DIY-Prefab

I have given some attention already to the concept that overall accuracy can be improved by the use of jigs and fixtures. I have also said some things about the desirability of breaking a design down into as few unique modules as possible. One other thing that I do not think I have mentioned yet is that the size and complexity of your jigs and fixtures is directly related to the maximum size of the modules that you have designed.

The framing table that we had in the panel company where I worked was large enough to make a wall panel that was at least 24’ wide. I think we had the capacity to make the panels up to 10’ tall but I am not sure now. That table was impressive but pretty expensive too. It had an array of LEDs along each side of the table that could be lit up under computer control to show where the framing members should be placed. We had software for designing the panels that created the necessary input for driving the LED display. I understand that there are panel machines now days that have laser projectors above them that can project the pattern of what is to be built onto the table surface.

My first exposure to panelized construction was with a small company that I visited in San Jose, California that had an approach where their panels were always 4’ wide. They were based on standard plywood sheet sizes. The operation was running in a shop that was maybe the size of a three-car garage. As I recall they had maybe 3 or 4 steel tables that they had made themselves that had various brackets to help them place materials. Some door and window openings would not fit entirely in a given 4’ wide panel so for these panels they would leave out the header that went across the top of the opening. They would of course pre-cut it but it would be placed in position when the panels were joined together at the job site.

Another issue related to the creation of jigs has to do with how you place framing members. One approach is to have brackets that hold pieces in place while another approach is to have some sort of visual indicators on your table or jig. Which approach makes sense to take depends to some degree on whether or not what you are making at that station is very specific or more general. For example if you were to discover that you had to make a lot of parts that were exactly the same you would want to lean in favor of a jig with fixed brackets into which you can quickly drop parts. A table for making wall panels needs to be a bit more generic if the walls vary in size and features. For example fixed brackets for placing wall studs might very well get in the way if a window needs to be added to a wall panel. There is an additional challenge for jigs that need to be more generic. Somehow you need to decide where to put materials each time you use the fixtures.

A framing carpenter will typically layout out a wall by drawing marks on both the top and bottom plates of the wall at the same time. They usually use symbols to indicate what is to be placed at each location. Here are a few variations on what might be drawn on an 8’ 2×4 wall top and bottom plate. The “X” indicates a full height stud, the “T” indicates a trimmer stud (supports a header beam for a window), and “C” indicates a cripple stud, which is the short member beneath a window opening.

Perhaps there is a way to build yourself a simple layout table that combines fixed brackets with visual indicators. I will attempt to describe such an approach in my next post.