Shortening a Bolt
Make what you always need when the store isn’t open!
Text, photos and video by Tom Hintz
Posted – 5-11-2011
If you are part of the real world you know that you will need the one length bolt that you don’t have when the stores aren’t open to go buy one. Sometimes the store doesn’t matter because what you built needs something in between the lengths stores carry. If you can cut and grind metal and maybe have a triangle file in the shop you can often make what you need in a few minutes right in your shop.
It is important to remember that to be effective (safe) a bolt must have 1-1/2-times its diameter engaged in a threaded hole or showing at least one full thread completely through a compatible nut. If you can’t meet either criteria you need to find another way to secure what you wanted this bolt to hold.
In most cases the home metalworker will be using a nut and that can actually help this shortening process a bit. Once the required length is determined mark bolt for cutting a few extra threads towards the end to allow some wiggle room during shortening. If the threaded portion of this bolt is long enough, thread on a compatible nut all the way down to the head or the edge of the shank to get it out of the way for cutting and grinding. If a nut won’t fit we can still shorten the bolt but it might be a tad more difficult to get it ready for the nut to start easily.
When you have established and marked the finished length cut the excess bolt off as square as you can. Pay attention while getting this cut started as the threads can influence the blade and try to add some diagonal to the cut. We will true up the cut end anyway but making the cut as square as possible saves a little work later.
Remove the Nut
If you were able to put a nut on the bolt before cutting start unscrewing that but stop when it gets to the new end of the bolt. The threads at the cut are often rolled over or have protrusions from whatever you used to make the cut. Work the nut back and forth to help re-form those threads. I usually us a box wrench and go about ½-turn forward and then back ½-turn or so several times until it feels good and free. Then go back it off a full turn and repeat the back and forth ½-turn again to get that spot cleaned up. Repeat this process until the nut comes off of the bolt.
Grind It True
At the grinder we want to grind the end where we cut the bolt off to make it reasonably flat and square to the rest of the shank and threads. We are removing major high spots that could make starting the nut more difficult. If the end looks pretty flat, that is enough grinding there.
Then turn the bolt so that it is roughly at a 45-degree angle to the surface of the grinding wheel and carefully grind the end of the bolt to create a small ramp. If you look at the end of a factory-made bolt you can see how small this bevel can be so don’t get carried away. I try to grind the edge just enough to add a small taper to the end of the bolt that will allow it to get started in the nut straight.
Extend the Thread
Now with the triangular file we want to open the beginning thread/s that helps engage the nut to get it started. To make starting the nut easier use the corner of the triangular file to clean up the end of the thread that runs out into the bevel ground earlier. Just clean and extend that thread AT THE SAME ANGLE that it runs around the bolts. Test the nut on the bolt to be sure that it starts easily. If the nut does not start easily try to find any marks in the thread that might indicate where the bind is. Here again, corrections should be made gradually. There will not be any major reshaping to do at this point. If you have a wire brush on the grinder use that to polish up the threads to make the nut run across them more smoothly.
Assemble the parts the new bolt is to secure and make sure that it has remained long enough to be tightened down normally. That’s all there is to it. Cutting a bolt down to size is not a difficult task but is one of those that seem difficult until you do it a time or two. Just take your time and keep the material removal to a minimum and all will be fine. And, you have a new skill that will make your custom projects a little more custom than they used to be.
Have a comment on this story? –Email Me