I have uploaded two sets of drawings that illustrate my thoughts on this problem in the Photos section of the board.
http://groups.yahoo.com/group/Homebrew_PCBs/photos/album/1427693708/pic/listThe first pair show my method for registering the two sides using alignment pins. The main thought here is that you do not need to use two ROUND holes. Two round holes are overkill for aligning the two sides and it is hard to punch and drill these holes at the same spacing on both the art work and the boards. Any differences in these spacings, even just a couple of thousandths of an inch, will result in problems. It is better to use one round hole and a second OVAL one.
An object in three dimensional space has only six degrees of freedom, the three longitudinal axis, X, Y, and Z and the rotational axis about these three longitudinal ones. The round hole restricts the art work in two dimensions (X and Y) or two degrees of freedom. The oval hole restricts it in a third one (Z rotation). Finally, the exposure frame provides alignment in three more degrees of freedom (Z, X rotation, and Y rotation). That is all that is necessary and any additional constraints can only result in problems if they are not consistent with the first six. A second round hole provides redundant constraint in the Y direction.
Check out the drawings to see the details.
A second pair of drawings shows the exposure frame that I have actually built and used for two sided exposures. They are at the same link as above.
It does not use any pins or holes. Instead, it uses two edges of the film/art work that are at right angles to each other and a pair of alignment straight edges that form a 90 degree corner. You simply push the board and then the film into that corner and close the clear cover over it. For the second side, you flip the board over and change the orientation so that the same corner of the board is in that corner formed by the alignment edges. This provides alignment without any holes or pins. I have used this frame for exposing boards with 0.1" grid spacing and feel it would work with components on a 0.05" grid with a bit of care. All drilling of the board can be done after both sides are etched.
Both of these methods avoid the problems associated with the need for precision when using two round alignment holes. Therefore they are well suited for home brew, DIY alignment of two sided boards.
Paul A.
--- In Homebrew_PCBs@yahoogroups.com, "crazy_geekchick" <amiegrl2001@...> wrote:
>
> I was wanting to make sure that everything lined up on both sides of a board -- easily. I thought a solution would be to make a registration hole punch. I know the pro ones have cameras looking down the bore of the hole punch, but I wasn't exactly sure where to begin making a diy version of that.
>
> I was thinking a solenoid with some sort of hole puncher, but I wouldn't know where to get precise equipment for the puncher itself.
>
> Any ideas?
>