Monday, November 26, 2007


Mould Parts Arrived at Last

Hi all well as always a longer journey than expected!

The above RepStrap is made entirely from moulded parts or off the shelf items for the couplings and timing pulleys. I intend to do mouldings of the pulleys to further drive down the cost but thought I should get something up just to let people know it was still happening!

The above RepStrap does not have the Extruder parts fitted but these are as posted before.

On Friday we will have a web site up so people can order any or all of the moulded parts, I’m still figuring out what the bits actually cost in resin and silicone mould! But expect a full set to be in the region of £100 to £120.

As they say watch this space!

Wednesday, November 14, 2007


New Firmware Release

Thanks to Andreas Unterluggauer we have a new firmware release. This improves the communications functions and reduces (indeed, on the Bath Darwin eliminates) comms errors.

You can get the .hex files from Sourceforge here:

or get the sources using subversion at:

Tuesday, November 13, 2007


Got the perfect repstrap ...

Hi all,

... after a hint in summer from ISEL, that one of their customers wants to exchange his CNC-glue-dispenser and is willing to sell his old system for low, i yesterday got the old 3D-system for 300 Euros - unfortunatelly without the Loctite-dispenser ...

The controller is still working, the mechanic and motors are in working status and needs only some cleaning and fine-tuning - so if i find a mill-head and an extruder, my 3D-RepStrap comes faster to real live, then i hoped :)

So if it's running as 3D-mill, i can perform and offer some 3D-milling at home for free or a low fee (for material and shipping) - best in germany and europe (after my desastrous experience with receiving a book from the USA and transferring the money, thanks Demented!)


Monday, November 12, 2007


Assembling the Tripod ...

Hi All,

... here the promised images of the parts and the assembly of my Tripod:

And here a video of manual playing with the kinematics ;)

Tripod-Test from Viktor on Vimeo.


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Sunday, November 11, 2007


Its Alive!!!

Finally got all 3 axes assembled and 3 stepper driver boards made up. I bolted all the electronics to a piece of wood. I used M3 nuts/bolts and short lengths of vinyl tubing as spacers (the same tubing that i used to couple the drive rod to the motor.)

Anyway, I'll let the videos speak for themselves:

Its Alive!!! from Zach 'Iowa' Hoeken on Vimeo.

McWire Cartesian Bot Running from Zach 'Iowa' Hoeken on Vimeo.

Saturday, November 10, 2007



Hi all,

... here i post some pictures of my Tripod-repstrap-demonstrator - when the last parts are ready, then i'll make some more photos of the separated components and the assembling ...

Here the complete Tripod - the toolhead and the ball-magnets-passive-joint-rods lays in the middle, i have to mill the fixings for the ball-magnets to assemble it completely.

The easy to assemble linear drives, actually i replaced the steel-nuts with milled PTFE-blocks for lesser friction ...

And here two of my (old, but still working ;) three microstep-drivers ...


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Wednesday, November 07, 2007


MCB Bearing Issues Solved!

Recently, I've been working on building my McWire Cartesian Bot. This design is based on a really cool, simple design posted on instructables about a month ago. It uses standard iron pipe + aluminum rails + roller skate bearings + acrylic stages to create a simple, elegant 3D positioning system.

I took it upon myself to build this design, and improve upon it if I could. There were a few major flaws I found with the design:

1. it used three aluminum rails per axis to keep it aligned. This created alot of friction, and aluminum rails are sort of pricey. I replaced one of the aluminum rails with roller skate bearings. This serves the same purpose of keeping the stage aligned, except it has less friction, and its cheaper to boot!

2. the next major problem i found was that the bearing arms to keep the stage up tight against the rail was only kept in place by a screw. If it worked loose, you basically had to keep tightening it down to keep it in place. I fixed this by adding a second bearing arm, facing the opposite direction and pulling them together with a spring. You still tighten them down, but if the arms loosen, the spring will keep the stage somewhat in place. It also helps with attaching the stage.

3. one of the things that really bugged me was that the acrylic stage just sat directly on the aluminum rail. even after just driving it for a short while, there were lines and scratches directly on the acrylic. seeing as how getting the acrylic stages lasercut is rather expensive (not to mention replacing a stage is time consuming). also, this was a source of a decent amount of friction in the system.

#3 was the toughest nut to crack. I tried a variety of things, that all failed for different reasons. here they are:

* roller bearings inside the rails. the idea was to have the stage ride on top of roller bearings. it was really hard to mount them, and really hard to get them all to the same height. failed.

* next, i decided to mount some sort of bearing on the stage itself. i thought about things that were soft, and came up with the idea of felt and/or ribbon. a trip to the fabric store and $7 later, i had some stuff to try. i had to glue them to the stage, but when i put it on the aluminum rails, it actually glided very smoothly. the problem with these is that i dont think they'd work very smoothly.

* after a couple days of mulling over it, i was thinking about what types of things have low friction when I remembered PTFE (aka Teflon) which has one of the (if not the) lowest friction ratings out there. i pulled up my favorite website EVER, and looked at their PTFE offerings. It turns out, they offer strips of PTFE for sale!!! I even found the perfect item for this: it is an ultra low friction grade PTFE strip: 1/2" wide, 1/8" thick, and sold by the foot. Fearing the worst, I checked the price: only $3.31 a foot!!!! Since my bearings only needed to be 2"-3" long this was absolutely perfect!

i ordered the PTFE strip and it arrived yesterday. i rushed over to the lab, cut it into strips. i then drilled holes in the ends of each strip for mounting. after that, i marked on the stages where each strip was to be mounted. i also numbered the strips and where they were to be mounted (i didnt really measure the strips much.) i then drilled the stages, and mounted the strips with self-tapping screws.

after that it was the moment of truth. i put the stages on the rails, and tested it out... it glided like butter!!!! i havent even sanded/polished the rails yet, but it seems like this is going to work really well.

the cool thing is that these bearings are *super* easy to replace. they are each just held on with 2 screws, so it would be a very simple process to buy another $3.50 strip of PTFE, cut and drill new strips, then screw them back on. simple, easy, awesome.

sorry about the novel, but i'm pretty psyched about this!

Tuesday, November 06, 2007


Non Blocking Stepper Control!

The biggest hurdle so far in developing an Arduino based firmware is controlling 3 stepper motors from one Arduino board. It doesn't have threading, so in order to control them separately you have to have to do time based checks. The Arduino libraries have support for getting milliseconds elapsed, but for driving these steppers at 200RPM+ speeds, I need sub microsecond resolution. I poked around in the forums, where westfw posted a sweet little function to get high precision ticks.. basically a counter that increments every 4 microseconds!

I took that function, put it into my Arduino RepStepper library, and tested it out. I had a few hiccups at first (which later turned out to be a ridiculously simple mistake) but once I got it working, I had a non blocking stepper controller class! The main benefit is that it allows me to control 3 steppers independently of each other. Here's a video of 3 stepper motors moving at different speeds, in different directions:

Crazy Stepper on Stepper Action from Zach 'Iowa' Hoeken on Vimeo.

Saturday, November 03, 2007


Pictures of Repstrap Darwin

I have started a Blog for my son and my selfs machine
we will be posting as we go but for now I posted a bunch of pictures.. We also hope to have some video of the machine in action before the end of the weekend..

Thursday, November 01, 2007


Furfuryl Alcohol and blocked catalyzers

Hi all!

Today I made some progress on the blocked catalyzer for Furfuryl resin mixes.
As a reminder, the catalyzer is para-Toluene Sulfonic Acid (pTSA), wich is a strong organic acid, in solid form. I have dissolved it in Isopropanol alcohol (a solvent quite similar to Ethanol).

This catalyzer, if added in this solved form to Furfuryl Alcohol (FAlc, the actual polymer resin) will cause it to polymerize extremely fast and under the release of heat. This should not be done, take my word! ;)

The trick here is to reversibly block the catalyzer just enough, so that you can mix it with the FAlc and the mix stays stable, for at least a day. The reversible word here means that with the application of heat, the catalyzer gets released again and the resin starts polymerizing on demand.

So this is what I have been testing today: I have tried different degrees of blocking and the temperatures necessary for unblocking the catalyzer.

The blocker I used is a strong organic amine base (N-Methyldiethanolamine, short MDEA).
This amine will react with the organic acid and form an organic salt, but that is reversibly bound. The fact that it's a base makes it possible to use the pH level (acidity level) to measure on how much it has been blocked.

I have tried pH 7 (neutralized), 6, 5, 4, 2 and 1 (strong acid).
What I have discovered is that MDEA blocks pTSA quite a lot and that the pH 7 down to pH4 samples were only marginally active catalyzers, even at heats up to 100ºC.
The pH 2 and 1 samples were catalyzers that reacted very nicely at 85ºC.

The method used to apply heat was a water bath. I used a small flat dish with 10g of resin and blocked catalyzer, wich I deposited on top of said waterbath. I couldn't use light-spot because i haven't been able to focus a light beam from a halogen bulb enough to produce a heat spot warm enough.

In future tests I will procede to try out the effect of filler materials such as Titanium Oxide (TiO2 probably replaceable with cheaper Zinc Oxide) and Starch (thanx for the idea Viktor!).
I will also have to fix myself a decent laser diode from an old CD burner I have or from a laser pointer. This will serve as a better proof of concept for the whole experiment than a water bath.

Cheers and keep posted!

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