Hi everyone!
This is my first post, but I've been lurking the forum every now and then for a while.
I am playing around with the thought of building a VMC from scratch. I have all the tools and machinery available; CNC-mills and lathes, welding tables, materials etc. I'm employed as a CNC-operator (apprentice at the moment).
[Have you got metal-casting equipment set up? All the VMCs I know of are based on heavy iron castings, not weldments.]
Before anyone ask why I would like to build my own if I already have everything available, the answer is simply I would like to build one to fit besides my computer desk at home. I'm not aiming for a small 200x300x100mm CNC mill - if I'm doing this I'm going all in. I am not looking to carve things in wood or plastics. I want a VMC being strong enough to cut stainless and other hard steels. I'm probably going to work a lot with aluminium - but I know I will regret not making it strong enough for harder metals.
Before anyone points it out, I would like to say that I have limited knowledge about the need for torque performing these tasks - which is why I am opening a thread. My hope is that some of you wise souls in here can shed some light on some aspects for me. Pardon me if I seem a bit to ambitious, and if it seems like I don't know what I'm doing. I'm still learning CNC, and I'm a talented CAD-designer. Everything is still conceptual at the moment, so I don't have any CAD drawings yet. Right now I'm just collecting information!
[Torque is important, particularly low-speed torque for the spindle, but the main issue is rigidity. Most of us here are building routers of one sort or another, which at the top end can handle aluminum. But something that can cut stainless and other hard metals effectively is another thing entirely,]
I'm picturing a budget of around 10 - 15 000 USD, with investments made over time. I am not rich, but I make an okay living.
I'm thinking I would weld the frame in 100x100x5mm squared steel tubing, with crossovers for stability.
To increase the work-area, I want to get the spindle doing the moving, while the table stands still. I know most CNC-mills have the spindle moving only in Z-axis to increase spindle stability - but I have some ideas.
[That's less rigid than a fixed-spindle moving-table design. Mills are all done the other way for a reason.]
I am thinking of using 4 HGW65 linear rails for each axis to increase stability - combined with some heavy duty ball-screws with double nuts to keep backlash to an absolute minimum. If I make it so that I can adjust the pressure on the rails, I can adjust it until it's dead-tight.
[I'm afraid you'll find that keeping all those rails absolutely aligned with one another is a total nightmare. If they're just a little off in any direction the whole thing will bind up. There's no way those welded tubes are going to be flat, rigid or stable enough for all those rails to mount orthogonally. And doubling up the nuts on a ball screw and loading them together makes it harder for the motors to turn them. "Dead tight" might as well just be dead. Less resistance to motion is the main reason you use ball screws in the first place. It's better to use a single nut and oversize balls.]
Regarding the axis control I started thinking about steppers, but after what I've understood these are inferior to servos. I've been looking around at some servos at eBay that I suspect should do the trick. With the little knowledge I have I see that it's capable of 15NM, which seems stronger than most servos for CNC online. I've understood that most DIY'ers build routers for wood and plastics, so I guess I need something stronger than usual.
Would this do the trick for something like this?
https://www.ebay.com/itm/3-8KW-NEMA5...EAAOSwWORbmI2s
[Servos would probably be better than steppers for a machine like you describe, although I don't know anything about those particular ones. You're in Australia?]
Eventually, would the use of matched speed reducers be sufficient to gain more power? What ratio would you suggest=
[Servos run a lot faster than steppers, so speed reducers are a good idea to get into their sweet spot. But you'd need to find ones that don't introduce backlash.]
Regarding the spindle, I've contacted a chinese spindle manufacturer, and I think they have what I'm looking for. They can also give me CAD-drawings of the spindle so that I can design the VMC to fit it before I order. I have the following demands for the spindle; water cooled, strong enough to cut steel, high quality bearings (preferably ceramic), automatic tool change, and ready to accept BT-30 tool holders. They've sent me a suggestion to a 7.5 Kw spindle that they say should do the trick. Picture is attached! Thoughts on this?
[I doubt a 36k rpm spindle will have much torque at all at the speeds you'd use for cutting steel, unless you're using very small cutters. It would be OK for wood, though.]
By all means guys; if you think I'm a way to ambitious dreamer with his head up his a** - I expect to hear it on a public forum as large as this. I've been using forums for too long to expect anything else. But please, any tips will be taken to consideration with gratitude.
Thanks for reading!