A jack is the opposite of a clamp, pushing things apart instead of pushing them together. If you build guitars using an outside mold there are times when you need to hold the rib assembly of the instrument securely inside the mold while maintaining access to the edges of the ribs. Guitar rib jacks are the tools for this job. These are simple shop-built tools and there are many different styles. The most common are made using turnbuckles. Mine are copies of those used by my colleague John Calkin, and they are made of scrap wood, threaded rod and through-drilled knobs.
First appeared: December 16, 2007
Last updated: September 11, 2018
Guitar rib jacks are used to hold the ribs securely in the mold. This is particularly important when the top or bottom edges of the ribs are being planed or sanded to conform to the domed shape of the plate that will be attached to them. In a small shop environment the edge of the ribs and the linings are sanded to shape using a dished workboard (also called a hollow form) which is covered with sandpaper. The ribs assembly is clamped into the mold, and the sandpaper covered workboard is scrubbed back and forth over the edge of the ribs until it is sanded down to the same domed shape as the workboard. Performing this operation when the ribs are not securely clamped down tends to move the ribs around in the mold, and the shape doesn't always end up quite right. In a production environment a spinning dished workboard is used, and the entire mold with the rib assembly in it is positioned onto the spinning sanding dish. This operation absolutely requires rib jacks to hold the ribs inside the mold.
For first time instrument makers I want to be clear that rib jacks should not be used to force ribs that have not been properly bent into the mold for gluing of the plates. Doing so will surely result in an instrument that will prematurely fail. Ribs should be accurately bent to shape, taking as much time and as many attempts as are necessary to get it right.
Here is a picture of my rib jacks in use:
Each jack is made up from two end blocks, a length of 1/2" - 13 threaded rod, and two knobs with 1/2" - 13 threaded through holes. The end blocks are made from scrap wood and have blind 1/2" holes drilled in them.
As you can see, the jack which is used to span from the neck block to the tail block of the rib assembly actually uses two lengths of threaded rod, connected by a coupling. Constructing the long jack this way makes it possible for the jacks to be used to support the ribs of an instrument while the back is glued on. After the glue dries the jacks can be disassembled and removed through the soundhole.
I rarely do this, but occasionally the ribs of an instrument are floppy enough so this needs to be done. It is nice to have jacks that can be used this way in a pinch.
All of the parts are readily available, and all can be ordered from McMaster Carr. Two jacks, one that goes between the blocks and the other that goes across the waist of the instrument, are usually all you need. But I would suggest that while you are building them that you also make a third jack that can span the lower bout at the widest point.
These are simple shop-built tools, but they are extremely useful when building guitars using the outside mold.