bouni's blog

A new eccentric tappet for the pcb drill

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The problem

I have built a PCB drill that works nice most of the time, but sometimes it occurs that the stepper motor don’t perform a full turn.

This is caused by the friction of the eccentric tappet. I’ve simply made it out of a piece of steel rod, with a hole outside of its center. That tappet pushes a brass screw upwards. I thought that this will work fine, but it does not (sometimes).

The solution

"My new eccentric tappet"

That’s my new eccentric tappet. It is a turned piece of metal rod, that has the eccentric hole (not yet done :-) ) and a ball bearing on it. That will hopefully reduce the friction and solve my problem.

When i developed the new eccenter i searched for a formula to calculate the distance the eccenter will lift the drill upwards.

"eccentric path"

  • red cross: marks the center of the circle before and after a half turn.
  • blue cross: the center of the rotation
  • grey dashed circle: the area the circle moves through on a full turn.

The formula is: distance [d] = offset [o] x 2


I’ve installed and tested the new eccentric tappet and it seems to work better as the old one. But I have to drill a complete PCB without problems to confirm that.

"The new eccentric tappet"