Of Shafts, Pens, and Bearings
This is part 2 of my N part series on building a DIY CNC machine. If you missed it, here is part 1.
One of the trickiest parts of building a CNC machine is the linear sliders. Each axis requires a rail to slide the carriage on. This is called a linear slider. The sliding mechanism must have very low friction but also be strong to support the weight of the other axis and the head (which could be as heavy as a router). The carriage must also grip the slider tightly to ensure it doesn't slip or turn. The accuracy of the final CNC machine is directly related to the quality and tolerances of the slider. Needless to say, professional slider mechanisms aren't cheap.
After researching what others have done I decided to build my linear sliders out of OpenBeam using #608 roller skate bearings. Skate bearings have a few advantages: they are produced in mass quantities so they are pretty cheap for the quality you are getting. They are also usually sealed to keep out road dirt, so they can handle any debris the CNC might produce. And, because they are a standard size, getting parts to fit them is fairly easy. Here's a quick look at two versions of my bearings.
Skate bearings typically start at 1 USD each in sets of eight and go up from there. However, if you can afford to buy a lot at once you can get them as cheap as 50 cents a piece. I purchased my bearings from Amazon as a pack of 100 for 50 USD. A CNC will end up using at least 20 bearings so it's worth going for the large pack.
To attach the bearings to the beam I used long M3 screws from my local hardware store, about 20mm. Of course the inner diameter of the bearings (the bore) is much bigger than the diameter of the screws, so I needed to fill in the space.
After playing around with a variety of spacers, sleeves, and bushings I found a combination that would work, as depicted in this photo:
This works and is what I used in the current version of the CNC machine, but it has a few disadvantages. The nylon parts are longer than the bearing is wide so I had to trim them by hand. Plus there is two of them needed for each bearing, which adds cost and assembly time. I also had to add some washers to prevent the screw head from going through the center and to give more space between the beam and the bearing. Not bad for a first try, but I need something better.
Some searching on Amazon turned up some better pieces:
This bearing assembly uses a slightly shorter screw, 16mm (also in cool anodized black). I also found spacers that the were the perfect length, no trimming required. The interior diameter is perfect for the screws. the outer diameter is a tight fit in the bearings but I can pop them in easily with pliers. Since they fit in with friction I don't need to worry about washers to hold the bearing on. To give more space I used some of my standard M3 nuts. Since I already have to buy tons of them they are cheaper than washers, and adjustable to boot. Score!
I'm learning that creative sourcing and constant redesign is the only way I will reach the 200 USD goal.
The next big challenge for a CNC machine is attaching the lead screw to the stepper motor. I would think this would be easy. I would be wrong.
The lead screw needs to be attached to the stepper motor securely so it won't slip, but it also needs a bit of flexibility to absorb vibrations. The two shafts also must be perfectly concentrically aligned or else the carriage will bounce up and down. Doing all of this for a decent price is very hard. Commercial solutions run 30 USD per shaft coupler.
I found several people online who use rubber tubing with clamps. Unfortunately I found the tube around the stepper shaft was too loose. If I tightened it with the clamp then the shaft would never be concentric with the lead screw.
My next attempt is what you see in this photo:
I used two pieces of tubing, one nested inside the other. The smaller tube is shorter and only goes around the stepper shaft. The larger tube contains the smaller tube as well as the leadscrew. The small tube is tight enough that it doesn't need a clamp, so that reduces the complexity a bit. The larger tube still slips, so the clamp must remain.
The two shafts are now more concentric but still not perfect. It's a good start though. For my next attempt I will switch to aluminum tubing from a hobby store.
I can't turn my CNC machine into a plotter without a pen of course. I started with a Sharpie rubber banded onto a piece of extrusion.
It almost works but not quite. The pen has no give. It is either pressing very hard against the paper or doesn't touch at all. When it presses hard it prevents the carriage from moving smoothly, so I get lots of skips and jumps. I'm still looking for a better solution that will add some spring.
That's it for now. Next time I'll show some of the improvements I'm making to the electronics side for V3.
posted Fri Aug 17 2012