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

Mix up some heatsink epoxy and coat the resistors with it. This will hold them inside the heater block permanently, so be sure to place them exactly where you want them.

Once the epoxy sets, 5 minutes or so, apply a drop of heatsink grease to the heater barrel where the block will sit. This will help heat conduction from the block to the barrel. Screw the block onto the barrel. I used a bit of fiberglass sleeving to insulate the resistor wires. If you don't have any you can use PTFE tubing or any insulator that can handle 250C. 

Attach the thermistor or thermocouple to the nozzle and wrap a bit of Kapton to keep it in place.

Slide the PTFE washer onto the barrel. Put another drop of heatsink grease on the barrel, then thread the heatsink on.

Thread the barrel into the thermal barrier, then put it back on the RepRap.

The resistors I used are 5ohms each, and in parallel this is 2.5 ohms. The power draw is a little too much for the extruder board, so I added in a 40 amp automotive relay to drive them.

And, it works!

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.

Here are some lookup tables generated using the script on the RepRap Wiki Thermistor page. The highlighted row (reading 54) is the turning point for these thermistors. Temperatures above this value will have lower resolution.  This low resolution is bad because each incremental value from the DAC is more than one degree; in fact it is quite a lot more. For each of the three thermistors below here is the actual temperature increment for one digital increment:

• 100k 1mm Makerbot Thermistor: ~5C
• 100k EPOCS: ~11C
• 100K RRRF: ~12C

If you were trying to use the PID to control the temperatures in this range, it would behave very poorly. The PID could never stabilize because the next temperature reading it received could be as much as 12C higher than the previous.  Notice the first thermistor's temperature at reading 54 is 189C. Anything above 189C will have a very low precision and cause the PID to operate poorly.  As a test, I used this thermistor and set my extruder temperature to 190C. The temperature would ramp up as expected to 189C and then instantly jump to 195C, causing the PID to turn off completely, instead of ramping down the PWM slowly to achieve steady state. With the heater off, the extruder starts cooling, and the temperature would fall to about 186C before turning the heater on again. It takes a few seconds with the heat on before the thermistor starts indicating the temperature rising again, by then the temp had fallen to 179C.

Think of the PID and heater control as you holding a long spring with a weight attached. Your goal is to move the weight to a specific height as quick as possible. Most likely, when you first move the weight up, you will overshoot the target height. When you move your hand down to make up for it, the weight will drop below the target height. Eventually you will find the proper place to hold your hand to keep the weight at the right height, achieving steady state. This height can be related to the PWM output, or power, of the PID control to the heater. The goal is to find the right power to keep the temperature at a steady state.

Now imagine the spring example, but you can't see the weight unless it is just below or far above the mark. We as humans could guess at where to hold the weight to achieve a steady state, but the microcontroller doesn't have that luxury. It needs to know the temperature with as much precision as possible to be able to achieve steady state.

If we selected one of the other thermistors, we could precisely measure temperatures of a much higher value, up to 255C or 266C. This is more than adequate for melting PLA, ABS or most any other thermoplastics the RepRap is capable of extruding.  There are other thermistors than these three, some with higher temperature ranges, most with lower.  Before purchasing a thermistor take a look at the datasheet, and run the createTemperatureLookup.py and see where the best precision range of that thermistor lies and if it will meet your needs.

If you still don't trust thermistors, you can always try a thermocouple! Next time I'll talk about PID tuning vs thermal mass and heater power.

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.

I've been using RepSnapper, and I think I'm starting to like it. It is so easy to calibrate your machine when it only takes 5 seconds to create gcode from and stl. There are a couple issues with it, but overall it is much more convienient to use.

Pros:

• Extremely fast slicing and converting to gcode ( < 5 seconds for most mendel parts)
• Easy to reposition/rotate objects and create RFO prints
• Easy settings and manual machine control
• Nice 3D preview

Cons:

• No bottom/top cap so objects are see-through
• Raft doesn't seem to work Added in new version
• Only 64 bit? It could probably be recompiled to work on 32 bit, but I haven't looked into it A new version has been uploaded to SVN and it works on 32 bit and 64 bit
• No reversing of extruder? Added in new version
• No early-start, the start of layers if you have multiple objects tends to gap

I haven't figured out if there are settings I'm missing or if these haven't been implemented yet. Anyway, thank you for this software, it is great!

Here are some photos of the Mendel building Nophead's pullys, being driven by RepSnapper.

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RepRap Stores will host your products, handle website maintenance, payment gateways and provide technical support. You can list as many products as you like and there is no limit on how much you can sell. You can also create blog entries and online instruction manuals using a WYSIWYG editor directly on the website. If you need help, a real person will be available to help you set up your account.

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For question and concerns please contact RepRap Stores, or leave a comment.

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 KL17H-2-47-168-4B 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.

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.

 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 photo of Mendel parts printed on the bed. Its about 25% of the RP parts needed.

Over a year ago, I built my original McWire RepStrap. From that point till now, I have build two complete upgrades to the electronics system, three (at least) different extruders, and numerous fixes to the cartesian bot.
I've had limited success printing in 3D so far, but hopefully this documentation will give me the inspiration to finally get this beast working!