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EAGLE vs Sherline CNC Mill: Maximum-size PCB Platen

I use the Standard edition of Cadsoft’s EAGLE schematic capture & PCB layout program, which puts a 160×100 mm upper limit on circuit boards. That meshes nicely with the capabilities of my Sherline CNC mill, which I use to drill component holes.

I’m currently making a set of PCBs that are pretty close to that maximum size. They’re awkward to clamp, difficult to peel off from double-sided tape, and require careful positioning to ensure they don’t hit the mill column. Been there, done that, time for something better.

The simple acrylic sheet platen shown here seems to work well. The PCB is a 5×8-inch sheet, clamped along three sides with some aluminum U-channel from the heap. That’s why two of the rails have random holes: it came pre-drilled for something else.

Platen with 5x8-inch PCB

Platen with 5x8-inch PCB

The rear edge (closest to the mill column) has three screws that serve multiple purposes:

  • They clamp the edge of the sheet firmly to the platen
  • The two end screws protrude through the platen and align it along the rear edge of the mill table
  • The middle screw is an origin alignment marker
Rear clearance

Rear clearance

My mill has slightly less than the absolute maximum Y-axis travel because I added a bushing to capture the end of the leadscrew, as described there. The picture shows the clearance between the back of the platen and the mill column: 2 mm, more or less. The 6-32 screw head is flush with the rear edge of the platen.

Alignment along the Y-axis is easy: jog rearward until the stepper motor stalls, ease away a smidge, then touch off at Y=3.8 inches. Stalling the motor is bad practice with servos or husky steppers, but on this sort of low-power machine it’s perfectly OK. (One could argue for limit switches, but in vain.)

Slap the platen on the mill table tooling plate (turns out that the Z-axis reach is marginal for the shortest carbide drill when it’s in a collet, oops), adjust more-or-less to the middle of the X-axis scale on the front of the table, line up the hold-down clamps, then crunch the U-channels down on the circuit board. That holds everything in place very firmly; the front overhang doesn’t get much torque because the mill can only reach 4 inches from the rear edge, just beyond the mill table underneath.

That center screw is eyeballometrically in the middle of the platen’s width, so X-axis alignment is also easy: put the laser dot (visible in the top picture if you squint) on the near-side edge of the screw and touch off X=3.2 inches.

That alignment puts the X=Y=0 origin at the front-left corner, about 1/4″ in from the left-side clamp and an inch behind the front clamp.

The mill’s X axis reach goes beyond the clamps, but the 160 mm = 6.30 inch extent of an EAGLE board fits neatly inside.

The Y-axis reach is barely over 3.8 inches, just shy of EAGLE’s 100 mm = 3.94 inches, but that’s close enough for what I need to do. Getting that last 0.14 inch would require a very, very thin clamp at the rear, minus the Y-axis bushing. There wouldn’t be much clearance from the holes to the edge of the board, either.

The generous Y-axis clearance on the front allows for the trickery needed to run toner-transfer sheets through the fuser; you want margins all around the drilled area. More about that there, plus search for PCB to unearth other posts.

Remember that the way I make PCBs, the holes act as alignment points for the toner transfer sheet. That means I don’t really care about absolute alignment with respect to the raw PCB sheet: just clamp it down and start drilling.

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  1. #1 by randomdreams on 2009-12-21 - 00:25

    I work with a LPKF milling machine at work, and what they do for circuit board hold-down is provide a masonite work surface for the drills and cutout router to terminate in, with two pins sticking up out of the masonite, right on the centerline. That way you can mill the top side, flip the board, and mill the bottom side by mirroring the bottom artwork around the centerline, and get pretty excellent top/bottom via registration, good enough that 40 mil od, 20 mil id rings are good.

    • #2 by Ed on 2009-12-21 - 06:54

      good enough that 40 mil od, 20 mil id rings are good

      That would certainly be good enough for my simple needs!

      I’ve been coming to the conclusion that my etched boards are adequate, but not great. I don’t have good control over the toner-transfer part of the process, which may…

      • not adhere, leaving broken traces
      • work perfectly!
      • have blurred traces

      While it’s easy enough to strip the whole thing off and try again, that’s getting old pretty quickly.

      But the amount of glass fiber swarf kicked up by a surface mill seems daunting; I’d have to conjure up a much better vacuum filter than I have now before trying that.

      After futzing around with various ways of clamping the boards, however, this platen seems to be just about perfect: the board is flat to within a few mils, even for slightly warped boards clamped concave-side-up. I’d have to add holes closer to the center for smaller boards, but that’s in the nature of fine tuning.

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