Are more flutes always better?

Categories: Knowledgebase

Theoretically, adding flutes should increase productivity because with the same chip load you will cover more ground, achieving a higher inches per minute.

However, physical and design limits of the tool may negate the feed rate benefit. A 5 flute endmill can handle a radial engagement, or width of cut, up to 25% of the cutter's diameter. A 7 flute, with less room for chips, is limited to 15%. 

Even with the increased feed, the overall metal removal rate might be less.

However, if we apply radial chip thinning, the MRR becomes comparable. 

With more teeth the time between tooth impacts is less, in this case 30% less.

Less time between tooth impacts means that the tool must deflect less to get that next tooth to arrive on time.

That requires reducing the cutting forces and you do that by reducing the depth or width of cut. 

Or, you can slow the spindle speed down to give the next tooth more time to rebound back.

Adding chipbreakers to the 7 flute endmill may reduce the cutting forces enough to match the depth of cut. 

The 7 flute endmill will have a large core diameter than the 5 flute making it slightly more rigid.

Of course all of the above is theoretical. The 5 flute endmill may not be able to cut at 25% engagement, the 7 flute at 15% and 6000 RPM may not be a stable speed for either. The ONLY way to know for sure is to tap-test.

This is illustrated in the above stability lobe diagram generated from a tap-test. The red lobe is a 1/2" 5 flute endmill with a 1" depth of cut. The light brown lobe is a 7 flute endmill (also 1/2" diameter and 1" LOC) in the same holder and same machining center as the 5 flute. The 5 flute can cut deeper than  the 7 flute.   

At 1" depth of cut and 25% engagement, and a feed rate of 0.005" per tooth, the 5 flute produces 22.81 cubic inches per minute.

At 1" depth of cut and 15% engagement, and a feed rate of 0.006" per tooth (radial chip-thinning was applied), the 7 flute produces 17.33 cubic inches per minute.

This isn't always the case. Here are different 5 and 7 flute endmills in a different machining center and toolholder combination:

The 5 flute is in red and the 7 flute in light brown. While the speeds are depths of cut are very similar, the additional tooth will allow for greater metal removal rate. Here are the Dashboards:

Despite having a lower radial engagement, in this comparison the 7 flute had a much larger metal removal rate than the 5 flute, 12.66 to 9.06 cubic inches per minute.