It has happened to all 3D printing practitioners at one time, and if not already will happen to you sooner or later. You have designed the perfect part that you know will fit nicely into your new assembly. Or you have a model that is exactly the size that you want and you are ready to print your masterpiece. Suddenly you realize….."Oh no!”. The model is too large for your printer.
What is one to do? You could borrow some print time on a friend’s machine. Better yet, you could send it off to a service bureau or prototyper to have it fabricated professionally. But the cost?! Plus you did get into 3D printing to “make stuff” yourself. Well, there is an option available for you to print your model on your own 3D printer.
The key is part segmentation. Segmentation is nothing more than dividing the part into smaller printable sub-parts. But before you take your virtual ax and start indiscriminately chopping your model to pieces, step back and consider the ramifications of what you are about to do. First, how many sub-parts do you make? Do you make the least number of sub-parts that will fit the print bed and be assembled into the completed part, or should the number of sub-parts be determined by efficient use of the print bed and printer time? With this in mind, segmentation is more than just cutting up a model/part haphazardly.
The most fundamental detail to consider is how the model/part is to be used. Is it wholly or part of an aesthetic piece, or is it a component that will come under some type of thermodynamic or mechanical stress? Ideally, you would not want to segment a part at a critical or structurally important intersection. For example, an airfoil/wing should not be segmented at the point where it comes into contact with the fuselage of an aircraft. If possible, segment the aircraft model toward the middle of the airfoil/wing or segment along the fuselage. Also, do not segment along narrow or small diameter sections of the model/part. Even if the segment is in a less critical section where stresses are minimal, it may be difficult to join the sub-parts back together while maintaining structural integrity. Just remember that component failures at the junction of segmentation usually indicate that the segmented location was a poor choice.
If your model/part is an aesthetic piece, then you may have some flexibility in the segmentation process. However, do realize that the junction areas where the segmented sub-parts connect may require post processing after finishing the print. Remember to segment your model/part in areas where segmentation would be less noticeable. Corners, overlaps and overhangs are often excellent places to segment your model/part. That way, junction areas may appear to be more natural in relation to the entire model/part.
Now that you have identified possible areas for segmentation of your model/part, next you need to consider how to shape the edge of the segmented model/part for proper assembly. For example, machined or 3D printed metal sub-parts/sub-assemblies are usually joined by hardware to include fasteners, bolts, nuts and screws. However, when working with the materials for desktop 3D printers, hardware may not be an option. As a result, joinery solutions and adhesives may be the only path forward.
Joinery has its history in woodworking and the construction of housing and boats before the advent of hardware or in cases where hardware was difficult to fabricate. The fundamental idea was to connect components using an array of joints that would provide strength and structural integrity. Many of these techniques are still visible today in ornamental woodworking or furniture making and has translated effectively to 3D printing. Upon consideration of how your model/part is to be segmented, reflect on the type of joint that you would want to utilize to assemble the sub-parts.
The common joints that you will find in 3D printing include laps, dados (with or without pins), dovetails, mortises and “puzzle” joints. These joints in addition to adhesives can result in an utilitarian solution not only for sub-parts but for assemblies in general. Designing these joints correctly often times takes practice and knowledge of the tolerances of the source materials being used (For a full discussion on part tolerances and assemblies, please refer to MatterHackers Lab: Design 3D Printed Assemblies). The bottom line is that segmentation into sub-parts is more than just drawing an electronic dividing line in the sand. Joint connections and tolerances need to be considered.
When working with cylinders, laps (see Figures 1 and 2) can be an effective solution to the joinery challenge. Often times it is possible to make laps with tolerances where the two sub-parts snap together and are held firmly through tension. Adding an adhesive to this joint may increase the strength of the bond but may not be necessary. Dados (see Figure 3) are probably the most common joint that you will find in sub-part and part assemblies. Again joints with dados that are properly designed may be held together through tension, so adding adhesive to the joint can only improve upon an already viable solution.
Dovetails and mortises (see Figures 4 and 5) are a little more complex than laps and dados and require a bit of art and skill to work in sub-part and part assemblies. Remember that tolerances play a huge part in the correct design of dovetails and mortises and you may need several iterations in order to be successful. Finally “puzzle” joints (see Figure 6) are nothing more than unique, stand alone joints much like the pieces of a jigsaw puzzle. Again, utilizing “puzzle” joints takes some practice and the iterative process of design most likely will come into play.
There are numerous adhesives that work with 3D printing source materials with most if not all being off-the-shelf products. Glues and hot glue guns are effective adhesive options and can may be fundamental to a successful end product. However, now there is an off-the-shelf option that uses the source material as the actual adhesive. This may be accomplished through the use of a 3D printing pen, such as the Crafty Pen. The Crafty Pen is an excellent device in which to repair or augment flawed or broken parts. In the case of sub-part assemblies, the Crafty Pen provides the best adhesive capability by far. If you have not utilized a 3D printing pen in your assembly process, now would be a good time to start.
Finally, one needs to consider how these sub-parts should be printed in order to optimally print a completed model/part. Remember not to use any section of a joint as the base of the print. Sometimes this is unavoidable, but most times it is best if another edge is utilized as the base of the print. Also, you may want to consider how the joint is printed in relation to the z-axis. As you well know, the weakest part of a 3D printed model/part is along the layers in the direction of the z-axis. If at all possible, plan to print you joint section of the model/part at an angle to the z-axis.
Printing a 3D model/part that is larger than the build volume of your printer can be challenging, but achievable if you follow these basic rules. The first rule is to segment your model/part in such a fashion as to mitigate stresses that will be placed on the completed model/part. Also, use existing contours of your model/part to “hide” well placed segmentation. Next, make sure to use standard joint solutions that that will enhance the strength and efficacy of the segmented sub-parts. These enhanced joints along with adhesive supplements will ensure that the finished assembled product will not suffer any ill effects from segmentation. Finally, use your design skills to properly segment and place the sub-parts on the build plate for efficient and effective printing. Following these guidelines will ensure that you will successfully print a part that is larger than your printer’s build volume.