Occasionally I have had the same issue with regular pen bushings. Perhaps it is just the tolerance of the bushings, or a build up of plating on the edges (electroplating can be a little wonky around sharp 90 degree edges). If a quick hit with my chainsaw file doesn't fix it, I simply switch to my "C" Drill Rod mandrel (0.242 inches in diameter). I think most standard mandrels are made from "D" Drill Rod which is 0.246 inches in diameter. Sometimes just a few thousandths (along with the tolerance) can make all the difference. I have even had 7mm tubes that were just too tight on my standard mandrel, but switching to my "C" sized mandrel fixed things perfectly.
"Way too small" though sounds like there might be something else going on. In addition to the mandrels I make with "C" and "D" drill rod, mandrels come in various sizes. Their designations are usually "A" and "B" (no connection to the letter sizes for drill rod).
When someone talks about a mandrel it is usually the "A" mandrel as it is the most common and widely used mandrel. It is sometimes called a 7mm mandrel because it is made to work with 7mm tubes. "A" mandrels are commonly specified as 0.247 +/- 0.002 inches (thus it encompasses both the "C" and "D" drill rod diameters that I use.
The "B" mandrel is 0.291 +/- 0.002 inches. It is typically used on Berea kits because the larger diameter helps prevent whip while turning. This requires bushings and accessories that are designed specifically for the "B" mandrel's larger diameter.
Regards,
Dave
PS Nylon does dissolve in Acetone, but at an extremely slow rate so cleaning nylon in acetone should be OK. HDPE on the other hand is not attacked by acetone (as is ECTFE (Halar) and PTFE (Fluorosint).
