Planning new machine

[creekrat82]

I’ve been lurking for a while and pretty sure I got the boss lady talked into a cnc router. Eventually I would love a 4x8 work area but figure I’d start smaller first and if I feel the need to go larger at a later date I can build a bigger beast. She wants to be able to do small signs and wood projects for craft shows and such in and a round our small town and I want it to be able to handle luthier work so I am thinking along the lines of a 2’x3’x7” cutting area. I will be using profiled rails and ball screws. Most likely will build a base grime 3x3 tubing with jack bolts for leveling on the feet. For drive motors I’m leaning towards Nema 34 with 2 of them on the x(long axis).

I’m looking for input and ideas for my gantry plates. I’m not 100% but hoping I’ll have access to at minimum a few Bridgeport knee mills with dro and possibly a cnc mill or 2. I want it built like a tank to avoid vibration and issues with harmonics so I will fill the base with sand and have my gantry built with most likely 1/2” plates and 1/4” skins, both from aluminum. Any help is greatly appreciated

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[creekrat82]

I'm going to try and work on some drawings this weekend but pretty sure i have a plan. For gantry plates I am thinking of 0.500" 6061, vertical of course, that will bolt with socket cap bolts to a piece of 0.750" 6061 that will ride on the bearing blocks. I would mill a press fit groove in the 0.750" plate that the gantry plates would index in and the bolts would be inserted from underneath. I can then machine a groove in the bottom face of the 0.750" plate and bolt a piece of 0.500" to it that has a bore and screw holes for the ballscrew into it. For the y axis, axis across the gantry, i would have a piece of 0.500" plate across the back of the gantry the the ballscrew supports would bolt to and use 2 pieces of the same material on the face of the gantry by the spindle which would leave a small gap for a dust shield.

For the grooves in the 0.750" plate, do you think that a depth of 0.250" would be sufficient so long as the edge of the surface being bolted to it was milled so that it was perpendicular to the uprights? I know this design will leave me with a fairly heavy gantry but that should also inhibit resonance.

[wizard]

Im not sure im following your design. Your uprights and gantry shouldn't be made from simple plates. You will be miles ahead using box sections especially for the gantry.

I wouldnt get too excited about gantry weight. Yes it is possible to be too heavy but you are only talking a small span if either 2 or 3 feet depending upon which way you go. It would be pretty hard to go too heavy especially if you want rock solid.

If you do get access to the machine shop equipment realize that you are light years ahead of what many have when building. Given that huge advantage id think seriously about welded steel structures. For example the gantry supports can be made with steel tubing with welded on flanges. These would be machined as match sets.

[awerby]

If you're prioritizing rigidity, then consider a fixed bridge instead of a gantry. A bridge can be as heavy as you can build it - more mass is better. It won't be depending on gantry plates, which detract from rigidity - more so the higher they are. The moving table will take up more room, though.

[creekrat82]

Space is at a premium so I think a moving table will be out of the question. I don’t need aerospace precision since I’d predominantly be using it for wood crafts for the mrs to hopefully sell at a premium while I’d be using it for guitar work.

Right now I’m just bouncing ideas around but would there be any adverse effects if I were to have the sides where the long axis rails mounted elevated so that the beam for the shorter horizontal axis simply rested on the bearing blocks?

[fretman_2]

Lots of good examples of builds on this site. If your gantry beam is of torsion box construction It'll be very rigid. Not that my build is a shining example of how to do things, it'll show a torsion box. https://www.cnczone.com/forums/diy-c...05340-cnc.html

[ger21]

Right now I’m just bouncing ideas around but would there be any adverse effects if I were to have the sides where the long axis rails mounted elevated so that the beam for the shorter horizontal axis simply rested on the bearing blocks?

That type of design makes it harder to load and unload the machine, but can be more rigid?
What will your gantry beam be made from?

From what you describe, I've seen several similar machines.

[creekrat82]

Ger21, I haven’t really decided yet but if I went this route I’m pretty sure I could get away with 2 pieces of 2x4 extruded aluminum profile bolted together in an L shape

[wizard]

Space is at a premium so I think a moving table will be out of the question. I don’t need aerospace precision since I’d predominantly be using it for wood crafts for the mrs to hopefully sell at a premium while I’d be using it for guitar work.

Sounds like you have interests other than processing sheet goods, if so you need to consider carefully your Z axis. You will need enough clearance and z travel to process a guitar neck (as an example). Depending upon what you want to accomplish that may require a fourth axis. I'd spend a good deal of time thinking about this because too little Z and you can't do some of the musical instrument work you want to do. Likewise wood crafts is a pretty open description.

Right now I’m just bouncing ideas around but would there be any adverse effects if I were to have the sides where the long axis rails mounted elevated so that the beam for the shorter horizontal axis simply rested on the bearing blocks?

There are all sorts issues to consider, how they apply to you depends upon what you want the machine to do. The number one problem in my mind is that mounting the bearings directly to the gantry beam means you will not have them spaced far enough apart. It is poor mechanical design and put excessive loads on the bearings. You would do far better to space them out some, say on a thick plate with a gusset or two.

Lesser problems or features depending upon your point of view is that such designs create a tube withing which the router must operate. The walls of the tube contain the swarf within the tube and the tube limits the travel of the router. If you are spaced constrained you may find that your design will end up being wider that it needs to be to get the same gantry capacity. In the end it really depends upon your intentions for usage. However the tube does make cleaning the table an issue between loads.