Binary Construct

It turns out a single resistor wasn't enough to drive the extruder, so following some ideas from Makerbot I decided to create a double resistor heater. I wanted to reuse parts that I already had as much as possible, so I started with a standard heater barrel. The heater block is machined from a single piece of brass.

Here are the build instructions.

Materials Required:

• Heater Barrel
• Nozzle
• Double Resistor Heater Block
• PTFE Washer
• Heatsink
• Hybrid Thermal Barrier
• 2x Resistors
• Heatsink Epoxy
• Heatsink Grease

Temperature Tuning Part 1

Thermistors work very well when measuring temperatures near the middle of the specified range. They operate nearly linear and have a high resolution (large resistance change per degree). These properties make it easy to determine the actual temperature using the RepRap electronics.

There are two parts to this problem. The first part is when you get close to the upper end of the range of the thermistor the resistance changes per degree are very small. Depending on the resolution of the DAC being used, this can allow for more error in measurements and loss of individual degrees (e.g. incrementing the digital value may be 2 - 3 degrees).

The other part is the resolution of the lookup table in the firmware. Since thermistors are nonlinear and ATmegas are relatively slow at float-point calculations, a lookup table is required to convert the output of the DAC measuring the thermistor to an actual temperature. This table approximates the curve of the thermistor’s change in resistance to actual temperature.

With careful thermistor choice you can reduce the effect of this problem. I will attempt to explain below.

I finally got my Mendel working, as it turns out almost all my problems were being caused by bad thermistor calibration. You can see from the screen shot, 250C indicated by the software is actualy around 225C in the real world. On short notice I found the correct look-up table here: RepRap 1mm Thermistor Lookup Table. Brian helped me fix a leaking thermal barrier. My spool is mounted below my machine, so I put together a Bowden cable to feed the extruder to keep it from jaming.

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I have assembled most of the Mendel now, just waiting on a new Z-belt from McMaster. The BoM for the Mendel has the wrong one listed, try this instead: 6484K512

The NEMA17 I have for the extruder is slipping. It is a KL17H247-168-4B rated at 63oz-in but I don't think I am getting that kind of torque using the Extruder Controler board. I have a spare stepper board I might try and drive it from.  I am also going to attempt to build a Bowden cable with teflon lined tubing (McMaster Part: 4mm I.D. 5557K33 or 3mm I.D. 5557K38). This will allow me to use a NEMA 23 motor rated much higher (282 oz-in).

I moved the stepper controller mounting plate outside the frame to allow for the 4th stepper controller. I will probably move it as it might restrict the Y axis movement.

The next step is to attach the Z belt and finish the extruder.

Photos below:

This is no April Fools' joke! All the metal hardware was delivered yesterday and I began assembly. I got a late start so only the X-Axis is assembled. The motors are not attached, because I forgot to make a few parts and the motors driving the McWire.

 A few of the holes were a little small so they were drilled out to M4. To fit the trapped nuts, I heated them with a torch and pressed them into the sockets with pliers. I used a razor blade to clean up any sloppy edges and the squished first layers on some parts.

Here are some photos of the assembly.

If you are experiencing extruder jams, here are the basic steps to follow to reduce or, as in my case, completely eliminate them! Jams can be cause by incorrect temperature settings, trash in your nozzle, incorrect extruder grip, long melt zones, poor spooling and any number of other causes. Here are some solutions to those problems and some additional tips.

 Needing a quick solution to a heated bed, I dug around my closet and found this $20 GE hot plate that I had used to do some SMT soldering.  I purchased the 8"x8"x0.25"aluminum plate at a local welding shop for $5 and covered it with Kapton tape. I used a bit of doublestick tape to hold the hot plate to the McWire platform and some more Kapton to hold everthing down.  I "borrowed" my wife's thermometer and taped it to the bottom of the plate for a quick way to measure temperature. 

 So far it has worked great and ABS parts come off with ease.  This solution isn't very elegant or complicated but it gets the job done, took less that 10 minutes to build and test and cost under $30 for all the parts including the Kapton. It probably wont fit on anything other that a McWire or gantry style machine with this size hotplate but you may be able to find a smaller one in your area.

Here is a short video I did for my friend at AMI Customs.

After building two more extruders, I think I've got it down. The body of this one is constructed almost entirely of MDF, with one aluminum angle to attach it. It only took about half an hour with a hand drill and some calipers to measure where to place the holes, but it is defiantly working far better than anything I have built previously. As you can see from the pictures it is a pretty simple design but is cranking out parts as fast as I can skeinforge them.

Click for larger versions of the photos.

 More after the break, including skeinforge settings.