Tool rigidity

[compact8]

I am planning to make a broaching bar which is to be used for cutting keyways on my milling machine. Obviously, the bar should be as rigid as possible and yet easy to machine. After some searching online, I found that the Young's modulus for all kinds of steel are very similar. For example, 12L14 steel, which is the easiest to machine has a Young's modulus of 190-210 GPa. 303 stainless steel is 193 GPa. High-speed steel is 200 GPa. This is very different from my impression that high-speed steel is much more rigid. Am I missing something here ?

 

[ddickey]

I think yield or tensile strength is what is higher w/hss.

 

[jwmelvin]

Yup, it’s surprising at first but the stiffness is essentially the same for all steels while the yield strength and ultimate tensile strength changes dramatically.

 

[JRaut]

Spot on.

The Young's modulus of steel is the same, regardless of the steel grade. That's the number that speaks to how stiff a material is when it's responding within its elastic range of response. You push any material beyond that limit and all bets are off.

Higher strength steels (tool steel, etc.) can support more force before exceeding their elastic limit.

But for a tool holder, you should never be exceeding the elastic limit. Therefore, any steel will behave essentially the same from a tool stiffness standpoint.

One benefit that higher grade steels do have when it comes to making tooling is that they're harder than mild steels. So they can stand up to a lot more use/abuse/dings/dents/handling.

As a point of comparison of Youngs moduli:
- Aluminum ------------ 1/3x steel
- Steel (all) ------------- 200 GPa (30,000ksi)
- Tungsten carbide ---- 3.3x steel


So there are essentially three ways to increase tool stiffness:

1. Make it shorter
2. Make it bigger
3. Make it from tungsten carbide

 

[Lo-Fi]

Just as interesting is that the state of hardness doesn't affect it either. Robin Renzetti on the subject at 6:30:

Yield points are another matter entirely, however.

 

[francist]

I know the above to be correct, I just have such a hard time getting my head around it! I think Robin explains it quite succinctly in his video.

-frank

 

[benmychree]

I learned that back in the 1960s from a book titled Tool Steel Simplified, published by the Carpenter Steel Co. It was one of our apprenticeship textbooks.