Conductor de Extrusor Adaptado Nema17
Esta página describe el extrusor estándar para RepRap Mendel. De la misma manera, mucha gente ha tenido suerte también con el Extrusor Adaptado de Wade(en inglés) que deberías echarle un vistazo.
Quería diseñar un conductor de extrusor sencillo que pudiera
- Be bolted to the X carriage of Mendel and run as a normal extruder,
- Be mounted somewhere at the side and drive a Bowden extruder (as invented by eD and developed by Erik), and
- Be mounted somewhere at the side and drive a Bowden paste extruder, as outlined here.
This is my first hack at a solution.
This design was inspired by Wade's neat geared extruder that you can find here.
- 55:11 gear ratio means that the motor runs on very low current
- No hobbing or knurling needed to make the filament driver
- Torque transmitted using a wing nut, giving low stress on reprapped gears
- Uses the same 624 bearings as the rest of RepRap Mendel
- Designed to work at higher extrude speeds than the old Mendel extruder, giving shorter build times.
What you need
This describes what you need to build an extruder to mount on Mendel. The Bowden variations will be added below when they have been tested.
The design files (AoI, STLs, and the complete RFO for immediate reprapping) are in the repository here.
This picture shows all the parts (some partly assembled) except for the two M4x20 screws that attach the extruder to Mendel's X carriage.
|M4 wing nut||1|
|M4 threaded rod||70mm|
|4mm diameter rod||25mm|
|4-way pin headers||2|
|4-way ribbon cable||130mm|
|16mm dia PTFE||50mm|
|M6 threaded brass||36mm|
|Nichrome wire||6 ohms|
|Araldite rapid||About 5 ml|
Here is a complete list of all the parts and materials.
The M4 Insert is what drives the filament and is the heart of the device. It is available from here, UK or here for Europe or these guys ship worldwide. It has an internal M4 thread and a locking grub screw. This means that it is very easy to mount on the 70mm length of M4 threaded rod, but that - once it is tightened up - it never slips.
The stepper motor is a NEMA 17 motor with a double shaft (front and back). The RepRap stepper motors page is here. I used the one from Zapp Automation, which works fine.
The 4mm diameter rod can be an old M4 bolt shaft (the unthreaded bit) hacksawed off. Clean the ends with a file.
Putting it together
The two 4-pin headers are used for the four stepper wires, and for the two heater and two thermistor wires.
Use the driven holder part as a drilling jig to drill two 4mm holes near to the bottom edge of the stripboard. Solder the two 4-pin headers near the top and towards either end. Split the tracks down the middle to isolate the two connectors from each other.
Note: some people have had trouble making extruders (like this one) from PTFE - they find it distorts under load. Others (including me) have no problems. This implies that there is more than one grade or composition (possibly with more or less polymerization) out there. We are investigating to find an authoritative answer. Meanwhile, the PTFE I used was this one, which worked for me...
Start by building the extruder nozzle. This is exactly the same as the one on the old Mendel extruder here except that the PTFE is longer at 50mm than the 35mm used for the previous design. I also cut the M6 thread slightly longer (18 mm) than the 15mm used for the previous design, and made a correspondingly longer brass nozzle.
The reasons for these changes (which may not actually be necessary) were conservative: I wanted to be really sure that the Araldite join between the PTFE and the base of the device would stay very cool and so not soften. And I wanted to make sure that the brass nozzle would not break lose, even under higher forces than the original extruder is capable of generating. One of the main reasons for designing this device - as mentioned above - is to increase the speed of extrusion. Doing that will obviously generate higher thermal and mechanical stresses, and so I thought it best to be prepared.
The longer nozzle does mean that you will have to put a longer Z-axis opto-flag on your Mendel to use this.
The gear drive
The large 55-tooth gear is the one that drives the filament. It connects to the 70mm M4 threaded rod using a wing nut, which spreads the torque load on it and thus reduces the stress that the plastic is subjected to.
Start by offering up the wing nut to the slots in the large gear on the opposite side to the projecting boss. Some wing nuts fit immediately. Unfortunately there does not seem to be a completely standard shape for wing nuts, so you may have to file down the inner shoulders of yours to fit, as shown here. The gear should sit flat on top of the inner part of the nut.
This shows the assembled filament drive. The sequence from the bottom is:
|M4 nut||M4 washer||624 bearing||M4 washer||M4 Insert||M4 washer||624 bearing||M4 washer||M4 nut||GAP||M4 nut||M4 wing nut||gear||M4 washer||M4 nut|
There are a couple of extra washers in the picture between the wing nut and the nut that locks it - these are not needed. The final M4 washer and nut are invisible behind the gear.
Get everything in the right sequence and hand-tighten it all. Don't use spanners and force yet.
This shows the assembly fitted in its slots in the extruder. Use a short length of filament down through the device to hold it in place while you build the rest.
The stepper drive
Now, frighteningly, you have to file flats on the shaft of your nice stepper motor. The shaft is 5mm in diameter and the flats you want take this down to are 3.5mm across.
Start by using Blu-tack to seal the entrance where the shaft goes into the motor's bearing. You really don't want iron filings going in there.
Then put the shaft in a vice as shown and file it down. Take care to get the shaft parallel to the vice jaws - this will make filing easier (though don't use the jaws as a sliding guide...) You will find that it is quite straightforward to keep things square because when you file one end or the other too much the rectangular flat that you are creating turns into a trapezium.
Measure the shaft frequently as you file. Stop when you get to 4.25mm (that is when you've taken off 0.75mm, which is half of the 1.5mm that you need to take off in total).
Turn the shaft over in the vice. Check it very carefully by eye looking at the end of the shaft to make sure that you will be filing parallel to the flat you just created, which is now on the bottom. Keep checking this as you file. Take the dimension down to the 3.5 mm that you want.
Next put the small drive gear on the shaft. The NEMA17 stepper that you are using is a double-shafted one to allow this. Clamp the back shaft, not the motor body, in the vice. That way you will put no stress on the motor's bearings when you put the drive gear on.
Drift (as engineer's say when they smack something with a hammer) the gear into place. Use a soft hammer, or, if you don't have one, some folded tissue or cloth between the gear and the hammer. When the top of the gear is flush with the end of the shaft, use a small length of scrap tube to push it down further. You want it to end up about 3mm inwards from the end.
Use the four M3 screws to attach the motor and gear to its reprapped mounting plate, and then attach that to the base using M4 screws in the slots provided. Those slots are to allow you to move the motor towards or away from the big gear so that you can get the teeth meshing well.
Adjust the nuts and wing nut holding the big gear that you left hand-tight on the driven gear assembly so that the two gears line up. Take the driven gear assembly out (remove the short length of filament you put in to free it).
Tighten all the nuts on the driven gear assembly pretty hard with a couple of spanners. Don't go over the top and strip the threads, though. Double check that everything lines up, then finally tighten the grub screw in the M4 Insert. This will bite into the 70mm M4 thread; you will be able to undo it again if absolutely necessary, but you don't want to have to do that too often (or at all). Fit the assembly back in and again retain it with the short length of filament.
Take the stepper and its mounting off.
Put four M4 nuts in the holes for them in the reprapped idler holder. Use an M4 screw with a washer under its head to drag them into their holes if they are a bit tight.
Put the final 624 bearing on the 4mm-diameter rod and put an M4 washer either side of it. Place it in the declavity in the idler holder.
Drop four M4 screws with washers down the holes in the driven holder, using the top two to retain the stripboard.
Offer the idler holder up underneath and screw the screws finger tight compressing the filament between the M4 insert and the idler bearing. Look down on the top of the device and adjust the screws to get the idler holder parallel to the driven holder.
Put the motor back on and adjust it so that the gears engage. Make sure that the large gear can turn freely through a complete revolution whilst neither disengaging nor running the gears too close together.
Put a small amount of silicone grease on the gears to lubricate them.
The finished device
The number of steps per mm-of-extrusion in the firmware needs to be changed from that of the standard extruder, of course. Firmware details are here. I used a figure of 1.8 steps per mm, with a half-stepped 200 steps/rev motor.
Video of it working