Choosing End Mills

Collets shrink as you tighten them, that's what they're for. I'd try the 6mm in the 6.35 and the 3mm in the 3.2, just in case. If the relief slots in the collet bind up before it puts pressure on the tool shank, then yeah, it's too big. "Real" collets (ER32, 5C, R8, etc) have ~1mm or so of actual capacity for using undersize or out-of-system sizes. Obviously the smaller the collet the less room it has to shrink, but you might get lucky.

As for sources, I no longer live there, so I can't help you with that one. Axminster was usually a good bet for quality (at a price) but Amazon/eBay will probably be fine for getting started in soft materials.

MDF can use regular 2-flute mills with aggressive cuts. Acrylic, especially extruded rather than cell-cast, is more finicky. I'd probably try and find a 4 or 5 flute mill, run at high speed, moderate feed, and keep some kind of coolant handy that won't craze or frost the plastic. It likes to get melty and/or crack. Best thing for acrylic is actually CO2 laser light. Someone here who CNC cuts acrylic plates might be more use on that though, my experience is more traditional machining.

 

1/4" and 6mm may look close enough, but I wouldn't dare putting a 6mm shaft into a 1/4" collet. You may be able to get away with it for 1/8" and 3mm.

You will always be able to find an endmill for your collet in whatever type.

I would do the exact opposite of what was suggested for acrylic. Infact, what was suggested would cause serious harm (from my experience) Acrylic cuts easy when you can remove material, which decreases heat. More flutes does not help remove material. With acrylic, I use a 3mm o-flute at about 20k rpm. 1000mm/min feed works perfectly fine for me. For a finer finish, You can turn your feed down to about 75-80%, but keep an eye on it. I can easily cut at 2mm depths, but much more and holding the material becomes the challenge.

He's using a router, not a low rpm spindle. My intention was not to be disrespectful.

 

Specifically a rotary tool/router collet, sure. They don't have much range of motion. Real 1/4" collets will happily take 6mm tools, I do it frequently. Even an ER-25 should, though I'm not sure about ER-20 and down. Might be a little tighter at that size.

No worries, that's specifically why I mentioned that my experience was in traditional machining- ie. 2000rpm and below. For that, to reduce the chipload per tooth (and reduce shock load on a brittle material on each tooth pass) the flutes get bumped since the RPM can't be. The higher flute count isn't to remove more material specifically (though it would technically allow you to increase your feed rate for HSM), but to allow you to take the same material in smaller bites. For HSM, I'd imagine the opposite is true. You're taking very small bites almost to the point of merely rubbing, so whacking the feed up to get the chips flying is critical.

That said, removing heat with material always applies to everything, if only for dimensional stability. I'm not sure what you mean by 0-flute, since that would just be a round shaft, but at 20k revs, I'd imagine even a 2 flute could be quite sufficient depending on feed rate, though low tooth counts on a low-precision spindle combined with a brittle material would make me nervous. As always, the triangle of flute count, speed and feed should be calculated out for what you're trying to achieve. I imagine there's also a difference in the machining data for extruded vs cast acrylic, but I'd also guess it would vary depending on flute pitch, flood/mist/thru-coolant, material thickness and workholding, etc etc. Plastics are tricky, I'm happier doing metals!

Often the best option as used on CNC centers in industry is the old "tighten till you hear the crack, then back a quarter turn"- ie. trial and error during the part setup phase, just keep turning everything up until something breaks, then drop it back down again to where it was good, and then you know you're near the limit of the setup and getting maximum capacity from the tool, rather than babying it and losing out on potential cycle time gains. Different machines may prefer feed over speed, or vice versa, depending on the components used, so playing around a bit is usually a good idea anyway, even when you've calculated the theory.

 

"oh"-flute, not quite zero-flute
Much different profile made to literally scoop material. They work great on higher rpm or heat sensitive materials.

As for the collet, you can, but do you really need to? You can always find your mill in whatever shank diameter.

 

Just looked them up. Very handy! Gotta keep those in mind for the next CNC build after the one I'm about to start.

Right now I only have a short set of imperial R8 collets and a random mix of ER32 collets, so it's generally the case that I have to just find the closest workable option. A complete range of R8 and ER32 (plus other, future spindle tapers) collets is a small fortune!