Need help with large format machine design

[metalchips62]

Hello members!

I need some help designing a gantry for a large format CNC machine. Due to shop size limitations and ease of inserting 8’ lengths of aluminum extrusions, this machine is designed sideways to go up against a wall.

It will be similar to this design:

The work envelope will be about 100” x 30” The gantry must be about 10’ long. This machine is designed to mill slots in 8’ aluminum extrusions.
We plan on using 30” ball screws and linear rails to move the gantry front to back (short axis) and R&P to move the spindle along the long axis (10’). Our Z travel will be about 4”

We prefer to work with 8020 extrusions to facilitate construction. We would like to use an upper and lower beam for the gantry to be able to mount an upper and lower linear rail. Our question is this: What size 8020 would you recommend and what sort of reinforcement would you suggest between the upper and lower beams (vertical or angled braces). The space between the upper and lower beams will be between 10"- 12”.

We understand that the more the gantry weighs the more beam deflection there will be. We are looking at a compromise between maximum strength with minimal deflection. Has anyone ever built a machine like this or have any suggestions? Any input would be greatly appreciated.

Thanks,
Brian

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[ger21]

Not sure why the 10" space between the extrusions?
You want the biggest extrusions you can find, which is probably 3"x6"
Just stack them up, and use a solid plate on the back the full length.

[metalchips62]

Thanks for the quick reply!
I was looking for light weight and strength and rigidity. I had seen several machines utilize the "ladder-style" gantry design (upper and lower beam with spaced out upright supports between them. I had assumed that this was a common large format gantry design. I was thinking about something like this:

I had assumed this design would be lighter and rigid.

The use of 2 3" x 6" beams as you suggested seems to make sense from a building standpoint. I had originally investigated that avenue of design, I'm just worried about beam deflection in the middle with all that weight.
Plus the cost issue. 8020 wants $3.00 per inch for a 3" x 6" beam - that's $360.00 for a 120" beam.
2 beams are $720.00 plus the reinforcing plate and the and the freight costs, I'm well over a thousand dollars just for the bare-bones gantry.

Any feedback would GREATLY appreciated!

Best Regards,
Brian

[awerby]

You'd save yourself a lot of grief (not to mention money) if you spanned the width of your table (3') instead of the length (10'). Yes, it might stick out a few inches further from the wall, but the gantry beam would be stiffer and weigh less, all things being equal, and would work better. Are you really that tight for space? If so, have you thought about a router that mounts vertically, like this one: https://buildyourcnc.com/Item/cnc-ma...ean-v1-milling ?

[wizard]

I'd have to agree with ger21 why the huge space between the two beams? That will only resist up an down deflection and wold do little for lateral deflection. Your Z is limited so you will not have huge torques being applied to the beam so that is good. Two extrusions does offer one advantage and that is the channel it provides for the leadscrew. In general though I'm not a big fan of this sort of approach to using T-Slots, if a single beam isn't large enough you need a different approach in my mind.

By the way 8020 will not facilitate construction of a gantry beam built like this. If any thing it will make the job harder. You may find yourself working harder than expected to get the rails parallel and in plane. If you must use aluminum, look for a single rectangular extrusion free of T slots and layout and properly drill and tap the rail mounting holes.

All that being said I really do think considering a steel beam is in your best interests. We are seeing some increases in cost with respect to steel However it is still very cost effective. I'd look for a rectangular tube maybe 10 x 6 depending upon how much deflection you can tolerate. 10 x 6 is just a wild guess by the way you will most certainly need to do some beam deflection calculations.

In any event I think you have cart before horse here. This is apparently a unique machine with a long beam and a very specific use case. As such the beam should really be designed for the requirements you have to get the job done. So you need to develop a set of requirements and desires for these milling operations. For example how much deflection (lateral) can be tolerated at the mid point on the beam. What is your surface quality expectations. The more specific a machines usage, the more mechanical engineer that needs to go into it to make sure it does what is expected of it. From my perspective this is far different than building a general purpose machine. Otherwise you might end up building a machine that doesn't do its only task in an acceptable way.

[TTalma]

If I understand what you are doing you are just milling extrusions. Why the table?

You could just set up a small machine that rolls the part along between 2 pinch rollers. You would probably only need a few inches to a foot of travel along Y, Your X would be the rollers moving the part.
This would be small foot print, (like 18" by 16' and would allow equipment to be placed in front of the roll out area. Probably cheaper and easier to build.

This idea may not work for your product. If you post a picture of the finished part someone may be able to give you an idea you hadn't thought of.

[skrubol]

Given that you're going to go R&P I agree with the others, skip the gap between extrusions, and probably skip the extrusions. The larger the better on the tube to make the gantry. Wall thickness doesn't help a whole lot (adds more weight than stiffness,) but larger outside dimensions are good.