on the bed, then print up from there. That'll give me great bed adhesion, and then it'll print support material as needed to print the curved and tapered parts of the knife.
That'll work fairly well, but one side of the knife is going to look pretty good (the top side) and the other side is going to need some touchup work. At best, there'll only be some marks from where the raft and support material were removed. There'll also probably be some drooping between the supports, where the should-be flat surface of the blade is obviously malformed slightly by gravity during the printing process. As a rule, I don't like to have an outer perimeter include the first layer of a bridge if I can help it, especially if it's a large face that should obviously be very flat.
This model is an excellent candidate for cutting before slicing. So, how should we cut it? Well, that depends on how we want the finished print to look. The obvious way to cut it is parallel to the flat of the blade, right at the edge. Then, we can flip the bottom and print it with the cut face downwards on both halves.
That's what I ended up doing, because my coworker said that he liked the print lines and wanted to make it obviously 3D printed. In this case, the "flats" of the blade had the least detail, peaking at the .4 mm flat line that my nozzle can extrude. The tip of the knife, on the other hand, was only .1 mm thick, since it was laying flat on the printer bed (we cut it ever so slightly below the edge so that the edge would be 1 layer thick). This cutting style also ended up with distinct markings on the handle, where the part near the cut was detailed at .1 mm resolution, but as it circled up to the top of the print, it eventually ended with a .4 mm flat line. The curve looked ever so slightly off as it was represented by the .1 mm layer heights on the sides, but by the .4 mm nozzle diameter on the tops.
How else could we print it? How about standing upright? In that case, the only "top" layers that we'd have would be the the tip of the knife (and the layers that would eventually get glued to other pieces to form the single kunai). The tip and edges would be more blunt (due to the .4 mm nozzle width), but the layer lines would be consistent throughout the print. The round handle would actually be a bunch of round layers stacked on top of each other, instead of parallel lines that slowly and then quickly grow closer together.While this sounds great, bed adhesion becomes a concern. We definitely don't want to print that entire kunai standing upright, as it's pretty much guaranteed to get knocked over. So, we're going to have to cut it into several pieces (I'd guess 3 would be about right, but more might be needed), so that no one piece is tall enough to lose its bed adhesion. We could use a raft (or at least a brim) to help alleviate this problem, but then we're back trying to cut off excess plastic and then hide the fact that we cut off a bunch of excess plastic.
So, how do we do all of this cutting? Depending on the complexity of the cuts, I find myself using anything from Slic3r to Meshmixer to Blender. Slic3r gives me a perfectly horizontal cutting plane that is perfect for cutting a rock column in half, so that it doesn't lose bed adhesion 90% of the way through the print. Meshmixer gives me more control, as I can draw arbitrary cutting planes and even exclude geometry from the cut (which is great for cutting arms off of minis!). Blender is my preferred tool if I am going to make a key to help me glue everything back together in perfect alignment (it can be done in Meshmixer, but I've run into situations where it didn't create sharp enough edges and then it was very difficult to get the key into the slot).
Slic3r is easy to use (and is how I generated these screenshots). Just select the piece that you want to cut and press Cut.... That'll open another window with a preview and a height adjuster. Just move that plane up until you're at a good cutting point, and press Perform Cut. You probably want to keep the defaults, Keep: Upper Part, Lower Part, Rotate Lower Part Upwards.
MeshMixer is also pretty easy. After you've imported your shape, go to Edit and select Plane Cut. You'll see a plane that you can move around, including pivots. You'll want to change the Cut Type to Slice (Keep Both) and you'll probably want to disable snapping (the green circle with an S in it, around the axis in the main display) so that you can get the plane exactly where you want it. Once you accept the cut, you can grab one half or the other, then do Edit and Transform to move it away from the other half. At this point, you can export the STL and move it into a slicing program to get it ready to print (or print directly from Meshmixer, if that's your preference).Cutting with Blender is a totally manual process. So why do it? I've had better luck with making keys in Blender than with anything else. A key, in this context, is just an oddly shaped piece of plastic that fits into a recess in the two halves of the thing that you're cutting, so that when you put them back together, you know that they'll line up. Here's how I cut with Blender:
1) Import my STL, then go to Add > Mesh > Plane. That will stick a plane in my workspace, but it's probably too small to see. So, I press S and drag my mouse around a bunch to scale it up until the plane is large enough to bisect my model. Then, I grab the axial arrows and move the plane to a position that I like.
2) I make my Key object. I go to Add > Mesh > Cube and then scale it up so that I can see it. Next, I press Tab to enter Edit mode so that I can change the shape of my key around a bit. When making a key, I like to use a trapezoid, so I press Ctrl-Tab to change to Face Select Mode and select one of the faces. Then, I scale it down along the "wide" axis (the X axis, in this case). I do that by pressing S to enter scale mode, then X to limit my scaling to the X axis. Once I like the shape of it, I press Tab to get back into Object mode.
3) Once my key is built, I move it into the middle of my model, somewhere large enough that it can leave a void without causing too much trouble. It helps to turn on Wireframe mode for this process, so press Z to do so. I also like to use Quad View so that I can get it exactly where I want it. Go to View > Toggle Quad View to turn it on. This will give you 4 windows: Top, Front, Right, and Perspective. I use the Top and Front views to get my objects lined up, then turn back off Quad View and Wireframe so that I don't go crazy.
4) Now you get to add a Boolean Modifier. Right click on the Kunai to select it, then from the Modifiers section on the right, click Add Modifier and find Boolean. Under Object, select the Plane and change your Solver to Carve. Flip back and forth between the Difference and Intersect Operations, to get an idea about what's going on here. Don't actually apply the Boolean action yet, though.
5) Now, we duplicate the whole mess. Back in the main window, press the A key to select everything (it might take a couple of presses, depending on what's already selected), then press Shift-D to duplicate it and drag the duplicates off to the side. Make sure that the Boolean operations on one of your duplicates is set to Difference and the other to Intersect, then apply them both. Now, delete those planes since they're no longer needed.
6) And we're going to repeat the process, this time cutting the trapezoids from each of the halved kunais. Once again, add a Boolean modifier to each Kunai half and select the trapezoid as your object (which is probably still named Cube). Do a Difference operation on both of them this time and ensure that you use the Carve solver. Now, you're safe to delete one of the trapezoids, but slide the other one out to where you can work with it.
7) We want to save that trapezoid, since it's the piece that we'll insert into the two halves to guide them together for gluing. If everything in the world were perfect, we could just export these pieces and we'd be good to go... but the world isn't precise enough for that. We need to give some wiggle room to our key; I find that .2 mm works pretty well, but you may need to increase that or decrease that based on your hardware. I select the trapezoid and press Tab to go back into Edit mode. I then Right Click the top face and press G to grab it, press Z to constrain my movement to the Z-Axis, then type -.2 to move that face down .2 mm. I then grab the Left most face, constrain it to the X-axis, and move it .2 to the right (so a positive move this time). Finally, I grab the front face, constrain it to the Y-Axis, and move it .2 backwards (once again, a positive move). In the end, you probably won't be able to tell that you've done anything, but now you've got some wiggle room for your key.
8) Once all of that is done, I go to File > Export > STL and save my modified STL file. Then, I can import it into Slic3r to get it ready for printing... except there's one more thing. You need to break the STL file up into multiple pieces so that each one can lay flat on the printing bed. Within Slic3r, press the Split button to split the STL into its component pieces (you'll probably see its alignment shift as everything gets moved down to the printer bed). One half of the kunai is still upside-down though, so Right Click on that guy, go to Rotate > Around X Axis and enter 180. That should flip it upright, and now, you're actually ready to slice the thing and then print it.
So, how do you slice this thing? Give me a break! No, really, I'm tired after putting this tutorial together, and I'm planning on going in-depth into slicing in a later installment!
Articles in this series (to be updated with links as the posts are written):
Getting Started with 3D Printed Minis, Part 1: Modeling Software
Getting Started with 3D Printed Minis, Part 2: Printing Software
Getting Started with 3D Printed Minis, Part 3: Downloading Models
Getting Started with 3D Printed Minis, Part 4: Cutting Basics
Getting Started with 3D Printed Minis, Part 5: Cutting and Supporting a Mini
Getting Started with 3D Printed Minis, Part 6: Slicing a Mini
Getting Started with 3D Printed Minis, Part 7: Finishing and Painting a 3D Print
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