How to carry out a R&R study on a checking fixture?GD&T — By Gladys on 30 March 2012 at 6:16 pm
This video explains very clearly the importance of the repeatability and reproducibility study for checking fixtures (R&R), and how to use the data on a R&R study sheet.
When measuring a part on a checking fixture, three variation factors affect the measurement:
- 1. The checking fixture in itself: the design of the checking fixture will be crucial to limit variations when having several parts measured by one or several persons
- 2. The user, that is, the person using the checking fixture
- 3. The part: several factors can generate variations when measuring a part on a checking fixture, from its design to the type of material used to manufacture it.
The function of the study is making sure that the main variations are due to the part measured, and not to the checking fixture – which would mean that the design of the fixture is not correct – or to its user – which would mean that the user has not been properly trained.
The R&R study aims at defining which percentage of variations is generated during the measurement process, and what these variations are due to. A proper R&R study will be carried out by at least 2 users, who will measure a minimum of 10 parts by dial indicator, at least twice.
At minute 1:08, the video shows you an example of a fulfilled sheet: in this sheet, the R&R study has been done by 3 users, that have measured 10 parts 3 times. The table is composed of three sections, corresponding to the three users that partipated to the study. The values in columns are the three measurements done for one part, and the values in rows correspond to the ten parts measured for the study.
To enlarge the picture, right click and select “Show picture”
Bellow this data sheet, you can see the different indexes that will enable you to determine which are the variation factors in the measurement process of your checking fixture, and whether the variations due to the checking fixture are acceptable. You can see these indexes at minute 1:44 of the video:
- the equipment index (checking fixture index): how much the checking fixture contributes to the global variation of the study
- the user index: how much each user contributes to the global variation of the study
The calculation generated by these two indexes indicates how much, or which percentage of the global variation is due to the equipment, and how much is due to the user : this is the repeatability index.
If the index is lower than 10%, it means that the variability in the measurements is low: the checking fixture is repeatable.
If the index is between 10% and 30%, it means that the repeatability of the checking fixture is acceptable, but that you will have to apply at least one of the following two measures to improve it:
- 1.train better the checking fixture user, to ensure a proper mounting and measurement of the part.
- 2.modify the design of the checking fixture to improve its repeatability.
But, how can you know if your checking fixture is OK? Bellow the variation indexes calculation sections, you can see two graphs:
- The first one, presented at minute 2:48 of the video, presents the measurement variations of the different users, which enable to detect a first possible cause of variation. In this example, you can see that two users have got very similar results, while the third one stands out. This can mean that it is necessary to train this third person, so that his results can be coherent with the ones of the two others.
- The second one, presented at minute 3:07, displays the variation ranges. In this example, you can see that the user represented by the blue curve has got rather coherent results; on the contrary, the user represented by the pink curve has a strong variability. Consequently, it is important to study why this user has measured in a different way, compared to the two others: because he has not understood the correct procedure for the mounting of a part on the checking fixture? Because the checking fixture fixation system can lead the user to make mistakes?
This study is part of a series of studies done for the MSA (Measurement System Analysis), aiming at detecting faults during the measurement process, in order to guarantee an optimum control of your parts.