Newbie- Fist CNC, Need guidance

[rcpilot82]

Hello, I am building a router to cut foam sheets, lite plywood (up to 1/4") and balsa wood with the occasional light gage aluminum cuts. my problem is that I have an x axis of about 5 feet, and from a little research on the zone here I think a belt drive system would be better and more cosst effective for my router. i have picked out what slides I would like to use from vxb.com. the router will have a moving gantry and be constructed out of aluminum tube, 2X2X1/8". the y axis will be just over 2 feet and a z axis of about 6". the frame is being built right now and I have 2 steppers that are 117 oz in tourqe. I plan on getting the 3 axis board from hobbycnc.com. ok, so what I would like to know is how well does this system work, what parts do I need, and will the steppers I have work, or do I need a bigger one for any of my axis? the belt system I like the best so far is the open loop system with the stepper mounted to the underside of the gantry and clamped at each end of the router. I estimate the weight of the gantry to be around 50lbs max. Once I get the frame finished welding I can post some pics, but anyone got some help with the info given?

[sonicwonder2000]

I would think 117oz/in steppers are going to have difficulty in moving a 50lb gantry; I would probably go wth the 305oz/in hobbycnc motors. That's what I'm using on my belt-driven machine.

Another thing to consider, is that your steppers will need to be geared down in order to drive the pulley. The gearing will depend upon the type of belts you are using (how many cogs per inch). Your resolution will be very coarse if you don't gear down. This ads complexity to the drive mechanism that is wise to engineer before you begin construction IMHO.

Check out David Steele's design at www.solsylva.com. A modification of his machine is what I am building.

[rcpilot82]

nice link, lots of ideas. I have attached the ACad file for the router to help out. the router is a modified version of twistedfuse's router. I figures I was going to need a bigger stepper for the x axis. will the 117 oz steppers work for my y and z axis? I plan to have acme screws on them. the speed of the router is not as critical to me (i'd like to be in the 250-200 ipm)range and the precision needs to be about .006 over 4 feet. I don't know if I am wording this all right as I am still learning, but I hope that helps. I will be cutting RC airplane parts out for the most part from balsa and up to 9mm depron for the foamies with the occasional odd-ball stuff.

[ger21]

Originally Posted by rcpilot82 (i'd like to be in the 250-200 ipm)range and the precision needs to be about .006 over 4 feet.

You might be able to get that with a HobbyCNC board, 300 oz steppers and 1/2-10 5 start. Maybe. Those 117 oz motors won't get you 100 ipm.

[rcpilot82]

I had a feeling that those steppers wouldnt work to well after lurking around on here. so i guess i'll ditch the 117's and just get the whole kit from hobby cnc. I will use the acme screws on the y and z axis but i wanted to use a belt system for the x axis, which is where I am stuck at. I saw a pic on here yesterday that looked exactly like what I had in mind and now I cant find it. i'll keep looking and post it when I find it.

[ger21]

Sorry, forgot about the belt drive thing. With belt drive, it can actually have more speed than with acme, but with less torque. All depends on how you gear it.

[rcpilot82]

I still can't seem to find the sketch I saw but it was a direct drive off of the stepper with 2 rollers (one on each side of the stepper) that made the belt run in a U shaped path. it looked fairly easy to make up and work with. dang I wish I could find that sketch!! I should have my frame home this week and get the full visualisation of the build. if anyone is intrested I can post some pics. this will be a slow build bacause of the whole cash issue.

[rcpilot82]

ok, here is my own little rendition of what I can remember, I know there has to be a tensioner somewhere in the belt system, but any thoughts on how this will work??

[harryn]

I spent a lot of time looking at the details of a belt drive system, similar in concept to what you have drawn. The basic conept is simple, but there are a lot of details of the belt and pulleys which can have a huge impact - such as backlash from tooth gaps, etc.

After adding up all of the parts needed, including fairly precise timing pulleys, etc, I am strongly THINKING that it is better and cheaper to use rack and pinion ( looking at boston gear, but there are others as well). My INTENTION is to use direct driven pinions from fairly beefy 1/16 th micro stepped motors.

Something like 20 - 25 tooth gears with 20 tpi rack and 14.5 degree pressure angle. (lots of more informated opinions in this area than mine)

In theory, this will provide something approaching 0.001 inch micro steps, and is similar to the mechmate concept. Backlash is a bit less clear frankly, so I am still trying to figure this out.

Bottom line, is that to get good (design) precision with belts, required really good belts and timing gears. Getting timing belts even close to rack size tpi is not feasible. It appears that rack and pinion is just really reasonable to produce especially if you are willing to accept some industry standard high volume part concepts.

[rcpilot82]

Hmm, I looked at the rack and pinion idea, but thought belt drive would be easier. Hey I am open to all options here, I just need to keep the cost down (thats why I am trying to get away from the ball screws) all good points keep em coming I will most likely not be able to afford any of the screws/belts/racks or whatever for a while. I just want to purchase the parts once and not twice.

[harryn]

From what I am seeing, the best way to keep costs down and obtain reasonable precision, is to focus your spending on as few of components as possible.

An example - the rack and pinion option. There are a number of assocated parts that go into a drive like this
- Rack (s)
- Pinion (s)
- reduction gearing
- stepper motor
- stepper motor driver
- one motor on each side, or a mechanical linkage
- Mounting the pinion on the stepper motor

Each part adds either to the precision, or frankly, non precision of the total system, so just like a chain, the most out of precision part, or installation of a part, can really harm the end result.

For my own project, I have NO fancy tools to build up the system, and I have poor craftsmanship, so it has to be constructed as simply as possible or it simply will not work.

First items on the list - get rid of gear reduction - nothing good can come from building this up myself - so I am going down the path of hefty stepper motors with lots of low end torque, direct driving a modest size pinion gear. This is done routinely by the mechmate projects.

Rack is actually quite cheap - esp if you stay with the one I pointed out.

Second item on my list - driver circuit - I don't have $ 100 / each for 6 stepper drivers (2 per axis) - so I am using linistepper drivers. They aren't perfect, but for $ 35 / each, it is ok. Even with only 1.5 amps, you can get some amazing torque from a nema 34 motor. That brings up another point - sometimes, a nema 34 motor is CHEAPER that a nema 23 motor for the same torque due to the cost of high end small magnets.

Third item - drive from center, or both sides - I have decided to drive from each side - this is my splurge area, but it gives me more confidence that the motion will be under control. I thought about a mechanical link between sides, but once again - lots of room for mistakes / imperfect components.

Linear rails - I am still deciding on this area, but it won't be a fancy linear truck concept - budget drive decision.

[rcpilot82]

HARRYN, you got a web-site for that boston place??