Friday, August 27, 2010

CNC axis design

This week I developed the design of the axis. I did a ton of research into the different methods of getting a carriage to traverse a distance. The difficulty is translating rotational motion into translational motion. The rotational motion is typically provided either by either a servo motor or a stepper motor. Servos have the advantage of providing position feedback, but are typically far more expensive for the same torque rating. Steppers are cheaper though slightly more difficult to interface. I knew that for me the priority was speed, torque and cost, not accuracy so I went for the stepper option.

By far the most common method used in CNC applications for converting the rotational to translational motion is through the use of a drive screw. The motor rotates a threaded shaft which causes the carriage to inch along the length of the shaft. This method is extremely accurate, but quite slow. The other less-common method is to have a belt stretched between the extents of the axis and use a rotating shaft with teeth to engage the belt and drive the carriage along the axis. This method is more complex to implement, but offers high speeds at the sacrifice of accuracy.

I bought a fairly high-torque stepper motor which has 200 steps/revolution and first of all worked out how to interface it with the arduino platform. There was a good tutorial on the web, which got me up and running quickly. With the stepper going I found a length of threaded rod and did a quick test to see how it performed. From this test I found out the threaded rod technique was completely unsuitable as I found the travel time, end to end of the axis would be in the order of minutes. This would be completely unacceptable as it would take about 5 minutes to output a simple word.

Now knowing that I would need to make a belt driven axis I began designing the structure of the gantry. I decided to build as much of the device as possible using laboratory standard materials (plastics, stainless steel and aluminium) because I knew that I would be re-using the finished cnc axis in the future, so I wanted to future-proof it for any application. I also have never worked with stainless steel, so I will be learning a lot from the fabrication process.

I managed to source a significant portion of the metal I would need from scrapyards who generally buy scrap. They were happy to sell to me, though they found it funny when they asked what it was for! In this way I was able to get a huge amount of stainless for under $30(NZD).

This week I also sourced the parts for the belt mechanism, including the belt itself and the sprocket which I will attach directly to the stepper via a connector I will spin on the lathe. I ended up getting the belt system from a hobby store which sold parts for rc helicopters. The belt I got is actually designed to spin a tail rotor.

Tags: fabrication cnc

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