- AGD American Gage Design
- ANSI American National Standard Institute
- DIN German Industry Standards
- MIL specs US Defense Department military specifications
When one of our customers insists that we follow certain standards, we had better comply. Chances are we won’t know all the details of those standards, and there is no reason why we can’t ask our customer to supply us with those details. And, if something gets out of hand, we will be blunt with the customer and ask what kinds of compromises can be made.
Here we list just a couple of the standards we may come in contact with, although it seems that new ones are being created annually. The intention being that we all use identical methods of measurement in order to guarantee uniform results and that one way of creating uniformity is to set certain standards that all measuring gages must have in common.
Professional literature on these topics is hard to find but checking Wikipedia will be a good source for starters.
AGD standards and specifications
All of today's dial indicators conform to AGD standards (American Gage Design) which, in 1946 established parameters for gage dimensions and accuracy. Most significantly, repeatability will be within plus or minus one fifth graduation at any point. Any dial indicator which is functioning correctly will also be accurate to within one graduation over its first two and one-third revolutions. Accuracy varies on gages with longer travel depending on manufacturer. Refer to their catalogs for details.
Further AGD specifications stipulate the range of the spindle which must be a minimum of two and one-half revolutions; the hand position which must be set at 9 o'clock when at rest; and the dial numbers which must always indicate thousandths of an inch regardless of magnification. (On metric indicators, hundredths of a millimeter.)
Finally, AGD specifies manufacturing dimensions. Dial indicators are classified in 4 size groups. Many manufacturers have arbitrarily introduced a fifth "size 0" group for indicators smaller than Group 1.
- Group 1 bezel diameter 1.375" to 2.0"
- Group 2 bezel diameter 2.0" to 2.375"
- Group 3 bezel diameter 2.375" to 3.0"
- Group 4 bezel diameter 3.0" to 3.75"
More significantly, the diameter of the stem must be .375" Oftentimes, particularly on metric models, the stem diameter will measure 8 mm as an exception. In these cases, adapter sleeves will fit over the stem to increase the diameter as necessary. Contact points will be interchangeable with 4-48 threads. Other dimensions may or may not be of importance to the gage user. Most manufacturers' catalogs give schematics of these dimensions.
Refer to our Index for information on specific manufacturers and gage types.
The US Department of Defense approved of AGD specifications (that’s a good thing) but sets forth some rules for test procedures. Calibration labs follow these stipulations as a matter of course, wherever practical. MIL Specs (A-A-2348) make the following demands:
- Repeatability: same as AGD, plus or minus one-fifth graduation but repeatability must remain reliable for up to 500,000 repetitions.
- Accuracy: same as AGD, plus or minus one graduation at any point over the first 2-1/3 revolutions.
- Calibration procedures: test 8 points, in equal increments, over the range of the indicator inward and outward for accuracy. Repeatability is checked at 1/4, 1/2 and 3/4 full range.
- Gage Durability: the indicator must survive half a million full scale deflections without showing significant signs of wear. (Only the military would test an indicator half a million times!)
- Contact pressure: the maximum force needed to move the spindle has been established for all kinds of test and dial indicators.
More recently, the American National Standard Institute (ANSI B89.1.10) has established specifications so that the end-user knows what to expect from the manufacturers. This applies to American-made products as well as foreign made products when “ANSI” is specified.
Without much to-do, the Institute has confirmed the 1946 AGD specifications and the MIL specifications outlined above.
There are two changes to note:
- Metric dial faces must be yellow. Keep in mind, this applies only to metric indicators sold in the USA. Everywhere else in the world, the metric dials are white. If you need yellow dials, be sure to ask for them, especially when ordering from Swiss manufacturers such as Compac, Tesa or Brown & Sharpe.
- The metric graduation of 0.005mm has been eliminated. This is no big deal. These indicators aren't easily available anyway.
Deutsche Industrie Norm (German Industry Standards) will be referenced by instruments made in Europe (including Switzerland). In order to find out what the standards mean, you will have to buy the book for about $65 in English. Apparently we are not permitted to reveal these copyrighted industry standards.
We'll give you a hint (without breaking the copyright laws, hopefully). The manufacturer's inspection reports may use these abbreviations. Here's what they mean, in short.
Deviation fe : maximum error over the entire range of the indicator
- example: maximum errors are minus .0002" and plus .0004", therefore fe=.0006"
Total Deviation fges takes into account the retrace error (fu) added to the maximum error (fe)
- example: fe=.0006" and fu=.00015", therefore fges=.00075"
Retrace error fu : (hysteresis) the difference between readings up the scale and then back down the scale. If these points are drawn as a line on a graph then a retrace error will be evident.
Repeatability fw : the inherent error in measurement of the same spot with repeated trials.
What has ISO got to do with it?
Not much. As we've all found out, ISO certification is a kind of smoke screen designed to give the illusion of integrity. We've had ISO certified companies send us checks that bounce. So much for any kind of integrity.
It is a membership in a fraternity of sorts and the membership dues are very steep indeed. $10,000 to $30,000 is not uncommon and then there are annual fees on top of it. Members of ISO are supposed to do business only with fellow members and that's how the organization maintains itself. It hasn't got much to do with the quality of the products but everything to do with red tape and paperwork. So much paperwork in fact that usually one or more staff member needs to be hired just to fill out the forms.
Maybe you are just curious or maybe someone is twisting your arm, here is an affordable paperback explaining ISO 9001 in easier to understand terms: