I've been experimenting with the relationship various parameters have on the rigidity of your tooling using the new rigidity calculator in my G-Wizard Machinist's Calculator:
It's fascinating to look at the impact of the different variables on the rigidity of the setup. We're taught to use rules of thumb having to do with ratios of the diameter to length. The reality is that rigidity is more complex than that.
Here are some random scenarios I played with:
- A 1/2" endmill at 1" depth is 1/2 as rigid as the one cutting 11/4" deep. Be careful with deep cuts and choke up on the tool as much as you can in the holder.
- A 1/4" endmill at 1" depth is only about 1/16 as rigid as the 1/2" endmill at 1" deep. Use the largest diameter endmill that fits your internal radii.
- A 5/8" endmill at 1" depth is 2.4 times more rigid than a 1/2" at 1" depth. I have a little 5/8" Iscar Helimill indexable cutter. Even though it adds a mere 1/8" in diameter, it is more than twice as rigid. That's why I like to rough with it.
Note that these "depths" are not cutting, they are the measurement from the toolholder to the tool tip. Rigidity is not related to how much you're cutting, but the latter creates cutting for that balances against rigidity to cause tool deflection.
This rigidity calculator is the first step of a more interesting journey, and that is the calculation of tool deflection. Deflection is even more interesting than rigidity. For example, the big tooling manufacturers suggest that when you get into the range of 0.001" deflection you're entering territory that can start to excite chatter. At the very least that much deflection will play havoc with surface finish and accuracy.
More news as I get closer to the deflection calculator. Meanwhile, everyone is welcome to sign up for the beta test and play with the rigidity calculator.
Cheers,
BW