In this brief overview of measurement uncertainty, we have arrived at a value for use in an uncertainty budget. There are several ways to use that value and the budget that created it.
An uncertainty budget is similar to a financial budget in the type of information it can provide. But unlike a financial budget where every element is in dollar terms, the uncertainty budget has to deal with various elements and convert them into linear terms, such as millionths of an inch or microns, when doing gage calibration.
Nothing provokes heated discussion like uncertainty. We see it in the global warming-now relabeled as climate change-debate and scores of other issues. We demand precise yet simple answers to complex matters, as politicians know only too well. And if we can find someone else to blame, that’s even better.
In dimensional measurement, we use numbers rather than fear to make a point. But sometimes the irrational does creep in. For example: “I paid a gazillion dollars for that thing and you’re telling me it’s not accurate enough to measure these parts,” or “we can put a man on the moon but we can’t measure this better than ...”
Those of you who have been following this series of columns know the caveats
that apply so I won’t repeat them this time around. If you don’t know them,
buy, borrow, beg or steal the previous issues for an update.
As
I noted in last month’s column, this will be an overview of the most popular
hardware used for calibrating gages. In case you are wondering why I didn’t
mention the use of coordinate measuring machines for this work, and won’t, the
answer is quite simple. Few of them are accurate enough for the tolerances
involved, and those that are rarely exist outside of national measuring
institutions.
Editor’s note: This is the first in a three-part series on gage calibration.
Part 1: Plain Plug Gages
This is not a how-to column but rather an overview of what is required in the way of hardware used for gage calibration. As with any measuring process, several types of equipment can be used; I will only deal with the most commonly used equipment. It is assumed that you have a proper environment, your equipment and masters are calibrated, and you have a skilled person to use the equipment.
I recently wrote about calibration reports and outlined some of the basics involved with them. This month, I thought it would be worthwhile to deal with documents that are often provided in lieu of calibration reports: certificates of compliance.
The original intent of certificates of compliance was to make sure that process steps were not overlooked if they could not be verified later. An example of this might be a product that has to be dipped in a specified solution at a specified temperature for a specified period of time before painting or plating. Usually, there is no way to verify that these steps were taken once the product has been coated. Testing after-the-fact can reveal that the correct thickness of paint or plating has been applied but not that all specified process steps were followed.
A year ago I wrote about the value of buying setting plugs along with adjustable thread ring gages, particularly special thread rings. One point I made was that setting plugs should be used for the resetting of adjustable thread rings in accordance with the standard. Taking measurements of them was not accurate enough.
The problem starts with buyers of special rings who do not wish to pay for proper setting plugs. When it’s time for calibration, the hunt begins for someone who has plugs and of course, it’s unlikely they will be found because calibration laboratories would go broke keeping special setting plugs on hand in most cases. Some laboratories are ready to cash in on this situation by offering to “measure” special adjustable thread ring gages so the technician won’t have to buy the right thing. And, wow, the cost will be a lot cheaper than buying setting plugs.
Quality Magazine’s publisher, Tom Williams, brought up the subject of metrication in the September issue-he wondered why the United States was not metric yet. This column is about dimensions of all kinds so I couldn’t let Tom’s questions remain unanswered. As Quality’s unofficial troublemaker, I offer the following.
