hy
some infos :
... if a drill shank is :
...... o7.8 , o7.9, use a o8 collet
...... o8.1 , o8.2, use also a o8 collet, but before use the grinder to taper the shank, in order to help it guide inside the collet (*)
...... o7.3, o7.4, use a o7.5 collet ( there are vendors that deliver also intermediate sizes )
...... o7.6, o7.7, use also a o7.5 collet ( grind the taper )
... check attached; it's grip is < collet chuck, but still, may deliver, as long as drill is sharp and cutting specs are low; however, this is more a solution for mills
... if you have $$$, get a set of .1 collets, er or for quick change hydraulic heads
most non standard drill shanks are hss, but, some vendors, also deliver standard shanks not only for their carbide, but also for their hss tools / kindly
* : a o8 collet works best with a o8 tool, so, once you deviate, the collet may get, just a bit, out of spec, so, for such non standards, use a set of, lets say expandable collets; for example i have a set of collets that i use only for standard values, and whenever i have a non standard, i use a collet from a separate set; this separate set is generaly subject to more stress, since it is clamping <>0 shanks, so i use here collets with -1 range; for tools with shanks 0, i use collets with h7 range; idea behind this, is that, the larger the range, the more flexible the collet is, thus should resist abuse even if a collet range is -1, tir increases with size difference, thus lowest tir is for o8 shank inside a o8 collet, o7.9 inside o8 collet has greater tir, and so on .... that's why i recomand using a o7.5 collet for a o7.6drill, instead of a o8 collet