Tuesday, May 13, 2014

Delta Star 3D Printer

Disclaimer: I plan on having monetized links throughout these documents to help fund more work with this printer design.

A friend started building a 3d printer a number of years ago and eventually it went into the dustbin of half started projects. When he moved, he dropped the half-printer off at my house. Last year, I decided to pick the project back up and got the early generation RepRap Mendel working. I found that all of the extra parts I needed had dramatically gone down in price from when the project started. Inspired by this and all of the new work in delta robot 3d printers (I had also seen one of the new style deltas in action at the Atlanta Mini Maker Faire), I decided to make a new printer with the help of the now functional cartesian 3d printer.

The result is the current generation of what I've been calling the Delta Star (design on thingiverse):



Design Goals

  • Low Cost - I wanted to get the price down to under $500. I really wanted something that would be affordable for someone even marginally interested in 3d printing.
  • Minimal unique parts- I wanted to eventually make a lot of these things for people and kits. I figured having a small number of parts would allow me to keep costs down and get better deals buying in bulk.
  • Predominantly 3d printed parts- I believe this allows the greatest flexibility in experimenting with new ideas and making new printers for others.
  • Speed and accuracy were secondary goals for me. The latter might sound funny, but I really just wanted something that would have repeatable performance. I figured shortcomings in these areas could be compensated for with patience and software.

Initial Designs

I combed the Internet with these goals in mind. Two major aspects of the work in delta printers caught my attention: aluminum extrusions and fishing line drive systems. I was especially enamored with the fishing line drive systems. I found the old Mendel design highly dependent on parts that were not as easy to source as I would like. I settled on OpenBeam aluminum extrusions and a fishing line drive system. The OpenBeam choice was driven heavily by the affordable cost and availability on Amazon Prime. I got a set of 6 meters in 2 days from Amazon. 


My build started using Johann C. Rocholl's old design of the kossel. I liked the 623 bearing on OpenBeam design of the rollers. I built the first iteration of the Delta Star using these rollers and some kossel frame designs.

Eventually, I wanted to try some new carriage designs and tried some ideas inspired by the Cerberus Pup. I liked the design but did not want to get involved with the custom wheels associated with it. I eventually landed on using some generic 623VV bearings which are just 623 bearings with V grooves around them. These let me lock onto the OpenBeams as well as use them as pulleys for the fishing line.

This design proved decent, but I was not pleased with the 'slop' introduced if the carriages weren't tuned just right (a testimony to the need the Cerberus Pup found for the custom wheels although I have not tried them). The performance of the carriages I designed led me to a new design with three main contributions:
  • Printed slider carriages which just use 3d printed sliders directly on the OpenBeam
  • Fishing line drive guided directly into the OpenBeam
  • Friction pulley system and winding to keep a consistent tension throughout the entire slider travel

Printed Sliders

Looking for information on directly sliding on the aluminum extrusions led to some discussions with a proof of concept video by Jay Couture showing some printed sliders being driven on a test beam (he gives credit to Bill Plemmons for the original idea).

The benefits of the idea are numerous. First, a well printed slider leads to very little wobble available for the carriage on the beam. All of my bearing based carriages were very finicky about how loose they could get before introducing slop and how tight they could get before introducing too much friction. Additionally, the problem of steel bearings on aluminum extrusions leads to a great amount of wear on the beam. I experimented with wire shrink wrap 'tires' on the bearings. This worked but would exacerbate the above problem of the tightness 'sweet spot'. Additionally, it was a pain to put the tires on the bearings. Second, this approach just removed 9 bearings from the bill of materials which at the budget I was targeting is not insignificant. Finally, the printed bearings are very simple in design and flexibility compared to the older carriages. I eventually settled on a very simple block with an extrusion sized hole in the middle with mounting holes. This allows me to experiment with different accessories on the carriage very easily. That being said, the sliders must be printed very precisely. Too tight and they introduce too much friction. On the too loose side, the wobble doesn't introduce slop as much as it introduces a significant spike in friction when the delta arms are in motion. The slider cants slightly when the arms move laterally making the slider get stuck on the beam.

Fishing Line in The Beam

My first design had the fishing line drive system arranged very much like a belt system would be arranged on a delta. I found this very annoying in that the line was just waiting to foul the arms in motion. I then started experimenting with threading the fishing line directly into the groove of the OpenBeam. This proved a lot harder than I initially though. First, it moved me away from directly driving the line at one end with the stepper motor. I thought this would be no big deal as I would just redirect the line with 623VV pulleys. It turned out that when you get to very intricate routing of the filament line, you start introducing very subtle changes in the fishing line tension. I eventually found a design that kept very strict routing of the line into the groove and up into the carriages.

Friction Pulley System

I initially used a drive system similar to the older kossel design with the motor spool at one end and an idler at the other end. When I started feeding the fishing line into the beam, I ended up routing the lines at the ends with pulleys which then went to a spool away from the vertical beam. These system used a spool system where the filament would actually wind and unwind onto the spool. This system works well but can introduce variations in the length of the filament on the rail which alters tension. This is due to both the path the filament takes feeding onto and off the spool as well as filament overlapping on itself on the spool. I found it problematic to get an ideal tension. Too loose and the carriages take on a vertical slop when changing direction. Too tight and you hit spots where the tension gets high enough to overcome the stepper motors. I eventually moved to a friction drive system which maintains a constant filament length. I came across an interesting proof of concept by Rob Povey for a friction drive system which inspired my current design. This has its own problems in maintaining adequate friction to prevent vertical slop from slipping but helps with maintaining constant tension.

Conclusion

I'm pleased with my current design for this printer and will continue to update this blog with improvements. I'm currently working on a unified cooling system and self leveling calibration. I will also follow up with a number of posts detailing the individual parts. I've greatly benefitted from the 3d Printer community building this printer and I hope to return the favor with these posts. Also, please let me know of any errors in attribution as I very much want to give credit where credit is due.