RepRap 3D Printer - Prusa i3 - Build Log: Part 3

Extruder Assembly

Fully assembled extruder with hot end attached to the x-carriage.

Start by cleaning up the printed components - Extruder body, Extruder block, Extruder Big Gear and Extruder small gear. In my printed parts kit, only the Extruder Body required serious cleaning up.

To clean the extruder body remove the printed support material and using the appropriate drill, ream out the holes. Your body should look like this -

Original Extruder Body
Extruder body after cleaning.
Now fix one of the 608ZZ bearings into the Extruder Block using the M8x20mm grub screw.

Extruder block with bearing installed.
Insert a M3 nut into the captive hole on the Extruder block. If required use a little heat from a soldering iron to ease it in.
Extruder Block with the M3 captive nut in place.
Now the Extruder block can be fixed to the Extruder Body. Using a M3x35mm screw fix the two components together. At this stage I also installed the two remaining 608ZZ bearings into the Extruder body as shown. These will hold the milled bolt at a later stage.
Extruder block fixed to the Extruder body. 
Now insert another M3 nut into the captive hole in the Extruder small gear. Take care to insure that the nut lines up with the bolt hole. I had to apply quite a bit of heat to insert mine.
Extruder small gear with M3 captive nut. Note the warping from the heat required to insert the nut.
Now slide the Extruder small gear onto one of the NEMA 17 stepper motors. Align the captive nut location with the flat of the stepper shaft. Don't fix the gear into position - this will be done after everything is together and aligned.
Next fix the stepper motor to the Extruder Body. Use three M3x10mm screws with M3 washers. Again, don't tighten at this stage.
NEMA 17 Stepper with Extruder small gear installed.
Now feed the Milled M8 bolt through the printer Extruder Big Gear. This is followed by M8 washers which act as a spacer - I required 6 washers. Then feed the big gear/washers/bolt assembly through the 608ZZ bearings in the printed Extruder Body. The teeth on the Big Gear and Small Gear might make it difficult to get the gears installed - if this happens, loosen the three M3 bolts holding the stepper.
Milled M8 bolt installed in Extruder Body.
The aim is to line up the milled filament grove with the hole in the extruder body, which feeds through to the Hot End. Add or subtract washers until these are aligned. Then move the Extruder small gear until it meshes with the big gear. Once you are satisfied with everything, lock the Milled M8 into place with a M8 washer and two M8 nuts, locking the second nut against the first. Lock the Small Gear with a M3 grub screw.

M8 lock nuts on Milled M8 Bolt

M3 grub screw locking the Extruder small gear

Finally lock the stepper motor into position by tightening the three M3 screws. The aim is to have the Big and Small gears meshing tightly together. To do this, apply a little pressure to the stepper so as to slide the stepper towards the Big gear (the three screw holes which fix the stepper motor in position are slightly slotted). Don't mesh the gears so tightly that they cause friction when turning.
Tightening the stepper motor in place, while the Big and Small gears are tightly meshed.

Next take two M3x40mm bolts. Feed onto them a M3 washer, followed by a 5mm diameter x 5mm long spring, followed by another M3 washer. I got my springs at my local hackspace, and they were 5mm diameter and 20mm long. I cut them down to length and they have worked fine. Place M3 nuts into the captive nut slots on the printed Extruder Body as shown and use the M3 screws to fix the Extruder block against the Extruder Body. My nuts did not lock into position very well, and this meant I had to use a pair of long nosed pliers to help the screw and nut to mate.

M3x40mm screws with washers and springs
M3x40mm screws in place with captive nuts.

Hot End Assembly and Installation

I installed the Hot End onto the Extruder before assembling the Hot End - I will document it in this order. In hindsight, it would have been easier to assemble the Hot End, then install it on the Extruder, but either option works.

The Hot End (J-Head in my case) can be attached to the Extruder a number of ways - but I chose to use screws rather than a mounting plate (which is another popular option). If I did it again, I would investigate the mounting plate option much more thoroughly, as screws turned out to be a time consuming solution.

Start by using a M3 tap to thread the holes either side of the Hot End mounting hole in the base of the Extruder. The Hot End sits inside the hole, with the screws holding the Hot End firmly against the Extruder body. I spent some time cleaning up the plastic in the Hot End mounting hole so that everything fitted snugly. I had to shave a little bit off my Hot End so that the screws could pass straight through the Extruder body and not clash badly with the Hot End. The Hot End's plastic is much stronger than the printed Extruder, and hence, if you force it, you will break the Extruder body before you squeeze the hot end.

Once I was happy that a screw would pass through the extruder, and hold the Hot End firmly, I tapped the hole again - this time with the Hot End in place. Finally, using M3x25mm screws I screwed the Hot End in place. Note that I didn't use nuts on the back, the thread in the plastic is all I used. The result is a very firm hold.

Tapping M3 threads in the Hot End mounting holes 
Re-tapping the holes, with the Hot End in place
The next step is to prepare the Power resistor (heater) and Thermistor (temperature sensor). Both of these came with my J-Head along with high temperature tubing.

Cut the high temperature (PFTE) tubing, so that it is about 5mm shorter than the length of the Power resistor leads, and slide the tubing on. Solder wires onto the ends of the resistor - I used some 16 gauge I had lying around, which should be able to cope with the current. Finally, use Kapton tape to insulate the solder. I have read of others using heat shrink to insulate instead of Kapton.

Repeat the process for the Thermistor. I used 26 gauge wire out of my wiring kit (see BOM).

Power Resistor (heater) with PFTE tubing and Kapton insulation.
Thermistor with PFTE tubing and Kapton
Now install the Power resistor and Thermistor in the Hot End. I used some silicone heat compound (the same stuff you use on heatsinks) to aid the thermal conductivity. You only need the smallest amount of compound. I have read of others having no problems while omitting the compound, so it's probably not essential.

Installation of the Power resistor, while applying silicone heat compound
The Thermistor sits in the small recess in the Hot End (See pictures). Use Kapton tape to secure everything in place. Don't be shy with the Kapton - if the Thermistor falls out, you're going to have a bad day.

Finished Hot End with Power resistor and Thermistor installed
Finally, using two M3x35mm and nyloc nuts, fix the finished Extruder to the x-carriage. (See picture at top of page). It's worth mentioning that I plan to add a cooling fan, but need to print out the fan holder. This will be one of my first projects once everything is running.

Heat Bed Assembly

Start by soldering power leads to the Heated Bed. I used 16 gauge wires (18 to 14 gauge is recommended) as the bed will draw about 6A. Make sure the solder connection to the bed is adequate.
Power leads soldered to the Heated Bed. 
Next, assemble the Thermistor (temperature sensor) in a similar fashion to the Hot End Thermistor (see above). Note, I used heat shrink instead of Kapton to insulate the solder joint (I may change this if the results aren't acceptable).
Thermistor with leads, PFTE insulation and heat shrink.
Tape the Thermistor to the back face (the side without traces) of the Heated Bed with Kapton tape. My Heatbed (version MK2a) has a hole in the centre of the bed for the Thermistor to sit in. This allows the Thermistor to measure the temperature on the top side of the bed (This temperature will not necessarily be the temperature of the glass however).
Thermistor taped to the underside of the Heat Bed with Kapton.
Now the Heated Bed can be fixed to the Y-carriage. At each corner, fit M3x10mm screws with nyloc nuts. Tighten the nuts such that they lock the screws in place but still allow the screws to turn in the holes. Now the Heated Bed can be screwed into the previously taped M3 holes in the Y-carriage (See pictures). Note that an alternative option is to use springs between the Y-carriage and the Heated Bed - this has the advantage of allowing the bed to collapse down if the Hot End were to accidentally impact the bed. We will level and set the bed height at a later stage.

M3x10mm screw fixed to Heated Bed (note- bed is upside down)
Heated Bed fixed to Y-carriage.
Now the glass print surface can be added to the Heated Bed. I used 2mm picture frame glass which I bought at my local thrift store and cut it (rather unsuccessfully) myself. Fix the glass down with bulldog clips.
Glass fixed to Y-axis frame. 

Final Frame Assembly

Now the completed Y-axis frame can be bolted to the complete X/Z-axis.

The Y-axis frame needs to be positioned such that the Hot End sits at the front of the glass when the bed is sitting at the rearmost position. This results in the Z/X-axis frame sitting slightly rear of centre. My X/Z-axis frame sits approximately 250mm from the front of my Y-axis frame and 110mm from the rear.

Once the frames are in position, sandwich the Z/X-axis frame between the M10 washers and tighten.

Hot End positioned at the front of the bed, while the bed is at the rear most position.
Z/X-axis frame bolted to the Y-axis frame.
Now go around all the nuts on the printer and tighten them with a spanner. Don't over tighten so much as to crack any printed parts. Also, using an accurate measure, such as a caliper, ensure everything remains square and free running.

Endstop Installation

My Endstop installation caused me a lot of headaches. What I ended up with is not pretty or elegant, but it works. I plan to completely revise my Endstop installation once my printer is up and going, and I have the ability to print out new Endstop Holders.

The primary reason my Endstop installation was difficult was the printed Endstop Holders which came in my printed parts kit. They are poorly designed and awkward and make Endstop installation difficult. To further the pain, my Endstops themselves are the Mechanical Endstop v1.2 design, which are quite large, and make installation while avoiding clashes difficult.

Each axis can have an Endstop at either end or both ends. I chose to have one Endstop per axis. There are several options for the locations of the Endstops, but the most common (and the option I adopted) is to have the three Endstops trigger at the axis minimums (i.e. home position 0,0,0), and for the home location to correspond to the Hot End at the front left of the bed. This corresponds to a standard graph paper type layout - with the x-axis increasing left to right, y-axis increasing front to back, and z-axis increasing with height. Another popular option is to have the home position set such that the Hot End is at the right-rear of the bed, this simply rotates the axis through 180degrees. And finally, you could set any of the Endstops to trigger at an axis maximum (i.e. position 0,200,0). All of the Endstop locations are set in the firmware at a later stage.

My Y-axis Endstop is located such that it triggers on the LM8UU linear bearing (see photos below). I used some leftover M3 screws with washers and nuts to fix the Endstop to the Endstop Holder and a M3x25mm screw to fix the Endstop holder to the frame.

Y-axis Endstop fixed to frame.
Y-axis Endstop triggering on the linear bearing at the rearmost location.

My X-axis Endstop is installed in a similar fashion to the Y-axis. The microswitch triggers on the stepper motor, and I had to bend out the switch's metal contact slightly to get a good trigger. This Endstop position does limit my bed area slightly (by about 10mm) - i.e. it triggers the X-axis minimum location about 10mm from the edge of the bed.

X-axis Endstop installed.

My Z-axis Endstop proved to be the most difficult to install. To avoid clashes, I had to remove one of my printed Tube Clamps on my Z-axis stepper and replace it with zipties. I also had to extend my Endstop microswitch trigger, with a M3x20mm screw which I soldered onto my microswitch.

Z-axis Endstop. Note the screw used to extend the microswitch trigger. Also the lower Tube Clamp is removed to prevent a clash. 

The exact positioning of the Z-axis Endstop is important, as it needs to trigger when the Hot End is only a fraction of a millimeter off the bed. I will refine the endstop position later when I calibrate the printer, and it only needs to stop the Hot End impacting the bed for now.