Spool clamps are simple screw based clamps that have many uses in lutherie. They are easy and cheap to make. Here's how.
First appeared: February 16, 2014
Last updated: September 11, 2018
Spool clamps are used to hold the plates of an instrument to the ribs during gluing. They are also used for a wide variety of small clamping operations. The clamps are made of two wooden spools on a carriage bolt with a wing nut.
Here is a picture of a small spool clamp:
The spools are made of hardwood. Any hardwood can be used. Here I am making them out of a cutoff of mahogany. Since you generally need a lot of spool clamps I try to bang them out as fast as possible. The quickest way I know of to make the spools is to use a hole saw with a 1/4" pilot drill in the dril press. A 1 1/4" hole saw makes spools approximately 1" in diameter. Use stock that is at least 1" thick. Set the stop on the drill press so the hole saw cuts 7/8" deep. Then drill the spools as close together as you can.
At this depth the center holes are drilled all the way through the stock but the spools remain attached to the stock. This makes it a lot easier than drilling them all the way through, because you don't have to pry each one individually out of the hole saw. Once the drilling is done, cut the bottom 1/8" off the stock using the table saw, and the spools just fall out, ready to use. Chamfer the edge all the way around on one end only of each spool. Then take a small 1/4" - 20 carriage bolt, insert it into the hole in one of the spools from the chamfered end, and pound it in with a hammer until the head of the bolt is flush with the top.
Flip it over and hammer the bolt out. Now you have a nice square hole in the end of the spool, which will receive the square shank of the carriage bolt in the finished clamp. Do the same thing to all the spools in the batch you are making. Note that it is really only necessary to have the square hole in one of the spools of a spool clamp. But I like to have the holes in both spools. In use, I often change out the bolts in a set of clamps for longer or shorter ones, depending on the clamping operation at hand, and I don't want to have to search around for a top and a bottom. With square holes in all spools, any spool can serve as either top or bottom.
The surfaces that contact the work are generally lined with cork. Cut out some squares of 1/8" thick cork sheeting, and punch a 1/4" hole in the center of each one. The really nice cork sheeting is made of ground cork and neoprene rubber. It is worth the extra money to get this, but hardware stores generally don't carry it, so you'll have to get it from an industrial supplier. Here are the parts needed to glue the cork onto one spool clamp. Note the short carriage bolt.
Push the carriage bolt through one spool. Apply some glue to the bottom surface of the spool. Push both pieces of cork onto the bolt. Apply glue to the bottom surface of another spool. Push that spool onto the bolt. Put a washer and wing nut onto the bolt and screw down firm but not too tight. So, we use the spool clamp itself to glue on its cork pads. Wipe up any glue squeeze out and let the glue dry.
Once the glue is dry the clamp is disassembled and the cork pads are trimmed with a small knife.
Now the clamp can be reassembled using whatever length carriage bolt you'll need for your clamping application.
There are a few variations to the general spool clamp which can be handy. When using the clamps for repair work on finished instruments, it is a good idea to either use partially threaded carriage bolts, or to slip a piece of pvc tubing over the bolt so the threads don't mar up the finish if it happens to touch a finished part. You can substitute sheet neoprene for the cork, which will provide a glue-proof pad. If you have an appropriately designed solera, it is possible to use just half of a clamp to clamp down plates, like this: