Self centering probing

[[Cl...]]

I need to check the depth of a drilled hole. It has a 140 deg drill point (cone) at the bottom of it.
We calculated the tangent point where the stylus makes contact with the cone and use that as
the nominal.

I'm using the self centering point option, but I'm not sure if I'm missing something. Hole diameter
is .112, using a .060 diameter stylus. Does the self centering point take into account cosine error?

[[Da...]]

No on the Cosine error question.
Zeiss use the feature geometry to remove cosine error. With self centering "Point" doesn't know the geometry just a center point in two axes. You should probe the cone instead of self centering.

Or you could do the math from a 140 deg drill to the center of you probes sphere.
You also need to change the Evaluation of "Touch Point" to "Midpoint"

Zeiss.JPG

[[Cl...]]

Thank you Daniel. I did as you suggested and my CMM measurement correlates very well with our hard Gage.

[[SH...]]

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What do you mean by self centring probe, creating a circle by pressing f11 and placing the probe in centre of the circle that method is self centering??.

May I know what is cosine error???

[[Cl...]]

I may not be explaining it quite right, but imagine using a test indicator to measure a height.
If there is too much/incorrect angle in the stylus tip relative to the work-piece, this will cause
the stylus to contact "not normal" to the work piece surface. This can give you incorrect results.

https://www.google.com/imgres?imgurl=ht ... egUIARDiAQ

[[SH...]]

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I think it is important in manual measuring but in cmm it is significant?. I am not using articulating probe head and I can't do anything with probe angle.

[[Cl...]]

There might be some subtle nuance's to my answer I'm leaving out, but If you are using CAD model, Calypso
will approach on the correct vector based on data from the model. Even without a model, Calypso is intuitive
enough to approach correctly.

[[De...]]

On the CMM all dimensions are calculated by the software from the center of the ball based on the distance from the point of contact to the center of the ball.

In order to do the calculation to be performed, Calypso uses the probing vector to determine where the point of contact would be.

If the surface being probed is a plane that is perpendicular to the probing vector (ie. probing along the surface normal) then this calculation is simple as it will only be the radius of the probe tip.

However if the probing vector is not perpendicular on a plane, or if is in fact a 3D surface being probed, then the calculation can only be performed if Calypso has an understanding of the surface that allows it to predict where the point of contact will be.

Somewhere in the documentation there is a document which shows the difference between different evaluation methods for points, such as touch points, plane points, space points, etc. which kind of explains this stuff.

So the answer to whether it matters on a CMM is complicated, but it essentially comes down to sometimes it does - it matters what you are measuring and how you are trying to measure it.

[[SH...]]

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I think this is the a core concept in cmm measurements, it is all about vector direction.if the surface is not smooth such casting or forging surface it became more complicated.

Thanks for the explanation.....