Hobbed Stepper Extruder

This is a process of making a groove and then teeth (“hobbing”) directly on a stepper’s 5mm shaft to be used in an extruder. It is a way of making a “Direct drive” extruder without needing a machined gear attached to the shaft. An advantage to this design is that a direct drive extruder can be made with a much smaller contact radius, giving more torque (it will take more stepper turns to push the same amount of filament). A conventional direct drive needs a diameter much greater then 5mm (often 8mm is used) whereas a hobbed stepper will have a diameter of less than 5mm. Also the only necessary vitamins are the idler bearing and springs. One disadvantage is that because of the smaller diameter less grip may be achieved since less teeth will be embedded in the filament at any moment.

It had been discussed of the forums before and someone claimed to of achieved it.

My design

My Mendel started with a mendel-parts v9 3mm direct drive extruder. It was clear that this extruder was only designed for 1.75mm. It used an 8mm brass gear and pushing the filament required the steppers max current (pot up just before jitter). The stepper is the mendel-parts one, 43Ncm torque. Notably extrusion was unreliable, especially when filament varied over 3mm. Being stubborn and liking the idea of direct drive we did not make a wades, instead we went about making a smaller drive diameter.

The groove was made using a hand file and driving the stepper (fast) through Pronterface. To protect the stepper from filings an eraser (with a hole in it) was pushed on the shaft and allowed to rotate. The teeth were cut using a tap mounted in a drill. Again the stepper was driven (slower this time) and the drill pushed against the shaft and rotated at the right speed. The variable speed drill needed to be matched to the speed of the stepper so that it wouldn’t drive forward or backwards or skid. This took some trial and error to get a good stepper speed to match the drill, but was not difficult.

So I’ve been running the design for quite a while now and it has not failed me yet. I had a period where I had extruder jams because I insulated the PEEK (seriously dodgy) and the gear would spin and chew the filament, but obviously more spring pressure could be applied. I can now run at way lower current then previously. The 8mm gears E Steps / mm were 67 and now I am running 147, so that’s 2.2 times more torque (thats 1/8 microstepping). This is all on 3mm ABS. Because the filament gear is made from good quality stainless steel, I don't see it wearing down like the brass ones.

As you can see the grooves on the filament are not exactly very good, but it does work well for me.

The extruder is custom designed to fit the mendel-parts v9's aluminum mounting plates. It uses a lever arrangement to multiply the force from the springs and pulling up on the lever releases the force. It also has a support bearing for the shaft so that there is not so much force on the steppers bearing as with standard direct drives.

So I'm not saying this is a great idea, just showing it can be done. It works great for me and I have no plans of changing it. I think that it is easier to make then a hobbed bolt and the direct drive has its advantages. Direct driving 3mm is normally hard because of the large gear radius so you're maxing out the stepper but some people seem to get away with it. This design allows for easy, simple and reliable direct driving of 3mm. I am seeing some geared stepper direct drive extruders being built now. So I think this is pretty solid and makes for a much simpler and compact extruder then the geared ones most people use.