This is nice. If we weren't located half a world away I would be interested in buying it off you (if you didn't need it)
As an example what difference a thou of runout makes I took two microscope pictures of test pcbs milled. Those were done with 10 degree 0.1mm v bit (2.5thou ),10k rpm, 130mm/min (5.1in/min) feed, 0.04 doc (1 thou) 2 passes so total depth 0.08mm (2 thou). I estimate the width of the cut is 0.11mm (around 2.5 thou) on the first picture and 0.15mm (3.5 thou) on the second. It is not a huge difference, but I think it will affect the tool life quite a bit.
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BTW, this on on 3018cnc "Pro" version using the same g code. The only difference is the height map. As the pcb is not flat and it is glued to a piece of mdf it wouldn't be possible without a height map.
I always considered 3018 an engraver rather than a "mill" (even in quote marks ;-) I assume we have the same device. I have the so called Pro version that uses thick bakelite as y supports rather than aluminium extrusions. I think they are OK. They feel very stiff in hand. I'm not expecting miracles from it, but with some upgrades it should be possible to cut aluminium with tiny cutters at high speed and shallow doc. My main issue with it so far was the runout and the flexibility of the spindle mount. Imbalance caused by motor's shaft cutout is enough to cause vibration in the mount... I think it is because they saved money by designing large empty cavities in the plastic. If it was a solid block of plastic it would be much better. I'm considering 3d printing a new z mount from abs with close to 100% infill. That should be much more solid. Of course aluminium would be better.
Here is a photo of the above pcb being cut.
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Coming back to the original subject
Yes, definitely. With the ER holder I hope it will fit the bearings nicely and I'll be able to "dust grind" the taper to make it true once it is assembled.
Having to wait for that adapter a long time I may end up making my own equivalent before it arrives...
Also, why magneto bearings? Did you just happen to have them, or are they in some way better for a spindle?