The Molly Project: Planning Stages

A friend contacted me a few weeks ago asking about the possibility of having me build something for her. Her name is Molly. We soon met to talk over the project and some of its details. It sounded interesting enough that I knew I had to find time to fit it in if I could develop a workable design based on her concept and requirements. I worked out the design, and recently met with her to discuss details and cost. We have a preliminary agreement to build as a result.

I am still working on that large shelving unit project for myself, but its project timeline can slip some though. This project is core to my workshop mission, and the first entries I write here will discuss the build process before I discuss the specific need that this project addresses. It is definitely one of the more interesting projects that I have been asked to be a part of.

Shelving Units: Expecting Too Much Sometimes

The machine pictured here is a combination planer-jointer. It can easily convert between one machine or the other. It is configured as a jointer in the photo here. I love this machine. I also hate this machine sometimes. I purchased it because I was doing smaller work in this not-too-large workshop of mine. It was perfect for that work.

I replaced the combination planer-jointer last summer by dedicated machinery with more accuracy and capacity to handle projects like the large shelving unit side frames pictured below. Yet the planer-jointer stays where it sits because it can surface a board flat that is 10" wide. I use it now almost exclusively in that capacity to surface one side of a board completely flat before running it through the dedicated planer or drum sander. Doing so helped create almost perfectly aligned mortise and tenon joints used in the construction of the shelving unit side frames pictured here. I love it for that reason.

This particular planer-jointer was designed with some limitations that make it more attractive to a woodworker like me with a smaller shop and budget. It has tables made from aluminum rather than cast iron for instance, and only the outfeed table is adjustable using simple set screws which periodically go out of alignment like they did today, and then are rather difficult to adjust. I hate it for that reason.

I like perfection in my machinery and in my work too, but with respect to the planer-jointer, I decided I had to accept the fact that it was not designed to offer perfection, but rather capacity and functionality within the reasonable price I paid for the machine. I therefore pay closer attention to its alignment now, and make adjustments when required. I accept this imperfection because of what the machine can do for me within the limitations of my workshop space and budget. It has a very welcome place here as a result.

Shelving Units: The Engineering Case for Quantifying Quality

Tolerance stack-up is an engineering concept defined as an accumulation of dimensional variations in a part or assembly made up of multiple parts. No part can be made perfect, and many mechanical drawings include dimensions with specified tolerances such as 15.000" +/- 0.005" to convey an acceptable range within which the dimension meets specifications.

The usually accepted tolerance range for most professional woodworkers is +/- 1/64" or  +/- 0.016". This degree of accuracy is not hard to obtain given experience and good equipment. The problem lies in a design that combines multiple parts in such a way that the dimensional variations add or subtract. The accumulation of dimensional variation can become significant if many parts are used to make up a single assembly as in the shelf unit side frames pictured here. Viewed in another way, if four sequentially assembled parts are off by only 1/64", the final accumulated dimension represented by their assembly will be off by 1/16". And that is significant. The same holds true for angular dimensions and perpendicularity.

I am not able to measure the shelf unit side frame diagonals during the glue-up process because the many clamps required interfere with the measurements. I rely on taking measurements with an accurate square in each of the internal frame openings and adjust clamping pressure where required to get good readings. The side frame diagonals are within 1/16" of each other or better when the clamps are removed.

Diagonal measurements within 1/16" or better are generally good enough to consider larger assemblies acceptably square. My designs often rely on ultra-accuracy to insure the overall object assembly comes together well. The Alice Table comes to mind as a good example.

Tolerance stack-ups are analyzed in terms of either a worst case scenario where an assumption is made that all dimensions will end up at one of the outer extremes of the specified tolerance, or statistically where a probable average of variation is assumed. I will often consider my woodworking technique and shop equipment good enough where I assume a probability of small average variation.

I should maybe consider playing it safer the next time I evaluate a particularly complex design by using a rough worst case analysis. It might save an unpleasant surprise. The computer screen might tell me I can build something, but reality might dictate otherwise. So far though, the shelf unit side frames are gluing up well.