Here is the design for my 3' x 4' x 6" steel gantry mill. Any feedback before I start would be great!

Attachment 183342 Attachment 183344 Attachment 183346 Attachment 183348 Attachment 183350 Attachment 183352 Attachment 183354 Attachment 183356 Attachment 183358 Attachment 183360 Vice to gantry distance:Attachment 183362
Z Back Plate:Attachment 183364 Attachment 183366 Attachment 183368
Z Front / Spindle Plate: Attachment 183370 Attachment 183372 Attachment 183374
Y Closeup: Attachment 183376 Attachment 183378
Exploded: Attachment 183380

This machine will be able to mill steel, aluminum, wood, plastic. This is a hobby (not production) machine for making various one-off parts. I expect to be doing a lot of 3D work. I plan to add a 4th axis at some point.

Why this design? I started designing an aluminum CNC router over a year ago, and just wasn't happy that you just can't cut steel with a router. I had several aluminum designs, but didn't want to build them. After a while I decided to just keep making it more rigid until this 9th design. Gone is most of the aluminum, for rigidity and thermal expansion reasons. I settled on a 3x4' envelope because that's about as big as you can get with ball screws. The 3' wide cutting area makes the gantry narrow enough to be a reasonable weight for this rigidity. The sides (x rails) will be raised so that there will be virtually no sideways gantry motion. The Y ball screw supports are attached to the gantry bottom beam, so the top beam gets no sideways force. I have incorporated triangles into the structure wherever I can.

Travel will be 37 x 49 x 12". There will be 6" clearance under the gantry, and 3" additional clearance if the bed is moved back. I plan to be able to mount a milling vise on the front horizontal beam. The spindle can move 1.5" past the front end of the bed so that I can cut dovetails. There will be lots of space between the linear blocks, so there will be no multiplication of forces. The Y Rails are 8" apart center to center. The Z rails are 5" apart.

Construction: Welded steel (stress relieved gantry), concrete filling inside the beams, self-leveling epoxy.

Rigidity: Horizontal deflection on one of the two 3x5" gantry beams is 0.00043 with 100 lbs force. This is for a bare beam, so it will be better when fully assembled. The top and bottom beams of the gantry will be joined by welded spacers. It will have a sheet metal skin on the back.

Dampening: Concrete in various places. Cast Iron Z if I can.

Accuracy: Linear rail beds will have self-leveling epoxy. The Z plates and gantry support bars will be ground. Linear blocks will have preload. Ball screws will have double ball nuts and AC bearings in the end support. I expect the accuracy of the ball screw to be the limiting factor, so .003 per ft. I expect that repeatability will be excellent. I will be using a KFLOP, so I may do glass slides and feedback later at least for the Z.

Speed (rapids): around 400 ipm (using 1610 (not 1605) ball screws, and low impedance motors).
Acceleration: Well.. A pair of 960 oz motors put out about 1300 LBS force at lower speeds... I'm not sure exactly how much gantry weight matters.

Materials:
Base: Front horizontal beam 3" x 5" x 3/8" steel, concrete filled, the rest 3" x 3" x 1/4" steel (750 LBS) + concrete. I'm thinking of maybe filling everything in the base with concrete. There are some recipes that include some aluminum powder to make the concrete expand slightly.

Movable Bed: 36x48x3" consisting of steel chanel around the edges, a rebar grid, 1" square stock in a grid pattern (drilled and tapped for hold downs), concrete, epoxy coating. 300 lbs. I haven't seen a bed exactly like this. It may be a crazy idea. This will probably be the first thing I make after I pour an epoxy pad.

Gantry (Y): Two 3" x 5" x .25" x 48" steel beams (5" is horizontal), 4" tall spacers welded between, sheet metal skin on back. 150 lbs. I am considering going up to 3/8" wall. I'm also considering pouring 2" wide concrete or EG bulkheads in the gantry beams.

Z back plate: 1/2" ground cast iron or steel plate. Girders up the edges to control flex.

Z front plate: mostly 1/2" cast iron. Box section. Z total weight (back,front,spindle,motor) 45+ lbs.

Linear Components:
CPC AR20 profile linear rails from Anaheim Automation.
CPC-AR20MNBZV1P blocks, light (middle grade) preload, "Precision" (middle) grade.
1610 Ball screws with double nuts from Linearmotion2008 (2 on the Y).
BK12/BF12 end supports (will replace bearings with AC).

Spindle:
Ah.. the spindle... That's an issue. I would like one that can do 800-15k with a couple of pulley ratios.
At minimum I will get an x2 R8 mini mill spindle from Littlemachineshop, and upgrade the bearings for 10k.

I'm also thinking of something like Mactec54's ER16 (But ER20). The current design shows this type.

Or dwalsh62's ATC spindles http://www.cnczone.com/forums/produc..._cartidge.html

Spindle Motor (so far) would be a 2HP treadmill motor
Motor controller: KBMM125

This is interesting but risky:
2 2KW CNC Router Metal Work Spindle Motor and Inverter | eBay

Tormach spindles are also a possibility. A full Tormach head seems kind of heavy at 120 lbs.

I'm also thinking of bolting on a router for cutting wood if the spindle isn't fast enough.

Coolant:
I'm thinking of using mist coolant when cutting metal. I'm not sure how to control flood on this machine.

Dust control:
Harbor Freight 2 HP

Noise control:
I'm thinking of building a wall with a door around the machine, air compressor, and dust control.

Electronics:
G201x Drivers from CNC Router Parts
Breakout board
960 oz/in, 2 ohm impedance NEMA 34 motors from CNC Router Parts. I will look into a kit from CNCRP when I get that far.
KFLOP
I have an old Dell computer

Software:
Alibre Design (have it)
Possibly Rhino
Meshcam
Kmotion. Don't know if I can get by without MACH3.
FsWizard
Free software for Vcarving

Tools:
I have a Chop saw, drill press, stick welder, metal lathe, small granite surface plate, height gage, dial indicator, test indicator, digital calipers, Master machinist level .0002 per 10". I will purchase a granite triangle for aligning the machine. I plan on getting an engine hoist. I will pour a 6' square pad of self-leveling epoxy to do layout on, build the bed on, and to put the machine on.

Work distribution:
I plan to have the gantry spacers and side plates water jet cut. I expect to have the Z back and front plates cut, ground, and drilled by someone.
It would be nice if I can get the tubing cut accurately.
I will probably make the smaller metal parts.
My brother and I will do the welding, aligning, drilling, tapping.

Budget: $5-6k US

Time frame: I will start after I get some feedback. I expect this to take up to year to build, mainly for financial reasons.

-Steve