Why are 24 TPI jigsaw blades recommended only for up to 3mm of metal?
As far as I recall, the usual recommendation is to have roughly 3 teeth in the material. That gets it cut and moved out quickly, without grabbing excessively. The finer blade does not have room to store 6-8mm worth of shavings (i.e. in the tooth gullets until the blade comes out the far side) so it does not cut as well as is ideal, since it is "packed full" before it can dump chips.