# Capability Study for Radii of laser cut pockets - 0.030" radius

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#### Bill Lewey

Hi,

This is my first post to the Forums but I've lurked here in the past and picked up quite a lot of good information. I'm hoping someone can help me with this problem.

I have a new product in my plant which has laser cut pockets about .85" by .25". The pocket corners have .030" radius. This radius is a KPC for the customer so I need to do capabilty studies for radius and R&R studies for for the gage.

We have radius buttons and comparators to measure this with. How would I go about doing these studies considering I don't have any variable gaging and I can't seem to find any type of variable gaging that will work in this application?

#### Paul F. Jackson

##### Quite Involved in Discussions

What is the function of the radius?

#### bobdoering

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Wow...you have a situation. First, you need a gaging system that will give you 10:1 - or .003" radius resolution - just to get started. What is the variation of sizes of the radius buttons in your set?

If they meet the above criteria, then pick 5 parts, pick a radius on each one, and have 3 people "measure" the radii with the buttons. Mark the chosen button size down for each one. Then, plug and chug in the GR&R calcs. They are variable, just a rough variable.

BTW - this is a great job for a vision system CMM (wishful thinking). fast with lots of resolution. Can lay out the whole part fast. OGP is my favorite...IMHO.

#### Jim Wynne

Staff member
Wow...you have a situation. First, you need a gaging system that will give you 10:1 - or .003" radius resolution - just to get started. What is the variation of sizes of the radius buttons in your set?
The 10:1 ratio applies to the tolerance, not the nominal. For example, if the total amount of tolerance is .010", the gage should resolve to at least .001".

#### bobdoering

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So true. (My bad) That makes it even tougher, I suspect.

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B

#### Bill Lewey

Thanks for the replies.

My buttons don't have that resolution. I've been attempting to measure it with a comparator and templates but it's a tough measurement to make.

The only function of the radius is help prevent breakage or cracking during operation. This is a highly stressed section of the part and a sharp corner would lead to cracking and failure.

Bill

#### bobdoering

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My guess is the comparator will not give you adequate GR&R results with that measurement approach either. You may be painted into a corner on this one with the gaging you have. You could send your capability study samples out for measurement, but that does not help your ongoing process verification gaging.

#### Jim Wynne

Staff member
Thanks for the replies.

My buttons don't have that resolution. I've been attempting to measure it with a comparator and templates but it's a tough measurement to make.

The only function of the radius is help prevent breakage or cracking during operation. This is a highly stressed section of the part and a sharp corner would lead to cracking and failure.

Bill
This is one of those cases that illustrates the reason that I don't like calling some characteristics "special." It seems that it's not the size of the radius that's important, but the presence of it. There are times when it's prudent to ask for relief from the customer in handling of KPCs, and this seems to be one of them. While it might not do you any good, it couldn't hurt to explain your dilemma to the customer and ask for suggestions.

#### bobdoering

Trusted Information Resource
This is one of those cases that illustrates the reason that I don't like calling some characteristics "special." It seems that it's not the size of the radius that's important, but the presence of it. There are times when it's prudent to ask for relief from the customer in handling of KPCs, and this seems to be one of them. While it might not do you any good, it couldn't hurt to explain your dilemma to the customer and ask for suggestions.
Good point! Also might not hurt to ask how they check them. It will give you an idea how important the GR&R is to them, if nothing else. If they use a comparator with templates, then...well, maybe it is good enough for you!

#### Paul F. Jackson

##### Quite Involved in Discussions
What is the tolerance of the .03 radius?

If you use an attribute gage strategy to estimate your capability then at typical customer defined levels of capability conformance the occurance of a defect would have to be extremely rare which means that your sample size has to be extremely large. You can reduce that sample size by comparing the actual radii to only a portion of the tolerance either side of .03". When/if a defect is observed to the reduced tolerance value it should be re-checked with either an attribute gage using the full tolerance or with equipment that produces continuous data.

It is good to know that the critical function pertains to the minimum value of the radius. Your gaging strategy should be respective of that and you may want to occasionally check the form of the radius to detect whether there are conditions (reversals, steps, cracks, etc.) which could introduce stress risers not detected by the radius measurement.

Paul

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