Modifying an M8 Extruder Stepper for Bowden Use

thingiverse

Update 6/3/19 The as-printed 2 mm hole at the bottom of the filament guide may be too small if you have to withdraw filament that has expanded just above the melt zone. I opened up the hole with a 2.5 mm drill and it is always possible to withdraw the expanded end of the filament from the extruder. Update 2/24/19 I added a filament guide that should make it easier to thread the filament below the drive gear. It is sized to work with the smaller hobbed gear. If a larger diameter gear is used, it can be installed and then filed-to-fit before the spring is installed. The guide has a funnel shaped hole with a upper dia. hole of 6 mm, tapering to about 2 mm. Make sure you check the clearance with the idler and drive gear before installing the spring. Also check the bottom hole diameter. The screw threads easily with a socket drive if the end of the thread is filed back a bit so the thread lead-in is easy to find. Update 2/21/19 I added a base for the spring which removes the bend in the spring that results if the spring sets on the lower shelf of the motor. The base is held in place by the 3 mm screw on the bottom that is cold-threaded into the plastic base. You may need to use a shorter screw or add a millimeter or two of washers to allow the printed base to secure the base of the stepper without penetrating the top surface of the print. See the washers in the photo. I also added a single 3 mm washer. ------ - - - - - - - - - - From my brief experience, the direct drive M8 extruder did everything it had to do. When you adapt the M8 Extruder stepper motor to a Bowden setup however, it is not ideal. It took me a long time to realize its shortcomings and even longer to come up with these modifications, which I believe are a near-necessity for good performance. If you only get occasional slippage (the ticking sound), and don't want to change the gear, consider adding a spacer washer under the spring to allow it to be compressed further to see if that helps. The changes include: 1. Replacing the straight drive gear with a hobbed drive wheel for pulling as well as pushing the filament without the ticking sound caused by gear slippage. 2. Replacing the idler wheel that has a groove for the filament with a bearing with a smooth outer surface. 3. Changing the printer steps/mm in the printer firmware (or slicer) required by the changed diameter of the hobbed drive gear. 4. Adding washers and and the shorter adapter piece to be able to use all or the available tension of the spring, which is required when using a smaller drive gear that reduces the tension on the spring. 5. Replacing the brass filament inlet connector with one with a large diameter that prevents bending the filament caused the increased angle of the load lever (caused by the smaller gear diameter). 6. Adding a filament guide to easily thread between the drive gear and Bowden tube. There are two choices: the one here, or the following filament guide, which has a smaller lead-in hole and may be preferable if you use flexible filaments. https://www.thingiverse.com/thing:3240122 7. Adding a slanted base under the spring to keep it straight when operating. DETAILS: 1. There are two choices for replacing the straight drive gear that I considered. They are the MK8 or MK7 hobbed drive gear, and they are available on Aliexpress.com and others. I chose the smaller MK8 gear because of the increased force it is capable of generating without slipping or stalling the stepper. See the article in the link below for a discussion of stepper motor drive gears. https://airtripper.com/1676/3d-printer-extruder-filament-drive-gear-review-benchmark/ 2. The grooved idler wheel has to be replaced with a smooth one or it will not work with the grooved, hobbed drive gear. There are standard bearings that work. Look for one with a 12mm outer diameter and an inner dia to fit the mounting screw on the loading lever. I think it may be 4 mm, but check it. 3. When you change the size of the drive gear, you also have to change the "steps per MM" parameter in the firmware. The default for the supplied gear is 95 in the Geeetech i3. I've found the steps for the MK8 gear is close to 154. The MK7 gear is close to the same size as the original gear, so it will likely provide some improvement and may still require a change in steps/mm, but you can compensate in your Slicer if you don't have the tools to change the parameter in the firmware. Repetier Host allows you to read and modify the steps/mm in the firmware of Geeetech i3 printers. It is likely not available in other printers that do not use Marlin or Repetier (or equivalent) firmware. 4. If the smaller MK8 drive gear is used, the angle of the load lever becomes larger, and some extra spacers have to be used around the spring to get enough tension to prevent slipping. The washers and the plug between the tension adjust screw and the spring will make sure you can compress the spring adequately There are two size plugs. I ended up using the version 2. 5. If you use a smaller gear, the angle of the load lever increases, and a standard inlet connector will cause a bend in the filament ahead of the gear, as well as making it more difficult to thread the filament into the Bowden tube. You can just remove the connector on the load lever, or use the printed one to provide a smooth and less restricted path to the drive gear. If you use the printed part, cut a piece of masking tape and place it around the bottom smaller diameter. Then you can easily thread it into the load lever by hand. 6. Add either of the two filament guides to the lower, filament exit hole of the stepper. The one included here or: https://www.thingiverse.com/thing:3240122 7. Thread the spring-base onto the screw in the lower shelf. It may be easiest to put the spring in place first to hold the base down while cold-threading the screw into the plastic.