Keybushing Caul FAQ
Q: What’s the difference between the two caul method and conventional methods?
A: With the two caul method, you bush one side of each mortise on a rail, then go back down the rail bushing the second side. While at first this may sound like it would take twice as long, it actually cuts time by half or more, while improving consistency. It does this through efficiency of tool handling and hand motions.
With conventional methods you use both hands to grab the ends of the cloth strip, then hold the ends evenly in one hand while picking up the glue stick with the other, apply glue to the cloth, then set the stick down. Next you transfer the ends of the cloth into separate hands and position it into the mortise at proper depth. Then, you hold both ends in place with one hand so the other can pick up and insert the bushing caul, then pick up the knife to trim. That’s a lot of tool handling just to bush one key! And, it is very difficult to judge proper cloth depth in the mortise when two pieces of cloth are filling it at once.
In contrast, with the two caul method you hold the cloth in one hand only, with about 1 1/2 inches of cloth extended between thumb and forefinger. With the second hand, you apply glue to this exposed length of cloth, set down the glue stick, and pick up a knife. With the knife in the palm of your hand, you can still pick up and insert bushing cauls with the fingers of that hand. You simply lay the cloth across the mortise, pick up an Intermediate Caul, insert it, and slice the cloth. Then immediately move to the second key and repeat the process. Every 6-7 keys you apply more glue to the cloth. When you get to the end of the rail, you remove the Intermediate cauls and repeat the process using the Final Cauls to install the second piece of cloth. It is very easy to maintain consistent cloth depth, and the entire bushing job for both rails typically takes 1 1/2 to 2 hours. See our Tech Tips page for illustrated instructions, and our publication Efficient Key Rebushing for additional details.
Q: What is the difference between Single Shouldered and Double Shouldered bushing cauls?
A: The single shouldered cauls are specifically made for the two caul method. Being flat on one side, they can be used for making “flush cut” (no shoulder) balance rail bushings, as most pianos have, by inserting with the flat side toward the cloth you are installing. Q: I already have sets of conventional double shouldered cauls. Can they be used with the two caul method?A: Yes. You can use your existing double shouldered cauls as Finals. To convert to the two caul method, just buy the appropriate Intermediate Cauls to go with them. Q: Do I have to buy complete sets of both Intermediate and Final cauls for each key pin size to use the two caul method?A: Not exactly. Caul sizes are specific to a given key pin size, since the idea is to mold the bushing to an exact fit to the pin. However, you can economize by using only 45 Intermediates for each key pin size, pulling the first ones out and reinserting ahead as you work your way down the rail. (You do need at least 90 Finals to work efficiently since the Finals need to stay in place several hours until the glue hardens to size the bushings.) Also, some sizes do double duty; i.e. the Intermediate for one size of key pin serves as the Final for another in some cases. See the bushing caul size chart for details.
Q: How do I know what thickness of bushing cloth to use?
A: Proper thickness is determined for each job by testing the dry fit of the cloth in a few mortises using the correct Final Caul as described in Step 2 here. The optimum thickness may be different for each rail as well. You need to have a range of thicknesses on hand to make this test.
Q: I notice that after removing the old bushings, the mortises vary in size somewhat and I have to choose an average cloth thickness for all the keys. Is there a way to make the mortises all the same size?
A: Yes, after removing the old bushings with steam or soaking (as described in our publication Efficient Key Rebushing and our Tech Tips page) , insert a Mortise Sizing Caul into each mortise while the key wood is still damp. Allow the keys to dry thoroughly and you’ll find the mortises are not only evenly sized but very smooth and firm. We manufacture these only for keys with pin sizes of .125″ – .133″, .146″ and .162″. However, you can use our key bushing caul size #4 (or DS-162 caul) as a mortise sizing caul for .087″ pin mortises.