Combating Chatter: 4 reasons to NOT reduce your feedrateApril 30, 2016, 6:58 pm
April 30, 2016, 6:58 pm
January 26, 2021, 12:44 pm
Sun November 28, 2021, 3:59 am
Sun November 28, 2021, 3:59 am
We all have heard hundreds of times that when chatter is happening during machining, we should reduce our feed rate. The same advice we also hear for compensating for extra-long tools and unstable setups.
Let me explain why I think this is mostly incorrect.
Let’s list the effects of reducing the feed rate:
- Reduces tool life.
- Reduces productivity.
- Increases deflection.
- Causes chatter.
Let me explain from my own experience and research I have made each of these points and a simple way to avoid chatter's adverse effects.
Reduction in chip-load reduces tool life.
This may seem counter-intuitive, but the tool gets the best cutting edge life only within a narrow window of chip-load range and cutting speed.
Reduce your chip load below a certain value, and you will be hurting your tool life a great deal.
Yes, your tool may run longer, but it will do a lot less work before it gets dull!
Reduction in productivity
This point follows right out of the previous one. Not only your tool now moves slower, thus reducing the material removal rate, but you also have to replace it more often. That adds extra setup time cost too.
Increase in deflection
Cutting action is a fine balance between a cutting edge plowing through the material, rubbing and shearing it. We want to do as much shearing as possible and less plowing, and ideally, no rubbing.
I recently did a test cutting 6061 Aluminum plate with a 1” long .25” dia Grooving tool and measuring its deflection at different chip-loads:
You can clearly see how below 0.0005” chip load (X-axis) bar deflection is double of what it should be if you were to follow the light-blue trend line. Here, we are getting 0.001” deflection!
Then once we increase the chip load to 0.001”, the tool is not rubbing anymore, and we get 0.0015” deflection - not double, but only 150% of deflection increase for 200% increase in chip-load.
The line continues steadily until we hit 0.003” chip load, and then it almost takes a dip!
Between 0.003” and 0.004” chip load, we get just 0.001” increase in deflection instead of projected 0.002”.
This means that the perfect chip load for the boring bar I was using is between .003” and .004” because right after the sweet spot, at .005” chip load, we get a sudden spike in deflection up to 0.010.”
As cutting edge (ideally) shears off a slice of the material, it hardens the work-piece right underneath. This layer of material will be sheared off by the next (or the same in case of a single flute) cutting edge.
If our chip-load is too low, we will be cutting right through the work-hardened surface, thus decreasing tool life (see point one) and actually increasing the cutting pressure.
Also, the cutting edge is not ideally sharp. It has an edge radius, half of which will be pushing the material up and the other one - down. So you will be having a lot of plowing and rubbing action too.
Because one image is better than a thousand words:
The red surface on the picture is the work-hardened layer. The green one - workpiece with the regular hardness. We want the cutting edge to shear off underneath the hardened layer.
In many cases, a lower feed rate may actually be the reason for chatter in the first place.
Have you ever heard an experienced machinist tell somebody to “load-up that tool” to prevent chatter?
You are most likely such a machinist, and you have noticed that increasing the feed rate may actually reduce chatter.
Why does chatter occur?
It is a combination of the above mentioned factors such as an increase in cutting pressure causing deflection and unstable cutting conditions.
The tool deflected by the cutting pressure tries to return to its original shape, and lack of chip load allows it to do so unobstructedly. The tool pretty much vibrates freely. When that happens thousands of times per minute, you get your tool singing.
When you increase the feed rate, you may actually create more stable cutting conditions and eliminate chatter.
So what should I do if I am already getting chatter?
First of all, make sure you are using proper speeds and feeds, to begin with.
Call your tool vendor to get proper numbers for your particular case, or try our HSMAdvisor- Advanced Speed and Feed Calculator.
HSMAdvisor will suggest you perfect starting speeds and feeds, and also it will suggest you proper Depth and Width of cut for your particular workpiece material and tool configuration.
If you used HSMAdvisor, chances are you are not going to get any chatter at all!
If you did not and want to go with your tool vendor' numbers - fine - try their speeds and feeds on your machine.
Did it help? If not, here is the simple solution:
Reduce the Depth of Cut!
While a reduction in chip-load leads to more rubbing, it does not reduce the cutting pressure in the same way the DOC reduction does.
For example. We might only need to reduce the DOC (and thus cutting pressure) by 20%, and the chatter will go away. The same effect would require us to reduce the feed by 50% or so if we are lucky. If we are not lucky, even further reduction in feed rate will not eliminate the chatter.
So in conclusion:
Whenever you are having chatter issues, get proper Speeds and Feeds and reduce that DOC!