4x cone diameters as datum

question for the pros out there
i have this casted part that has a datum target of 4x diameters.
these diameters are in a conical hole, and they re boxed 8.2mm down in the cone hole.

to get this datum
do i measure 4x circles 8.2mm down in the hole, and bestfit to them
or do i measure 4x cones, construct 4x circles using cone feature and 8.2mm down, and bestfit to them

ty all
love u
boy hol

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Mike Ruff over 2 years ago

As BKulpa stated, you need to measure these as cones. If you probe a circle at a depth of 8.2, your diameter will measure .052mm smaller than it actually is at that depth (assuming perfect form) when using a Ø1.5mm probe. The error will increase with a larger tip and decrease with a smaller tip, but the error will always be there. See picture below for clarification. This happens because a circle is probed at 90°, while your surface is at 75°, so you are not probing normal to the surface. You are actually probing below the intended depth of the circle (.194mm below the specified 8.2mm when using a Ø1.5mm probe).

HTH
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Mike Ruff over 2 years ago

As BKulpa stated, you need to measure these as cones. If you probe a circle at a depth of 8.2, your diameter will measure .052mm smaller than it actually is at that depth (assuming perfect form) when using a Ø1.5mm probe. The error will increase with a larger tip and decrease with a smaller tip, but the error will always be there. See picture below for clarification. This happens because a circle is probed at 90°, while your surface is at 75°, so you are not probing normal to the surface. You are actually probing below the intended depth of the circle (.194mm below the specified 8.2mm when using a Ø1.5mm probe).

HTH
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