Tuesday, 27 December 2011

My Prusa experiences

Five years, or so, ago I was trawling the web and found the RepRap site. I was intrigued, the Darwin caught my imagination. However, work, travel and a lack of time meant that it was impractical for me to build one. Every so often I would dip into the site to rekindle my enthusiasm. February 2011 I dipped in again and discovered the Prusa. Mmmm, building one was now an achievable possibility. I visited eBay. Nophead was selling a set of printed Prusa (V1) parts with a Wade extruder at an affordable price, I was hooked.

The printed parts came from Nophead, the basic vitamins came from ThingFarm.org and the electronics, steppers, opto-endstops etc. came from mendel-parts.com.

The assembly was fairly straightforward, I have been building electronic and mechanical devices for years, so no real problems that a little thought, plus some dismantling and rebuilding could not solve.

My big problem was getting host software to communicate with the machine. I use a MacBook and either the software did not work well with a Mac, or it did not work well with the Gen6 electronics.

Eventually I found Pronterface, and it works very well and is constantly being updated.

The other problem I had was getting the machine calibrated. Originally, I started with Skeinforge 41, I got it calibrated fairly well, but was not happy with the consistency of the prints.

When in doubt pay attention to the details!

My original setup used PLA bushings and whilst watching closely I noticed that sometimes the Y axis would wobble up and down as it moved, I also noticed that the extruder nozzle would oscillate from side to side as the carriage moved up and down. Between them it meant that the layers would be uneven and inconsistent.

Once you have a machine that prints something reasonably well you are laughing! Time to use the machine to print modifications for itself.

Stage 1 was to print a new set of Y-Carriage LM8UU bearing holders.

Fitting these sorted the Y-axis wobble.

The next problem was caused by the Z axis drive shaft couplers. When I built the machine I did not understand the reason why the Z drive stepper motors were only taped into position, so I bolted them down. A mistake. A couple of months later one of the Z axis drive couplings snapped. The very helpful and generous nophead sent me a new one.

The reason that the stepper motors should not be bolted down is because the drive coupling does not hold the stepper motor shaft and the Z-drive shaft in perfect alignment, this means that as the motor turns the shaft wobbles. If you bolt the motor down this places the couplings under stress, hence they broke.

On my machine when I removed the bolts and taped the motors down this misalignment was transferred to the lower end of the z-drive shaft which traced out an elliptical orbit as the Z-axis moved up and down, hence the wobble.

I solved this by printing and fitting a Prusa Z Rod Constraint to the bottom end of each Z-axis drive shaft.

Now the machine was starting to print well and consistently.

During this process I designed and printed a mount and fan holder for my Gen6 electronics, the original mount consisted of some cardboard and duct tape.

Along the way Alessandro Ranellucci released Slic3r to the world. Once I started using it I was hooked. It is much easier to setup and use than either Skeinforge or SFact and the prints are just as good, if not better, without a lot of tweaking.

It is very much a work in progress, there are constant updated to it. It is a lot less forgiving with badly created STL files, but Alessandro is constantly improving that, and so far I have not had to drop back to using SF.

Recently Josef Prusa released V2 of his design. Whilst the machine was working well there were a few things that I wanted to change. Occasionally I had problems with my X-carriage tilting. Again I had the original designs PLA bearing, so now I printed the new Frost/Prusa V2 X-carriage that used L8MUU bearings.

When I fitted my original hotend I had to make a plate and fit it in place, not easy to change, and I wanted to be able to take it off to mount my Pen Holder etc. So I printed the new modified Wade extruder too.

I have missed out the testing and trials of various firmware and driver software, but now I have standardised on this tool chain:

OpenSCAD, or Alibre Express, for the design stage.
Slic3r for the slicing.
Pronterface to control the machine.
Marlin Beta for the firmware.

It works well and I am very happy with it.

My next modification will be to add an adjustable Z-axis opto holder. I find that trying to manually move the opto up and down by 0.1mm, or less, is pretty hard, so I have a design created that will allow adjustment using a screw, now to print it and try it out.

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