Scratch Milling Jigs

The key thing that has to be right when making two sided PCB’s, is both sides must line up accurately. This is where a milling machine shines. With 1 mil/0.025 mm resolution and the precision of CAD software, it is fairly trivial to get a perfect match on both sides — as long as you use a good jig. And who doesn’t love a good jig.

When going for high precision in machining, you first must establish what measurements you can trust. You can not trust your eye or a ruler. You can not trust the dimension of a copper clad board. You can not trust a screw to not wobble things. You can not trust waste board to be flat.

So what can you trust? You can trust CAD and the objects it creates. You can trust a pin and machined hole. And if you have a reasonably good machine, you can trust its homing positions and tool measurement system.

With that in mind, you must find a way to create an accurate PCB holder that allows you to scratch, mill, and drill on both sides of the board. You want a minimum of measuring and zeroing out, you want low fast clamping, and you want it to be easy to set up between jobs. Oh and cheap and easy to make of course.

The best thing I’ve found so far is rivet nuts, which work remarkably well. Rivet nuts have a smooth outer diameter, which can be used as a very accurate pin. The inner diameter is threaded, so it is simple to hold down your board with minimal clamping height. Lastly there is a small flange on the bottom with can be used to set the desired height very accurately – I set mine to stick above the board but still slightly below the PCB to allow perfect placement and strong clamping.

To create a PCB jig using rivet nuts, you first mill a flat area into MDF board (or something waterproof like Delrin if you use liquid in your process). Then drill four holes the size of the outer diameter that will fit just inside the corners of your PCB’s – these will be your locating pins. You will need to drill matching holes into each PCB you use, but more on that later. These locating holes should be a known distance from your homing location – that way you can just home (G28) and type in offsets for you design, rather than doing a whole bunch of annoying measuring and zeroing out.

You may need to drill a few test sizes to find the perfect fit for the rivet nuts – I use a bore operation in Fusion 360, with a range of four or five sizes to dial that in. If you have multiple sizes of boards, align the bottom left hole and drill as many locating pin holes as you need. These rivets will be pulled up into the PCB when you tighten them, but unused ones can be pushed back down into the MDF or removed completely.

Lastly, flip the board and drill holes the size of the flange to the depth needed for the stick-out needed (the flattened board height, minus the rivet height, plus the flange height, plus the PCB thickness, minus a wee bit for clamping. Whew. This allows you to insert rivet nuts from the bottom, and perfectly locate a pre-drilled PCB when tightening them. Because these holes form a rectangle, the board can be flipped to fit. Note: if your boards are not perfectly cut, be sure to leave extra space around the holes — otherwise the flipped board may not fit into your leveled area. Also drilling a small hole along the expected edge can give you a little extra leverage if a board is hard to remove.

For drilling the PCB blanks to fit into the jig, you need to insure the board doesn’t move and the holes are the correct size. As long as the hole positions are correct relative to each other, they don’t need to be perfectly centered — remember, you can’t trust the edges to be perfectly cut and aligned on cheap PCB blanks, the holes are the alignment truth. It is imperative to measure the rivet-to-hole fit using different sized holes in a test, just like you did with the waste board. It should be snug but not super tight, otherwise the boards are hard to remove and can bend. Be sure to re-measure every so often as well. A routing bit will wear over time which changes the hole size slightly – you can either adjust for this in CAD, or use a new bit.

Tape is enough to hold down boards if you go easy on the speeds and feeds, though you can of course get fancier and faster. You can put the PCB hole cutting jig on the same waste board as the carving jig — you can even overlap them if you push down the rivet nuts. Separate jigs are probably easier in the end though, as swapping jigs only requires re-zeroing the height (and not even that if the jigs are the same height). Once you have the setup you want, it pays to drill 10 or 20 blanks in a run.

With your jig and the stack of blanks that fit it, you are ready to start carving.

Next: FlatCam and bCNC for Scratch Milling