Fluffy mdf

[Neale]

I think you probably have an ER11 or ER20 collet on the end of that spindle. Extra collets aren't very expensive (take a look at Arc Euro Trade, for example) and it's well worth using the right ones. In theory, one collet will handle nominal size shanks or up to 1mm smaller but especially in the smaller sizes they don't like compressing that easily so I also use a mix of metric and imperial size collets. 6mm shank cutters go in a 6mm collet and the 1/4" cutters (nominal 6.35mm) go in a 1/4" collet rather than using the larger collet for both. You only have to save one broken cutter to make up the cost of a collet!

[Jonathan]

Yes, Arc Euro is a good place to get individual collets. I got a set in 1mm increments off eBay, then got 1mm, 1/8" and 1/4" collets from Arc. You can put a 1/4" cutter in a 7mm collet, but it'll wear out the collet faster and takes longer to do up the nut!

With the ER collet system you should never try and clamp a cutter that's bigger than the diameter of the collet, even if it's just 0.1mm bigger. Depending on the size, they can clamp 0.5-1mm smaller than the nominal diameter.