Helen and Don,
As the latter has said, he needs them to remove metal and glass( enamel!) at the same time so needs justify the cost.
Interestingly, the way these files, disks, core drills cut is different.
Metal files cut one way ie like a saw blade having teeth pointing in one direction(OK! there are wood saws which cut in both directions! I know that!) whereas
diamond, Tcarbide and other grits cut in any direction. being random particles bonded or sintered or encosed in a matrix.
You can get these needle files in sintered Tcarbide which is more than adequate for most jewellers needs.
Don's applcation to remove both metal and glass is similar to the grind stone and water way used by enamellers since time immemorial.
Theres another cutting way tho, for enamels and metal at the same time.
I use 320 grit abrasive belts for just this ,cutting dry,(with vacumn!) as my enamels on copper are fired much higher than normal and the copper and enamel reacts on the edge of pieces to form a very hard oxide/enamel crust. When your making 30 items a week hand finishing is way too slow what with all the other work involved.
Then i refire at a lower temp to gloss the ground enamel edge at the 45 deg grind. angle.
Recently, I came across abrasive belts coated with diamond. 3M make them. . There likewise seriously expensive, used for finishing hardened material(hard facing!) like plastic extrusion feed screws.
My machines for belts, ive several, all run vertically, and have a free area as well as a supported back plate. mostly 1in wide by 48in long.
Problem is they get used for just about every other metal removing task !!..Even sharpen HSS drills,
sharpening wire brushing wheels, derusting tools deburring blanked metal prior to using 3M consolidated scotch brite wheels. thers only one type that cuts titanium without balling up thats T carbide 1000 mesh wheels. there 6in dia and 1 in wide and last for ever. bought mine some 30 yrs ago and 90% of the wheel is left.
Hope this background info is useful for someone.