0136// Measure surface profile by Dial Test Indicator without false rejection (Type 1 Error)

Today, we will discuss about how to measure surface profile with the most basic tools -- Dial Test Indicator.

Before we straight to the topic, we need to understand what is false acceptance without false rejection and why this is important in production floor.

What is false acceptance without false rejection?

"False acceptance" mean we established a measurement method which may accept the bad parts (which cannot meet our functional requirement). "Without false rejection" mean the measurement method which established by us impossible to reject a good parts (meet functional requirement).

Why measurement method with false acceptance without false rejection sometime is important to production floor?

Sometime, production may not have a proper tool which able to measure a specification without false rejection and false acceptance. For example, you are asked to measure a surface profile but you realize that CMM in your plant unable to establish true profile for measurement. If this example is happen during fire fighting in production, you are given limited time and tools to perform this task. I'm sure that you will figure out a way to measure it indirectly.

So, in today topic I will share with you, how to measure a surface profile control without CMM but with known false acceptance and without false rejection. (help you do the first screening for the bad parts, get them off from your plant within few hours !!!)

What is surface profile control?

To use simpler tool such as Dial test Indicator to perform surface profile measurement. We need to understand, what is this control is controlling? Based on Y14.5M-2009, 8.2, this control is controlling size, form, orientation and location.

If the measurement is done by measure each of the control, that is possible to measure the surface profile control. Table below shown the possible basic measurement tools for each of the control. Please take note that, not all tools are always applicable to your measurement.

How to measure surface profile control?

In this post, I'm only want to demonstrate the possibility to measure complex control by using basic tools. Therefore, I will just share an example here. If you have any other doubts for other scenario, feel free to drop an email to gbwonggbwong@gmail.com

Figure below shown a cross section of a supporting block for precision height locating.

From the specification, we know that. This spec can split into three controls. First control is flatness, this is the primary specification which require to meet to meet parallelism requirement. Second control is parallelism, this is prerequisite for the location control. The last control is location control.

For me, I will skip the flatness measurement. As you know that, flatness is difficult to measure compare with parallelism.

The third control, you can measure it with height gauge. But, the error for this measurement will be huge due to lower accuracy and repeatibility for height gauge for location measurement.

Suggestion from me:

I would rather skip the location control measurement due to unreliable data. Unreliable data which cannot repeat by your supplier (if you want to fault your supplier unable to produce parts which meet specification), may damage the relationship and trust between your supplier and your SQE!

Basically, in this example, we only need to perform parallelism measurement as the quick screening method for you problematic inventory. If the parts unable to meet this parallelism requirement. There are no chance for them to meet the surface profile control!

All in all, flow chart below shown our approach =)