I am starting to play a bit with brass, and it seems the best surface finish comes from using a fly cutter.
I am running a 3HP (ISO-30) spindle so I can drop the RPM down a bit, but fly cutters like slow speeds … 500-2500 RPM
Now the depth of cut is going to be small, so I don’t think HP will be an issue.
Anybody have recommendations? (TMToronto, looking at you 
)
Thanks
Carl
Hey, interesting topic. I think the challenge is achieving appropriate surface speeds at low RPMs and little torque. I have also read that these tools are not really balanced for high RPMs so it may harm bearings over time, as well as being dangerous. If not balanced the surface finish may suffer as well.
On a related note, I have had great success with OCD endmills and inserts, followed by polishing - that is aluminum. I have often thought about the large mirror like surfaces that I see fly cutters produce, but I just don’t think I have the right machine.
Sorry, OCD is me…PCD are the diamond endmills 
Ironically, this just up on my YT feed (mirror finish on copper)…
https://www.youtube.com/watch?v=pSnWD7SQHS8
…and this is the tooling from Datron that they use with their high speed spindle
https://shop.datron.com/product/stepped-double-flute-end-mill-with-edge-radius/
I would like to get more Datron tooling but the cost to get it to my door from their USA based distribution centre is extremely high. I had my eye on the above tooling in the 14 0r 20 mm diameter.
I have heard / seen the Datron bits in a NYC CNC video:
Machining Golf Ball Markers on the Tormach 24R and 1100MX!
It appears SMW is quite happy with the Datron’s… to quote them “they will treat you right if you treat them right”. He used the 14MM 2-Flute Facemill at about $150. I agree the bits are a probably double what I would like to pay, but if I end up doing more brass, they maybe in my future.
My reading has lead me to believe that why single cutting face cutters work because there is a very tight tol on that ONE cutting edge. When multi faced cutters are used they were talking tols in the ten thousandth of an inch.
I had read in forums as well that multi-flute surfacing cutters, like those using inserts, work best when all flutes are within incredibly tight tolerances. Some people said they had luck removing one of two inserts to avoid this problem, essentially making a mini fly cutter. Some said that the re-cutting of chips causes the ‘less than mirror’ finish some experience with multi flute/insert tooling.
Using flutes with ‘wipers’. radiused edges, and/or flood coolant have also been mentioned as important to getting mirror finishes when facing, and good finishes in general when machining.
For interest, and in case they are new to you, here are a few videos I had bookmarked in the past when looking into facing aluminum for some of my projects - these two both compared the Datron endmill we discussed above…
to get a good finish with a family cutter is a combination of speed and feed you may have to play with that also you need a little weight or Heft To smooth out that fly cutter interrupted cuts I hope I’m explaining that right so that it’s understandable