“Corncob” Roughers are endmills that have serrations on the flutes to help break up the chips.  You’ve probably wondered what the advantages are for these, so I thought I’d put together a post and talk about them a bit.  These roughing endmills are only useful for roughing as the serrations leave tracks on any walls machined on the workpiece.  So, unless you don’t care about those tracks, you’re going to need to put up with a toolchange to a finishing endmill and another pass to finish your part.  Because of these extra steps, it’s reasonable to wonder about the value of these endmills.

What do they do that regular endmills don’t?  There are at least two strong benefits to these endmills.

Their first advantage is that they often seem to minimize chatter.  If you’re hogging hard and getting chatter, try one and see if it doesn’t go away.  The configuration of the teeth with the serrations changes the characteristics of the vibration and can often move the frequency out of the critical zone for chatter.  For more about these sorts of effects, see our Feeds and Speeds Cookbook chapter on chatter.

The second advantage for these roughers is they can run considerably higher chip loads than comparable finishing style endmills.  The serrations chop the chips down to much finer sizes, which means more of them can be packed into the flutes without jamming.  In addition, the smaller chips are lighter making it easier for your coolant, air blast, or mist system to clear the chips from the cutter.  The combination of these effects is significant.  At the behest of a customer, I recently revisited the chip load bonus our G-Wizard Feeds and Speeds Calculator gives to roughers.  I went through the offerings from 10 different endmill manufacturers and reviewed the chip load multiplier for their corncob roughers versus the closest equivalent finishing endmill for aluminum and steel.  The results were impressive and ranged from a minimum of 1 to 2.75 at the top end.  The average was 1.48 and the median was 1.23.  When doing these kinds of analysis, I have a statistical model I use to clear out the far outliers and get to an amalgamated value that is a better than a simple average.  It’s proprietary so I won’t go into it.  When all of that was said and done, I wound up increasing the bonus G-Wizard awards these roughers from 20% to 38% (a change that will appear in the next release of GW shortly).

Is it worth a toolchange to get a 38% increase in Material Removal Rate?  You be the judge, but I really like using this style cutter.


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