General GD&T Questions

Common Questions:

I have been slightly confused with how to correctly determine perpendicularity for a part. If the user measures a small plane on a surface, the value is relatively low. If you measure the plane over a larger area, the feature length increases, and the gap created at the end of the plane is increased creating a relatively larger error. I am not sure what the correct feature length is supposed to be. I believe that if there is not a feature length on the print, we use the default length which is the largest length across the 3-dimensional plane. Therefore, the plane is translated to the center line of the cylinder (in my case) and the value displayed is at the farthest point away from the centerline (cylinder) on the plane. I am not exactly sure how the software is calculating this value.

1. What is the default feature length using when no feature length is keyed in?
2. Does the software use the overall length of the feature (the diameter) or does it use half of the feature (the radius)? Using a plane and a cylinder?
3. Should I be taking points over the entire surface (the plane) of the cylindrical part, or should I only check the call-out using a small plane on the surface? Depending on how I check the part, different results consistently obtained.

Answer:

Many Geometric Dimensioning and Toleranceing (GD&T) calculations are based upon the size of a feature so the size of a feature has a large effect on the calculation.

1. When no feature length is keyed in then the software uses the measured points to determine the calculation. Originally the Key In length was added because a measured plane added 1" to each side of the part. If a 1" surface area was measured the plane in Measure would be 3" long and the perpendicularity/parallelism value was 3 times higher than it should have been. The Key In was added so you could enter the length of the feature being checked and avoid this problem. In Measure 4.0 and later, the individual readings are used in the calculation rather than the overall geometry so the extra size of a feature doesn't come into play. So the key in option is a legacy hold over from older Measure software. If you key in a length on a feature it will begin at the center point of the feature and extend each direction to a value of the entered length/2. If the Key in is used the operator should enter the longest side of a plane or the length of a cylinder (depth of hole).
2. A plane compares each reading in the parallelism and perpendicularity command, not the overall geometry. A cylinder will use the length of the centerline to calculate perpendicularity, concentricity and parallelism as these commands are based upon the centerline, not the diameter/radius. The length of the centerline is determined by the two furthest points taken during the measurement.
3. They should measure the entire surface as that is what the print will call out. If only a small section of the plane is measured then you are not accurately representing the feature.

Keywords:

plane to plane, accuracy, gd&T, perpendicularity, length, parallelism, parall, perpendicular, measure, plane to plane