How to 3D Print Replacement Gears

3D printing a replacement gear can be a cost-effective and efficient way to fix a broken or worn out gear. It allows for the creation of a custom, precise replacement part without the need for expensive tooling or long lead times for ordering a replacement from the manufacturer. Additionally, it can be useful for older or discontinued equipment for which replacement parts are no longer available. In some cases, the ability to print a replacement gear can enable you to fix and prolong the life of a piece of equipment, rather than having to replace it entirely.

There are a few hurdles to this though, and it can seem daunting to try create a high performance part from scratch, but hopefully after reading through this guide you'll be more comfortable creating your own replacement parts.

Modelling

• Fusion 360
• GF Gear Generator
• Calipers (or other measuring tool)
• Camera
• Your gear

FDM Printing

• FDM Printer
• Filament
• Slicer

SLA Printing

• Resin Printer
• Resin
• Slicer

Before you begin it's important to know some of the words used to describe gears. These are all set out in ISO 1122-1:1998, but you don't need to know all of this to print your own replacement gear.

Pressure Angle: angle between the tooth face and the gear wheel tangent

Module: reference diameter of the gear divided by the number of teeth

Helix Angle: acute angle between the gear teeth axis

Measuring a component is not totally straightforward, but luckily most gears come in standard sizes so you don't need to be perfectly accurate.

• To find an approximation for the module, measure the total diameter of the gear then divide by the number of teeth. The module is defined as the reference diameter divided by the number of teeth, so your result will be a bit larger than the actual value.
• To find the helix angle, take a side-on view of the gear, then measure the angle between the teeth and the flat surface, subtract this value from 90° to get the helix angle.
• The pressure angle is much harder to measure, but most gears either use 20° or 14.5°, so select whichever looks closest to your gear.

The internal geometry of your gear will differ from model to model so take note of any dimensions that seem relevant.

Before starting make sure you have GF Gear Generator installed for Fusion 360.

• Click into the Utilities tab then click on GF Gear Generator drop-down menu to select what type of gear you want to make. You will then be prompted to fill in the desired dimensions.
• For the module, select the value that is closest to, but still less than, the result you found in the previous step.
• Fill in all other values and click OK to generate the gear.
• Before going any further you need to double-check that this gear is dimensionally accurate. Click the ruler in the Utilities tab to begin measuring, and compare to its real life counterpart.
• If the diameter of the model is too small, go back and increase the size of the module you chose and vise versa.
• If the gear teeth look too pointy, go back and select a smaller pressure angle.
• If everything looks good you can move on to modifying the gear to fit your needs.

To edit the gear, open the Solid tab and create a sketch on the face of gear, draw the relevant shapes, and cut them out as needed with the Extrude tool. Note that you don't need to replicate the internal geometry to exactly match that of the original part, you just need to be able to fit it in the same place as the original.

Now you can move onto 3D printing.

Depending on the application, FDM gears can work great as a replacement. There are many different material options to choose from, so you'll need to choose the best material for you, based on your needs. But, note that different materials pose different challenges. For example:

• Nylon needs to be kept extremely dry, and needs to be printed at high temperatures. You should only use this if you are already comfortable with the material.
• ABS is a strong plastic, but can be hard to print if your printer doesn’t have an enclosure. ABS is prone to warping, and if that happens your gear will end up being unusable.
• PLA is easy to work with but it easily deforms so it’s ideal for high-torque applications.

To increase the strength of your part, you may be tempted to increase the infill density, but this isn't optimal. Instead you should increase the wall count, that way the outer-profile of the gear is reinforced more.

Lubricating an FDM gear is practically a necessity. This is to overcome the friction caused by all of those prominent layer lines. Unfortunately, you can't use oil as a lubricant as it has a tendency to seep through the layers, being absorbed by the plastic itself.

Printing the gears with resin is similar to anything else you might use SLA for. Follow the standard technique you use for SLA printing, and you'll end up with some accurate and sturdy parts.

However, there are some things you can consider if you have the time.

• Printing solid resin parts will often be heavier than your original gears, so consider printing hollow parts to save on weight and resin.
• Printing the gears flat against the build plate, while fast, risks giving the parts an "Elephant's Foot", which can cause friction in the mechanism. To avoid this, you'll want to print the gears at an angle as you would any other part.
• While the layer lines on the final part are unlikely to be noticeable to you, the minute surface roughness will cause more friction between parts. This can be negated by using plenty of lubrication.
• When you remove the part from the printer make sure to wash all uncured resin off the gear-teeth so nothing will cause a jam when you install the gear.
• Be careful not to warp the gears before curing them fully.