For bolted frames then the process is timely and the Dowel pins take some time and effort, but if welded steel frame and you use an adjustable top rail then it's quite easy really. Just careful setup and epoxy putty. Beauty of epoxy putty is you have plenty of time to do this and it's not messy, can also be sanded, shaped, drilled & tapped just like metal when dry so it's good for filling gaps and holes. If in use you find you just quite didn't get the rails 100% on same plane or parallel then it's very easy to shim into plane with fine shims. (Tin foil works a treat)
Damning the top rails with a bridge and flooding with epoxy also works well but takes more time and effort can also be messy, so it's important to have good seals other wise leaks will happen and then shrinkage in places. It's also not so easy to fine tune if you haven't quite it the mark.!!
I have no personal experience of building and using a metal-frame CNC router. However, I do have a "working" MDF machine in my garage, built to a slightly modified version of the open-source JGRO design (Google will give you plenty of references). I have indeed used it to cut some useful items. After 6 months or so, I have decided that I need to replace it with a decent metal-framed machine. Does that tell you something?
I've learnt a lot:
- I have a box of electronics that works well and is sized for a bigger and more powerful machine, complete with steppers also reusable.
- I've had a lot of practice with CAD and toolpath-generation software. Free does not necessarily equal best...
- A 2KW water-cooled spindle provides much more cutting power than the machine structure could possibly cope with. And its weight is more than MDF can cope with, as well. See below.
- A home-made anti-backlash delrin nut assembly works fine. I can't say the same about M10 stainless rod used as a leadscrew. Not sure what the theoretical critical speed is, but in practice it works out at about 800mm/min for the X axis.
- A "rapid" feed that is slower than most people's cutting speed isn't too important when the max cutting speed is (depending on cutter, etc) around 200-300mm/min. At this point, I hear the experts laughing, holding their sides, and saying, "I told you so..."
- You can get an enormous amount of satisfaction out of building something that works. I've cut a presentation plaque in hardwood, engraved some Christmas presents very acceptably, and even sold a few small items. I just had to do it very, very slowly, and with a lot of shimming of spoil boards to get something to approximate a level bed. I even improved the original design by replacing some MDF components with pieces made on my 3D printer (also home-built) but this just shows up deficiencies elsewhere.
- MDF is not the most stable constructional material. Actually, it's not stable at all. Or even suitable as a constructional material. OK, I should have varnished or painted it, but I doubt that it would have made much difference. Just before Christmas the bed had a sag in the middle of around 3mm in 300mm, and with the recent high humidity it's probably worse than that and still going. My son has built a very similar machine and stiffened the bed with a couple of lengths of 1" square steel tube which has helped, but the Z axis plate is now a few degrees off vertical due to MDF sagging under the weight of the spindle.
There are people who have built a JGRO design more or less as per the plans and used it for some fairly serious work. Well, not in my local micro-climate! On the other hand, if I reuse all the steppers, control electronics, spindle motor and VFD, etc, I shall be throwing away less than £100 of materials, mainly MDF and steel pipes used as linear bearings. Even the skate bearings for the three axes altogether probably cost less than a tenner off Ebay. I always had the thought in mind that this would teach me what I, personally, could do with a machine like this and that the first machine would have to be considered disposable.
I am currently putting together an outline design for a slightly larger machine, to be steel framed with decent bearing rails and ballscrews, and this will be presented to the community for comment. In particular, I have some idea of what is possible with a somewhat crude and flexible machine, and I don't want to go the other way and grossly over-build. But if you want a sensible size machine, don't think about wood. Ply might be better than MDF, but for a machine for commercial purposes in any size? Don't go there!
Thanks again for all the ideas and suggestions. I have done *some* reading through the build logs but by no means complete. I have begun writing lists of specs in categories. I've posted them here for any early comments people would like to make:
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