Add-on CNC build log

[bikepete]

I hereby nominate this as the 'longest-ever-dead build log finally revived'... evening all!

Life intervened in a big way along with pretty much zero spare cash or time available for CNC. Now getting together a little of both I hope! Have been looking in occasionally and lurking but not letting myself get really back into it until other things were a bit more sorted...

Did manage to get the control box finished and tested - see attachment - went a bit OTT with blinkenlights and meters but I like it :-). Don't like sticking out cables so put on the alu channel for the XLR sockets. The other bit of channel on the right hand side is where the mains comes in from the back (and there are also two mains outlets for spindle power etc which are also controlled by the main isolator switch). Did an OTT e-stop scheme with time delay relays, links out to e-stop a future spindle VFD, etc etc. Will do a proper write up at some point...



Anyway, the plan now is for a self-contained milling machine not an add-on as originally planned. I seem to have gone a bit more OTT with the frame than I'd anticipated too - managed to get a massive 180x180x12.5 steel beam off ebay from a friendly farmer for £50, and he torch cut it into bits for me. It weighs approx 64 kg per metre so handling it is an issue. The short bits at 750ish long are not too bad but the 1150mm column is tricky. Cleaning up the ends of the short bits is about at the limit of my milling machine... not yet sure how to handle the long bit. Will find a way...



Main reason for this posting is to help me decide between two frame designs. Ruled out welding as with such heavy section would have to farm it out and also don't want to deal with the distortion/creep issues.



Quick and dirty mock-ups in Alibre. The PDFs attached should let you rotate/zoom etc.

The first is my original plan: two lengths support the X-Y table base, with the column passing down through both, braced with 75x50x8 unequal steel angle top and bottom, all bolted together with masses of M12 bolts (super cheap at Screwfix). Lots of holes to drill (may get a mag drill for this) but should be seriously rigid. Don't need to accurately machine any faces particularly (a potential problem as I have short travels on the milling machine, plus the difficulty of mounting the heavy column). Offers good support for the X-Y table base.

The second design I recently thought of and am rather tempted by. Column butts onto a single horizontal beam and would be bolted to it into tapped holes from underneath - then the side pieces are bolted on too with many bolts. I like the simplicity of this and it saves buying the angle steel. Not as many holes to drill. Easier to mount onto a bench or stand. Machining the (shorter for this design) column end and its seat flat should be just about do-able I think. Not sure it's as rigid though, especially for front-to-back loads on the column, and the X-Y base overhangs rather. The long diagonal cuts for the side pieces should be do-able with some repositioning, with the mill in horizontal mode.

On both the X-Y base plate is actually made up of three thicknesses of tooling plate which I bought cheap from the chap who recently advertised on here (Sutton Coldfield) after seeing them on Ebay. Can't really afford the equivalent thickness in new Ecocast at the mo (ground flat steel would be even better!). Wanted to use tooling plate to give an accurate base for the rails - max travel on my mill is 250mm so would need to reposition for 400mm rails. Plan is to 'laminate' the three thicknesses with epoxy, then also have this sit on a bed of epoxy to provide a level surface on the tops of the steel beams. Might be good for damping as well as avoiding the need to buy new materials :-)

Talking of damping, plan is to fill (partly if not fully) the main beams with sand to damp vibrations. May use plastic guttering down the middle of the beams to reduce the volume required. Will also have to make a very solid stand (weight could be approaching half a ton eventually!), but waiting to find some cheap steel for this.

Alignment (tram, nod) will all be taken care of in the Z axis, details to follow. Spindle also still to be decided - will likely make it interchangeable between three different ones (stepper driven for tapping, plus brushless DC type for engraving, plus VFD/three phase motor for general purpose). Keen to avoid the weight of a big motor being driven up and down with the Z, have some ideas for that...

So - anyone got any thoughts on the two frame designs? Main objective is to maximise rigidity - may as well make the most rigid machine possible with the materials I have. One issue is that the surfaces of the steel beams aren't terribly accurate or clean - the beam has been bashed around a bit in its previous life - so bolting sections together will not necessarily result in very even contact, though I'm pretty sure it'll be sufficient with multiple M12 bolts torqued up tight. Epoxy between the surfaces might be an idea if necessary, though I'd rather try without it first simply to avoid the mess involved...