Hi,
I'm interested in getting in to hobby CNC milling with the end goal of producing a CNC able to make small parts from aluminium. I've started this thread to get an idea of what sort of things I ought to do along the way.
My intention is to build something small and reasonably portable for cutting auminium parts. I'd hop to achieve a precision comparable with or slightly better than that achieved with typical consumer 3d printers, I am not after extraordinarily fine tolerances for ultra-precise parts. I'm interested in being able to mill aluminium simply because it can make parts so much stringer than 3d printed plastic, things like gears, shaft couplers, small mounting brackets and plates...
I have a lot of 3d printing experience but CNC is something I've mostly just seen discussed online. A lot of what I've seen online is geared towards big routers for wood cutting, or very heavy knee mill type machines cutting steel parts. I'm particularly interested in making a personal project of building my CNC machine, more satisfying than buying one, and also far more able to tailor it to my use cases.
I thought I'd start with a few questions, focused mostly on the mechanics, I've some thoughts about CAM software which I'm working through at present, better to work out a few more things there before making that the topic of discussion.
I've a list of questions below of things which have occured to me so fr:
1. Spindle head. I can see these available online as large chinese made motors with fitted collets and often coming with a drive electronics box to convert mains power to high voltage DC and give speed control. But I've also seen a lot of CNC machines built using something like a dremel or corded router tool. Which is the better option? Some of the videos I've seen seem to sugest that even for milling metal the power tool router type can be better as the bearings are lower down and more able to handle side loads. My use case is milling Aluminium.
2. Holding the workpiece. How is this usually done. Clamping a block of metal would seem fine for most of a part, but at the very end you've got a situation where the CNC is cutting away the very last pieces of metal which hold your machined part to the bigger block, at which point I would guess the part would come loose, collide with the milling bit in all manner of funny ways and wreck everything. I've seen examples of wood cuting CNCs using a spoilboard with self-drilling screws going up from beneath to anchor in to the part, so the part is held from below. This ofcourse leaves screw holes in the bottom of your part and isn't so practical for screw-driving in to aluminium.
3. Liquid profing for coolant? Is coolant essential from the very most basic CNC design upwards? Does that mean everything below the working area of my CNC needs water(or other coolant liquid)proofing? Does one effectively need to have a small tank structure around the bed to contain coolant and stop it flowing on to stepper motors lower down. The same goes for catching chips from the milling and ensuring they don't end up in the leadscrews/ballscrews?
4. General machine geometry, for cutting small 2.5D and 3D (no undercuts) parts out of aluminium what is the best form to use? 2 axis "bed slinging" with the spindle moving on z only? Spindle moving on x, y and z(assumedly a bad idea as stacking so many moving axes after one-another ruins stiffness and puts much bigger weight loads on the first one to carry)? Gantry router(surely for big workpiece requirements only?)? Workpiece moving on x or y, spindle moving on z and the other of the horizontal axes?... I'd be expecting to construct the CNC from aluminium extrusions, 3d printed brackets, and I can probably order laser cut metal plates from an online service too. All construction would be with machine screws, no welding. Depending on advice I could open some future threads in the T-slots forum about this.
5. Milling bits and behaviours. Is it the case one needs very different types of motion up milling and down milling? Dotoolpaths all have to account for whether the cut is up milling or down milling at any given point? As well as many bits being unable to do plunge cutting? This must really complicate toolpath planning if you have to swap bits every time one needs to start cutting a new pocket? Or does this only apply to larger bits, I'd be expecting to probably work mostly with 1mm and 2mm.
6. Electronics control board. I understand that milling is a much more user interactive process than 3d printing. A printer you just load a file from an SD card and it spends hours moving around to print it, but I understand a mill needs a lot more manual processes at points during operation. With a printer you can just run the same gcode again for a copy of the part, whereas I understand with milling you're often looking at very different gcode for the same part if milling it from a different sized initial starting block for example. How does this affect the choice of control board? My understanding is that the main open-source options are GRBL running on a microcontroller versus LinuxCNC which needs a full computer dedicated to it(single board computer like a Rasp Pi?), what fully are the advantages and disadvantages to each.
Thank you
P.S. Forgot to mention, when I say small I'm imagining I'd never do any single part > 10cm x 10cm x 5cm (z), although if it feasible i'd be interested in getting a working volume of 15cm x 15cm x 15cm (might open opportunities for making parts I'd never previously considered). Most of the parts are probably going to be sized within 5cm x 5cm x 3cm (z). I'd be after a precision somewhere similar to or slightly better than typical budget 3d printers, somewhere around 0.05 to 0.1 mm, tighter if it is easy to do so.