Capability Analysis (Cpk) of torque driver process

Hello all,

This is my first post but I have been looking around these forums for some weeks.

The post I want to start is about a torque driver porcess capability. I am in charge of doing this on my company but I am not sure how to do it correctly. Normally what we have been doing is taking the torque driver tool and doing measurements in order to see how near of the target is that tool calibrated. Then, after 80 measurements, we calculate the Cp and Cpk.

I would like to know if this is the correct way to perform a capability study for torque driver process.

Thanks,
JP

Hello fellow Juan and welcome to the cove.

The first step is to perform a normality test of your data, THEN you can proceed to perform an adequate Cpk analysis of the data according to the approriate distribution of your data.

Juan Dude said:

Hello fellow Juan and welcome to the cove.

The first step is to perform a normality test of your data, THEN you can proceed to perform an adequate Cpk analysis of the data according to the approriate distribution of your data.

Click to expand...

Hello Juan,

Thanks for your feedback. What I am trying to find out is how to make those measurements. Currently we are using a torque meter (same equipment that we use to calibrate the torque driver).

I read somewhere that this capability study has to be performed during the assembly (while the screw is being installed).

The problem with torque drivers are that people get very confused over what gets an MSA study and what gets a capability study and so on. Because many use their calibration lab to set a torque driver, some confuse it with a gage, but it is not. It is more like setting a temperature controller.

• The torque analyzer is a measurement instrument. Perform an MSA on the torque analyzer.
• The torque driver is a process controller. The torque setting is a process parameter. Perform a process capability study on the torque setting using a torque analyzer with a run-down adapter.
• The torque of the fasteners is a product characteristic. Perform a product capability study on this torque.

Now this is where it gets really complicated. You and your customer need to agree and define what this is. Static or dynamic torque. Break-away torque or torque to continue tightening? Pass/fail to a proof torque? And so on. Your last post seems to imply that you have a torque monitoring system. If so, reach an agreement with your customer on the appropriate measurement. Number of turns would be another option with this type of system.

Note: A Torque monitoring system is also a measurement system needing an MSA. Since torque measurement on a product is essentially a destructive test, i.e., you cannot perform repeated torque measurements on the same part, the usual methods such as finding homogeneous parts or subdividing will not work. This forces you to use a substitute such as a selection of torsion springs in the approximate torque range of your process. See this White Paper from Instron on a similar approach using compression springs.

Thank you for your response. I understand the need to perform an MSA on torque analyzer and is something that we will start.

So, from your comments:

Miner said:

• The torque analyzer is a measurement instrument. Perform an MSA on the torque analyzer.
• The torque driver is a process controller. The torque setting is a process parameter. Perform a process capability study on the torque setting using a torque analyzer with a run-down adapter.
• The torque of the fasteners is a product characteristic. Perform a product capability study on this torque.

Click to expand...

I think we are not as lost as I thought. The way we are doing right now is your second point on the list "Perform a process capability study on the torque setting using a torque analyzer with a run-down adapter."
The other way I have been thinking is to use a torque transducer in order to measure the torque while applying it but this seems to be very expensive specially if we have too many torque drive tools.
So I think we can continue doing it the same way we have been doing plus start with the MSA.
Thanks for your support.

Is very important define the type of torque that you use for the studies. Dynamic torque is the peak torque measured while the fastener is being driven with a power or hand tool. Static torque is the torque required to break away a fastener in the tightening direction after installation, if you are using a computerized torque process you can use the data to evaluate the capability, other method is use a transducer and readout device on the output spindle of the tool, develop a correlation study between the systems, if you use torque-angle the procedure change.