Introduction: PCB Ruler
I was in the midst of some coding when I realized I had a lot of scrap PCB laying around and wanted to put some of it to use by turning it into a custom made ruler. The one I made is 3" long, 1" wide, and graduated in 1/32". This instructable will also briefly cover PCB etching.
One thing to note is that the ruler is only as accurate as you make it, so take care during the layout of the graduations. No one likes a ruler that doesn't measure properly.
In order to make one of these at home like I did you will need the following supplies:
-1.25"x3.25"PCB (or desired size) NOTE:This is the oversized blank and will be cut to 1"x3" after etching.
-laser jet printer (ink jet WILL NOT work)
-PCB design software
-buckets for etchant and water
-220 grit sandpaper
-a square for checking the ruler
Step 1: Design Your Ruler
After deciding to undertake this project, the first thing you will need to do is figure out a design you want to use for your ruler. For mine I chose a simple design with no numbers to make it easier to read. Now its time to load up whatever software you chose (I used eagle since it was easy and free), and start adding in however many notches your ruler requires.
I found it easiest to do each inch, then each half inch, then each quarter, and so on until each graduation had been drawn. Once you have drawn all the graduations, simply copy and paste for each other inch until your ruler is filled. It is also helpful to include a textbox indicating what each graduation means. For mine, each graduation is 1/32".
Once you are done your design you can print it out, but make sure you do so in reverse or else your ruler will be backwards!
I have also included the design I used to save you some trouble in getting started. Feel free to modify it to your own needs.
NOTE: If you choose to use a double sided PCB, then you can affix information such as conversion tables or a second ruler. However the second ruler scale is complicated since it is quite difficult to get both transfers to line up perfectly. You could even attach your name, phone number, and company logo and use them to hand out to your customers.
Step 2: Prepare the PCB and Apply Transfer
In order to ensure the best chances of success, first rough up the copper on the PCB using 220 grit sandpaper. Once you have a uniform pattern, clean the PCB VERY thoroughly with electrical contact cleaner or brake cleaner. After this, try to minimize touching the copper directly as oil from your fingers can affect how well the toner transfers to the copper.
Set the PCB aside while you cut out the transfer. Ensure that you are able to tape the transfer paper to the backside of the PCB so it can stay secure while you transfer the image.
Fold the paper around the PCB and ensure that the sides are square and tape the backside so the paper is tight around the PCB.
Step 3: Can't Take the Heat...
Now the fun part.
Take your iron (preferably not your moms) and set it to the highest temperature. Once it is hot, apply pressure over the entire PCB for a few minutes. After this lift the iron and apply pressure using the tip of the iron to rub the spots on the paper that have toner. This will help to ensure a good transfer and a successful etching.
There is no real time limit to this and as long as the paper doesn't catch fire you should be just fine. I've found after 10 minutes of heating and pressing every few minutes that I get a good transfer from the toner.
If the transfer didn't go the way you had hoped, simply redo the previous step and print out another transfer sheet.
Step 4: Etching
In order to make that transfer useful, you must submerge the PCB in an etchant for the transfer to resist. For this I used ferric chloride since it was the only thing I had laying around, and have had very good results with it over the years. One of the tricks to using the stuff is to make sure the entire PCB is coated, and the solution is kept warm while etching.
To do this, I made a simple double boiler from a large measuring cup (scrap of course) and and old pickle jar (also scrap). Pour the etchant into the pickle jar and add the PCB. Then, put the pickle jar in the measuring cup and add hot water until it is full.
The etching will begin as soon as the PCB is immersed and will stop once all the copper is gone. This process usually takes less than 15 minutes and you must take care to see that it does not go through the transfer, as this will ruin the PCB.
Once all the copper is removed and only the transfer remains, wash the PCB off with contact cleaner and finally water.
WARNING: FERRIC CHLORIDE IS TOXIC AND SHOULD NOT BE INGESTED OR COME INTO CONTACT WITH EYES, MOUTH, SKIN OR CLOTHES AS IT ALSO STAINS.
Step 5: Cut to Size and Polish
You can now trim the PCB down to its final size of 1"x3" or the desired size. This is done after the etching to give the most accurate results in relation to the ruler. To do this I used a belt sander for the rough sizing, then did the final sizing using various files and then a light sanding on the sides with 400 grit sandpaper.
After the etching process, only the copper that was under the transfer remains. However, this has a rough finish and could use a polishing. While sanding to 400 grit or steel wool achieves a nice finish, I found that polishing with autsol on a cloth buffing wheel creates a really shiny finish, and makes the exposed fiberglass as smooth as well, glass.
After you finish polishing, give the PCB another spray of contact cleaner to clean off any polish, and buff out the copper by hand.
Congratulations! You're all done and have a new ruler you made all by yourself!
Step 6: USE AS a DIGITAL READ OUT (DRO)???
I was just thinking about how this ruler was designed, and the fact that it can be used as a VERY poor mans DRO!!!
If you were to connect all the graduations by a common trace and run a voltage through it, you could conceivably drag a sensor (or a pin head hooked up to an arduino) over each graduation and get a voltage change. You could count this voltage change, and do the math to figure out how much it has moved for each count, and VOILA!!! You now have a semi-accurate, cheap DRO for whatever you want, along with being able to customize the length to your needs.
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