541,768 active members*
3,605 visitors online*
IndustryArena Forum > Mechanical Engineering > Mechanical Calculations/Engineering Design > Semi-Cylinder Gantry for Fiberglass CNC Build

Thread: Semi-Cylinder Gantry for Fiberglass CNC Build

1. Semi-Cylinder Gantry for Fiberglass CNC Build

Hello,

I am in the process of designing a CNC router with a 4’x4’ cutting area. It will be built mostly with fiberglass cloth, epoxy, and foam.

The design question I have is in regards to the gantry. The geometry that I have chosen for the gantry is a hollow semi-cylinder (which will be fabricated with biaxial fiberglass at +\- 45 degree angles over a foam core).

This is my reasoning:

1. The weakest link in a router is often the gantry.

2. The weakest link in the gantry is often torsional strain.

3. A circle has a much better polar moment of inertia than a rectangle. However, using a whole cylinder makes it difficult to mount the linear rails to, and offsets the spindle from the gantry’s axis of rotation, giving the spindle extra leverage.

4. A square tube has a worse polar moment of inertia given the same perimeter as a semi-circular tube and stresses the skin unequally, putting a higher proportion of stress on the corners of the tube.

5. A semi cylinder gives me a flat surface on the front to mount the rails/ball screws to, and gives me better resistance to torque than using a rectangle tube.

For now I am only soliciting advice for this one problem, I.e., the merits of a semi-cylinder versus a rectangular tube. Discussing other aspects of the build would require many cans of worms to be opened. I will post a full build thread once I am ready to commence the build in order to get feedback on the overall design.

Am I missing something?

Thanks!

2. Re: Semi-Cylinder Gantry for Fiberglass CNC Build

Update: I've decided to go with steel instead of fiberglass, but I'm still really intrigued by using a semi cylinder profile as apposed to hollow rectangle. Is there a drawback here besides difficulty of fabrication? Thanks.

3. Re: Semi-Cylinder Gantry for Fiberglass CNC Build

Hi Paul - There are some commercial machines that use semicircular gantries. The most troublesome issue to achieve good machine stiffness is a good Z axis. These are usually underdone by DIY builders. In terms of gantry torsional stiffness its more the issue that the loading point is far away from the shear centre of the gantry vs its shape. If you use a square gantry for instance and have one rail at the bottom front and one rail at the top back this puts the action line of the bearing reactions closer to the shear centre of the beam and it behaves stiffer as it is more efficient. If you place a force at the shear centre of a beam it does not twist it only bends... This will be the same issue with a semicircular beam, getting the forces to pass thru the shear centre which they can't do with any flat front gantry with bearings on it.... Many gantries are also triangular and I think this is the way to go. Look up the Milli thread it covers a lot of territory with the gantry...Peter

4. Re: Semi-Cylinder Gantry for Fiberglass CNC Build

Hi Paul - You have to consider maximising every parts geometry (dimensions) to maximise its stiffness. The semicircular shape has 40% less torsional inertia then the RHS in its envelope. Then there is the issue of getting the applied torsion to act at the shear centre. This can be done by using a "flat" rectangle rather then a tall RHS see attached. The shear centre issue is a secondary issue but worth dealing with if you can. A gantry is under shear, bending and torsion. Each of these have primary deflection and secondary deflection effects that can be minimised if you understand the engineering mechanics... Peter