Wringing is the phenomenon of adhesion of two flat and smooth surfaces when they are brought into close contact with each other. The force of adhesion is such that the stack of a set of blocks will almost serve as a single block, and can be handled and moved around without disturbing the position of individual blocks. More importantly, if the surfaces are clean and flat, the thin layer of film separating the blocks will also have negligible thickness. This means that stacking of multiple blocks of known dimensions will give the overall dimension with minimum error.
Fig.1 Functional features of a slip gauge.
When two surfaces are brought into contact, some amount of space exists between them. This is because of surface irregularities and presence of dirt, oil, grease, or air pockets. Let us assume that the two surfaces are perfectly flat with highly finished surfaces, free from dirt and oil, and firmly pressed together. Now the air gap becomes so small that it acts in the same way as a liquid film. The thickness of this film can be as low as 0.00001 mm. Now a question arises as to why the blocks stick together so firmly that even a high magnitude of force acting perpendicular to their surfaces will not be able to separate them. A combination of two factors appears to ensure this high adhesion force. First, as shown in Fig. 2, an atmospheric force of 1 bar is acting in the direction shown by the two arrows. This is contributing to the adhesion of the surfaces of the two slip gauges.
Fig.2 Wringing phenomenon
Secondly, the surfaces are in such close proximity that there is molecular adhesion of high magnitude that creates a high adhesion force. Since the slip gauge surfaces undergo lapping as a super finishing operation, material removal takes place at the molecular level. Since some molecules are lost during the lapping operation, the material is receptive to molecules of the mating surface, which creates high molecular adhesion.
These two factors collectively ensure adhesion of slip gauges with minimum air gap between them. Therefore, a stack of slip gauges will have a length equal to the sum of the individual heights.
Technique of Wringing Slip Gauges:
The ability of a given gauge block to wring is called wringability; it is defined as ‘the ability of two surfaces to adhere tightly to each other in the absence of external means’. The minimum conditions for wringability are a surface finish of 0.025 μm or better, and a flatness of at least 0.13 μm. Wringing of slip gauges should be done carefully and methodically because a film of dust, moisture, or oil trapped between gauges will reduce the accuracy of measurement. The first step is to clean the slip gauges immediately before wringing because any gap in time will allow dust and moisture to settle on the gauges.
A very fine hairbrush can be used to clean them. Some people are under the false notion that a thin film of oil should always be applied to the gauge surfaces before wringing. Most often, the application of oil itself may introduce unwanted dust and oil in between the gauges. The need for additional oil film is felt for worn out gauges where there is reduced metal-to-metal contact resulting in poor molecular adhesion.
Fig.3 Technique of wringing slip gauges (a) Step 1 (b) Step 2 (c) Step 3 (d) Step 4
The following are the preferred steps in the wringing of slip gauges:
1. Clean slip gauge surfaces with a fine hairbrush (camel hairbrushes are often recommended) and a dry pad.
2. Overlap gauging surfaces by about one-fourth of their length, as shown in Fig. 3 (a).
3. Slide one block perpendicularly across the other by applying moderate pressure. The two blocks should now form the shape as shown in Fig. 3 (b).
4. Now, gently rotate one of the blocks until it is in line with the other block, as in Fig. 3 (c) and (d).
Combining Slip Gauges:
As pointed out earlier, gauge blocks are available in standard sets of 31, 48, 56, and 103 pieces. While figuring out the slip gauges that are required to make up a given dimension, a procedure must be followed to save time and, more importantly, to ensure that a minimum number of gauges are used. Please remember that more the number of gauges used, more is the separation of gauges by a thin film, which can cumulatively contribute to substantial error. In addition, the accuracy up to which a dimension can be built depends on the gauge that can give accuracy up to the last possible decimal place. For instance, while the 103-piece set can give an accuracy of up to 0.005 mm, the 56-piece set can give up to 0.005 mm.
Thus, whenever we need to build slip gauges to the required height/dimension, the primary concern is the selection of a gauge that gives the dimension to the required decimal place. This is followed by selection of gauges in the order in which they meet the dimension from the next last decimal place towards the left until the entire selection is complete.