Intro: Re-purposing an Old Scanner Into a Tabletop Saw
I have been working with solar cells for about six months now and needed a way to cut them into smaller pieces, to get higher voltage when stringed together in a panel. I first started with a wooden jig fixing a Dremel tool with a diamond saw attachment, and manually cutting the solar cell across the saw blade. But a slightest movement of the platform or hand would break the solar cell which is very brittle. I needed something more sturdy and some device which could move the saw blade in a straight line with no jitters, and I thought of the humble scanner.
Here is a youtube video of this table top saw cutting a solar cell :
The scanner has all the necessary mechanical components for to and fro motion and all I needed to do is attach a super-high speed motor to the “Read-head” which is the main black box that moves and scans the document, and rewire the circuits inside and the buttons on the front panel to make the stepper motor that controls the read head to go back and forth.
I use an Arduino microcontroller and some existing components on the main circuit board to control the Read-head.
The final product is shown in the Main Image
Step 1: Start With a Used Old Scanner You Might Have or Buy on Craigslist or Ebay
Buy or start with a used scanner that you can take apart. I bought a UMAX Astra 2200 Flatbed scanner from Craigslist for $5. You can easily find one on Craigslist or Ebay. Now open the front cover.
Step 2: Replace the Glass Top With a Slotted Acrylic Sheet
You need to replace the top glass cover as you will have to make a slit through the center for the saw blade. Open the top cover and remove the glass plate, by removing a couple of screws and prying the glass out which is usually stuck to the body using a mild glue.
I use an Acrylic plate as a replacement, which is easier to cut and work with and is a little more scratch resistance than say Polycarbonate. But you can use any plastic that works for you. Cut the Acrylic plate to the same size as the existing glass panel and make a slit through the middle leaving enough margins on the top and the bottom.
Step 3: Connecting the Slot Motor to Diamond Saw Blade
The next step to fix the motor coupled with the diamond saw blade to the Read-Head. This step is complicated and it took me a long time to find a good super-high speed motor. But after a lot of searching, I found that the slot car motor works best : http://en.wikipedia.org/wiki/Slot_car motor. You can readily find it on Amazon, Ebay or your local hobby store.
The size of the motor is important, as the outer diameter OD of the motor body or the narrowest portion of the motor body must be smaller than the diameter of the diamond saw blade by at least ¼ of an Inch – so as to have a significant portion of the blade sticking out of the top plate of the scanner.
Connecting the Dremel diamond saw blade to this motor is tricky. This is what worked best for me: Dremel saw blade is usually connected on a Mandrel like you see in pictures below. Then follow the steps below :
a. I used the Dremel tool with a regular metal cutting saw blade to cut the Mandrel at the intersection of the body which carries the screw and the shaft with smaller diameter (talk about using a thorn to remove a thorn).
b. Drill a hole in the center of the body which carries the screw, from the end opposite to the screw (of-course). The diameter of the hole should be slightly smaller than the shaft of the slot motor. This requires a little finesse, patience and some trial and error. You can see the tools and the bits I used in the pictures below.
c. Once you have the modified Mandrel, you can hammer it on to the shaft of the Slot motor and connect the saw blade. Be careful as the motor is pretty delicate. This setup helps in easy removal of the blade should it need replacement and to attach different types of blades to the motor.
Step 4: Completing the Assembly of the Slot Motor and Connection to Read-head
Now comes the setup to mount the motor to the Read-head of the scanner. I got an L shaped bracket (Something similar to picture shown) from OSH with a center slot one side, and a center hole on the other (if there is no hole you can drill one) -but Home Depot or ACE hardware should also sell something similar. You will have to cut the slotted side using the Dremel tool to match the height of the Read-head.
a. You will also need a U-Shaped clamp (see picture) and drill a hole to match the size of the hole on the L-Shaped bracket , so that you can mount the U-shaped Clamp on top of the L-Shapped bracket to hold the motor. I then line the inverted U-shaped bracket with some double sided sticky tape and push the motor down the U-shaped bracket to complete the set-up.
b. Now mount the assembly above to the Read-Head carefully so that the top of the motor (not the diamond saw blade) is flush with the topmost portion of the Read-head. The slotted side of the L-shaped bracket will be helpful in making the height of the assembly adjustable.
Step 5: The Electrical Connections
Now that the mechanical components of the Table-top Saw are ready and rolling (literally .. make sure that it is ), it’s time to look at the Electronic components so that you can re-purpose your buttons on the front of the scanner to make the Read-Head of the scanner move back and forth. The circuit board of the scanner controls the functioning of the Stepper motor among other things and has a lot of components you can reuse. If you are adept at writing device drivers , you can probably use the entire circuit board to make the buttons move the Read-Head. I am not, so I use an Arduino microcontroller to control the stepper motor and use the buttons to make it go forward or backward. Here are the steps to accomplish that :
a. You can find the setup for controlling Stepper motor on the Arduino help page
but where is the fun in that.
I started with understanding the basics of Stepper motor : E.g. http://library.solarbotics.net/pdflib/pdf/motorbas.pdf, and Arduino programming to understand how to send pulsed waveform to the Stepper motor .
b. Next, while the scanner was functional , I read the waveforms of the Pulse Width Modulated (PWM) inputs to the Stepper motor using an old oscilloscope. And it looks as seen in the picture below. This is for the 1st two armatures with a 90 degree phase shift. The remaining terminals or armatures receive an exactly inverted (180 degree phase shifted) waveform. The voltage is about 12V.
c. The idea of using an Arduino board is to mimic this waveform to control the Stepper motor. Now, the Arduino board output ports that generate the PWM cannot supply the 12V that the Stepper Motor requires, so we need to use a Darlington Array like one available from TI (see : http://www.ti.com/lit/ds/symlink/uln2003a.pdf) which takes the input from the Arduino board and supplies the required 12V PWM to the stepper motor.
Luckily, the Darlington Array IC UN2003A exists on the Scanners controller board (PCB), along with the power supply and the inputs from the Scanner buttons. All I had to do is cut out (using Dremel tool) a small section of this controller board and repurpose it to take the inputs from my Arduino controller:
The various important components of the small section of the board are explained in the picture below : The Red Lines show the connections underside the board to a connector I found in an electronics surplus store for 75 Cents. The connector supplies inputs and outputs to Arduino board. I also supply the power to Arduino from this board.
Please note the numbers on the Red lines which indicate the wire numbering . I1 –I5 indicate the input wires, and O1-O4 indicate the output wires that go to the stepper motor. I will use the same numbering to mark the connections to the Arduino board.
The next step is to connect the input and output pins of the Arduino microcontroller to the inputs from Scanner buttons and the Stepper Motor. The connections look as shown in picture below showing the Ardunio board.
The electrical connections to the Slot motor should be pretty straightforward and if you have come this far i will let you figure out how to do it. I would recommend that you connect it to a separate power supply so as not to mess with the logic controls.
Step 6: Making It All Work With the Arduino Controller
The last step to make everything work is the code for Arduino that runs the stepper motor forward or backward based on the inputs from the FWD or REV buttons (indicated in the first picture)
Attached is the code I used.
Once the code is uploaded to the Arduino board, the FWD / REV buttons should control the to and fro motion of the Read-head as seen in the Animated GIF.
Enjoy the table saw!