Inspection Standards

I'm sure that different countries could have slightly different standards, but then you also get into different types of standards such as Mil-Spec which is generally more stringent and has higher specifications. "Shells" could almost be 'anything' but... Anyway, more info would probably help someone who knows much more than I do. My wife is a consultant/engineer for pharmasutical companies and she prepares them for FDA inspections/audits, so that's where my knowledge comes from lol

If there wasn't enough links

http://www.qualitydigest.com/magazine/2008/sep/department/skilled-metrologists-remember-them.html

I will tell you. But it will cost you the sum of 4 yrs college tuition and 5 yrs annual salary.

Start with personal study of books on the subject. I know of several and will post their authors, titles, publishers etc. if you are interested. Perhaps other CR4 members will do the same; especially members from your own country with suggestions for books readily available to you. I do suspect some good information on techniques for solving specific measuring problems can be found on the Internet. Many machinists like to share such information.

Your best bet for facilitating your lab will be to obtain a modern coordinate measuring machine (CMM) and placing it in a location where you can maintain a temperature of 20C +/- 1 deg C and keep the humidity below about 75%. Some additional fixturing for the CMM will be necessary to mount the parts you intend to measure. There are a number of other small inspection and measurement tools that you will need to equip the lab. Someone already posted a reasonable list in this thread. I'd suggest you obtain several manufacturers' catalogs and price lists and establish a source through which these tools can be purchased.

You will also need to have a good understanding of geometric tolerancing. There has been much written on the subject ranging from tutorial texts to standards documents like ANSI Y14.5.

If your work requires you to verify measurements closer then about 0.05 mm then some additional understanding of metrology theory and practice will probably be needed. For many this comes with experience in the mechanical inspection business, although there are a few texts on the subject. A background as a precision machinist offers a major qualification for this kind of work.

Ed Weldon

Thanks for your suggestion Ed. Already we bought 7 Axis CMM Machine for inspection.Please let me know why the Humidity and temperature are important for that?

thanks

-Nava

Many inspection standards require that all inspection take place in a climate controlled environment.

Say you are testing for flatness of a surface or parallelism of 2 surfaces at 90F and 45% humidity. Everything looks good using the best test equip money can buy in your lab.

Now your customer performs an incoming inspection in his lab. His lab is controlled at 72F +/- 2 deg and less than 20% humidity. If his lab can not verify your results YOU have a problem.

This is also why it is important to allow incoming shipments to stabilize prior to inspection. I have seen shipments come straight off the truck and rushed into inspection where much frustration is encountered as a result of the temp diff.

Nava --

All materials expand and contract with temperature. Your eyes cannot generally see these effects; but your measuring equipment certainly can. Texascharley mentioned that. Humidity is less of an issue for metals since they don't absorb water like some plastics and composite materials. But from a practical standpoint you want to keep the humidity in the lab low enough that your steel precision tools don't rust and the electronics in the CMM are not affected. Keeping everything free of dust is an important factor here also. Dust retains condensation of atmospheric water on steel surfaces promoting rust formation.

Take some time and do some simple calculations of how much various lengths of steel and aluminum expand and contract with temperature variations of 5 or 10 degrees C. What you find will be enlightening.

Following are some common practices in USA machine shops that produce parts for precision machinery in tolerances in the range of 0.01 to 0.20 mm:

1. Inspection is done in an air conditioned room separate from other manufacturing activities. Temperature control prefers to be 20 +/-1 degree Celsius. Air conditioning, especially in humid climates, should be properly sized for the application so sufficient humidity is removed from the air to keep it in the range of 50-60%. Air filters should be higher quality than usually used for ordinary comfort cooling. Exterior walls and ceilings of the inspection room should well insulated with no windows to allow sunlight in to avoid sources of radiant heat.

2. The machine shop is also air conditioned although the standards are not as high as in the inspection room for economic reasons. But, very importantly, machine tool coolants are temperature controlled to within a degree or so wherever tolerances are in the .005 to .02 mm range.

3. Parts are not brought into the inspection area without being cleaned of visible coolant and chip residues. Large parts need to be held in the inspection room, sometimes for several hours to reach uniform temperature. Actual experience with the items you manufacture should be your guide in this area.

4. Avoid prolonged body contact with granite surface plates. Also no coffee or tea cups. Granite is a good insulator and a warm spot will be a high spot for a surprising length of time.

5. Calibration standards are necessary. 10x greater accuracy than the instrument to be calibrated is appropriate. Traceability to recognized measurement standards is appropriate for any manufactured products entering into world commerce. Clean cotton gloves should be used for all standards such as gauge block sets and the calibration ball for the CMM. They should have their own special storage area.

One additional comment -- ANSI/ASME Y14.5M-1994 (R1999) is the current US standard. You should obtain a copy of that or the equivalent ISO standard if such is common practice in your country. You will personally need to become well versed in it's content if your role is management of the inspection activity in your enterprise. Copies of Y14.4M are expensive; but necessary. A good source is: http://webstore.ansi.org/RecordDetail.aspx?sku=ANSI%2FASME+Y14.5M-1994+(R1999)

I hope the above comments are useful to you.

Ed Weldon

Ed Weldon has done a great job here in highlighting the basic needs, and I concur throughout.

Ed mentioned traceability. This is important. Typically (ISO) it is required for all final inspections to be performed with traceable equipment.

As the manager of this functional area you must consider how you wish to achieve this requirement should it in fact apply to your organization (CMM's are not exempt).

An approved in house program (my preference) or 3rd party service can assist you here. I have found that there is always a need for some 3rd party calibrations - ring gages and electronics primarily.

It is also important to note that just because a tool or instrument is purchased new that it is automatically "in calibration". More on that later.

Another thing to keep in mind is inspection times.

Depending on the product configuration, features to be inspected and tolerances, a CMM can measure many things, but a Height Gage, Calipers, Micrometer, Snap Gage, Plug Gages… will be quicker.

More and more customers are wanting a CMM involved in the ISO driven inspection reporting. Of course most anything can be measured w/out a CMM. However data recording and single set ups as well as repeatability are greatly increased w/ the use of a CMM.

We used a great old Mity CMM that was DOS driven and as far as I know it is still in use today, only 5 yrs later. Loved it in our little shop. Other shops I have seen with more CMM than they will ever need.

I firmly agree that a shop should have all the tools necessary to inspect whatever they manufacture without the use of a CMM. Mechanical or electrical failure of the equipment is a real possibility. The notion that no shipments or goods go out because no one has the tools or knowledge to inspect what is being made, is to me, absurd.

When I first started machining there were no CNC's. Punch card and tape reader were just hitting the market. The first piece of "automated" equip I used was a 10 tool tracer turret lathe.

We had an instructor named Mr. Battreal who made us learn how to perform square and cubed roots long hand. We would buck and huff at the notion of ever needing this skill. Calculators were showing up on wrist watches at this time. His response? "Men you just never know when your gonna need somethin' "

I have cherished that notion ever since.

I am setting up a laboratory for the qualification of mechanical parts. The parts are slim, shell-shaped, and CNC machined. I would like to know the standard procedure for checking the width, straightness, concentricity, circuit and other GD&T parameters of the parts and the needs of the measuring instrument for the same.