When advising a new maker about grinding /filing/sanding the bevels, the use of a familiar object is easier, and more likely to be visualized than an abstract measurement. Saying "Sand/File to about .040" " will produce many different thicknesses, as most folks have no idea what that thickness looks like. Saying "about the thickness of a dime" will give a mental picture, and a physical object they can pull out of their pocket, that will get close to the desired edge thickness. A dime is approx. .050", and since most folks will over sand/file their first blades, trying to get them to stop there on the filing usually ends up as .030-.040" by the time they are ready to HT the blade.
I usually recommend that the first filing stage stop at the thickness of a nickel, then the sanding at a dime. With those approximates, the post HT edge should end up .010-.020 after sanding down the scale and decarb.
If enough edge thickness isn't left to remove in final sanding and polishing, the edge may end up sharpened, but with a layer of decarb still on it. It will seem to be a bad HT, and not hold up well until the third or fourth sharpening,.....when suddenly the knife starts cutting much better.
Now, as to cultural history of the "thin" dime":
When I was a boy a dime was different than today. First, it was a reasonable amount of money, often about ten minutes work. It was likely the highest value coin a person had in his pocket, as quarters, half dollars, and silver dollars, were heavy and not often carried.
The term "thin as a dime", "One thin dime",and "slick as a dime", come from those days when dimes were made from real silver (either sterling or coin grade). The silver wore down with wear in ones pockets and from handling, and before the dime was very old, the coin was often nearly .030 thick or less. The surface was smooth and the features of the coin and date would often be unreadable. I have seen them where the edge was worn down to almost nothing, maybe .010" .
This wear made for problems with the banks taking in rolled dimes from people. A roll of thin dimes could be as much as 1" shorter than a roll of new dimes. Most dimes with normal wear would be about 1/2" shorter. The tellers would feel they were being cheated by being given a "Short Roll". This made the tellers have to hand count the coins and re-roll them every time a short roll came in. If the roll looked OK, they would skip the re-count and take it in for $5. It was not uncommon to get a roll from the bank in the daily change pickup for businesses, and find the dime rolls were two or three dimes short of $5. Register ladies in stores and diners ( few were men back then), would count the rolled coins as they filled their till, as they had to pay any shortage at the end of the day from their own pockets.
As one who grew up realizing that a dime...and even a penny, had value, it makes me sick to see the young people today walk out of a 7-11 and throw their change on the ground.
When my daughter was a young teen ager, I noticed there were pennies in her bedroom trash can . I took them out , but the next time I was doing the trash, there were more. I took them out, too. When Saturday came around and she was expecting her weekly allowance of $5, she didn't get it. She came to me when going out with friends for the evening to get the money and I told her that she would get no more allowances. She looked shocked, and asked why? I told her if she felt that it was worthless and would throw it away, I was not going to give it to her, because it was still valuable to me. She said that she valued it a lot. Then I mentioned the pennies to her, and she understood. I let her go that week with no allowance. She though I was mean. The next week I noticed a jar on her dresser with change in it. To this day she saves every coin she gets in change and puts it in a big jar. Once a year she rolls it and has a big night on the town....just from saving a few pennies, nickles, and dimes.
