I don’t always get have fun fixing old radios, restoring vises, or cleaning up antique tools. Sometimes I actually have to fix stuff around the home. Here’s a short post on one of these occasions. A couple months ago we bought a new light fixture to replace one over the kitchen table. From the bottom of the fixture, a threaded rod sticks out and on the end of it there is a decorative plastic nut that holds another piece on. I noticed the other day that this plastic nut had cracked somehow. Of course they don’t have a replacement for this at the store and I’m not going to buy an entire fixture. So, I wandered around the store and came across something that I could modify to work.
Here’s a picture of the parts. The one on the left is the broken plastic piece while the one on the right is the aluminum part that will need modifying. Notice that the threaded holes aren’t the same size…so that’ll need fixing.
The first thing to do is figure out what size threads are on the light fixture. To do this, I use a thread pitch gauge. To use it you flip through the leafs until you find one that matches the thread. Once you’ve found the pitch of the threads, you can figure out the diameter by measuring the diameter of the threads or looking it up on a thread chart. Below is an example of how to use the pitch gauge. This picture isn’t from the lamp though. In the picture below, you’d read that the thread pitch is 20. From there you can measure the diameter to determine that it is a 1/4″x20. The threaded rod on the lamp had a thread count of 27 per inch. Referencing a thread pitch chart, I determined that it was a 1/8″x27 NPT (national pipe thread). This is very annoying because there is not much in the stores that is this size. Luckily I have a pipe tap!
Before you can use the tap to thread the hole you have to enlarge it to a specific size. Most thread charts list the drill diameter required. For the 1/8″x27 NPT size, a “Q” size (0.3320) drill bit is required. To hold the cap I came up with this…contraption. Yeah, it’s not pretty but it worked. Necessity is the mother of invention and all that. As an aside, you don’t want to try to hold parts in your hand while you’re drilling into them. Never hold metal parts because the drill bit can grab it and may cause you harm. I found a drill bit that was a little larger than the existing hole and started increasing the diameter of the hole using successively larger drill bits.
Once I had the right sized hole drilled, I placed the cap in the vise between two pieces of wood to keep from marring the surface. Carefully start the tap making sure that it is perpendicular to the surface. For every half rotation of thread cutting, back the tap out one turn to clear the threads. Then advance the tap until you feel it starting to cut again and repeat the cutting and backing out steps. Pipe threads are different from regular threads because they’re tapered. So, you can only go so far before before the tap gets really tight and you’ll have to quit.
I removed the tap, blew the chips out of the cap, and went into test it on the lamp. Well, it turns out it didn’t fit. It wouldn’t even start on the threads. What now? Remember how I said pipe threads are tapered? I went back and enlarged the hole two steps to a “S” diameter and taped the cap again. With a larger hole the tap could go in farther. This time the cap fit the lamp!
Here’s a picture of the lamp with the new cap.
So, there ya go a “simple” fix! OK….maybe not too simple if you don’t have the right tools but if not you’d have an excuse to go buy more tools.