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How Effective is MQL for Drilling Titanium/Aluminum Stacks?

Titanium expert Mark Larson describes how coolant properties affect the cut. MQL might work, but water-based coolant is typically best in titanium.
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A reader recently used the “Ask an Expert” feature of our Titanium Machining Zone to ask a question about MQL in titanium.

 
Question
 
What is your opinion of the efficacy of using minimum quantity lubrication (MQL) to drill a total of 308 holes through a sandwich stack of titanium and aluminium where the hole diameters range from 2.4 mm to 15.8 mm and the stack thickness ranges from 7.5 mm to 60 mm?
 
Response from Mark Larson, manager of titanium process development for Makino
 
My first rule is that anytime you have a closed feature such as a milled pocket or drilled hole, through-tool delivery is going to give you the best result. Evacuation of chips is critical for tool life and productivity.
 
Other factors are these: The better the lubricity, the better the life. The better the cooling, the faster you can run.
 
So, if you are “cooling” only, then life may suffer. If lubricating, cooling may suffer. Higher speeds create higher heat in titanium (Watch your surface speeds on these varying diameters to control heat buildup), and only a very tiny amount of that heat goes into the titanium chip. Therefore, cooling the tool is important. The fluid with the best “heat removal” parameter is water. That means use a water-based system if possible.
 
If you do use an MQL system, make sure it can deliver through the tool. Even at 3:1 L:D, through-tool will help.
 
As for the “stacked” material condition, make sure there is no possibility for movement or slippage of the layers. Tool geometries from cutting tool companies often share geometry between aluminum and titanium. Check with the tool supplier first to make sure the cutting geometry is applicable for both materials.

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