Joe's 1000x600x200 steel and aluminium router started

[devmonkey]

Tool deflection chaps, tool deflection!

So I received a bunch of single flute endmills from China, these things cut aluminium and SRBP nicely. However I'm having some problems, and would like your diagnosis,

So I've just cut a bunch of parts that need a hole that is a reasonable sliding fit on a 25mm shaft. I used one of my new 4mm single flute endmills, 2mm DOC, 1600mm/min into 10mm SRBP. It cuts like butter. However when I measure the hole afterwards it is larger at the top than the bottom by about 0.15mm, effectively the hole is tapered. If I cut the same hole with my usual 6mm 2 flute it is straight.

I assumed this was the single flute deflecting on subsequent deeper passes but it could also be that the endmills are themselves tapered?? What do you think?

I guess the solution is to rough with the single flute and finish with the 2 flute, just a bit of a hassle.

Also could it be that i'm spinning them too fast a 24krpm and they are bowing out due to centrifugal forces?