Need Help!- Opinions/suggestions for moving gantry & table build?

[metalworkz]

Hello,
I am just starting the design of a DIY CNC router/mill and have some design considerations I would like to get your expert opinions about.
I have limited space in my garage and would like a machine with a 4'x4' minimum machining capacity. I was thinking I could make a moving worktable machine with just over 48" capacity in the X(table moves min. 24" each way) and say 50" long in the Y and have the minimum travel capacity and also have a smaller footprint(about 5'x3') so storage when not in use would take up less room. I did some searches here but did not see any builds utilizing both a moving gantry and moving table so I wondered what the pros and cons to a design like this might have? I realize the build might be easier with just the table or the gantry moving but that requires a larger footprint and the main reason for this thought is that it will take less storage space. Also the extra cost for ballscrews and slides probably might be a factor. I appreciate any and all input or advice on this idea to help me decide if it is OK to proceed or perhaps I need to do something else.
I also want to include a 5th axis B/C head on this machine so if that will make the idea less feasible it may be good to know that up front.
Thanks for taking the time to help me with this!

[ger21]

So, if you do a G1 X1, does the gantry move 0.5" one way and the table 0.5" the other way?

[metalworkz]

Originally Posted by ger21 So, if you do a G1 X1, does the gantry move 0.5" one way and the table 0.5" the other way?

Hi Gerry,
That is what I thought might be possible, but now that you point it out I wonder how feasible it will be for programing, and if it will only complicate things?
Maybe I could devise a more simpler way to reposition the table and then restart the toolpath after repositioning? I could possibly move the table manually to specific locations and lock and this way avoid the need for motor and ballscrew for the table?

[ger21]

Just slave them together and set the steps/unit to double the correct amount. Programming would be the same.


Edit: Not sure if that messes up the feedrate. I'll have to think a little more.

[jsheerin]

The feedrate and acceleration should be okay if the steps/unit was set correctly (compared to the feed rate of the non-compound axes or whatever you want to call it). Mach would put out half the number of step pulses per unit time that would normally be required for a given velocity but because of the compound action this would be the correct movement per unit of time.

I'm not sure this approach would yield a much smaller machine though... If you have a 48" table that moves 12" each way with a 24" bearing spacing and a gantry that moves 12" each way, your foot print is 48" when not in use. If you had a moving gantry, you could typically get away with about 60". A moving table would need 72". So while your idea would be the smallest, I don't think you'd be getting down to 36" unless you are wanting a smaller table that can move longer distances. If you are planning a 5 axis machine, this could be exactly what you want so you can move the head around the part with a long tool in the spindle.

[metalworkz]

I've looked at this idea a bit more and it appears that if I try to get the larger travel from 2 smaller opposing axis there will be an area of material unsupported at each end unless I can figure a way for there to be some points of support at the ends of the X axis. I was thinking maybe something like some bearings on vertical posts set at the height of the top of the table or maybe a roller support. The actual cutting may be supported so this might not be too difficult to do?

[jsheerin]

Just make your table the full size of your material to be cut and strong enough to support cutting forces with acceptable deflection with whatever overhang of the linear bearings you pick.

[metalworkz]

Hello,
I'm thinking of moving forward with a build around this size(36" x 48") but have not decided if I should try and implement a moving table and gantry as we discussed here. I think if I plan the base carefully with optional features in mind I should be able to get started and perhaps decide a bit later whether to include the compound motion X axis.
I have read many threads pertaining to epoxy/concrete type machine bases and thought I might be able to use say a 2" thick polymer concrete inner base with an outter shell of wood or aluminum to basically provide a barrier/shield and a sacrificial work table top for the base. I wanted to get your input as to if this sounds like something that will add to the machines vibration absorbing qualities and if my design is missing something or needs changes. I realize I need to add some vertical bolting scheme for mounting a sacrificial plate or aluminum shell etc. but wanted to post what I have so far to get some input from you guys.

[metalworkz]

Hello again,
I have been doing some sketches for the gantry design lately and I had another 'different' idea for a method to drive the X axis. I was thinking why not turn things upside down and put the ballscrew in the center of the working area in the Y direction on top of the framework? As long as the Z travel is limited so there is no interference with the ballscrew above it this might work. The slides for the X will be fully supported without the need for a cross member running below the worktable. It may cause some other problems I am not thinking of but I figured you guys might foresee some other things to take into consideration before moving forward with this idea. Any comments or advice?

[jsheerin]

You probably could do that. Disadvantages would be that it would probably be pretty far away from the rails and the cutter, so there would be a large moment arm there. The screw would potentially be exposed to more dust, so you might need to give more thought to shielding it. The screw would block a dust collector hose from dropping from the ceiling to a dust shoe. You could potentially route one so it didn't interfere, but it would be more difficult than usual. Last, the screw would probably be in the way every time you tried to put material on the table. I wouldn't do it for that reason.

[metalworkz]

Thanks jsheerin,
Those are some very good points and perhaps I will not go this route for the X axis ballscrew. I wanted to find a better alternative for the screw beneath the worktable but not when it involves other disadvantages. Thanks for taking the time to post the reply! Maybe a rack and pinion or roll pin gear drive would be better with each side driven? I will have to give it some more thought.

[jsheerin]

I would go with the R&P on each side. Then you can square your gantry when you home and get the higher speed of R&P. I wish I had that on my router - eventually I will.

John