Toolholder taper contact of 85%?

Hi all,
While looking at tooling specs, I see reference to 85% engagement of the taper. Here's the comment: "The toolholders also maintain an 85%
toolholder taper contact against the inside taper of the machine spindle."
What does this mean exactly? Does it mean that the balance of the engagement is achieved via tension on the drawbar? If so, that would explain the benefit of HSK tooling which I understand has a machined flat that the tool is pulled against. TIA.
Cheers, Peter
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Peter,
It's very difficult (and expensive) to make two tapers so precise that you get 100% contact. The odd 15% is probably accounted for by surface finish "micro" irregularities, and angular tolerances. 85% is probably pretty darn good.
Mark

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