Friday, February 19, 2010

Where to start?

Hobby CNC Machines. Where to start?

Google Search (or Bing if thats what you're into) might have some helpful hints on basics etc. There are some excellent forums out there as well, CNCZone springs to mind instantly.

But, where am I starting. I'm going to outline the goals of my latest CNC Machine project and then maybe I can get in to some cad here later on.

First off, I want a spindle that can hit 10,000 RPM. I would like to have 1-2hp behind it, but quite frankly I'll settle for 1/2hp if I have too. (Oh, sorry yes I'm from that one country that decided not to go with metric, so my drawings will be in inches...But I'll dual dimension things for the other 90% of the world who adopted metric) Oh, and I would really, really, really love, if I can build an ATC.

I don't plan on using large End Mills, 1/4" - 3/8" will probably be the biggest I use. So, hopefully spindle power will not be a huge thing, but I will need a pretty quick spindle, hence the 10k RPM requirement. Going along with that however, I plan on cutting aluminum which is pretty easy to cut, and at high SFM. With some luck I will be able to support chip loads of 0.002" - 0.005", which at 10k RPM will require feed rates of 60 to 150 IPM.

I'm going to aim for 100 IPM cutting, and 150-200IPM rapids.

Next the working volume, this is the design feature that is most likely to change because it has such a huge impact on the cost of the machine. Towards the end of the design when I start looking to order parts, well, this is going to become and issue. I've designed several machines up to this point but by the time I get to the end I realize they're hugely expensive and I just won't have enough feasible, disposable, cash in order to start building. I already have a decent selection of machines, and at least up until now, its simply been a better idea to re-invest in them.

I would like to have a working volume of 14" x 18" x 12" ... but we'll see how that goes.

Finally there is the form factor, and then we can start talking about the 'incidentals'. I have not fully decided on the form. There are quite a few styles of vertical milling machines, and I have fully decided is, that it will be, a vertical milling machine. Whether that be a Gantry (see router style), Fixed Gantry, Bed Mill, or Knee Mill I'm not entirely sure yet, but I'm really leaning towards the Fixed-Gantry, we'll see what FEA says here in the near future.

The Incidentals:
Some of the design will end up going in a certain direction just by the performance criteria already mentioned. To hit 100-200 IPM movements reasonably, I will almost certainly have to use Linear Rails. I will most likely be using open loop stepper motors/controllers at least in the beginning because well, quite frankly they're cheap and they work reasonably well. Since steppers develop most of their torque at low RPMs, I'm going to need a low TPI lead screw, thus I'm probably going to go with 5 TPI ballscrews in order to get the position accuracy/efficiency needed to post those kind of rapids.

Notes: I said in the first post that this not really an 'entry-level' blog, my choice to use steppers may in fact lead some to believe the opposite. The stepper vs servo debate has been raging for eons (well eons for the tech world) and I'm not really going to choose sides. I already have mid sized (270 oz-in) steppers, control electronics, and experience working with both of them. Because of that, I'm going to reuse them for this project until I can save up the money that will be required for a servo upgrade later.

Time to sum it all up. My 'design criteria':
  • 10k RPM Spindle - With ATC if possible
  • 100 IPM cutting, and 150-200 IPM Rapids
  • 14" x 18" x 12" working volume
  • Linear Rails
  • Ballscrews
  • Stepper Motors
  • Enter form factor here
Now its time for some research.

No comments:

Post a Comment