This gantry is part of a 40''x18'' CNC dremel project that is currently in the works. The design of the gantry and its fabrication are in dire need an instructable so the design can evolve.
Please tell me what I could do better in the comments!
There will eventually be an instructable on the entire project (An arduino-based, GRBL-controlled, CNC dremel mill for cutting parts from soft woods and foam) so be sure to check out my page for updates on the entire project.
Some info on the project:
The gantry (and the rest of the CNC) is built almost entirely from .25'' plywood, with the obvious exceptions of the linear motion parts and hardware. For the most part, it is bolted, epoxied, and tab/slotted together because these are the easiest construction methods for assemblies cut with a laser. Most of the structural members are 1/4'' plywood layered together for the correct thickness, because that is the greatest depth the CNC laser I used can cut.
The tool-holder plates and supports for the dremel are not included in this instructable, as I will likely change those as soon as I try my first cut.
I am not going to be cutting anything more than a few inches of foam or 1/2'' soft wooden boards, so the z axis motor is relative small. I used a NEMA 17, but any motor around that size will work if the top and bottom plates are changed accordingly for proper bolt hole spacing.
CAD files are pictured in step 3 but not downloadable at the moment. The finished CNC machine's instructable will have more details.
Scratch building these machines is incredibly rewarding- should you choose to build a similar z axis, hopefully this instructable removes some of the bumps in the process.
Step 1: Specifications/ Design Parameters
For hardware and linear motion parts, see the next step.
The gantry's overall profile is an 8''x2.5''x4'' rectangular box without the protrusion of the NEMA 17 motor (which is ~2''x2''x2'').
Z axis travel is 4'' with an 1/8'' of offset at either end for safety. I'll be using less than a quarter of this movement for the majority of cuts, but it's nice to have the z-axis travel to drill sizable holes in a work piece should I need to. This design is built to be very simple and accomplish basic milling. I don't have the need at the moment for extreme precision- I'll be upgrading if I do.
Linear Motion parts are all 8mm (sc8uu and 200mm rods)
Gantry is driven by a NEMA 17 stepper motor with a 5mm to 8mm coupler for the 8mmx2mm leadscrew (RepRap type)
The most effective use of this gantry would likely be for CNC routers with dremel-sized rotary tools as that's what I intended, however I can see size modifications being made to use this as a pen plotter, PCB mill, or a 3d printer.
On to the materials!
Step 2: Required Materials and Tools
Here's the part where I bypassed the most difficult part of the project, cutting parts accurately, by using a laser cutter. The dimensions are all in Autodesk Inventor, so the parts can easily be made with simple power tools, but it would be a tedious affair, as the parts need to be fairly accurate, roughly within 0.05'' for most dimensions for it to have reasonably smooth travel. It can be done, but the project goes much quicker if with a CNC laser or mill to do the tricky bit for you.
- Hex/Allen keys for bolts
- Crescent wrench or socket set
- .25'' hobby grade plywood (mine is 6 ply because that is the largest depth the laser can cut, use whatever is machinable for your equipment)
- Epoxy (I used 6 minute)
- 4x 3mm x 8mm length screws and washers for NEMA 17 motor installment
- 16x 4mmx19mm length screws for attaching linear bearings
- 4x SC8uu 8mm linear bearings (8 if the same method is used to mount it to y axis)
- 4x 8mmx 200mm chromed shafts
- 1x 5mm to 8mm shaft coupler
- 1x 8mm x 2mm x 200mm length lead screw and nut
- 4 x 2 mm screws/bolts for installing lead nut
Optional: CA glue (superglue) for touching up the assembly
Step 3: Designing the Laser-Cut Parts
All laser- cut parts were done in Autodesk Inventor. I'm familiar with a handful of CAD programs that could have been used for this project, but I used Inventor because of the ease of making .dwg and .pdf files for the laser. The software doesn't even necessarily have to be 3d CAD (2d drafting could work), as the laser takes a simple PDF file anyways. However, I needed to make an assembly to check all of the fittings and dimensions. These files will be up on the full CNC's instructable. I'd like to see the machine make a few cuts before I upload any CAD.
The design intent can still be seen from the pictures however, so I'll leave it at that for now.
Step 4: Assembly
Assembly of the gantry was pretty simple- All I needed was some epoxy and screws. As you can see from the first few pictures, the sides of the box are epoxied to the rear plate, and then epoxied to the top and bottom plates. Like I've stressed before, these joints wouldn't take heavy loads, but I really don't need them to (If anything breaks, you'll see it in the CNC's full instructable) because I'm cutting very light materials.
Before I glued the motor plate to the sides/top of the box, I screwed in the NEMA 17 and put the two 8mm rods and the leadscrew into their positions to ensure a good fit. Everything fit well, so I glued the whole mess together.
The lead nuts were simply bolted to their retaining plates with 3mm bolts. Since there are two of them, I needed to screw the Y axis's leadscrew through one finished side to make sure that the threads were aligned for the other nut. The spacing was perfect, even though admittedly I didn't think through this initially. I'm just glad it worked!
Finally, the plate for the Y axis is bolted/glued to the rear of the gantry box for the Y axis leadscrew to fit through. I realize now that this is probably not a good way to do this as there is zero room for bad tolerances and there is no way to adjust the height or angle of the rear plate if it is off for some reason. I'm not too concerned with replacing parts or mild inefficiency in the leadscrew, but this is a notable area for improvement.
Step 5: Areas of Improvement in Design and Construction
The following are design aspects that I didn't particularly like:
Weakness in assembly- This is due to the fact that I was only able to accurately cut parts of 1/4'' depth on the laser, so I was constrained in build materials. But hey, it gets the job done. If I had access to a proper machine shop this gantry (and probably the whole CNC, for that matter) would be aluminum.
The plates are mostly attached together with epoxy. While not all joints are necessarily stressed directly against the epoxy, a mechanical joint such as a tab and slot or bolts/screw would have been preferable. None of the members take particularly high loads during operation, but if I were using even a small spindle this would need stronger joints. In its current state, it will handle the loads of a handheld rotary tool cutting very light wood and foam.
Size- For the applications I'm using this for, the gantry is quite bulky. There isn't too much I could do about this for the sake of the linear motion parts (linear bearing & lead nut size), but again, it gets the job done. This project was meant to be quick and easy so I could get my machine running.
Tolerances- The tolerances on overall dimensions for this gantry are well within acceptable limits, but I mention this because the four rear-mounted linear bearings required some minor shifting to get smooth motion on the z axis. This is in part due to dirt-cheap linear motion parts. It isn't perfect by any standard, but this is something to note as I'd probably be doing things much differently if I wanted extreme precision.
This is one of the first steps in the whole CNC build, so please follow me if you'd like to see the project through to my first cuts on the machine. Thanks for reading and happy building!