Introduction by B.B. Pelletier

Guest blogger
Today, we’ll start a new tradition, where I periodically turn the reins over to a guest blogger. We have done similar things in the past, but it was never formal. Now it is. If you would like to write a post for this, please contact me at

Bloggers must be proficient in the simple html that Blogger software allows, know how to take clear photos and size them for the internet (if their post requires them) and they must use proper English. We will edit each submission, but we won’t work on any submission that contains gross spelling or grammatical errors.

Today’s guest blogger is Nick Carter. Nick has his own blog at The Mechanical Philosopher, where he talks about machining, airguns, making the jewelry that he and his wife sell, and his sons. This report was originally posted on his blog, and I asked him to share it with you as our first guest blogger. Now….

Making Plastic Front Aperture Sight Inserts

by Nick Carter

I recently bought a Diana 75 T-01 match air rifle. It came with only one front sight insert with an aperture of 3.5mm diameter. I was having a dickens of a time getting a good sight picture, so I noodled around on the internet for information and found that most shooters recommend around a 4mm aperture for 10m target shooting. I could have bought either metal inserts or a set of plastic inserts, but I figured I’d save some money and have some fun making my own plastic inserts.

Some squares cut out of 1/8″ thick lexan. Acrylic might be slightly better optically, but Lexan is what I had. I used a drill press to drill a hole in the center of each blank with a #32 drill bit (.116″ – just under 3mm) for a #4-40 screw hole.

I mounted several on a mandrel held with a #4 screw, chucked in the lathe and running true.

I turned them down to a few thousandths under 17.5mm (.689″) diameter, which is the size that fits the front sight mounted on the Diana. This gives a stack of round blanks.

While the blanks were on the mandrel, I chamfered the edges using a vee cutter, as seen above. This removes the sharp edge of the blank and makes it easier to insert in the front sight tube.

I removed the blanks from the mandrel and mounted one of the blank discs in the 3-jaw chuck, which was equipped with soft aluminum jaws bored out to hold the disc. I lightly chamfered one end of the hole to provide centering for the drilling operation and to deburr the edge of the hole.

Then, I drilled with a 4mm bit at relatively high speed to get a round hole, which is hard sometimes with a drill bit, as they want to make a Reuleaux triangle. It’s better to use a reamer if you have one in the correct size. I’ve made them with different hole diameters for different lighting situations.

I flipped the disc in the chuck and put in a 90-degree countersink with a 90-degree center drill. This is what gives you the dark circle of the sight. The thickness of the ring, assuming that you use a 90-degree countersink/center drill for making the conical countersink, is equal to the depth that you drill with the countersink minus the diameter of the aperture hole.

The finished insert with the old metal insert beside it.

As you can see, I get a nice dark ring without the added distraction of the little arms on the metal insert sight. I snapped off a quick 5 shots to test and shot the smallest group thus far. It works! All told, it took about 20 minutes to make the one from start to finish, and I had several blanks left over.