## Step 6: E=mc2

(this is a long one so get coffee)

Now we can get down to the really fun part. We are going to use parametric equations to define the dimensions for the back of the Bookcase.

To save us the boring task of drawing another rectangle, constraining, dimensioning and entering all the dimensions that define the bookcase, we will instead open the Side part (if not still open) and do a Save As under the new part file name 'Back'. This will allow us to go directly to re-defining the size of the rectangular part that will now become the back of our bookcase.

The normal orientation for the back of a cabinet or bookcase is that with the grain running vertically. It does not matter that we have created the part for the back laying on its side, because it will be oriented vertically when it is constrained into the bookcase assembly. What is important here, is that the Width of the Back will be related to the Width (Breadth for my husband) of the bookcase. In this design, the back will be inset between the two side/end panels of the bookcase. So, the Width of our Back panel will be the Width of the Bookcase, minus the thickness of the two bookcase side panels.

Open the Equation Editor, and MODIFY the Width parameter for the part, and replace the current Equation with a new equation that defines the Width of the Back. I am sure you are already way ahead of me, but that Equation will be 'Bookcase_Width - 2*Bookcase_Thick'. Bookcase_Thick is the value used to define the thickness of each Side panel in our first part.

Maybe you can already see how all the parts for an entire cabinet can be defined if you create all the parameters to describe the cabinet. Counter_Height, Counter_Thick, Toekick_Height, Toekick_Depth, Cabinet_Width, Cabinet_Depth... all can be defined (and easily modified) to provide all the information required to specify the dimensions of every part. The Top of a cabinet is sandwiched between the two cabinet ends would have the same Length formula as the Width of our bookcase back. The equation for the Length of the Top would then be something similar to 'Cabinet_Width - LeftEnd_Thick - RightEnd_Thick' where a thickness is defined for each end in order to allow differing thicknesses if desired (which we have encountered far more than you would think). While it may appear at this time that it is more trouble than it is worth to go through every single part and change the values that define the cabinet itself, there are techniques that make this a reasonable task. From the use of Configurations in the Professional and Expert versions of Alibre to create multiple parts in the same part file, to Spreadsheet driven parameters, and even custom programs written using the Alibre API interface. Creating parametric driven models can make creating common woodworking designs quite simple and quickly.

Xpress users should not loose heart because you too can benefit from configurations. A properly constructed model using configurations will allow you to manipulate the defining parameter values and use a model composed of a significant number of parts, while only be regarded by Xpress as a single part because all the parts are contained in a single part file. But, you can tell I have been listening too much because my husband prattles on about configurations and modeling. Time to finish the back to our bookcase.

Open the Equation Editor again, and MODIFY the Length parameter. Since the back of a bookcase usually ends at the bottom shelf and does not go all the way to the floor, we will subtract the Toe kick height from the Height of the bookcase in order to get the length of the back. Replace the Equation for the Length of our part with 'Bookcase_Height - Toekick_Height'.

Since the thickness of the Back does not change, save the part file and we are done with the Back part.
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Great instructable!&nbsp; I designed and built the physical shelf with this, and I used your&nbsp; molding extrusion instructable for the corner lock miters.&nbsp; Thanks!<br />
I know this may be premature, But I am interested in making a knockdown version of a similar bookcase. Is it possible to set up precision drill holes for cam-locks with this program, so that everything stays snug? Ps. Know any good woodshops in Jacksonville, Fl looking for talented furniture design and build?
I didn't like how it was put in computer format
Would love to see you add this to my new group.<br/>Hope to see you there.<br/><a rel="nofollow" href="https://www.instructables.com/group/fixit/">Home Repair, Refurbishment, and New Projects</a><br/>
HI. I'm just getting into alibre and these tuts are great But, on step 6 when I go to modify the width parameter the ok button grays out on me after I enter any words.I'm using Alibre Design. I don't know if this is the right place to ask this . If not just send me to the right area to ask questions. Thank you.
The OK button will be grayed out if there is an error in the parameter name or value. There can not be any spaces or dashes in the name (which is probably not the case here), and the parameter names in the formulas have to match over valid parameter names. I will check for a problem in the instructable, but I suspect a misspelling of a parameter name or lack of math operation to separate parameter names. I am glad the tutorials are helping, and we are working to prepare more aimed specifically at using Alibre for woodworking.
VERY nicely written! Great job!