Repeatability for an Electrical Screwdriver Torque

[Ed-Bal]

Hello to everybody:

Recently I was asked by the repetibility of a electrical screwdriver. I made a capability study (I calculate the Cm index), but I really don't know if it's valid or not.

Additional Information:

I work on a company where car door latches are produced. We use a small screw to fix a frameplate. On the drawing, the spec for the screw is: 0.70 +/- 25% Nm (0.525 - 0.875 Nm).

My problem started when I tried to obtain the repetibility of the screwdriver. With the use of a torque analyzer I got 30 readings taken directly from the screwdriver (I call to this dinamic torque), getting values similar to these: 0.95, 0.83, 0.92.
If I measured the torque applied directly on the screw latch (I call to this residual torque), I got values similar to these: 0.65, 0.70, 0.60.
As you see, the specification match perfectly for the torque found on the product, but not for the torque applied directly by the screwdriver.

So, I don't know if it's valid to calculate the average of the readings taken directly from the screwdriver and apply the % tolerance defined for the latch product (not for the screwdriver)? Once having the tolerance I could easily calculate a capability index.

As a second question, I don't know if 1.33 is the minimum value acceptable for Cm capability index or could be less?

I would appreciate all your point of views to clarify my doubts

Thanks & Regards

Eduardo

 

[BradM]

Re: Repetibility for an electrical screwdriver

Hello Eduardo!

My problem started when I tried to obtain the repeatability of the screwdriver. With the use of a torque analyzer I got 30 readings taken directly from the screwdriver (I call to this dynamic torque), getting values similar to these: 0.95, 0.83, 0.92.

You stated you took 30 readings, but you only list three above? Were the other readings identical to the above three readings?

Were you testing counter clockwise-torque or clockwise torque with the torque analyzer?

Now, these are some really basic questions, but did you assure to minimize distance, any wobbling and the like between your screwdriver and the torque analyzer? There are several factors that can contribute some pretty significant error.

If I measured the torque applied directly on the screw latch (I call to this residual torque), I got values similar to these: 0.65, 0.70, 0.60.
As you see, the specification match perfectly for the torque found on the product, but not for the torque applied directly by the screwdriver.

How did you measure this? Did you estimate the counter torque it took to remove the screws in the latch? Did you apply the identical torque clockwise, then measure removal torque?

How many readings did you take with this application?

Sorry for the questions, but I was a little interested in your setup here, and from experience, torque can get tricky.

I know your question was specific about gauge R&R, and some of the other experts can help you with that. However, you might be interested in some of the other threads on torque. Here is one:

Verifying Applied Torque

Where one of the Covers talked about developing a relationship between applied and removal torque. Also, you can enter torque in the search, and there are several really good threads on torque, if you are interested.