View Full Version : Making decisions about my first CNC

04-01-2015, 09:07 PM
Hi all.

Apologies if these questions have been asked (individually) before but I thought I'd post this as I'm having trouble deciding on a few aspects of my forthcoming CNC build.

The intended purposes of the machine are:

- Cutting plywood/MDF and acrylic
- Occasional cutting of sheet aluminium (making cases for electronic projects) and
- PCB milling, cutting and drilling - through hole stuff so not massively fine work

I am primarily an electronics hobbyist (so electronics won't be a problem) but I don't have a huge amount of experience with the other required skills - I can do rough carpentry (to the point of having built a small timber framed building and several workbenches) but I'm not so experienced with finer work; I have a bit of metalwork experience such as drilling and tapping aluminium and some welding but again, not much.

I only have a limited range of tools - jigsaw, circular saw, drill and drill stand.

Thought it was worth mentioning these things as I know they all have a bearing on the decisions I make. So...

I am trying to build a small (maybe 30x30cm work area, bigger is better but I only have a tiny workshop) CNC router on a budget (I bet that's what they all say, but seriously, I don't want to spend too much money on it).

I already have five NEMA 17 stepper motors from an old 3D printer project, so I want to re-use those. I think these motors should have enough torque to run a relatively small machine like mine. Is this correct?

There appear to be a few options for the framework of the machine. Quite a few people seem to have gone with MDF. Will this be rigid enough for my purposes?

The other popular options for DIY appear to be aluminium extrusion (expensive) or even welded steel frames (I'm concerned I won't be able to do this with the required accuracy, so I might as well write that off right now, and also it may be overkill for a relatively tiny machine). Are there any other options I should consider?

Linear slides. Again, there appear to be a few options here:

- Hardened steel round rails and linear bearings. These are attractive to me as they are relatively cheap and you can get linear bearings contained within a mounting bracket (SCxUU). What are the disadvantages of this? If I do go this way, should I choose 8mm rails or would it be prudent to go bigger (and, although I've not looked into the cost, presumably more expensive)?

- Hardened steel rails with bronze bushings. The bronze bushings are slightly cheaper than the ball bearings and presumably more accurate.

- Rotary bearings (i.e 608 bearings or similar) arranged at an angle. This seems to be quite a popular option for DIY machines but I have concerns about being able to drill the brackets accurately enough to get everything to line up.

Can anyone advise?

Thanks. :)

04-01-2015, 10:34 PM
I think maybe the motors are a little small perhaps? I'm assuming that you will use leadscrews, unless you use ballscrews then friction could be a bit high, especially with homebrew slides. Steel rails with bronze bushings could be a good option for the slides as they could have less play than cuing cheap linear bearings, but they will have higher friction. Could be worth investing in some beefy type 23 steppers.

04-01-2015, 10:53 PM
You have conflicting goals for your limited budget.?

Woods plastics etc are no problem as accurecy isn't critical but PCB works requires accuracy and this means building to higher standard which means higher quality components and these cost money.

Also like John points out the motors you have probably won't help you anyway if your going the threaded rod, skate board bearing and MDF route as efficiency is very very low so friction high.

Cutting aluminium on limited budget is doomed from the start so I'd forget that instantly.!

In a nut shell what you get from CNC is directly proportional to what goes into it and if your limited on budget with limited tools then expect limited results and accurecy.

Whats your budget.?

05-01-2015, 12:05 AM
I have built and use an MDF router, although with a larger cutting area (around 900x400). I use skate bearings on 2" steel tube for X and 1.5" tube for Y. I would try to avoid using the skate bearing approach as they are a fiddle to build and set up, and probably a bit big and clumsy for this size machine anyway. Supported rail and linear bearings would be much easier to install and set up. I've seen working routers using this approach and although it is not as good as profiled rails, it's probably a good engineering compromise for this purpose. I would go up a couple of sizes from your "8mm" comment above, though - my 3D printer uses these and they are only just enough for a machine that hardly generates any cutting forces!

08-01-2015, 10:26 AM
I would say that you have the skills to build a decent machine. You have the big advantage of having an electronics background. On the whole, it's easier to pick up "mechanical" skills as you go than to pick up electronics skills although you may be a skilled machinist/welder. Trying to explain to someone the how and why of, say, an NPN open-collector output or ground loops is harder than how to level rails with epoxy or even basic welding! Just go for it - there's plenty of advice available on this forum.

Boyan Silyavski
08-01-2015, 08:21 PM
Here and there you could do cheap. But if i were you, Nema 23 motors and profile rails like hiwin, new or used from ebay, these 2 are a must.
On such a small machine you actually need 4 rails, 2 left right, gantry, Z. Yeah, the Z will work ok with even 1 proper rail. With spindle 0.8kw i mean, no bigger.
Moving table and fixed gantry will even give further possibility to your build.

You could perfectly make the machine using belts without problem, though the 3 ball screws for that machine will be like 150. So not so much.

Having a drill is a must.

So if you are clever you could build a beauty on a budget. Spend more time designing and considering and less money wasted.

Don be shy from steel, cause thats the cheapest way to go. You will spend how much , 50-70 for the steel.

Better dont be in a rush, buy when you can and build slowly something that can make you money.

Or if you want your hair to be white soon, do a crappy machine that will make you swear every time you look at it.