Portable CMM Accuracy Definition - Stating CMM accuracy as a volumetric accuracy

[Phil Fields]

I am in the process of evaluating portable CMMs for use in our facility. Both companies I am looking at are stating their accuracy as a volumetric accuracy. I have information as to how this is calculated, but my question is how does this stated accuracy (.0068 to .0027) relate to real world measurements.

 

[Hershal]

Others here are better qualified for CMM specifically, so they will likely provide better answers.

Basically, specs provide a guidline for comparison, and what you may reasonably expect in a nearly ideal arrangement. That is, tightly controlled environment, well trained operator, etc.

The specs do not generally include the meaurement uncertainty.

Hope this helps.

Hershal

 

[gard2372]

Phil,

Welcome to the cove!

First off I have to ask, what are you measuring? Are the portable CMM's replacing or completmenting standard hand calipers, and depth micrometers pin gages etc...? Are you looking to increase productivity throughput without losing quality?

If you're using a FARO arm, or some other type of portable measuring device, then I would first off ask what is the variation from inspector/operator to inspector/operator when taking manual measurements? If you have identified this then you have some base line as to what measure of uncertanty you're engineering department or customer is willing to accept.

If you could provide some more detail as to what type of parts you're inspecting to, their tolerances and an industry, I think we may be able to help you brainstorm this issue a little more.

 

[Phil Fields]

gard2372

Thank you for your response and questions. Not to make this a long story, this company has been around for over 100 years, and has gone though many changes (owners/managers). I have been on board for 3 years. I have been in quality over 10 year but have not dealt with this in the past.
The portable CMM is not replacing anything, it is adding capability that we do not have. We purchase pressure vessel for our overseas parent company, machine parts locally, sheet metal in-house. We utilize micrometers, calipers and TAPE MEASURES to evaluate product.
*our tolerances are mainly in the ±.020” range, pressure vessels can be 4’x6’x8’
Management has asked me if a portable CMM would be useful when evaluating suspect pressure vessel during product build.
I can see this becoming a tool we use for the suspect pressure vessels to receiving inspection to design validation.
These CMMs are advertised with several different accuracies, Single point .0017, Cone Test .0019, Volumetric Length Accuracy .0027 (12 ft Romer ), .0034 (12ft FARO). I have been told that the volumetric is the most important to look at, the sales reps. At ± .020 is this accuracy acceptable? Do these listed accuracies say that no matter at what point I measure I have the chance of the result being off by ±.0027”? Oh yeah, all of these accuracies come with a price, so I want to be able to determine the best product for our needs and communicate to management the best way to spend the dollars,
I hope this answers the questions, thank you,
Phil

 

[gard2372]

I am in the process of evaluating portable CMMs for use in our facility. Both companies I am looking at are stating their accuracy as a volumetric accuracy. I have information as to how this is calculated, but my question is how does this stated accuracy (.0068 to .0027) relate to real world measurements.

Phil, from my experience and general understanding of pressure vessels, there are a few important criteria to note when inspecting the and manufacturing them. They are the "Certified Welders" that should be welding to ASME Section 9 of the BVP (Boiler and Pressure Vessel) code and the Certified Wedling Inspectors "CWI's" that inspect the welds or the NDT technicians for possible X-rays of weldments and the overall material integrity of the material (certs from suppliers). Dimensional inspections are important too, but as you say you're using micrometers, calipers and TAPE MEASURES to evaluate product then why wouldn't you try a portable CMM?

Now when you say...

"The portable CMM is not replacing anything, it is adding capability that we do not have. We purchase pressure vessel for our overseas parent company, machine parts locally, sheet metal in-house. We utilize micrometers, calipers and TAPE MEASURES to evaluate product.
*our tolerances are mainly in the ±.020” range, pressure vessels can be 4’x6’x8’
Management has asked me if a portable CMM would be useful when evaluating suspect pressure vessel during product build."

I have to ask right there, is not the portable CMM better in accuracy, even volumetric @ (.0068 to .0027) thousanths or mm whichever the case may be than the "TAPE MEASURES"?

I say go for it! We once had a FARO sales rep actually bring one out to our facility and we let him set up the machine and actually inspect some of our parts to let us evaluate the set-up time and actual run time of the inspection. We comapred his initial data and times with our previously recorded data and times (we inspected and recorded the part prior to their arrival). We based our decision based on the initial findings. It has not only increased out initial production capacity, but has freed up some inspectors to focus on other areas. This is a common practice to use the equipment a sales rep is trying to sell, usually called a product demonsteration. Many of them are aware of this and mostly oblige your request for this as they know the customer must feel comfortable in order to purchase the equipment. I don't know of many sales rep's who would risk not having a sale because he/she didn't want to let the customer have a hands on demo to see the "real world" application.

Hope this hepls a litttle.

 

[Phil Fields]

gard2372

Thank you for your reply. Yes our supplier is ASME certified and the vessels to come with complete paper work. The issue of the measuring equipment has been an issue since I arrived at this company, but "this is how we have done it since our last management change", is the answer I get.
Yes the CMM is better than a tape measure, my main question, just to have full understanding is, what does the accuracy mean in real world measurements.
The reason I am asking is we are looking at this equipment for one purpose, inspecting suspect vessels. I can see this equipment being used for design/development where we may need better accuracy. So I want to be able to communicte to managemnt the best machine for the accuracy and cost. I do not want to buy the equipment then get one year down the raod and say I wish I had a more accurate machine to do the work that is being asked of me.
Thank you for your help, have a good week end,
Phil