So I am a a college student currently studying mechanical engineering. I'm working on a team with few of my good friends for our senior project and we are building a CNC Foam/wood cutter. The school doesn't have a machine capable of cutting the 4' x 8' x 2" sheets of foam. We are building it for future students to use to cut foam mostly for fairing design and building.
We are all new to building a CNC machine from scratch and we are trying to gather advise and information on building one. We have an initial design but it needs refinement and re-evaluation. Are current machine is going to be designed to have the capabilities to cut wood and foam.
-Our work space we are shooting for is to cut in a 4' x 8' x 12"-18" range. So the actual machine will be about a foot bigger on each axis.
- The table sits on 6 legs that are about a foot and a half high.
- We are using 10 mm pitch 1" diameter ball screws for all three axises. We found motor packages which range from $500 for steppers and $900 for servos. We currently want to use 425 oz-in motors and hope to achieve 300 ipm.
- We have a budget of about 4,000 dollars and may be getting more donated by the school.
These are the basic specs and are subject to change but our major questions are:
- Basic materials, we are split between using steel, Aluminum, or 8020 extrusion. Some of us are hesitant on using 8020 and are wondering would it be feasible with our size of our machine. We want to avoid using it for the table frame due to cost efficiency. Would the gantry be strong enough if made out of pure 8020 or would we run into rigidity and strength problems? or does it need to be reinforced by other materials or frame?
- Frame assembly, since we are cutting foam we decided to keep tolerances to 1/8". I'm a certified welder and our other team member is good with metal fabrication, would welding the frame be a bad idea? If so would other options be to machine holes and bolt them together? We have some concerns of warping if we decide to weld it.
- Ball screws, I've been looking around and noticed it's pretty safe to use ball screws on 4' long sections. My concern is the x-axis would probably require a 9 foot ball screw. We have only one ball screw on the x-axis and it's supported by the gantry and one bearing on each end. I haven't seen a homemade machine use a ball screw bigger than 4' and my concern is whipping. Our machine will be cutting at both ends of the table. Will having a 1" ball screw whip, when the gantry is at the opposite end and trying to feed back to the middle? We are looking at about 7' of ball screw being supported on it's own. So I figure with it's own weight it will have a chance to sag and then start whipping.
These are our main concerns and we are looking for insight/advice or alternatives. We are all new to this and it would be appreciated if anyone knows some advice. If more information is needed I can post more defined specs later tonight, i just don't have them on me at the moment.