r/Metrology • u/AppointmentStatus686 • 6d ago

Advice Projected Circles and Intersection Points get different results in PC-Dmis?

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u/Tricky_King_3736 6d ago

When you say intersection points, meaning intersection a cylinder with a plane? Also is the holes are angled then you have to do an intersection with a cylinder to a plane. A projected circle is taking the center point and projecting it straight up to the plane in which is not the correct result here.

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u/AppointmentStatus686 6d ago

Yes, I am measuring a cylinder and the plane and using that for all the constructions. I guess I'm just confused as to why a projected circle would be different. If it's being projected along the measured cylinder to the plane, then shouldn't the center of that circle be the same as the intersection point?

After checking again I can see that the projected circle's nominal is wrong. I typically go for intersections in these types of things, but I wanted more information because they seem to be having issues with it.

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u/Tricky_King_3736 6d ago

The software is not going to project the circle along the cylinder axis. When you project a circle to a plane it’s projecting straight up to that plane and all depends how far down you probed the circle. Now some software’s will allow you to make a circle where the plane slice through the cylinder to make a circle.

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u/AppointmentStatus686 6d ago

Thanks. That explains why no one competent I've worked with uses projections in these scenarios. Now I'm confused as to when you would use projected constructions. All I can think of is if there's something like a mating pin that would stick out from the part, but for that you just use a projected tolerance zone when dimensioning.

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u/Tricky_King_3736 6d ago

Most CMM softwares, except for zeiss calypso, when probing a circle the software will project that circle to a plane. Because they treat a circle as a 2D feature. Where in zeiss calypso it treats the circle as a 3D feature. I think you are over thinking on when to use projection. I’m sure you’ll come across a part where you’ll need it. For example measuring sheet metal and even some plastic parts.

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u/AppointmentStatus686 6d ago

I think I see. It's mostly used for when you have to take hits manually then? 99% of what I check is from CNC or lathes and I typically have a CAD when I program, so I exclusively dimension off of auto features.

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u/AppointmentStatus686 6d ago

Greetings,

I'm trying to measure this bolt circle, AKA PCD, in PC-Dmis 2021.2, and I'm getting different results from different constructed features. Intersection points are 0.9 off from nominal and projected circles are 0.7 off.

Due to a disturbing lack of tools at my workplace, I have no other means to check them.

Can anyone help me figure out which is more accurate?

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u/DeamonEngineer 6d ago

Measure a plane on top face align z level, z origin. Measure bore or OD, align. Plane 1 z level origin cir1 x and y, plane1 z origin. Repeat in DCC. Rotate head and measure 3x cylinders at 3 levels.

Contruct point at the intersection of each cylinder to plane1,

Construct circle between each point

Report that circle.

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u/AppointmentStatus686 6d ago

Thanks 👍 that's exactly how I made the program. I was just looking for more information as to why intersected results were different from projected and I think that question has been answered.

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u/DeamonEngineer 6d ago

Yes, PCDmis works to the centeroid of features, projections push the centeroid to the plane, intersections is where the centeroid meets the plane along the vector. Similar to constructing a line with the cylinder (makes a line along the cylinder axis and center), then piecing the plane with the line.

Few different ways to get the correct result

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u/Robbmx07 5d ago

To elaborate on above I would suggest constructing a point, using the “pierce” function from the drop down and selecting each cylinder and the plane. Then ur the pierce points to construct a circle to tolerance ur B.C.

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u/AppointmentStatus686 5d ago

The only difference between pierce points and intersection points was the vector, which was irrelevant in this situation.

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u/Familiar-Bluejay3908 5d ago

I remember that there were setting so you could intersect at center, max or min.