Coordinate Measuring Machines (CMMs) and Digital Bore Gages

Gage R&R, Torque Wrence

Q: When inspecting diameters with tolerances of .0005 and below, are there any studies relating to the accuracy of different inspection methods, such as a coordinate measuring machine (CMM) versus a digital bore gage with setting ring combination?

A: The answer to this question can often be one of opinion and/or personal preference.  What I will present are my opinions, along with some known facts.

Non-contact measurement systems such as optical and laser equipment are bulky, expensive and impractical.  With these systems, the part must be taken to the system. This is not much good in a production environment.

While a CMM is without a doubt very accurate, they are also slow.  Like the optical or laser equipment, the parts must be taken to the system.  In many production situations it is more practical to check the part in the machine.  Also, even though CMMs come with reticulated heads, measuring at abstract angles or various depths is not always an option.  It is also wise to keep in mind that deeper bores would require longer stylus probes.  This is a situation that can introduce concerns of error and rapid movement can generate false contact readings with longer styli simply due to the motion.

A final thing to keep in mind is the high initial price of a CMM, as well as the maintenance costs.

Two and three point contact measurement is readily available.  Popular digital bore gages are calibrated to a master ring.  The rings themselves can be verified with a CMM or sent out for certification traceable to national standards. Most digital bore gages can be set up to interface with a statistical process control system. This is important when process control is vital.

Cylinder bore gages (generally two point contact) can sometimes have problems with linear accuracy. Analog versions can be more prone to operator error.

While two point systems will more readily detect ovality, where this is not a major concern, three point digital systems are, in this quality technician’s opinion, the best all-around option.

When I am inspecting parts in which ovality could be an issue, if the parts are readily portable, I will check a percentage with a CMM to verify their roundness.  However, for speed, accuracy, practicality, and price, a three point digital bore gage would be the way I would go to verify product with tight tolerances.

A final note: If parts are relatively small and can be in contact with other materials, robotics is often used with air gage instruments.  This is another expense but can be introduced in high volume manufacturing.

I hope this will help.

Bud Salsbury
ASQ Senior Member, CQT,CQI

ANSI/ASQC C1-1996 Supplier Testing

Schedule, calendar, timeline

Q: I need clarification on the following, please:

ANSI/ASQC C1-1996 — Specification of General Requirements for a Quality Program — has been included in the required specifications from a prospective customer. Section 3.3.4 states (in the last sentence) “Furthermore, the validity of certifications shall be periodically verified by the buyer through independent testing.”

What criteria (time-frame, suppliers, mills, etc.) should be used to comply with “periodically?”

What testing is to be performed for the required independent testing? Is it to be only a chemical analysis, or are mechanical tests to be performed as well?

Does this standard require independent testing of materials in purchased components such as gaskets, glass, bolts and fittings, or is “raw materials” only meant to be the base materials such as plate and sheet steel that we purchase?

A: To begin with, most establishments, including your customer, already know that materials most often come with material test certificates.  For example, when you order a sheet of steel from EMJ Metals or another supplier, they will supply a test certificate along with it.

The certificates include that data which would be most important to your customer such as chemical analysis, mechanical properties, ASTM specifications, etc. You are probably already aware of all this.

As for “periodic” and “independent” testing, here is my opinion:

If you have, in writing, a document stating that all purchased materials will be subject to receiving inspection and such inspections will verify that customer requirements have been met, that will be step 1.

For step 2, if you go to the web site of almost any materials supplier, they will have documentation (quality manual, ISO certification, etc.) which you can use as evidence they are a qualified supplier.

You can then contact that supplier and ask if they will verify, in writing, that they also test the material they are sending.  Steel suppliers, like most material suppliers, sell what they receive from the original mills.  The material certs they provide to you are made of tests the mills run.  A company such as EMJ, which I mentioned earlier, uses what is called a Niton tester to verify chemical make up of the product which they buy and in turn sell to their customers.

Finally, step 3: as with any quality management system, you must “do what you say you do.”  So, if you say that part of your receiving inspection includes hardness testing, be ready to provide evidence of that (incoming inspection reports).

In closing, I feel confident that if you prepare the steps noted above, or something similar and communicate this to your potential customer, they will be doubly satisfied with your company. Doubly because all of this would display evidence of an organization with a mature QMS.

Bud Salsbury,
ASQ Senior Member, CQT,CQI