How to Stop Tool Chatter and Deflection

How to Stop Tool Chatter and Deflection

by Bob Warfield

In a word, “Yes!”  Let’s talk about why.

The reason we use fewer flutes in some materials has to do with the behavior of chips in those materials.  Simply put, aluminum creates bigger chips, all other things being equal.  This has to do with the way the material curls as well as other factors.  The space created by the flutes of the endmill is where the chips have to go as they’re being cut.  If there is not enough space relative to the volume of the chips, you’re going to have problems and may wind up with a broken cutter.  Therefore, we typically dial back the number of flutes for aluminum because it creates a larger volume of flute space to carry away the oversized chips.

This productivity issue, where more flutes can be more productive, has to do with two factors: Material Removal Rates (MRR) and Surface Finish.  One matters more to roughing (MRR) and one obviously matters more to your finish passes.  This all has to do with what I’ll call the Tyranny of Surface Speed.

Let’s put aside this issue of flutes and talk about single point cutting on a lathe.  It’s so simply, it helps shed light on what’s going on.  For every material there is a best surface speed that the manufacturer recommends.   This recommendation largely has to do with spinning to the (or workpiece on the lathe) as fast as possible without harming tool life.  The limiting factor is heat.  Tungsten Carbide will tolerate a lot more heat than High Speed Steel before it begins to soften.  If the material your tool is made of softens, the tool’s sharp edge quickly dulls and your tool life is shot.  So, you want the tool to handle as much heat as possible, and that’s why Carbide can often beat HSS.  Since we can’t beat the speed limit, we have to fiddle with other factors when we’re up against it.

Well, then it’s time to delve into times when they make the best sense.  To do that, we have to consider the phenomenon of rubbing when chip loads get too low.  You can read more about it in our feeds and speeds tutorial, but suffice it to say that if you move the tool too slowly, eventually the chips are so thin relative to the cutting edge, that it is unable to cleanly slice them off.  It plows at them and can even skate a long for a couple revolutions before it manages to pull out a rough dirt clod of a chip.

The answer is to use a single flute endmill because it halves the necessary feedrates without rubbing.  So there is one case where it helps, when the machine just can’t feed fast enough to keep up with what the spindle is putting out and maintain adequate chip loads.

Here is the other case:  Whenever the extra chip clearance is of benefit.

There are lots of chip clearance challenging scenarios out there:

–  You’re cutting really gummy cast aluminum plate.  Definitely go to 2 flutes instead of 3, but you may find you have to go all the way down to a single flute.

–  Micro-cutters have terrible geometry compared to larger cutters, it’s just life in the world they live in.  You can only make the cutting edge so sharp, and at micro-scale it isn’t sharp enough.  So the flutes are beating their way through the material like a cold chisel and 5 pound sledge instead of cleanly slicing.  The tendency to chip welding is much higher and chip clearance is problematic when micromachining.  Switch to single flute.  Datron has been recommending this for roughing for a long time and even makes special singles flutes with a geometry allowing them to be balanced for high rpms.

–  You’ve got to cut a brutally deep slot or small deep pocket and it is extremely hard to pull the chips up out of the hole.  They are just hanging out down there clogging up the works.  Ideally, you’d try through spindle coolant, but failing that, give single flute endmills a chance.

–  You’re doing deep relief 3D profiling where the cutter is dropping into a lot of narrow spaces without much clearance.

Okay, that’s two good cases for single flute end mills, and we’ll end on a third:  Some materials just come out better with single flutes.  Typically, these are softer materials that are easily scratched.  Making it easier for chips to get out of the way so they don’t go back in and scratch things up is a good idea.  Many plastics fall into this category, although a two flute will polish some plastics.  A lot of wood products will also benefit from a single flute cutter.  Soft woods and MDF comes to mind.  Any cutting of stacked sheets can sometimes benefit from a single flute cutter as well.  These differences are typically not very profound, and usually will only come to light at higher rpms.

There you have it.  A bunch of information to help you decide when it makes sense to use a single flute cutter.

On my visit with Datron, I learned they recommend single flute endmills for many applications.  Fewer flutes means less chip recutting and the finishes just come out better.  I walked away from the demo with a mirror finish mold part straight off the machine that sure made me a believer!

Since then, Datron has started recommending our G-Wizard Feeds and Speeds Calculator to their customers.  If you haven’t tried it yourself, check out our Free 30-Day Trial.

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I know that certain material prefer certain # of flute to be used to rough out material. can someone tell me what is a good number of flute used to rough out 316 ss

At a previous employer we had many Single Lip Cutter Grinders and made many ball and bullet nose cutters for Mold work. The rigidity supplied by the minor diameter often out performed any bought cutter. As to heat, the single flute with extra rake and clearance but without a thin flute also dissipated heat well. The finishes were superb!

Great article! I don’t know what I’d do without a single flute in 3003 or 5052 aluminum. Two and three flutes have a tendency for chip welding. I can even run the single flutes dry!

Datron micro cutters are great. But you have to learn on the job. I’ve been using them over and 15 years and they offer no data to help with speeds and feeds. It’s a case of suck it and see. Once you find the sweet spot though, the longevity is fantastic

Original Article: https://www.cnccookbook.com/afraid-tool-deflection/

 

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