I had two c-clamps which I made many, many years ago when I had access to some workshop tools like welding machine, and bench grinder and could find many off-cut discarded steel materials. My intention at that time was to practice my mechanical fitting skills. My original c-clamp had a solid nut welded at the end of the threaded clamping rod.
They were usable for most purposes, but when the pieces to be clamped must not move, it could not perform very well. Because of the turning movement of the threaded rod during tightening, the work pieces tend to move slightly when they were pressed together, and so were not accurately held in the intended position.
Later, when I observed that commercially-made ones had screw swivels at the ends of the threaded ends, and that they were useful for keeping the work pieces in place, I decided that I too needed to modify mine so that it would do the same.
Contact surface at the step of threaded rod
However, it was not as simple as it seemed. The commercially-made ones seemed to have custom-made clamping shapes which could fit in exactly over the threaded ends. For me, I had to fabricate and cut from raw materials.
For the c-clamp, the threaded rod would be screwed in so as to transmit the rotating force into a linear force. From the threaded rod, the force must be transmitted to the clamping face. From what I could observe, there were basically 2 types of design:
- The linear force acting on a step, cut into the threaded rod being transmitted to the clamping face. In order to make this, I would need to fabricate some sort of cup-shape face with a hole in the middle and then grind the threaded rod to a smaller diameter so as to be able to push through the hole in the cup-shaped face. To prevent the swivel face from coming out, I needed to expand the end of the threaded rod by punching it like a rivet.
- The linear force acting on the face of the threaded rod being transmitted to the clamping face. In order to do this, I need to grind a slot near to the end of the threaded rod so that the side edges of a clamping disc could be pressed into the former. I have to be careful with my measurements here. If I made the slot too far from the edge, then I may find that the side edges of the clamping disc are too short to fit into the slot and so not able to prevent it from dropping off. If the slot is ground too near the edge, then I might find it difficult to bend the edges of the clamping disc. Also, the clearances between the threaded rod and the clamping disc face must be sufficient so that when in use, the end of the threaded rod will touch the clamping disc face. Furthermore, I would want to minimize the height of the clamping disc because I want the c-clamp to be able to clamp as big an object as possible. If the clamping disc becomes too big, it will severely limit the capability of my c-clamp.
Contact surface at the end of the threaded rod
Another factor was the thickness of the threaded rod. It was only 6 mm in diameter. The challenge was with welding of thin steel plates when making the clamping disc which would be located at one end of the threaded rod of a c-clamp. All my welding efforts yielded poor results. Since I do not have a lathe, I would have to do all the fabrication by welding. I have tried using super glue, but it could not stand the forces encountered when in use.
After many attempts, including using plywood, I came out with a workable solution which did not require a lathe or welding. It just needed a steel pipe, hand drill, file and hacksaw. If you have a bench vise, it would be helpful, but otherwise a hammer would do.
This solution uses option 2 described above. Below are some basic steps:
- File (or grind) a slot at the end of the threaded rod.
- Cut a short piece of steel pipe, the size you want the clamping disc to be.
- Drill a hole in the center.
- Cut a slot over the hole.
- Pry open the slot.
- Insert the threaded rod into the hole on the pipe.
- Close back the slot by clamping with a vise, or just by hammering on it.
- Flatten the pipe by clamping with a vise, or just by hammering on it.
- File (or grind) the corners to make them rounded.
That’s it.
https://youtu.be/TQM8KsjHUQc