I've been using the same 2 larger files for a few years -- trying to preserve them by using a grinder or lesser files on rough castings first, where necessary to get below sand and scale. My best file is used last, the duller of the two gets first dibs. But zinc alloy and brass work lately have pointed out the fact that it's probably time to move on to a new one.
My two most often used files are both big 14" flat double-cut Nicholson bastards, and not easily replaced, at least locally. Online prices seem to put similar files in the 25+ dollar range, even on Ebay, not including shipping, which in several ads brought the total to $40.
However, I was able to find a Nicholson catalog online, and through that, get a part number for the file. A new lookup on Ebay with that number found them available from Zoro tools for $14, plus free shipping if I made up a $50 order. So I plunked down for two of them. Looking through the Zoro products available I found a few useful items so I added a tube of Dykem Hi-Spot engineer's blue (for scraping), a bottle of Loctite 860, and a commercial size pack of JBWeld epoxy, all totalling almost exactly $50.
I realize files aren't the most glamorous shop tools to report here on the forum, but I sure can take a lot of metal off easily with those big files, and when dealing with castings they really make life eaasier. They also work extremely well for quickly bringing a surface to flat before final filing, scraping or finishing. Or conversely for rounding square corners with a few strokes.
I do use a lot of other kinds of files, and finer ones, too. But these are real workhorses in my tiny shop, and surprisingly versatile. They have also outlasted many other tools, though you'd think they'd dull quickly, considering what they have been used on. I do try to treat them well. I file in one direction. They hang from the wall, I clean then if needed (seldom since they are coarse) with a card.
Both my old files have a very slight bow, but I use this to good effect where needed to take off different parts of a surface, and where I want a true flat, I reverse them every few strokes, check frequently and adjust as needed. A common beginner mistake is to assume that flat files are true flat and just file away to finish hoping to flatten a part. When checked, disappointment sets in. Happened to me early on until I realized how to compensate.
Anyway, I'm kinda excited to get these in the mail. I know they will make a difference.