Tuesday, January 29, 2008

Early Adopter

I just ordered a molded parts kit from BitsFromBytes. That'll tack another $400 onto my costs, bringing the total up to about $1200. Now, that may seem like a lot for something that may or may not work and that you have to put together yourself. On the other hand, you could look at RepRap as the 3D printing equivalent of the Apple 1 or Altair. If you adjust for inflation, those kits would cost about $2000 today.

Sunday, January 20, 2008

Thermistor Mounting

I've been trying different methods of mounting thermistors. I tried JB Weld-ing one to a 3/8" washer but it dissipated too much heat. So, I resorted to the second most common engineering material (after duct tape): a bent paper clip. This seems to work quite well. It holds the thermistor in place without affecting the heater performance.

Saturday, January 19, 2008


I spent the afternoon measuring thermistors. I got 5 100K thermistors from DigiKey. They were the 5% ones ($0.80 each). Here's the technique I used:
1. Wrap the thermistor with PTFE tape and attach to the tip of a cooking thermometer. The one I used is good up to 230C.
2. Attach wires to the thermistor, keeping them separate. If the wires cross, they might short out when the insulation heats up.
3. Attach a multimeter to the wires to read the resistance.
4. Let the whole thing sit for a while to bring it to room temperature. Note the temperature and resistance. Use this as T0 and R0.
5. Put the thermometer and thermistor into a toaster oven set to 225C.
6. Wait for it to heat up and note the temperature and resistance. Use this as T and R.
7. Plug the values into the calculator and get beta and Rz.

The betas came out around 3900 for all five, with Rz around 336000. I put a 0.22μF electrolytic cap in for C3. As far as I can tell, that'll give me 1 degree increments up to about 280C.

Sunday, January 13, 2008

Models Undergoing Plastic Surgery

I've been using Art of Illusion to alter some of the Darwin parts. My aim is to make as many of the parts single-head printable. That means eliminating any overhangs greater than 45 degrees. There are quite a few. I've put together a spreadsheet of the models used in the Darwin Cartesian robot and the extruder here. The files marked in red are ones I can't figure out how to fix. I have a zip file of the altered STL files here.

I've been inflicting these models on Vik, who's been attempting to print a Darwin set in PLA. He gave me some tips on how to export STL files from Art of Illusion:

  1. Make sure the object doesn't have any children
  2. Convert to triangle mesh (0.1 accuracy is OK)
  3. Edit mesh
  4. Simplify mesh
  5. Save
  6. Export
I used 0.1 on the "simplify mesh" step as well. The export process can be either ascii or binary; can't be compressed, though.

Friday, January 11, 2008

Keeping Busy

Waiting for RRRF to get RP parts in stock. Meanwhile, I've been playing with Art of Illusion. Figured I might as well make myself useful, so I started cleaning up some of the Darwin RP part models to make them single-head reprapable.

Also looking at coming up with a thermistor configuration that has decent accuracy at high temperatures. I juggled some numbers and I think that a 1uF cap with a 1000K, 3890 beta thermistor might do the trick. Ordered 5 thermistors and a capacitor assortment from DigiKey so I can try some experiments. Also ordered another PIC, just in case.