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PT Doc

Active User
Active Member
Dec 27, 2012
What is the general recommendation for a 3hp knee mill when considering a face mill with intergral r8 shank ? Option available is 2.5” diameter with 5 flutes or 3” diameter with 6 flutes?

This will be a new purchase so I can’t try both out. They both use SEHT inserts so 4 cutting edges are a plus.

Thanks in advance for the help.


Carbide Guy
Active Member
Dec 15, 2014
PT Doc:
Insert milling cutters are great! And your choice is particularly good for most materials. I was a technical resource for Kennametal salesmen for 35 years and milling was always fun. The 20 deg. positive insert with the "H" tolerance is tough to beat. Now, the R8 platform is very good for small diameter milling cutters, but, they do have some limitations. I would recommend the 2.5" cutter with 5 inserts, but if you need the 3"diameter and plan to take a full width cut, you may want to remove 3 of the inserts so you don't run out of horsepower and stall the spindle. Bad news on a knee mill since the feed motor may continue to drive the table.
There is a simple metal removal calculation that will work for just about any application you will ever encounter. Only materials that are very high strength will need reduced parameters. Most metals, including pre-heat treated 4140, 17-4PH, 15-5PH, stainless steel, and many others will machine at 1 cubic inch per minute, per horsepower. To calculate a cube we only need three dimensions, length in per minute IPM), depth of cut (DOC), and width of cut (WOC). To calculate hp. requirements multiply IPM x WOC x DOC.
A 2.5" cutter has a periphery of .6545 ft. A 2" Width of cut on 4140 bar stock would run easily at 350SFM, 535RPM. With a feed per tooth of .005, the IPM would be 13.375" (5 x .005 x 535). A 2" WOC would allow 3 hp. to be consumed at a depth of cut of .112DOC.
13.375 IPM x 2" WOC x .112 DOC = 3 cubic inches per minute and at 80% efficiency, that equals roughly 3 hp. and gives you a little wiggle room for positive vs. negative, dull vs. sharp, etc.
For a couple of other materials:
Aluminum will net 2 to 4 cubic inches per minute
Cast Iron will net 2 cubic inches per minute (G3000, G3500, G4000 CI)
TiAlN/ALTiN coatings are universal and work well on many materials. Since your knee mill has fairly limited rpm compared to CNC machines, choose tough grades for best durability. Grades described as 30, 35, 40 will give excellent service and they will take a beating. Higher the number, the tougher the grade, but you will sacrifice a little wear resistance and a little heat resistance. For Aluminum go uncoated, TiCN, or TiB2 for best results.

Good luck with the new cutters.
Best Regards, Gary
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