In this tutorial I will help you with the basics of modeling a slide rule. It is pretty simple, but there is a lot to have in mind while modeling.
I have corrected the wrong name, instead Slide Rule, this is actually called Vernier Caliper. So, thanks to Chuck Hoffmann for pointing that out
Resource files
Images
Blueprints (pdf)
Video tutorial
http://www.rhino3dhelp.com/video/modeling-slide-rule-video/
Introduction
I’ve decided to make a slide rule as my first tutorial because it will be the most used tool to measure object I will model. That way I can provide you very accurate blueprints and models for you to use and download.
Step 1
First, we will set up our scene with some background image that we will use as reference for modeling. Please note that I will not follow the curves in background image all the time. It is solely for reference, and my modeling is strictly based on my taken measures. Before we start, you need to open the centimeters template in Rhino. As we will have grid laid out as centimeters.
First we will measure the length of our slide rule. I don’t have another slide rule, so I’ll use regular ruler. If your Osnap is not enabled, please enable it now as it is the tool you will use the most! You just can’t live without it, and sometimes with it. Something like having a girlfriend.
I have measured 22.4 centimeters, so lets make a nice polyline long 22.4cm starting from origin of coordinate system (0,0). You can click on Snap so your cursor snaps on intersection of grid lines. That line is mostly for orienting the background image we will insert next.
So, lets put the background image. Go to View->Background Bitmap->Place and browse for your reference image (you can download it from this page). Again, if your snap is not on, click on it so it gets bold (that means it is on). You can either write in the command line 0,0 or you can click on origin. That is the first corner, and next you need second corner. Just move your mouse and stretch the box so it is a little wider than your 22.4 line. After that you are still in the command, and you can choose some of cool options, like grayscale image. It is default to yes, so you can click on no. I like it that way.
Now, we will adjust the background so it fits our reference line 22.4. First we need to move it a little to the left. Go to View->Background Bitmap->Move and move it. I believe the image is self explanatory. Next we need to scale the background image down, go to View->Background Bitmap->Scale.
This is what you need:
Step 2
In this step, we will be making the base top view for our model. You need to have the documentation preferably printed out on A4 format for better comparison and readout of dimensions.
Ok, we’ll start from the upper right corner of our slide rule, and we’ll use polyline to create the base closed curve. Lets create first 5 segments of this part. Select polyline (or type in command line _Polyline) and click on that far upper right corner, and type in the command line 1.5 and press enter. Next input (you are still in the command) 21, then 1, then 0.1 and in the end 5th segment is 2.8 in length. You have something like in the picture.
(please note that in technical world point “.” is used for decimal point, and comma “,” is used for separating the coordinates. i.e. 5,4 is a coordinate with 5 units as x or abscissa and 4 as y or ordinate, while 5.4 is length – if you are interested you can refer to wikipedia’s Cartesian coordinate system)
We’ll continue with the segments, this time on the left upper side. We can start with putting the point in the start of our first reference line we created in step 1.
That point we need to move along the reference line for 2.2cm right. So use the move tool, click on the point, and input 2.2 in command line and make surein the osnap settings you have Near checked. Or, you can click on Ortho and move the point. Next, we will again use polyline and this time we will start from the moved point. Again, make sure you have Point checked in osnap settings. Input 0.3 in polyline command, and go up, then click, input 2.2 go left and click (it is a lot easier if you have Ortho on the whole time you make this) and then input 1.8 and go down and click. End the command by hitting enter, or clicking right mouse button. Now, you need to disable Ortho and make that angle line. You can use polyline again for that. Now you can position the line according to reference background bitmap. Start from the end of last line, and end where the part is starting to curve according to background bitmap. 
Next, you will create another line, starting from the end of right line, and make it parallel to x axis and length random. Use Ortho. Then, using blend command, create blend between that parallel line you just created and that angled line. You should get something like on the image below:
Using Trim command you can trim that reference line and that Point we created earlier. Join all lines and curves, and you should have closed curve which you will use as a base curve for creating the model.
Step 3
Lets create the little holes for screws. Using Circle command we will create two circles according to position like on the image. You can either create two circles one by one, or you can create one and mirror it so you are sure the proportions are ok, and that both circles are exactly the same distance from the base closed curve. I have used the second method. Activate the Circle command, and point in the center of upper screw. For the radius input 0.15. Now, make sure in osnap settings you have cheched Mid, and select the Mirror command. We will mirror that circle according to the Middle point on the right line, and now again we will use Ortho.
Finally we are moving from 2D to 3D. So, select (Hold Shift + Click = Adding to selection || Hold Ctrl + Click = Removing from selection) the closed curve, and both circles. Type in the command line Extrude and 0.3 for extrusion height.
Next we are moving on the opposite side again, and we’ll make little cuts on the curved part. The cut itself is 1cm wide, so again, you can use the same method with Point to start the line. So create the point in the far bottom point, where curved line ends, and move it up by 1cm. Make sure the point is on the top plane of your model. Start the polyline from that point and move left and down according to image.
The shorter line needs to be adjusted a little. You can select the L polyline, and using PointsOn command turn the control points on. After that, control point that is on the edge of the object needs to be lowered down by 0.14cm. Be careful, as there resides the Point you created earlier, so when you click, you will be prompted which one to select, and select Curve Point. Using move tool, and Ortho, from Front View you can move it down. After you’ve done that, using PointsOff command turn off the control points, and using Explode command explode the L line, into two lines. Next using Sweep1 sweep the shorter line along the longer or vice versa. extrude the shorter line up (use the direction [d] for specifying the direction of extrusion if it is not as we need it).
Now, join those two surfaces. You can mirror that joined surface, even though I didn’t do it that way in video. Using trim command trim your base object with those two flat surfaces (actually it is one, as we joined it).
