Re: going into the unknown of CNC PCB milling
« Reply #11 on: January 11, 2010, 07:07:20 AM »
For what it's worth, I have some extensive experience with CNC PCB milling. I purchased a purpose-built mill (Proxxon MF70) with a tidy CNC interface and software, a couple of sets of tool bits of various sorts, and a kit of accesories to hold PCBs down, etc. Came to around Euro1000.

I've also worked with iron-on toner, spray-on developer, and several other methods.

My conclusion: The CNC mill has some significant limitations.

The good things:

1. I can make a PCB when I feel like it.
2. I don't feel like I am getting chemical poisoning when I make a PCB
3. I can make really interesting shaped PCBs, milled out to precise dimensions. Did one in a crazy Xmas tree shape a few years ago for the festive season. Went down a treat.
4. Most important - I can use the CNC for lots of other projects, like milling front panels, and a bunch of other little bits and bobs for other hobbies and stuff around the house which would otherwise be impossible to do. Wins big praise from my wife and family.

The bad things:

1. Much depends on the software you use. I use PC-NC from a German outfit. It was cheap (Euro70?) but it is not easy to use, and it requires a bit of relearning each time I go back to do another PCB. I don't make many - maybe one or two each month - and I have to just go over a few things each time to make sure the settings are right each time. Yes, other software is available, but it takes a considerable time to get a package up and running. It takes less time to review where I was up to with this stuff and make the board than mess around with something new.

2. The PCB must be absolutely dead flat i.e. within about 0.1mm from side to side, corner to corner, centre to edge, you name it. On small PCBs (30mm x 30mm) this is no problem. And this must be repeatable i.e. sustained as the mill head goes up and down, maybe a few hundred times on the average PCB. Backlash can be an issue, but mostly, its getting the milling table and the PCB(!!) flat, and keeping it flat. Any offset larger than this will likely result in a PCB which is not going to work out. I aim for 0.05mm. Yes, really.

3. Tools must be sharp, and the table/mill must not vibrate while in use. My carbide tools seem to last about five times (PCBs avg of 50 x 100mm in size, fibreglass) Phenolic PCBs will let you use the tools for maybe up to twice this time.

4. Every PCB needs a clean-up with a very sharp pointer. This has to be run around every track by hand while you look at it though the biggest magnifying glass you possess and under the brightest light you can find. Otherwise you will be plagued by tiny, near-invisible scraps of copper which seem to be left by the mill. Some PCBs are better than others, sometimes this doesn't take too long. Other times, it's an absolute curse. Keep Valium to hand.

5. It takes time. On average, a couple of hours. Yes, I see you laughing. How can it take so long? Well, usually it takes longer! you have to take the PCB layout, convert it into a suitable format for the machine, set up the PCB, check every setting and the flatness, and then let it run. It takes time.

6. There are limits to the size of PCB that the machine can handle.

7. It makes a mess. Milled copper and PCB material go everywhere. I use a vacuum cleaner hose held close to the job to keep this under control. So, having given up poisoning by chemical etching, I now expose myself to lung cancer from particulates. Go figure.

8. The boards do not look like the perfect ones you see in the ads. Sometimes, maybe, but mostly not. They tend to be a tad rougher in my experience. Once they are in the final box, it's all out of sight. Still, I'd like them to look nicer.

9. There is a limit on the sort of PCB that can be made. 15mil conductor widths are just not possible. Sorry, no. Not even on the really expensive CNC mills, despite what the ads say. Doing surface mount stuff on a CNC mill is a good way to stress yourself out. But, hey, don't believe me. Go try it for yourself.

All in all, I'd pause before going down this avenue again. I'm thinking about trying laser milling, myself. Maybe that's the answer....

But, hey, maybe you guys have answers to all these things....