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This is our old divider. Notice the missing thumb screw that locks in the radius.
I use dividers regularly in my work. They’re indispensable for spacing out and dividing lengths—whether laying out dovetails, bisecting a line by drawing intersecting arcs, or sketching circles and curves.
At my school, we have a few old dividers, some over a century old, that are still functional. However, one was missing the retaining screw that locks it in place. And since I refuse to let a good tool sit idle just because of a missing screw, I decided to fix it.
This story will walk you through the process of sourcing or fabricating missing fasteners. So, the next time you lose a screw, you’ll know what to do. More importantly, it’ll give you the confidence to pick up an old, incomplete tool at a flea market, knowing you can restore it with minimal effort. And if all else fails, I’ll show you how to drill and tap a hole in steel, allowing you to fit a new thumb screw if you can’t find a match for the original threads.
Step 1: Check Your Thumb Screw Collection
A good habit is to collect orphaned thumb screws whenever you come across them. Keep them sorted in a small box—you never know when one will save the day. Over the years, I’ve accumulated a variety of miscellaneous thumb screws salvaged from old equipment, lamp fixtures, and more.
The first step in fixing a missing fastener is simple: try matching one from your collection. Luck was on my side this time—I found an old metal thumb screw from a discarded lamp that fit the divider’s threads perfectly.
Step 2: Identifying the Threads
But what if you don’t have a match? In that case, you’ll need to determine the thread size and buy a replacement from a hardware store or an online supplier like McMaster Carr catalog.
A thread identifier plate. This is a helpful tool, but it only has holes and internal threads to check for external threads founds on screws.
The best way to identify threads—especially internal ones, like the hole where the screw goes—is by using a thread gauge. I keep a few in my shop: some are wire-connected sets, while others are individual screws and nuts stored in a dedicated box. I highly recommend getting both metric and imperial thread gauges in small, medium, and large sizes. When buying, opt for steel rather than aluminum, as aluminum is too soft and can strip when forced into a steel thread.
Once you’ve identified the correct thread size, you can order the part and call it a day.
This set includes nuts and bolts to probe for internal or external threads.
This is a handy set that has many rare thread identifiers.
This steel thread gauge includes studs with both interior and exterior threads. It is a great set for the shop.
Step 3: When the Threads Don’t Match
But what if none of your thread gauges fit? This is common with older woodworking tools, which often used non-standard hardware that doesn’t correspond to modern industry sizes.
In such cases, you’ll need to take a more drastic approach—drilling and re-tapping the hole to fit a standard thumb screw. Stay tuned for Part 2, where I’ll show you how to do just that.
Here are some supplies and tools we find essential in our everyday work around the shop. We may receive a commission from sales referred by our links; however, we have carefully selected these products for their usefulness and quality.
