I wanted to jump in here and add something. Full disclosure, It's my project.
The machine is designed to print things out of the sort of manufactured sheet materials we use in most construction these days so things like plywood, sheetrock, MDF..etc. Knots don't tend to be an issue because they are only in one ply of the plywood, meaning the force is very consistent even when passing through a knot. We've never seen an issue. Materials like drywall and MDF of course don't have knots at all.
Awesome project btw! I'm glad you've not seen any issues. Re-reading what I wrote, it sounds more pessimistic than it should. I am sure everyone that uses one of these will have lots of successful "prints" and will make many interesting things.
If the problem does occur you could simply make the thing heavier and run a bit slower. Inertia is a big killer for speed but this isn't a production machine.
I wish it were that simple! On a lot of materials, going too slow will cause problems as well. You want your "chip load" to be within a band; too slow, and you'll start burning the wood instead of cutting it.
I, too, do not want to come across negative about this. I'm very excited about it. If it isn't sufficiently strong to handle some types of wood, even just being able to drop a V-cutter into the router and having it very accurately draw out all the cut/guide lines is a win for the target price. If my work flow is "draw accurate cut lines, do a rough automated cut (leaving some extra around the edges), and finish on the scroll saw and belt sander", that's great!
Yes, obviously if you're moving so slow you're not longer cutting then that won't work but that's so slow that I highly doubt it couldn't be improved on.
Setting the rpm of the router is an ordinary setup/configuration step for doing 2d machining. I used to run 2d router systems at sign shops for years. You're always looking to optimise the speed of travel and router rpm to get the best cut qualities - fast travel with high quality cut - too fast can result in chips/burrs, too slow results in burning/melting and damage to bits, etc.
I always had a list of settings to use for various combinations of materials, thicknesses, router bit sizes/types. It was a bit of a black art actually - sometimes the results of changes were counter-intuitive because of specific material properties (heat retention, etc), and that cheat-sheet wound up containing the accumulated knowledge of years of trial and error experimentation. "The only difference between Science and screwing around is writing it down" :)
I don't like that the ropes do not meet at a point. For any given pair of lengths, you have not entirely constrained the position of the router bit. Rotation of the hanging cradle/holder will change the position of the bit. Gravity is nice, but it seems like this problem would be overcome if the ropes met at a point coincident with the bit - or as close to that as possible. That and a Z-axis would make this a pretty cool device that I could get behind.
I would say that it depends heavily on the feed rate, expected bit life, and what wood. Soft mahogany, basswood, poplar might be done on a machine like this. I would NOT expect good results from hard, flamed maple, or other heavy, dense woods.
source: have made guitar bodies on a normal CNC router. Material holding is difficult at the best of times.
This looks like an awesome project. Do you have an idea on when Z-axis will be available? As a (single user) data point, 3D motion is a huge benefit -- if it ends up under $1000 with 3D movement I will likely just buy it to play with. Specs on accuracy would be useful, but unless it is really horrible, not likely to affect my buying decision. Good luck!
Would it be possible to tether the machine to four corners instead of two to avoid this potential? It complicates the pulley assembly (probably doubling it), but should make the bit more stable.
I wanted to jump in here and add something. Full disclosure, It's my project.
The machine is designed to print things out of the sort of manufactured sheet materials we use in most construction these days so things like plywood, sheetrock, MDF..etc. Knots don't tend to be an issue because they are only in one ply of the plywood, meaning the force is very consistent even when passing through a knot. We've never seen an issue. Materials like drywall and MDF of course don't have knots at all.