I am working out the details on building a sphere cutting cnc machine. I have worked out a lot of the mechanical challenges and am working on the “Drive Mechanics” The link below shows the prototype I built as a proof of concept. It works, now on to the all metal build. Problems to work out. Inexpensively. The turn table will have at times a maximum load of around 1,000 lbs give or take. And the Gantry arm that swings up and down will be very heavy, with a counter balance, but it is still a lot of mass to start and stop moving. I’m thinking of making a large “sprocket “ made from alum. For the teeth I would us the GT3 belt glued or attached by whatever means are necessary, with the ribs facing out. I would make a drive belt from the same material. I am wondering if the belt can be joined and remain flexible by bonding a second section of belt to the back side instead of using the belt clamps. The joint may need to go around the drive pully. The gantry arm would have two of these assemblies and the turntable one. I know it might not be fast but it needs to be accurate for carving details into projects. The finished project can carve sphere’s, bowls and plates, I may even figure out how to carve 3D items. Open to all opinions and ideas. Below is a link to the prototype in action along with a description. Patrick Parsons | Facebook
Hi I would drive the gantry arm with some sort of worm drive at the largest diameter possible. Are you planning to make the arm out of steel? This will be heavy, but maybe C-beam type extrusion can do it instead, for a much lower weight? For the turntable, how about a wheel/axle assembly from a car? The stub axle from the rear of any front wheel drive car should do just fine. Again I would look at a worm or some other 'large reduction' on that drive though a belt with large ratio can do it, of course. You will need a lot of angular resolution to achieve carving at that diameter. I did do the calculations in one of my other posts, basically as the diameter increases it moves the circumference further for every step of the drive motor, so for large diameters you need a large reduction to get fine control over the circumferential movement.
Hay David, Yes I will probably go with and automotive wheel assembly for the turntable, it's strong and fairly easy to get. I had thought about a worm gear, with a LARGE dia on the gear, I just will have to research how to make one, I might be able to cut one out on my cnc router. It would have great holding power, I'll look more into that. I have been thinking that a large dia.gear ( around 30" OD) driven by a small drive gear ( standard R&P setups for cnc routers) would give acceptable control over the circumferential movement( I had a good idea what this was, but googled it just to be sure!) I plan to make the machine out of mostly 8020 extrusions, maybe with a mix of V-Slot or OpenRail thrown in for good measure. While the machine can put out large pieces I also want it to be able to do small items like 2" dia balls. The cutting head will need to be able to move inbound past the center pivot point , then it can cut dish and bowl shaped items, hints a movable or re-configurable gantry/z-axis setup. Thank for the input and keep the ideas coming!
You might consider a bicycle chain and sprocket type setup on the armature and then have the smaller sprocket driven by a worm drive reduction gear. Cutting a large diameter sprocket for such a system should not be a big deal. If you can't do it yourself, it could easily be water jetted or plasma cut.
