I put the calipers to my Granberg chain the other day just out of curiosity to see how consistently I had been maintaining even tooth height with my hand filing. The way I measured it was by measuring the length of each tooth, since each tooth has the same downward slope on it I figured that would be a good proxy for the height.
It was a bit fussy to do, but eventually I was able to get consistent measurements that I had some confidence in. So apparently my tooth length (and therefore presumably tooth height) varies by about 10%. I was wondering at what point do I need to go knock them all down to the same height? I am thinking 10% is probably past time to do it.
I used to religiously grind my chains after 5-7 days of milling to get the cutters back to the same lengths. I did this to minimise chain vibe especially on the longer chains, but it's nearly 2 years since I have worried too much about cutter lengths without any increase in chain vibe.
I put the lack of chain vibe down to switching to progressive raker depth setting. Chain does not cut flat up against the bar so it's not just cutter height that determines the final amount of "wood grab" but cutting angle (raker-cutter-wood angle) which is a combo of raker height and cutter length.
Thus consistency in cutting angle is probably more important than consistency in cutter length. This consistency and the fact that the chain porpoises up off the bar during the cut enables each cutter length to vary a little but still grab about the same amount of wood. I have still found that when the cutting angle varies too much this produces a lot of chain vibe and it can also produce wear spots on the bar.
I set the raker for a cutting angle variability of about +/- 10% in cutting angle so theoretically the cutter heights could then differ by as much as 20% but in practice this is rare when cutters are new - if I see a cutter is clearly too long I do bring it back a bit more. A 20% variation in cutter length will be more likely when cutters are near the end of their life (ie much shorter) and at that point it does not seem to matter that they differ in length by that much.
In terms of filing cutters I file to remove glint and leave it at that length unless it is obviously too long and then do as I said above. During the next cuts the taller cutters may wear more and thus show more glint and thus need more filing.
Of course this does not mean that once can run any cutter length. If I replace cutters on a chain I do drop the length to be approximately the same as the others. If I rock a few cutters and they need to be tidied up I don't bother to file or grind all other cutters back to the same length - just file for no glint and set the correct cutting angle. These cutters will wear slightly less than the others over time and eventually the others will catch up to it. I haven't rocked a chain so badly that the majority of cutters need attention.
All the above works if the left and right hand cutters are of slightly varying random lengths. If all or most of the RH cutters are a different length to the LH cutters this can lead to other problems. In this case it's a good idea to pull them back into line.
