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.