Variation on Cylinder due to probe touching angle

I'm not sure what you are talking about when it comes to using vertical touch points. I'm interpreting this as the 101 course said to measure the cylinder with the probe oriented in the same direction as the cylinder. This may be ideal, but it is not necessary. Quite often you just have to use the best probe angle you can get to measure a cylinder. Having the probe at a different angle may mean that you can't measure all the way around with one probe angle, but depending on your needs, you may get by with measuring just half of the cylinder, especially if you don't need to evaluate the form. If you need to measure all the way around and need 2 or more probe angles to access all the feature, that is fine, just be mindful that you are adding a few microns of error from the probe head's imperfect repeatability and the larger span you will be using of the machines measuring volume.

You may be concerned about the probes actual touch accuracy if the probe is at a different angle from the cylinder. This is quite negligible, especially if you have an analog probe. If you have a touch trigger probe it takes a little more force to take hits that are in the same direction of the probe vs hits that are square to the probe, but with an analog probe that is all compensated for. If you ever find yourself using a touch trigger probe and want to see how much this affects your measurements, you can measure your qualification sphere as a sphere with several hits at several levels and report/graph the form error. You would be able to see how much form error the probe and stylus combination has due to the different amounts of force needed to trigger a hit a different vectors. Normally this is just a couple of microns.

As for taking your first hits at the same level, that is just something to do to help PC-DMIS figure out the orientation of the cylinder when you are using manually measured hits to define a cylinder. It helps if you take a lot of manual hits at several levels. The more hits you take the more likely the software will correctly identify the type of feature you are measuring and the proper location and vector. You will know right away if the software misinterprets your intent. You'll end up getting the wrong feature or it will be pointing the wrong direction in the graphic display window. You may be able to override the guess, but in many cases you will just need to delete the feature and try again with more hits.

All that said, louisd gave some good advise for measuring that feature accurately. Considering how small that feature is and how much it is allowed to shift in position, a local alignment will help a lot.

I'm not sure what you are talking about when it comes to using vertical touch points. I'm interpreting this as the 101 course said to measure the cylinder with the probe oriented in the same direction as the cylinder. This may be ideal, but it is not necessary. Quite often you just have to use the best probe angle you can get to measure a cylinder. Having the probe at a different angle may mean that you can't measure all the way around with one probe angle, but depending on your needs, you may get by with measuring just half of the cylinder, especially if you don't need to evaluate the form. If you need to measure all the way around and need 2 or more probe angles to access all the feature, that is fine, just be mindful that you are adding a few microns of error from the probe head's imperfect repeatability and the larger span you will be using of the machines measuring volume.

You may be concerned about the probes actual touch accuracy if the probe is at a different angle from the cylinder. This is quite negligible, especially if you have an analog probe. If you have a touch trigger probe it takes a little more force to take hits that are in the same direction of the probe vs hits that are square to the probe, but with an analog probe that is all compensated for. If you ever find yourself using a touch trigger probe and want to see how much this affects your measurements, you can measure your qualification sphere as a sphere with several hits at several levels and report/graph the form error. You would be able to see how much form error the probe and stylus combination has due to the different amounts of force needed to trigger a hit a different vectors. Normally this is just a couple of microns.

As for taking your first hits at the same level, that is just something to do to help PC-DMIS figure out the orientation of the cylinder when you are using manually measured hits to define a cylinder. It helps if you take a lot of manual hits at several levels. The more hits you take the more likely the software will correctly identify the type of feature you are measuring and the proper location and vector. You will know right away if the software misinterprets your intent. You'll end up getting the wrong feature or it will be pointing the wrong direction in the graphic display window. You may be able to override the guess, but in many cases you will just need to delete the feature and try again with more hits.

All that said, louisd gave some good advise for measuring that feature accurately. Considering how small that feature is and how much it is allowed to shift in position, a local alignment will help a lot.