[CitizenOfDreams]

It is not enough to simply "flatten" the PCB. No matter how you mount it, the height variations will still be enough to spoil the traces and/or break the bit.

You need some way to ensure the constant depth of cut - either a depth ring, or surface probing. AutoLeveller is the program many people successfully use.

Spindle runout is also critical. It does not take much runout to spoil the cut and/or break the tool tip. Many entry-level "spindles" - DC motors with ER11 chucks slapped on - are nowhere near accurate enough for repeatable PCB milling.

Also, a homebrewed machine with limited rigidity may require VERY conservative feeds and speeds. My current settings are 4000RPM and 100mm/min, for instance.

[HPB]

Spindle runout is also critical. It does not take much runout to spoil the cut and/or break the tool tip. Many entry-level "spindles" - DC motors with ER11 chucks slapped on - are nowhere near accurate enough for repeatable PCB milling.

I must agree spindle runouts is a nightmare for milling precise PCB's, but we all have slightly different out comes with DC motors with ER11 chucks,
below are pictures to prove otherwise.

Attachment 464588
Testing new 20º 'V' bits, Spindle is DC motor with ER11 Chuck.

Attachment 464590
Milling PCB with DC motor with 20º 'V' bit

Attachment 464592
Repeatability guaranteed, some claim no success with DC motors with ER11.

Will add that no need hold down PCB with bolts & nuts, double sided tape should do, I use a 100um tape at ONLY three points

Cheers,
Hanspeter.