Will it do any damage if I use 3mm and 6mm collets for 1/8 and 1/4 mills?
ive done it with both those sizes in an er25 chuck, it does work in a pinch, but its not practical. you need to put the nut on first or the collet wont fit, which makes the whole thing a bit clumsy because especially on the 6mm you have to pry it open a little to make it fit.
basically, unless your really desparate and cat wait, but a properly fitting collet.
I have ER16 and Kress spindle. The collets on my Kress has been divided in 4 grips and is only split in the front.
The Kress spindle is on my router and I need to use a 3/4 planner bits with 1/4 shank in order to plane wooden surfaces.
Most brands of ER collets are designed to collapse 1mm from the open size. The 1mm collet is good for 0.5 mm collapse.
So for 0.125 use a 4mm collet and for 1/4 use a 7mm collet.
For best grip and concentricity you do want a collet the same size as the tool but for wood working it should not be a problem.
A 6mm ER collet is usually specified as 6-5mm, meaning, in theory, that you can use it to grip anything between 5 and 6mm diameter. For a quarter inch ( 6.35mm ) shank you should use a 7mm collet ( 7-6mm ). If you force a quarter inch shank into a 6mm collet it will permanently expand it and reduce the gripping power over the proper 6-5mm range thereafter - don't ask how I know this! Same considerations apply to an eighth inch shank in a 3-2mm ER collet. I must admit I never make use of the theoretical gripping range of an ER collet and always use the exact size for the job. I would advise imperial collets for imperial shanks. My tuppenceworth.