[dinkyguitar]

Hi All,

I'm planning to build a similar gantry design using 3/4" thick 6061 aluminum and 20mm supported round linear rails. The top piece is 34.5in long and 3in wide. The supported rails will be 1.5in short on each side.



I'm wondering whether or not the top will bend any with the weight of the gantry. I'm estimating about 70lbs. for the z axis.

Thanks,
dinkyguitar

[neilw20]

Plug some numbers in here.
Structural Beam Bending Deflection Stress Equations / Calculation Supported on Both Ends Single Load at Center - Engineers Edge
The vertical need some diagonal bracing too.
Everything bends. It is just a matter of how much.

[fizzissist]

Perhaps you should be equally concerned about rigidity, not as it relates to bending, but vibration...

[louieatienza]

I'd be more concerned about the narrow spacing of the bearing blocks on the uprights, acting as a big lever against those blocks, which are already mounted in the second strongest position.

As I understand you'll be using this to cut hardwoods for guitars, and you'll want all the ridgidity you can get to get the best cuts.

if you're using Glacern rails, you could probably get 6060 or 60120 extrusion, and you should be able to bolt those rails onto the extrusion directly using the t-slots. I guess the design here relies somewhat on the linear rails itself for ridgidity, and I'm not sure that's the best way to go at it.

[dmalicky]

That design doesn't make very efficient use of material -- it's stiff in some directions, but flexible in others, and pretty heavy. A heavy base is good, but aluminum is an expensive way to add weight. 80/20 would give much more stiffness for the same $ since it's a box section.

The main gantry cross-member should be 1 big box-section, and well-connected to the uprights. For example, if using two 3/4” x 3" cross-members, the total cross-sectional area is 4.5 in^2. A 6 x 6 x 3/16" square tube would have a bit less cross-sectional area and mass, but a bazillion times more stiffness in bending and torsion.

The tall and thin gantry uprights lack lateral stiffness (parallelogram mode) -- C-channel, Box, or Angle would all be better than flat plate. Or add a diagonal brace, as Neil mentioned. Or make them a lot shorter and a little thicker.

As Louie also mentioned, the X-axis bearings need to be spaced apart more.

[dinkyguitar]

Thanks all,

That picture is from a member on this forum, and although I was thinking of a similar gantry design, I was hoping the design wouldn't be criticized since my intent was to find a picture when I didn't have one.

But thanks again for all your suggestions and thoughts.
dinkyguitar