Hello!

I would love to get feedback on this design that I'm working on. I'd like to order parts as soon as possible, but figured it would be a good idea to get community feedback first in order to avoid any unforeseen problems.

General specs

Footprint: 4’ x 4’
Y Travel: ~30"
X Travel: ~32"
Z clearance: 6” (Vacuum chuck/spoiler board will take ~2" of that)
Transmission: 1610 ball screws
Guides: 25mm linear rail
Motors: Nema 23 w/ DM556 drivers (already have on hand, plan to upgrade later)
Electronics: Arduino Uno w/ GRBL (already have on hand)
Spindle: 2.2 kw generic Chinese spindle (already have on hand, would like to get a low speed spindle later)


Design principles
- Identify and strengthen the weakest link (don’t over-invest in areas that are not the biggest problem).
- Where possible, achieve rigidity by widening cross sections rather than thickening skins (doubling skin thickness x2’s rigidity, but doubling cross section x4’s rigidity).
- When making design choices, consider cost, tools at my disposal, and ability to assemble accurately.


Materials of construction

Composite skins: steel
Joining plates: aluminum
Table core: extruded polystyrene foam
Core for Y-beams and X-beam: polyurethane foam

I plan to use epoxy to level the table, X-beam face, and Y-beam sides. I haven’t put much thought into what to do about the meniscus yet. I’ll either use a regular high-viscosity laminating resin from US Composites or a specialized leveling epoxy from Precision Epoxies. I’ve put off researching this since I have been focusing more on other parts of the design.


Table

The table is a 4’x4’x2’ polystyrene foam core with 1/8” steel on stop and bottom, and the lightest gauge steel I can find for the sides. I can’t make the table thicker than 2 ft or else I won't be able to get it out of my basement. I would like to make a vacuum chuck for this, and I’m not sure if I will have pipes come up through the table or if the vacuum ducts will come in from the side.

Reading this manual by Hexcell (https://www.hexcel.com/user_area/con...Technology.pdf) made a few things clear: high performance cores are only necessary in thin panels where the core will be subjected to higher shear stress or if the core will be subjected to point loading. Affordable polystyrene should work fine as a core for the table, since the shear stress will be spread out over a large volume. Local compressive failure won’t be a problem because the workpiece, spoiler board, vacuum table, and steel skin will all help spread out the load. The steel on the sides will be thin and only serve to protect the foam from impact damage. There will also be edge and corner protectors.


Y-Beams

My goal with the Y beams is to cut down on the length of the X-beam arms in order to reduce the moving mass and increase the rigidity of the machine. I went with a triangular shape because I think its main weakness will be bending in the X direction. The triangular tube will be made out of 1/4” plate and will be filled with polyurethane foam.


X-Beam

This is my thinking: The semi-circular X-beam handles torque better than a square tube, and the flat front face allows me to mount the Z-actuator assembly. One great thing about this gantry is that instead of resonating at one low frequency, like a cylinder or square tube, it will be broken up into several higher pitched nodes. Like the Y-beams, it will be made from 1/4” steel plate and filled with polyurethane, which will not only help with damping, but also significantly raise the natural frequency (not so much by adding mass, but by raising the rigidity of the flat faces).


Ball Screws

I haven't completely worked out the mounting yet, so please help me out. If you have better ideas or suggestions for the mounting plates, I would love to hear them. Note: I have to use two screws on the X-axis, otherwise the Z-assembly will be susceptible to torque.