Opinions sought on cheap desktop PCB milling machine

[HankMcSpank]

I've spent the guts a few a months trying to get consistent pcb milling for reasonably fine pitch SMD....I actually only go need down to SOT23-6 compoinent size (which is about 0.6mm space between pins). I've got there in the end, but not without an astonishing amount of hair pulling & frustration.

Here's my general musings...

1. Level the machine bed - if you don't have a level bed, then you are up against it from the start - forget a V cutter unless your bed is level (in which case go with a small endmill instead)

2. Choose the right V bit for the board - for example if you intend going really fine - it likely has to be 20 degree V bit....careful though, these tips snap like buggery & are expensive!

3. All V bits are depth sensitive - the wider the angle, the more robust the tip is(less prone to snappig the tip), but with wider angle V bit, just taking the bit down a little in depth really make the milled cut a lot wider - personally I'd never now use anything wider angle tha a 45 deg.

4. Calibration, calibration calibration - eliminate backlash from your Z - I knocked up a simple g-code file to raise/lower the Z axis about 30 times....and made sure at the end of it my dial meter still showed 0.01mm or less error after 30 Z ascents/descents. Also, missed steps are the scourge of pcb milling (where incredible amounts of accuracy are needed - so make sure that your axis consistently move the distance they are meant to ie use a dial meter again)

5. If you are going really fine pitch, then seriously consider some form of Z autoleveller program- (this is where you first probe your copper board to pick up the height irreguarities (even if your bed is level, just clamping the copper down will raise the board slightly in the middle region) ...and then apply the probed Z inconsistencies into your g-code Z info prior to the cutting run. You can get a free autoleveller in pcb-gcode (an eagle plugin), but I went the way of CNC-USB which has this feature integrated (it's called warp - http://www.youtube.com/watch?v=0jGY92S8bxM )

6. Use a CAM program that imports gerbers...then you can really fine tune your V bit tip offset to suit...and see the projected cut before you go near the machine itself . I use CAMBAM (so create pcb in eagle ,export tracks as a gerber, import gerber into Cambam...group select all the pcb track outlines in there, tweak your machining parameters to suit....some say you should use an engrave mop for milling the tracks ...this is wrong...if you do, there is no offset applied (it cuts along the pcb track outlin line...and therefore you lose PCB track width = the radius width of the V tip at the actual cut depth ...you need to select a 'profile' cut (which then allows you to apply an offset away from te pcb outline- you want the maximum offset possible but without fear of then cutting into a neighbour pad when milling your finest pads - you can see the visulaizations within cambam - the trial is free )

it's been a long journey, but worth it....being able to knock out a consistently good proto pcb in short order...epanelled, mount holes in the right places etc (vs chemical etching) is a major win for me.