Tooling Rigidity

[BobWarfield]

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

[Crashmaster]

I am curious to hear what you find out about deflection, please keep us posted...

[BobWarfield]

The deflection calculator is completed, and now part of G-Wizard. In essence, it let's you optimize your depth of cut or width of cut by locking one or the other, and then finding the maximum value for the floating parameter that is within your tool deflection allowance.

Let me elaborate.

When roughing, you can tolerate a lot more tool deflection. The biggest issue is that when deflection exceeds 0.001", you start to invite chatter.

When finishing, the operation is more sensitive to deflection. A value more like 0.0002" is needed for best surface finish, not to mention accuracy.

The tool looks like this:



Note the Optimizer line. So, you enter the basics--material, type of cutter, diameter, flutes, etc.

There are basically two common cases. Either you are slotting, in which case you just hit the Slot button. It'll stick the diameter in as width of cut and calculate the max depth while maintaining no more than the roughing (0.001") tool deflection. Full width slotting is pretty much a roughing op, because you really want to individually cut the sides for finishing.

In the second case, you are cutting something less than full cutter width. Here, you enter your depth and it will figure out the width of cut given that you want to either Rough (0.001" deflection) or Finish (0.0002").

Here is what the Optimizer popup looks like:



You can see how you can lock depth or width and so on. There is an option once you optimize, if you still don't hit your desired depth, you can click the feedrate button and it will try slowing down the feedrate to hit the deflection allowance.

Note that it can't slow too much, or the tool starts to rub and that's a problem, so it has limits there.

All in all, I'm really happy with it. I have never seen a tool quite like it for optimizing milling parameters. More details are here:

http://www.cnccookbook.com/GWCutOptimizer.html

Cheers,

BW

[John Welden]

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

Cool stuff, thanks.

I don't quite understand how to do the math to figure out how much more rigid one tool is than another. Say .5 em vs 1.0 ?

[BobWarfield]

John, hence the calculator so you don't have to work the math.

Take your 1/2" EM versus a 1". We need to know how much the tool is hanging out of the holder to give a real answer, so let's make some assumptions. If the 1/2" is hanging out 1.25" and the 1" is hanging out 2", then the 1" is only about 1.3x as rigid as the 1/2".

Tool stick out makes a BIG difference!

OTOH, if we stick them both out 1.5", that 1" tool is now 5.3x more rigid. If your machine can support the extra rigidity (1" is pretty big for a lot of hobby class machines), that means you can go to town roughing on that part with the bigger cutter.

The step that G-Wizard takes beyond that, is to build a tool deflection calculation into the Feeds and Speeds calculator. You can set up a deflection allowance and see just how much faster you can cut with the 1".

Cheers,

BW

[John Welden]

Thanks for reply.

I appreciate the calculator, but what I’m trying to understand is how to do the actual equation. Lets say, ½ em vs 1” both sticking out the same amount. The 1" is X times more rigid. How do I figure that out?

I’m a math idiot and just trying to understand.

[TOTALLYRC]

Hi Bob,
I was out in the shop the other day and I went to G-wizard as usual to check some speeds an feeds on an unusual setup. I noticed that I needed and update and I got the rigidity update.

Seeing as how I was using a xl 3/4" endmil and had to put the quill down almost 5 " it was interesting to see the effect on rigidity.

Very nice work and keep it up.

Mike

[BobWarfield]

Thanks for reply.

I appreciate the calculator, but what I’m trying to understand is how to do the actual equation. Lets say, ½ em vs 1” both sticking out the same amount. The 1" is X times more rigid. How do I figure that out?

I’m a math idiot and just trying to understand.

John, check the screen shot, the basic math is in the comment to the right of each parameter.

Cheers,

BW

[Eurisko]

Great job, Bob.

The relationships between rigidity, length and diameter should be posted in every shop. The actual numbers are surprising, much greater than I would have expected.

In our shop, tool lengths are specified on the process sheets. A tool, built to a standard length, which is used on a variety of jobs.

Unfortunately, the length is usually too long, and we end up fighting the process until we give up and shorten the tool.

Which crashes on the next job because the operator didn't check the clearance.

So much for standardized lengths.