So I just received an LMT Onsrud 57-285 Downcut bit today. It can be referenced here.
Since the default Shapeoko Feeds and Speeds chart isn’t set up for this bit, I’m trying to reference the Onsrud tables from their website to calculate feed. But the formulas from their page don’t make it easy to calculate feeds and speeds because you need to know 2 out of 3 things: Chip load, RPM, or feed rate. Well technically I want to calculate RPM and Feed rate, so this becomes difficult. Their chart for hard woods is: http://www.onsrud.com/files/pdf/2012%20LMT%20Onsrud%20Production%20Cutting%20Tools%20Hard%20Wood.pdf
Using that table, I’m trying to get a chip load of .005-.007 for that series mill in 1/4" diameter. If I assume a 19000 RPM speed (from the base Shapeoko chart for bamboo), I get a feed rate of 190 IPM (FR = 19000 x 2 x .005) which seems crazy.
But now after searching around the internet some people use a feed rate formula of FR = Speed x # cutting edges x tool cutting diameter x chip load. That gives me a much more reasonable 47.5 IPM (FR = 19000 x 2 x .25 x .005).
So I guess the question is which one seems right to you?