Thursday, February 16, 2012

Preparation for double-sided PCB - sacrificial surface

A replaceable backing system is necessary since the drilling will damage whatever is behind the PCB.
To hold the PCB, I use a 3/4" MDF (Medium Density Fiberboard).  The PCB backing board needs:
  • flat and true
  • inexpensive
  • easy to replace
  • fast alignment
  • fast leveling
MDF clearly meets the first three requirements.  To meet the last two, I decided to use three 1/4" registration pins and "T" nuts with set screws for leveling.

0.250" Registration holes in milling table
(Click for larger image)
I drilled 3 holes into my machine table. I made an "L" shaped layout.  The pattern (in inches) for the three holes are:
  • 0,0
  • 12,0
  • 0,8.5
These holes are small and do not interfere with any operations when I'm milling large stock.

The actual sacrificial table is 3/4" MDF, 10" x 14" and is routed from the back to accommodate:
Underside of sacrificial table
(Click for larger image)
  • Three alignment pins - the reverse of the dimensions above (1/4" aluminum rod stock)
  • Three "T" nuts with inset hex head set screws for leveling
  • Two alignment holes for homing the tool (setting x=0 and y=0)
This part is milled from the back side (as shown in the image the above) - this allows all holes to be milled at one time.

Close-up of alignment pin and
leveling screw
(Click for larger image)
Here is a close up of one of the alignment pins (epoxied into place) and one of the "T" nuts with the leveling set screw visible.

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