Form Factor

Early I mentioned the one last undecided was the form factor.

I'm only considering three of the ones I listed earlier. I don't believe there is any reason for me to attempt a Knee Mill...

So of those three we have Moving Gantry, Fixed Gantry, and Bed Mills remaining.

First off lets look at the Bed Mill:

General features of a bed mill are the overhanging spindle that rides on a single column. The Z-Axis is along that column and spindle simply moves up and down. The X-Y table below the spindle is where work is fixtured etc. These are very simple designs, and there are quite a few very successful homemade bed mills out there. The one built and designed by 5Bears jumps to mind instantly. Most Asian hobby mills are of this type as well. Its a standard repeated over and over, but there are some geometry considerations.

The most major is that the Head is cantilevered out over the table. This causes a torquing action against whatever linear bearing surface, whether that be a dovetail, box-way, or linear rail, and usually requires a counter-weight to offset this torque. The cantilevered design also requires more material to keep the head rigid while cutting, thus adding weight to the machine, and further exacerbating the need for a counter weight. Keep in mind though weight is not necessarily a bad thing, more mass is increased dampening (as the cutting vibration must overcome more inertia before the machine starts bouncing around) and counter weights are not exactly uncommon on any machine type.

Next the joint between the X-Axis and Y-Axis may be subject to extra deformation due to lack of material (since the X-Axis 'floats' on top of the Y-Axis there just isn't a lot of room there). Often times on bigger VMC's this area will be heavily reinforced with ribbing etc. This is really just speculation, but with limited material there, it does appear to be a point of flex.

Bottom line, it appears at least to me, that pound per pound, inch per inch (of working volume), bed mills require more material in order to be rigid. Cost, to me, is an issue, and anyway I can lower it is a good thing. I believe I may write off the Bed Mill.

Next up, moving Gantry Mill:

This is a Datron High Speed Milling machine, have a look at Youtube, its a nice one, pallet changer et al. Moving Gantry Mills mount the spindle on a moving table, mounted to a moving Gantry. The Gantry effectively becomes the Y axis, while the spindle is mounted on a 'X-Z' table on the Gantry. Gantry Mills are extremely space efficient allowing for really large working volumes. However they suffer from the same geometry issues a bed mills, and a few more.

The Gantry is usually held by two uprights, these uprights can become the limiting factor as far as rigidity is concerned when the milling force is perpendicular to the axis the slide on...In other words when you're pressing to the side of the uprights there may be a tendency for them to flex if they are not big/strong enough. Since the entire gantry moves theres some concerns with weight as well. The linear controls have to be able to accelerate the entire mass of the gantry, which includes the spindle, X-Axis, and Z-Axis motion control equipment as well. Further, the Z-Axis invariably is cantilevered, and because of the weight concerns, there usually is not enough material to keep the Z-Axis rigid.

Gantry Mill's are space efficient, but they aren't usually that rigid. They're typically used for plasma cutters, laser cutters, wood routers etc, typically applications that don't require a lot of rigidity.

Lastly, the Fixed Gantry Mill:

The Gantry is, well, fixed... The spindle, is still mounted to it, however the entire Gantry ends up being the Z-Axis, so the cantilevered issue is resolved. Since the gantry does not need to move, the columns holding the gantry can be very large and strong. This resolves the issue with them bending when cutting along the X-Axis. Lastly the table itself becomes the Y-Axis. All and all this a much more balanced design then either Bed Mill or Gantry Mills, at least in my opinion. Unfortunately, they end up taking up much more space then either Bed Mills or Gantry Mills.

Which am I going with? I have no idea. Most likely the Fixed Gantry Mill, mainly because of the rigidity factors.

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.

Hello World

I'm so sorry you had to stumble across this blog! This is (or will be?) a hobby CNC site. I plan on posting my rambles here for machine design. Hopefully some of it may be useful to others, and at the very least it should help reinforce my ailing memory.

I plan on starting a 3-axis fixed gantry CNC design with a full ATC, here in the near future with full access to FEA it should be an interesting ride.

So some starting points. This is not really an entry-level sort of blog, I'm not terribly interested in machining wood or building MDE routers (no disrespect to those who do, there are some really brilliant machines out there I just hate saw-dust, don't know why I think aluminum is any better though) primarily I want to design something that is, not necessarily cheap, lightweight, or even good looking. I just want it to cut aluminum (maybe some steel) and do it reasonably quickly. And, if I can get rid of some tool changes thats a plus too.

Further, this is about the machine, and quite frankly I don't care if I can buy a used VMC for $10,000, or a small/mid-sized Asian mill for $1,000-$2,000. This is about machining and building a machine. Yes, sometimes just building a half usable machine is reward enough. That said, I can't promise I'll actually ever build something but I really do enjoy Cad work.