During the course of home repair or machine repair, you will occasionally encounter a bent cylindrical metal part. If it matches the industry standard, a warped screw or bolt can usually be replaced with a new off-the-shelf part. That’s the cheapest and most reliable solution.
However, sometimes a bent piece is unusual and can’t be easily replaced. Or, perhaps you’re in a remote location or a temporary repair is needed until a replacement part or machine can be obtained.
A couple of weeks ago, I noticed that my wife’s footstool was upside down with a leg removed. Upon closer inspection, I saw a bent double-ended screw that connected the wooden leg to the metal base. The leg had probably become partially unscrewed over time, causing an imbalance in the footstool, with the greater torque on the elongated leg resulting in a bend.
You can manually straighten a bend, although results will vary based on the type of material, complexity of the part, and the severity of the bend. But, be warned that it is nearly impossible to restore the part to absolutely perfect original condition.
I couldn’t immediately find a matching replacement part for my wife’s footstool screw. I could have looked a little harder or I could have made a replacement part on my lathe. However, I have an antique drill with a bent screw that I eventually want to restore using as many of the original parts as possible. So, I figured the bend footstool screw would be good practice.
Here’s the before and after picture:
Top: Bent screw. Bottom: Straightened screw.
If you look carefully, you'll notice that the straightened screw still has a couple of smaller ripples. Even so, a nut can travel the length of the machine screw threads and the piece is repaired well enough for satisfactory operation.
The machine-threaded end of the screw fits into a threaded metal hole in the base of the footstool. The delicate threads need to be protected during the straightening process, or else the screw won’t fit into the footstool base again. I could repair the threads with a special tool or with a die, but why not prevent damage in the first place?
I want to hold the bent screw in a vise while bending it straight. Ordinary hex (six-sided) nuts will adapt the threaded end of the rod to the flat jaws of the vise.
To prevent the screw from rotating in the nut while it is in the vise, I had the idea to split one side of a nut so that the vise jaws would force it tightly against the screw.
Left to right: Standard hex nut, a hex nut altered by cutting through one side, and a Dremel with a cut-off disc cutting the hex nut in a vise.
The nut was easy to split with a high-speed rotary tool with a cut-off disc. The split nut fed onto the screw without difficulty. However, after tightening the nut around the screw in the jaws of a vise, the split nut didn’t seem to grip any more effectively than an unaltered nut.
Perhaps a nylon or brass nut would flex and deform better than stainless steel? Or, perhaps I should use a nut with a nylon insert.
In the end, the screw didn’t rotate in the vise that much, so I decided to just use a bunch of unaltered plain nuts. I decided to include these pictures in this article in case a split nut is applicable to your particular project.
The footstool screw is complicated in that both ends have screw threads. To grip and protect the wood-threaded end, I decided to use a piece of wood. Wow, how obvious.
I selected a 5/8-inch diameter hardwood rod to hold the wood-threaded end. The outside diameter of the wood rod almost matches the inside of a 1/2-inch inner diameter steel pipe that I intend to as leverage during bending.
You might wonder why I wouldn’t use a 1/2-inch diameter dowel for a 1/2-inch inner diameter steel pipe. Well, it turns out that the “labeled” dimensions of steel pipe are the “industry accepted” values, not the actual measurements. The 1/2-inch standard pipe (schedule 40) actually measures 0.840 inches outer diameter and 0.622 inches inner diameter (less inner seams, tapered ends, and rough insides).
It’s bad enough that the US still uses the imperial system, but even worse that our 1/2-inch pipes and 2x4 lumber neither measure 1/2 nor 2x4.
As I was saying, the 5/8-inch diameter wood rod almost fits inside a standard 1/2-inch steel pipe. After a little hand sanding, the wood fits snugly.
Left and Middle: Sanding and drilling the dowel. Right: Screwing in the bent screw by gripping the machine-threaded end with RoboGrip pliers and a protective cloth.
The wood-threaded end of the bent screw is designed to securely cut into wood. To generate the necessary torque to feed the screw into the temporary wood rod, I wrapped the opposite end of the screw with a protective cloth and gripped it with a Sears Craftsman Robo-Grip pliers. Ordinary pliers would work as well.
Some people attempt to repair a bend by hitting it with a hammer. Although I suppose this might work in a pinch, a hammer is not really the correct tool for this job. The blows from a hammer:
We’re looking for a technique and tool that presents a gentle, controllable counter-force to the original bend. In this case, a long metal pipe fitted on one end of the bent screw can apply considerable force while being easy to guide by hand and eye.
Steel pipes are inexpensive, rigid, readily available, and tough enough to not bend or break when applied to smaller diameter screws.
One end of the bent screw has a bunch of nuts threaded onto it. The nuts are secured in a vise. The other end of the bent screw is protected by a wood rod. The wood rod is inserted into a long steel pipe.
Even with a rubber pad, the vise can’t hold the nuts well enough to straighten the bent screw with this setup.
Unfortunately, as soon as I pushed down on the far end of the steel pipe, the nuts slipped in the vise. I tried tightening the vise as much as possible and I even tried using a rubbery pad for added gripping power.
The force needed to bend the screw is greater than the frictional force of the flat vise jaws against the stainless steel nuts. The bent screw and nuts can rotate without being stopped by anything. There needs to be an unmovable physical object that blocks rotation.
For my second attempt at bending, the force is applied perpendicular to the vise jaws, rather than parallel to them. Instead of the vise holding the nuts, the nuts secure the screw to a thick piece of metal that has a hole in it.
The front and back flat square jaws of the vise securely hold one end of the bent screw while force is applied by a steel pipe.
For the thick piece of metal, I used a block from the Micro-Mark clamping set for my milling machine. However, any thick piece of metal with a hole in it should be fine. If the hole in the metal block is about the same size or larger than the hex nuts, you'll need to add washers to mate the nuts to the metal block.
With this arrangement, it is easy to gently bend a screw, rod, or other malleable material into alignment. The direction of the pipe controls the direction of the bend.
Assuming the material is of equal strength throughout, bending (or unbending) occurs between the nut and the metal pipe. By screwing the part further in or out of the fixture (or by adding a fulcrum), the location of the bend can be adjusted.
If I had to do it over, I wouldn’t have used a wooden rod inside of the pipe. The wood cracked and broke down during the bending process, making it difficult to predict how far the pipe needed to be tilted to straighten the screw. Instead, I would have used a thick piece of rubber or a rubber stopper (cork) that would rebound after force was removed.