There are a lot of things at play here...
Material choice: What material is used for the jig. Some metals can get harder than others. Some alloys are more wear resistant than others, even though the hardness is the same. And many times this has to do with the process used to make the jig or the target cost of the jig. In addition to that, there is the quality of the alloy used. A block of metal in the heat treat furnace is no different than a cake in the oven. What comes out depends on whats inside. Lower quality (ie cheaper material usually) has a higher probability of content variation. Just as certified material is notably more expensive. Its usually a battle to find the best mix of quality and price.
Heat Treating: Heat treating is just like anything else, sometimes things go wrong, or the timing gets missed, or poor quality material is used which affects the heat treat. Leave a cake in the oven too long or not long enough, and it may not be a cake. Use the wrong ingredients and who knows what comes out. Heat treating is a skill, just like call making. There is much to learn, equipment, processes, etc... A good heat treater is worth double the cost of a bad one... at least in my eyes.
Tempering: Heat treating is a two part deal... bring it up to the specified temp, for specified time in relation to thickness, and cool per requirements of the material. Then, its time for tempering. Bring up to temp (usually much lower than the initial temp), again, for a specified time in relation to thickness, and let cool. The first process brings the material to max hardness. The second, actually makes the material softer. Harder = brittle and Softer = durable. To hard, and it could break if dropped. Too soft, and well... you can file it.
Jigs: There is a balance that has to be kept with the making of the jig. It has to be hard so that it lasts. It has to be durable, so it wont break in to pieces the first time it gets dropped. And it has to be affordable. Add in the variable hardness of files, people using abrasives on the jig, and it turns into quite the battle to come up with a long lasting high quality jig, that is affordable.
Files: From what I have learned over the years... one can expect a good quality file to be in the 55 HRc range (aka Rockwell hardness scale) - which is right on the edge for some jig materials. The cheaper files, including ones with a good name, but manufactured elsewhere for cost savings - I have seen many instances where people are reporting them to be either softer and dull more easily - or in the case of some Chinese (only ones Ive seen specific info on - but would guess its not just those) files HARDER than expected and also more brittle - up in the mid 60s HRc... which is as hard or a touch harder than most, if not all jigs. But the wear caused by a file being equal hardness as the jig is pretty minimal so that at least helps maintain the life span of the jig to some degree.
I would estimate a properly made jig (material, heat treat, tempering) that one could very well expect it to last easily for 10's of thousands of calls. But, that does have to account for proper use. Im sure there is a "shelf life" just because of physics (friction and wear) but would guess that its long enough that a properly heated jig and files would last a lifetime to two - even with pretty heavy use.
If youre concerned about the life span of your jig, knowing that the hardness of files is an additional variable, I would try each file on an unimportant area of the jig with every file you plan to use on the jig, if the file skates along the surface, its okay. If it feels like it grabs in the slightest or removes material, I would set that one aside and not use it on the jig - just use it for hand tuning.
Ive found with the nicholson files Ive used... you can get about 10 calls per file before the file is annoyingly dull. Might be another variable to help identify if the file is harder or softer than it should be.
Keep in mind, on my jigs, since they are cut after heat treat, the EDM leaves an recast layer of the eroded material from the cut on the surface - the dark area on the surface of the profile of the jigs when you get a new one - that will be easily removed and is about .0002-.0003" thick. That is normal, once that is removed, the file should skate right along the surface. I would imagine on the jigs that are cut prior to heat treat (which I think are most if not all the jigs that are made in two pieces) that there will be a slight discoloration that will "polish off" with use that came from the heat treating process. But if you can knock a corner off the edge of a jig with a file, then it might be worth looking into the file or the jig.
Wade
