View Full Version : Belt driven CNC mill - some questions

29-04-2014, 04:46 PM
seeing how a lot of people successfully build their very own CNC routers, I'd like to jump on the band wagon and try it myself. I've read a lot about different designs, looked into build logs etc. but whenever I have specific questions it's hard to find precise answers, because most people do not explain why they went for the option they chose instead of using an alternative.
I thus decided to try and formulate my questions such that I may receive helpful answers. If any of my questions have been answered in detail elsewhere, please feel free to point me in the correct direction instead of repeating things that have already been answered thoroughly. But if you feel as if my questions matter and have to be answered properly, please do so.

Some general information about my plans:
- My CNC is supposed to have belt driven x and y axis accompanied by a screw driven z axis. I want to mount the belt driven axis' motors fixed to the frame / gantry instead of moving them in the gantry and z-axis sled to reduce the amount of weight on the gantry and create some space between viable parts.
- I want the work surface to be separate from the frame such that I can add to the work surface from below without changing the whole frame (i.e. vacuum setup, drainage, ...).
- I am a programmer and prefer modular solutions instead of designs where lots of independent vital parts are incorporated into combined sections. This increases the amount of work spent on some smaller parts but introduces a lot of "cleanliness" (in my opinion). This means for example that I would like being able to remove the belt drive at any point and replace it with a screw drive without changing the whole machine.

1. Does it matter where the belt is located in relation to the sliders?
I see 2 possible ways of positioning the belt:

a) Right next to the supported linear rail.
This implies that the mounts holding the pulleys must be on the same level, therefore in front and behind the linear rails respectively. In addition to increasing the machines and the belts' lengths, but would reduce the overall height and move the point at which the gantry is being accelerated closer to its center of weight. Also setting this up on a rigid frame would be easier, since both the linear rails and the pulley mounts could simply be bolted on top of it.

b) Below and to the side of the supported rail.
This alignment would kind of lead to the reverse effects. Putting the pulley mounts below the rails results in a shorter machine but it will be slightly higher. The belts' lengths will be closer to the required minimum but the point at which the gantry is being pulled would be further off. Additionally the pulleys would have to be either mounted to the frame itself or the frame would have to be altered such that it does not stretch the whole length and allow for the pulley mounts to be attached to its ends.

Some quickly made pictures to illustrate my point (rails, pulley mounts, pulleys and frame as seen from their side) ;D

2. Does it matter if the belt's length exceeds the required length?
I would imagine that a longer belts leaves more play for errors. However machines with larger work surfaces have longer belts too and do not automatically suffer from worse precision.

3. Would connecting the non driven pulleys using a single shaft be beneficial?
My current plan for the x-axis linear rails including belts as shown from above (The work surface would be in the very middle.):


The grey bars on the sides are the belts, next to them are the linear rails including mounted sliders. The driven pulleys are connected to a single shaft, while the others are mounted on separate shafts. Is there any reason to not have them separated?

I hope my questions and pictures can be understood :D

30-04-2014, 12:39 AM
I've built a few belt drive machines now and can I say while they work very good they don't workout any cheaper than machines with ballscrews bought from china. By the time you have applied a ratio needed to get decent resolution the cost of good quality pulleys and belts required is the same or if not more than screws from China.

The cheap class 7 ballscrews are much better and far less hassle than even good quality belt drive with better efficiency. So my advice is Don't even got there just go straight away with ballscrews you won't regret it.

If you must go with belts then either of your designs will work but you will need a ratio applied, how much depends on the pulley size and resolution required.
One bit of advise is don't go too small on pulley size to increase resolution as the number of teeth engaged is important and smaller pulleys means less teeth engaged so higher chance of teeth jumping and belt wear.