The other way around blog - Mendel almost done - Details to come later

Usually a Mendel blog will show you all the details of creating, assembly trouble shooting and more, and eventually reach completion. As I have started blogging way too late in the process, the mostly done Mendel pictures follow here, and details will appear later. Maybe also combined with pictures of some of the next Mendels that are being assembled in my hackerspace: Labitat.dk




The almost finished Green RepRap Mendel

And a few closeup

Front view

Rear view - with better view of the Heater and Temperature control board

The design of the Heater and Temperature control board has two on/off (PWM) channels for the Heated Bed and two for the extruder heater. I plan to add a fan and light on/off (PWM) board with 4 channels. I use MOSFET HUF75337 as it has several built-in protection features - and can easily handle more than 20Amp (specifications say 42Amp continues and 75Amp bursts), so I expect the PCB wires to "blow" way before the MOSFET.

The PC power supply hook up.

There are two separate 12V rails, one for the Heated bed, and one for the rest. Designed with the intent to make sure that less than 10Amp is drawn on each wire/socket pin (preferably much less per wire/socket pin). Currently the heated bed can draw 2x7Amp (rail 1 on two wires at max 15Amp), the extruder heater 2x2½Amp, all the steppers run fine on a handy 2Amp test power supply that used to be an external USB drive power supply (rail 2 on three wires at max 8Amp). So far no wires or power connectors should overheat.

Calibration table system for NTC temperatur sensors

Here is a picture of a calibration system for NTCs I have made.

Items used:

• Arduino
• AD595 K-type temperature sensor (@watterrot)
• K-type sensor
• Breadboard
• Breadboard wires
• LCD Display
• NTC sensor
• Insulation material (Rockwool, Glasswool, cork, and more can be used)
• Kapton tape
• Constantan wire
• A piece of aluminium
• Thermal paste

Other links:

Thermistor (NTC) info and Thermistor tables at RepRap.org

The Arduino sketch used packed in a zip file.

Update:

Here is the table that came out of the experiment

The first column is RAW data, that is the value read from the analog port,

 the second column is temperature.

Raw numbers for temperatures above 280-290*C are not usefull, but that was to be expected, at the NTC is only supposed to go until 300*C

#define NUMTEMPS 72
short temptable[NUMTEMPS][2] = {
   {7,355},
   {7,350},
   {8,345},
   {8,340},
   {9,335},
   {10,330},
   {10,325},
   {11,320},
   {12,315},
   {13,310},
   {13,305},
   {14,300},
   {15,295},
   {17,290},
   {18,285},
   {20,280},
   {21,275},
   {23,270},
   {25,265},
   {27,260},
   {29,255},
   {32,250},
   {34,245},
   {37,240},
   {41,235},
   {45,230},
   {49,225},
   {54,220},
   {59,215},
   {64,210},
   {71,205},
   {78,200},
   {85,195},
   {95,190},
   {105,185},
   {116,180},
   {129,175},
   {142,170},
   {158,165},
   {174,160},
   {193,155},
   {213,150},
   {238,145},
   {271,140},
   {295,135},
   {322,130},
   {354,125},
   {385,120},
   {418,115},
   {453,110},
   {490,105},
   {529,100},
   {566,95},
   {605,90},
   {643,85},
   {681,80},
   {717,75},
   {752,70},
   {782,65},
   {814,60},
   {838,55},
   {863,50},
   {883,45},
   {904,40},
   {919,35},
   {934,30},
   {946,25},
   {958,20},
   {966,15},
   {974,10},
   {983,5},
   {992,0}
};

So for practical use, I would trim some of the rows out. 

The following table is probably more useful with a RepRap.

#define NUMTEMPS 30
short temptable[NUMTEMPS][2] = {
   {7,355},
   {14,300},
   {20,280},
   {27,260},
   {37,240},
   {54,220},
   {59,215},
   {64,210},
   {71,205},
   {78,200},
   {85,195},
   {95,190},
   {105,185},
   {116,180},
   {129,175},
   {142,170},
   {158,165},
   {174,160},
   {193,155},
   {213,150},
   {238,145},
   {271,140},
   {385,120},
   {529,100},
   {681,80},
   {814,60},
   {904,40},
   {946,25},
   {974,10},
   {992,0}
};

Other pictures of interest:

This is what temperatures of at least 350*C will do to kapton tape. 

The constantan wire was probably a slight bit warmer than the temperature at the thermistor/K-type sensor, to compensate for some heat loss through the insulation.

This is what happens to Glasswool at 315*C.

.

On the right is what fresh Glasswool looks like, on the left is what it looks like after exposed to 315*C. From the amount of smoke coming from the test bed setup, I did not dare go any higher with Glasswool.

The scorching is about 5mm deep

At 350*C even Rockwool suffers some discoloration, but the smoke was not near as bad.

The kapton tape from the Rockwool pad, suffered some wrinkling in the center, but was not burnt like the kapton that had direct contact with the heating wire.

I really love that kapton tape. It my new favorite kind of tape.

Closeup of the Vero board with the RepRap style NTC sensing, and the MosFET on/off controller.

Introduction

For some time now, I have been part of a group of RepRap builders at the hackerspace: labitat.dk

Documentation and Internet communication about the project has primarily happened through the labitat.dk wiki (see: https://labitat.dk/wiki/RepRap - Mainpage -> Projects -> RepRap), some kinds of documentation are however easier to handle via a blog, thus this blog about my experience at labitat.dk, building a RepRap.