I wanted an extruder that keeps its tension setting when I release the filament and I wanted it to be easy to use and maintain while looking more finished. To do this, I chose a concept that closes around the drive gear and the tension mechanism. The filament is found completely encapsulated in the extruder and protected from external elements. The beauty of 3D printing is its ability to transform ideas into reality and here is the result:
Reliable MK8 Drive Gear:
I have used a drive gear MK8 for almost a year and I am very satisfied. This gear has a good grip on the filament to push it efficiently. It has an effective diameter (7 mm) close to the one on the motor shaft (5 mm) which creates the best motor torque.
pushing force = motor torque / (effective diameter * pi)
For more information on drive gears and suppliers: http://reprap.org/wiki/Drive-gear
The direct drive offers several advantages over gears reduction. Unlike gear systems, there is no backlash that could affect accuracy and fewer moving parts which could break. You should avoid plastic gears for this type of application, such as small gear teeth used with a high load and precision. A metal gearbox would be a better fit, but are more expensive.
A direct drive system needs a powerful motor. I recommend a minimum torque of 45 Ncm and maybe even more. I use a 55 Ncm motor at 66% of its max intensity and I get good results.
The stepper motors typically used by RepRaps (NEMA 17) generally have 200 steps per revolution and are used with 16 microstepping controllers. This gives us 3200 microsteps per turn. With an effective diameter of 7 mm (MK8) it gives a feed of 0.0069 mm per step (or 0.00027 inch/step), which is accurate enough for this kind of use. For the same drive system, the smaller the diameter of the filament the more accurate it will be. In this case I recommend you use a 1.75 mm diameter filament, which is widely available.
Release the filament without changing the adjustment:
It is sometimes necessary to relieve the filament in order to change it or to do maintenance on the hot end. For any reasons, if there is a filament jam anywhere on the line (the hot end for instance), among other possibilities, the gear can chip the filament and fill itself with plastic until its grip loosens. It must therefore be accessible for cleaning.
To facilitate access, the idler can be released by sliding the clamp forward. To replace it back in position, just pull up and slide it backward on the idler.
With a little practice, I’ve realized that the pressure affects the penetration depth of the gear’s teeth. This varies the effective diameter of the gear and thus the amount of plastic extruded. By allowing release of the filament without touching it to the tension screws, it keeps the pressure adjusted. For easier use, I place the springs in a box to keep everything together and it’s more pleasing to the fingers.
However, it may be more difficult to find springs matching the size of the box, in this case I can modify the box size. It is possible to use the springs and washer directly instead, in which case you should adjust to the length of the M3X50mm tension bolts.
Clean looking while protecting the filament:
I used the MK8 for several months in an excellent extruder AirTripper. I noticed that once properly adjusted, you can spend entire rolls from MK8 without fault. In fact it is reliable enough to stop worrying about its operation and start worrying about its usability and its appearance. Its closed form, based on the dimensions of the motor makes it compact and discreet. The gears, springs and nuts are hidden and the relief mechanism stays together.
By sealing off the mechanism, there is a smaller chance that dust or debris will contaminate the filament extrusion. With guide tubes in and out of the extruder, the filament can be protected from the spool to the hot end.
On both sides there are places to screw a tube connector (look for “Pneumatic Tube 4 mm Push In Fitting M5”).
It can be used in both directions giving you an option to place the hinge up or down. You can also connect two guiding tubes on each side and protect the filament entirely from the spool to the hot end.
The PTFE (teflon) tube offers less friction and is recommended to guide the filament to the hot end.
Brackets are placed on each side in order to ease the assembly on the 3D printer. The holes are spaced 68 mm and have a diameter of 6.5 mm to fit M6 or 1/4 ” screws.
Its compact design makes it possible to install multiple extruders side by side or in opposition to extrude different colors or different materials at once.
Bill of Material:
Regarding metal parts to buy, I tried to use standard and easy to find parts as much as possible. If I am using different diameters of bearings, fittings or springs, I can perform small modification to the design of plastics parts on request.
A- 1x Spring compressor
B- 1x Clamp
C- 1x Tension cylinder
D- 1x Main body
E- 1x Bearing shaft 8mm
F- 1x Idler
G- 1x Drive Gear Mk8
H- 2x Compression springs, wire diameter: 1mm, outside diameter: 9mm, free length: 12mm
I- 1x Ball bearing (skateboard) 8x16x5mm 688-2RS type or equivalent.
J- 1x Pneumatic 4mm Tube Push In M5 Fitting
K- 3x Bolt M3x30mm
L- 2x hex nut M3
M- 2x Bolt M3x50mm
N- 1x Tube PTFE (Teflon) outside dia: 4mm, inside dia: 2mm, length: as needed (not shown)
O- 1x Stepper Motor NEMA 17 (not shown)
P- 2x Bolt and nuts diameter M6 or 1/4″ as long as necessary to attach the extruder on the printer (not shown)
* Recommended Stepper Motors:
You can check this page to get good info on choosing a stepper motor. You should choose a motor strong enough for direct drive extrusion, I would say 45 Ncm minimum, maybe more. For example, I personally use the SY42STH47-1504A model that can provide 55 Ncm, but I use it with only 66% of the maximum current intensity. Note that beyond one amp, you have to think about a cooling system for Pololu controllers otherwise they can overheat and turn off.
- Position the springs on the pads of the compressor (part A) and insert it all in the clamp (B). The compressor (A) should be on the side of the bigger holes inside the clamp.
- Insert the M3x50 screws (M) in the clamp. The screw head should fit inside the holes of the clamp and press the compressor (A).
- Insert the hex nuts M3 (L) at each end of the tension cylinder (C).
- Insert the cylinder (C) in the main body (D), taking care to align the holes of the cylinder (C) with the grooves of the main body (D).
- Screw the M3x50 screws (K) with the clamp and the tension cylinder (C) through the grooves of the main body (D)
- Screw fittings (J) on the main body (D). Take care to align them perpendicular to the surface and keep a pressure while turning. It is possible to screw fittings (J) directly into the plastic, but I prefer tapping the threads first with a tap M5 x 0.8.
- Insert the bearing shaft (E) in the bearing (I) and put it into the idler (F), squeeze a bit if necessary.
- Place the drive gear MK8 (G) on the motor shaft (O), taking care to orient the set screw on the motor side. Lightly tighten the screw with an Allen key to hold the gear (G) in place facing up.
- Use three M3x30 screws (L) to screw the base (D) on the stepper motor (O) without forgetting to place the idler (F).
- Open the extruder and insert a filament to align the drive gear (G) on it. Fix the gear in position with the set screw (Allen key required).
- Install the extruder on the 3D printer by using the brackets and M6 or 1/4″ bolts.
- Insert PTFE guiding tubes in the fitting(s) to the hot end and/or the spool.
Before trying the extruder for the first time, the firmware should be configured properly. If you don’t know how to do this, please read http://reprap.org/wiki/Calibration.
The calculated feed rate of the extruder should be written in the firmware. For example a 200 stepper motor with 16 microsteps per steps, MK8 (7 mm eff dia), should give something like: 200*16/(7*3.1416)=145.5 steps/mm
Insert the filament in the extruder
Ensure the hot end operates properly and the filament is not overly restrained.
Close the idler with the clamp (spring box)
Tighten the idler by screwing the tensor screws 2 to 3 mm deep.
Start the 3D printer to test the extruder with the free area under the nozzle.
Heat and send the command to extrude a few centimeters. If the filament is not moving but the motor is turning, tighten the screws until it moves. If instead the stepper is not turning or skips steps, you can release some pressure. If you do not get satisfactory results, please inspect the hot end or check for something restraining the filament.
For more details on the calibration of the extruder you can visit this page: http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide