How to Design Parts in SketchUp that Connect for 3D Printing
So you cut your SketchUp model into parts. Now you need a way to get it to all stay together after it’s printed. Here, you discover different strategies and features, mechanical and otherwise, that you can build into your model for attaching its parts together.
Tolerance and clearance in SketchUp
Before you can delve into mechanical connections, you need to understand two more of those pesky realities that crop up when you move out of SketchUp’s idealized environment: tolerance and clearance.
- Tolerance is the difference between the measurement of your part in SketchUp and the measurement of the part produced by your 3D printer. If you draw a 10mm cube in SketchUp and 3D print it, none of the measurements of the printed cube will be exactly 10mm. The differences are small — just a few tenths of a millimeter more or less — but they can cause problems if your 3D printed parts have to connect to parts that already exist in the real world. The 3D printer’s manufacturer provides a number for the machine’s tolerance as a plus or minus value, usually something like pm0.05mm. This is the maximum variation for that machine, and you should be aware of it as you work.
- Clearance is the extra space you need for parts to slide past one another. If you try to install a 10mm peg in a 10mm hole, you’re in for a bit of a surprise when the two won’t go together. The peg and hole can fit perfectly in SketchUp, but that’s not what’s going to happen in the real world where you have to contend with friction. The surface of the peg is so much like the surface of the hole that the friction between them will keep the peg from ever going in if the fit is too exact. You need to add a small amount of space called clearance so the two parts can slide past each other. How much clearance you use depends on how the part needs to move. A spinning shaft, for example, needs more clearance than a simple snap fitting.
You’ll come to an inherent understanding of clearance and tolerance as you do more designing in SketchUp for 3D printing. As you use your 3D printer, you’ll be able to find values that work well with your equipment.
As a starting point, add 0.2mm of clearance to all holes and 0.5mm of clearance to any points of rotation.
Using Glue in SketchUp
Glue is the universal way to stick something to something else. Unfortunately, it’s also the weakest and most unreliable method. Glue joints in plastics have very little strength and will tend to break under stress, in response to temperature change, or if you look at them funny. If your part is meant to be anything more than a visual model, use one of the other attachment systems.
- The plastics used in 3D printers — ABS, PLA, PVA, nylon, and PETT — all require special glues to bond. These glues are available, but must be ordered from an online retailer.
- Biodegradable starch plastics such as PLA have a crystalline structure that doesn’t work well with liquid superglue. The glue tends to stay liquid and migrate unexpectedly across the surface of the part.
- When in doubt, use epoxy. Its messy stuff, but will stick to pretty much anything.
- There are no glues that will stick to the flexible materials that are available for 3D printers. To attach parts made of that stuff, you’ll need to look into a process called thermal welding.
Snap fittings in SketchUp
Snap fittings are an awesome way to take advantage of your 3D printer. Properly designed, they’ll let you pop your model together as soon as the parts come off the printer. Snap fittings can also be removable, so you can change out various parts of your model as the design evolves.
In SketchUp, you create snap fittings by following two general steps:
- Create the tongue with the Line and Push/Pull tools.
- Create a matching capture point on the opposing part.
As you create your snap fittings, keep these points in mind:
- Be sure to leave enough space for the tongue to bend backward as it slides into place.
- Include a clearance of between 0.2mm and 0.5mm, depending on how tight you need the joint to be.
- Always position snap fittings so they print horizontal to the 3D printer’s build platform. Snap fittings printed in the Z direction, perpendicular to the platform, tend to break off.
- If you want your snap fitting to be re-openable, make sure that you provide a way for the tongue to disengage from the capture. This can either be a ramp in the geometry that forces the tongue backward as you pull on the joint, or an access point that lets you release the tongue manually.
- Don’t make the tongue too thick. It has to bend for the joint to go together.
Press fit in SketchUp
A press fit is when two parts are designed with very little clearance between them. When the parts are forcibly pressed together, friction keeps the joint together with no additional hardware or glue.
To make a press fit, use the Offset tool to create an outer lip on one side of the connection and an inner lip on the other side. Include a clearance of 0.2mm or less between parts, so you can still assemble them. Keep a few points in mind about press fits:
- For a press fit to work well, it needs to be a tight connection. You might need a small hammer, large clamp, or your whole body weight to press the parts together.
- A press fit is usually a one-way connection. After you put it together, don’t expect to ever get it back apart.
- Press fits don’t scale up well. Always design them at the size at which they’ll be printed.
- The tight clearance of a press fit can make it difficult to get started. Running a hobby knife or deburring tool around the edge of the hole will widen that area a bit and help you get the parts together.
Bolts, screws, and hardware in SketchUp
The strongest connection you can make between 3D printed parts is one held together with metal hardware. These connections are great for things that are more than just prototypes — for example, parts for robots or mechanisms.
When integrating hardware into your design, include the hardware in your SketchUp model. Take measurements of the parts you’re planning to use and model them in SketchUp. After you make these components, save them to the component library so that you can use them again with one click. Keep these points in mind:
- Remember to include enough clearance in your holes for the hardware to be installed.
- If you’re using bolts or screws and don’t want to include a matching nut, a trick is to slightly undersize the holes. The threads of the bolt will cut into the excess plastic and hold it firmly in place.
- Online hardware suppliers like McMaster Carr and Amazon Supply stock every fastener known to the human race. If your local hardware store doesn’t have what you’re looking for, these online stores will have it.