When you'd like to start building more complicated objects (http://reprap.org) with your printer the dimensional capability becomes more important. The spacing between holes for instance may determine whether you are able to bolt it to another part.
Printing a calibration block is a good way to get your feet wet on a printer you haven't used before and will set a good foundation for future printing, knowing that the printed size is correct.
I made it at TechShop!! The Detroit Tech shop (http://techshop.ws) location has a Makerbot Thing-O-Matic and will also be setting up a new Makerbot Replicator with dual Extruders in the near future.
Step 1: Selecting a Calibration Block to Print - the Universe of Thingiverse!
If you go to the Thingiverse home page (http://thingiverse.com) and type "calibration" (no quotes necessary) into the search bar and hit return you will get a long list of calibration and test objects to select from.
Depending on the type of objects you plan to print after your calibration check you may want to select something that offers thin walls, overhangs, bridges, or holes. But my suggestion if you are just getting started is to chose one that is simple, but will give a good indication that your X, Y, and Z dimensions are working correctly.
I selected the Calibration block by kludgineer (http://www.thingiverse.com/thing:4203) which provides 10, 20, and 40mm lengths in all three primary axis to check. This will help determine if any issues are due to printer scale or if they are related to other issues such as material expansion.
You can download STL files from Thingiverse by selecting them from the LH column of Thingiverse item pages. I've also incuded Kludgineer's Calibration Block instructable (as of 8/11/2012) under the sharing terms allowed by the Creative Commons GNU/GPL license. I'd encourage you to always check the Thingiverse page for updates, particularly on more complicated projects.
Once you have your STL file downloaded to your local computer we will get ready to print it.
Step 2: Getting Ready to Print
1. Turn the computer you are going to use to print ON.
2. Make sure the USB cable and Power Cable are connected to the 3D Printer
3. Turn the Printer ON.
4. Launch ReplicatorG program on the computer. The latest version of ReplicatorG is available at http://replicat.org/download/
5. Check to Make sure the correct machine is selected under Machine->Driver->. The machine I was using was Thingomatic w/HBP and Stepstruder MK7.
Steps 6 & 7 are specific to the machine I was using, your's may vary in the zero'ing process
6. Using the controls found under Machine ->Control Panel position the nozzle over the center of the print bed. You may notice that the X/Y axis seem to move the opposite of what you intuitively think. That is because it is moving the bed relative to a fixed nozzle position. Once you have centered X & Y slowly move the Nozzle down in Z. DO NOT CRASH IT INTO THE PRINT BED. Use progressively smaller steps as you get closer to the bed. Position the Nozzle so that you can slide a regular piece of paper between it in the bed.
7. Using the LCD Interface run the tomcalib.s3g file to run the printer thru it's zero'ing routine.
8. Load the STL file acquired in the previous step using File -> Open. At this point you should see your part on the computer screen and the status bar at the top of ReplicatorG should be Green, showing that the machine is connected and ready.
Step 3: Create G-Code and Print
1. Make sure your model is centered on the virtual build area in the ReplicatorG window. If is not there may be something wrong with the model that you downloaded or created. ReplicatorG does have capability of moving, scalling and mirroring your part, but that's outside the scope of this Instructable.
2. Build G-Code. The fourth button on the top bar will begin the generation of G-code for the current model, opening an option window.
a. Some of the options to check on the machine I was working with are "Use raft", no Support Material. Object Infill (%) doesn't need to ne any more that 25-30%
b. Under the Plastic option tab make sure the correct type of plastic and size are selected.
c. Click "Generate Gcode" button.
3. The first button from the left on the top bar will start building your part on the printer. Click "Use existing Gcode" to ensure that it uses all the options you just selected in the previous step. The machine will most likely move to a starting point and wait until the print head and bed come up to temperature. The temperature of both are displayed in the status bar at the top of ReplicatorG.
4. You may want to familiarize yourself with the location of the "STOP" button in case anything unexpected happens. It is the sixth button from the left side of the top bar in ReplicatorG. You may want to have your mouse cursor hovering over it initially until you are fairly convinced things are working properly.
5. Wait for your object to print. Grab some free TechShop Popcorn, but don't wander to far, keep an eye on the machine while it's running.
Step 4: Reviewing Your Calibration Part
Before removing any calibration object from the printer it is a good idea to mark which direction represents each axis. On the Makerbot Thingomatic the X-Axis is Left to right, the Y-Axis front to back and the Z-Axis going up from the print bed. (This are all while looking at the front of the printer)
After marking the axis's you can carefully remove your part from the print bed. If you used the raft option you can remove the raft by peeling it off the bottom and inspecting the part.
This particular calibration object is good for determining if there is a problem with the scale the software uses to translate Gcode lengths into turns of the servo motors. If you find an error in length that changes in proportion to the length being measured it would indicate that a scale correction might be useful for improving the dimensional capability of the printer.
For instance if the part measures 10.5mm, 21.0mm, and 42.0mm across different sections on the X-Axis would indicate a consistent 5% error in length that could be corrected in the calibration.
Chances are if you are using a Makerbot printer you will not find this type of error as the printer calibrations for all of their printers have been well developed and are included with ReplicatorG.
In my particular case I found the part to be fairly consistent in the three lengths on the three axis.
Now on to printing some useful parts!! Thanks TechShop!!