a few days ago, I posted the very first pictures of my mpcnc.
Now I use it already pretty regularly for working off a long list of wooden parts I had collected for the day when I have a CNC… So far, so good. N.b., this is still a build of Ryans initial design, I guess I was among the first hundred or so who started that very enthusiastically. Long story cut short, as expected, the stepper motor holding plated, printed in PLA, have developed a pretty impressive change of their once plane nature.
Ryan had provided a dxf file of a motor holder plate to be milled from 6mm aluminium. What a nice job for the MPCNC, me thought.
First attempts to cut aluminium went, well, pretty well. The usual balance between feed rate, tuning by touch, feel, hear, until there is a decent track. Until…
Until some strange things happen. At certain places, certain directions, the machine started rattling, shaking like mad, snafu.
My next step was to reduce the z-step size to 0.1mm (!). Big progress from 1.2mm. I cranked up the spindle to 25 kRpM (looking at the values recommended by Proxxon for their MF70 (not in this MPCNC, but standing next to it) and observing the behaviour. Tool speed in X-Y was found to work good @~200mm/min.
After some hours, I could pick the first motor holder plate from the MPCNC.
The reason for the change in quality was identified as a losening of the belts due to a stronger bending of the PLA motor plates. Oh well. [attachment file=91936]
Then the shot hot the fan. Rattle and shake again, and then the very unpleasant sound of a breaking cutter.
New cutter, taming the tool path a bit, everything went smooth, until again a new rattling and shaking appeared when milling out the hole for the stepper axis. And only at the topmost point where Y changes its direction. Not at the lower minimum. While I watched that and hoped for the best, the next entry into the danger zone ended fatally for the cutter. [attachment file=91938]
The second in 30 hours. Starting to become an expensive hobby
Is the Makita RT0700 generally too fast for this? Its minimum speed is 10.000 rpm, max 30.000.
The Proxxon BFW 40/E, on the opposite, runs between 900 and 6.000 rpm.
Any recommendations from you people out there who have stepped though this already?
Should I build the latest gantry assembly first to get more stability into the machine?
The usual formulae for setting the rotation and forward speed suggest very different parameters for doing this. But would a 3mm cutter really cut @ 500 rpm? Can’t believe that. I did not find a source that specifies minimum values for the rpm settings.
QCAD, Estlcam 11.102, Linuxcnc, own stepper driver assembly.