Today we are going to evaluate just when you might use the MIN, the MAX, and the DELTA features on an indicator. However, let’s first just run through what each of these mean, along with a few other important terms, so we know we are all on the same page.
Every part or piece of material that is measured will have a certain type of shape to it. This shape can be planar, or flat; cylindrical, or round; or contoured, or of a particular curve. By simply placing an indicator against the surface of a part, and then rotating that part, you can read off the MIN and MAX measurements from the indicator across the whole surface. What you are doing through this process is finding the lowest point and highest point of the material making up your part. If you think about it, this must be important because if for example the planar surface of a part is supposed to be perfectly flat so as to fit into a bigger machine, discovering that the MIN and MAX readouts are different can help you to adjust the part accordingly. Then, the DELTA measurement from an indicator is the same as the total indicator reading, also known as TIR. The TIR is the difference between the MIN and the MAX measurements that were taken. Therefore, TIR is a measurement of the deviation from the intended flatness, roundness, concentricity, or any other shape or texture.
The term total indicator reading has gone through a bit of an evolution. Originally, the term used was total indicated run-out. Total indicated run-out referred to any imperfection on a part that resulted in the part not rotating with perfect smoothness, where “run-out” was the imperfection. The most commonly used term today is total indicator reading (TIR). Total indicator reading is the same concept of finding the imperfection on a part, but is also inclusive of all types of shapes and features. Finally, the term full indicator movement (FIM) is again the same concept of measuring the imperfection found in the rotation of a part, but is intended to emphasize the actual distance traveled by the tip of the indicator, rather than the readout difference between MIN and MAX. The difference here is subtle, and we will continue to refer to TIR as it is more commonly used.
Alright, so now we know what we are talking about, but why? When should you be using the MIN, MAX, and DELTA measurements from your indicator? An indicator is used to measure the relative position of a part, and the MIN, MAX, and DELTA (or TIR) measurements tell you more detail about that relative position, specifically how much the part is shaped the way it is meant to be shaped. Each of these measurements, particularly the TIR, is necessary when you are assessing the precision of a part that will eventually fit into a bigger mechanism. For example, think of the axle in a car. The axle is a very particular shape that requires a precisely smooth surface to fit into place properly as well as to function properly. By knowing the TIR you can see whether the axle was made correctly and then predict if it will fit where it needs to, whether it will wear evenly over time, and whether based on this wearing if it will continue to function or lead to eventual failure in the car. In a similar way, using TIR to check whether an already used axle is shaped the way it needs to be can help identify if the axle is the source of malfunction in a car.
Knowing the MIN, MAX, and DELTA measurements of a part is another way in which to check the precision of your work. Additionally, using these readouts to know that a part if shaped the way it needs to be will help you ensure that it functions properly once it is functioning as a part of a bigger machine.