Hi Mahmood,

My panels are held down by friction with the registration pins. My machine has a two pin registration system. The two pins form an axis around which the board flips to mill the other side. You absolutely must have some sort of registration system if your going to mill more than a single layer. Anyway, the pins are 2.95 mm diameter, and they fit in 3.00 mm holes in the panel. The friction is more than enough to keep the board in place.

You also must put sacrificial underlayment material beneath the panels to protect the machine bed during drilling. I use a panel of high density cardboard material. It's much less expensive than copper clad board material. Whatever you put under your panel, it should be unblemished before you drill. You don't want to drill down and intersect the edge of a preexisting hole.

PCB drill bits are usually made of carbon steel. If not, the fiberglass (FR4) will dull them quickly, which will have a deleterious impact on plating your holes. The tradeoff is that the bits will not withstand even a tiny amount of lateral stress without snapping like dried spaghetti. If your underlayment has a preexisting hole (from prior use), and your shiny new bit intersects the edge of that hole, the differential force will produce a lateral strain that will break your drill bit immediately. You can buy a lot of underlayment material for the cost of one drill bit.

Joe