I wish to have a mini circular saw to cut my pcb, but sometimes it's enough to have a modified cutter and a proper frame to drive cutter into a straight line. I've designed one in this instructable, it's very cheap and it cames from ordinary objects.
Because you can't simply "cut" the pcb with an ordinary blade, you have to modify it to scratch the two surfaces of the pcb, so to snap it.
This method works well if you draw on pcb two lines on the upper and underneath surfaces, and check that they're exactly aligned. Then slide the pcb into the frame and scrath more and more times the surface until you have a rut which runs over all the lenght, then turn upside down the pcb and make the same on the other side. Now take out pcb and make a little force by hand to break it into two pieces. Round off the pieces with some sandpaper.
Step 1: The Cutter
Let's begin with the cutter. You need a strong one, better with some additional blades. I've bought this aluminium one, it came with 10 blades for about 4$. You can extract the blade acting on the little lever, and when blade is in position it's very tough, also if you pull hardly.
Step 2: The Blade
You need to modify the blade so it will form an hook shape, with a spiky point. You can easily make it with a grinding wheel and refine it with a dremel mini drill. Insert back the blade into the cutter and you'll have a perfect tool to scratch the pcb. Maybe you want to make the same modification to other blades to have the ready when this one will be consumed.
[UPDATE: I discovered that you can buy these blades]
Step 3: The Rulers
Now let's make the rails.
This was a 40 cm steel ruler, you can find it at less than 2$, which I've cut at half. I also made three more holes at far ends. The holes' diameter is the right size to keep the bolts with hexagonal head you already have, or you just could easily find, the exact misure is not so relevant. You also could use four short screws which length is less than board thickness.
Step 4: Drill the Board
The board is sold to cut salami as the one I've used for my column drill stand, they sell these boards at groups of six for a few $.
To set up the exact position of the holes in the board you need to put in place the two halves of the ruler, and leave between them about 1mm or less. Then with a pencil mark the center of the holes. This has to be very precise.
You see the bottom size of the board, where I've already inserted the four bolts. I've drilled a larger hole from bottom to about 1/3 of the thickness, to keep the bolts' heads, then I've screwed them, because the rest of the hole is a diameter a little smaller than the bolts' thread (see detail).
Step 5: And Insert Screws
On the top side of the board you've to thread some washers on the bolts. These will make the right thickness to let your pcb slide under the rulers. Probably you'll have to change them or add or remove some, because height varies depending also on the rubber sheet thickness.
Step 6: Assemble the Rulers
Here you see the rulers in position, but if you don't add something to make grip between wood (or rulers) and your pcb this will slip when you'll try to scratch it with the cutter. So I decided to use a nice transparent rubber sheet of which I've bought some meters to protect the bottom of my tools drawers. Before adding the rubber check that rulers are perfectly parallel and that slit is narrow enough. If something went wrong you can enlarge the holes in the rulers so to shift a little them.
Step 7: Glue the Pad
Measure the inside lenght between the bolts, and also the width of the board, and cut a piece of rubber sheet about 1 cm shorten in both directions. Then round off the corners. To attach the rubber to the board I suggest to use a transparent double faces tape. I've made it turn all around the border and also, this is important, put two stripes exactly under the steel rules.
Step 8: Test the Tickness
Now test the thickness of the space which remains under the rulers. The pcb should slide with no resistance under them, indeed you'll keep in place the pcb with a little pressure on the rulers when you'll scratch it, then it will be free to exit also if it's smaller than the rulers width.