9) CMM errors

Question 1

What is the Accuracy of a measurement?

Answer 1

deviation of reading from known output

Question 2

What is the Precision of a measurement?

Answer 2

ability to reproduce a certain reading with a given accuracy.

Question 3

What is the sensitivity of a measurement?

Answer 3

ratio of instrument scale change to change in measured variable

Question 4

What is the Error of a measuring instrument?

Answer 4

quantified deviation between instrument reading and known output

Question 5

What Is the Uncertainty of a measuring instrument?

Answer 5

deviation between readings when a known reference value doesn’t exist.

Question 6

What are the 2 types of error within measuring instruments?

Answer 6

Systematic error: constant or change in a regular/ predictable fashion

Random: difficult to predict (e.g human, environmental)

Question 7

What are the 2 CMM error categories?

Answer 7

Spatial errors: errors in the measured position of a point.
- Accuracy of components of the CMM.
- Environment in which the CMM operates.
- Probing strategy used.
- Engineering characteristics of the workpiece.

Computational errors: errors in the estimated dimensions

• CMM software used to estimate the workpiece geometry.
• Precision of the host PC
• Number and relative position of the measured points.
• Geometry departure from the ideal geometric form.

Question 8

What is ISO 15530? Explain parts 1,3&4.

Answer 8

The standard to provide framework for evaluating uncertainty from setup and use of CMM.

Part 1: Overview and metrological characteristics

Part 3: Use of calibrated workpieces or measurement standards.

Part 4: Evaluating task-specific measurement uncertainty using simulation.

Question 9

Explain Interim checking.

Answer 9

Full CMM verification (calibration) - once a year, expensive

Interim machine checking - machine checking gauge, once a week, health check

Question 10

What is a Probing error?

Answer 10

Known as ‘R’

‘R’ is the error within the stylus. Calculate radius of r (distance from sphere centre).

Question 11

What errors can occur from small styli?

Answer 11

Small stylus - influenced more by surface finish, stylus can sink into surface imperfections.

Question 12

What error can occur from large styli?

Answer 12

Large stylus - mechanical filter, average out surface.

Question 13

Explain how combining x and y tolerance zones can hide an error value.

Answer 13

Combining x and y tolerance zones means that a max x and max y error could be out of the x and y zones but not show up as an error. Diagonal maximum (far corner of square)

This is called a Positional extended error.

Question 14

What temp should all measurements be referenced to?

Answer 14

All measurements to be referenced to 20 degrees Celsius.

Measurement tools should be calibrated at same temp.

Temp change changes length of material.