Making Plastic Front Aperture Sight Inserts on the Taig Lathe

I recently bought a Diana 75 T-01 match air rifle. It only came with one front aperture sight insert, 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 recommend around a 4mm aperture for 10M airgun. 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. Acylic might be slightly better optically, but Lexan is what I had. I drilled with a #32 drill bit (.116", just inder 3mm) for a #4-40 screw hole.
Mounted several on a mandrel with a #4 screw
I turned them down a few thousandths under 17.5mm (.689") diameter, which is the size that fits the front sight mounted on the Diana.
I also chamfered the edges as the blanks were on the mandrel, using a vee cutter, as seen above.
First op. was to gently and lightly chamfer one end of the hole, both to provide centering for the next op. and to debur.
Then I drilled with a 4mm bit at relatively high speed, so as to get a round hole (hard sometimes with a drill bit as they want to make a Reuleaux triangle. I'm going to make a bunch with different hole diameters, for different situations.
Then I flipped the disc and put the 90 degree countersink in with a 90 degree center drill.
The finished insert, with the 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. So it works! All told it took about 20 minutes to make the one from start to finish, with several blanks left over.

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 in with the countersink minus the diameter of the aperture hole.

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