Usual preface, I’m with PreciseBits so while I try to only post general information take everything I say with the understanding that I have a bias.
If that was the issue you were having be sure to check your collets for damage. If the taper got damaged they can damage the nut and then the other collets used in that nut. Here’s another thread with pictures and discussion of the same topic. Proper Collet Use - Whoops - #9 by rb_designs
While not the issue here this is definitely something to watch for. It’s always in the smaller direction too so you could (depending on the ER size) use a 1/4" collet for a 6mm shank, but never the reverse. Additionally, runout is only spec’ed to the actual collet bore and typically increases even when going down in diameter within spec.
I’m extra bias here so I’ll only address a few things. A lot of it depends on what you are cutting, tooling, and tool life. A big part of this is that collets are a source of runout (tool spinning off the center axis of the spindle). One of the issues with runout is that it can functionally add and subtract to your chipload (feed) in multi-flute tools (also always effects it in straight plunging). This can effect the tool life, limit your feeds, or just straight break tools.
Examples: In metal cutting even with bigger tools better collets pay for themselves eventually. The usual number thrown around is for every 0.0001" (0.0025mm) of runout reduction you pick up 10% extra tool life. For small diameter tools there is very little chipload available so runout can significantly reduce your feeds or break tools. If you are cutting soft media with decent size tooling you’re probably fine with anything decent but could pick up some tool life, feed, and a better finish with better collets.
One thing I usually recommend if you are looking for high end is getting collets that are actually spec’ed for runout. This is pretty hard though as most collets list a “typical” number which isn’t very useful. There’s 4 brands I know of that actual measure the collet YOU get:
- Us (PreciseBits)
- Techniks “UP” grade
- Rego-Fix (creators of ER collets and what the R in ER stands for)
- Pioneer
Although, we all have different standards for runout.
I’ll stop here as I’ve already gone long winded on this. If you want more info here’s some of the post I’m aware of (posted in) where this is discussed more.
The last one goes off the deep end…
Hope that’s useful. If there’s something I can expand on or help with let me know.