GD&T Success Stories

Listed below are excerpts of a regular column featured in the quarterly TDC newsletter entitled the "GD&T Rag". The column is entitled "GD&T Success Stories", and offers GD&T users the opportunity to share their GD&T successes with others.

Excerpt from GD&T Rag, number 37;

My company has produced a certain industrial pump for many years. I believe the pump drawings may be older than GD&T itself. The end bell has been a manufacturing problem for a long time. Since geometric tolerancing was not used it is unclear where the datum features are. Also the plus or minus system was used to located the fastener and component holes.

After completing my GD&T training I was asked to update the pump drawing to reduce these problems. I explained the GD&T drawing symbols and concepts to the tool designers, machinists and inspectors. They understood how datums gave the entire team the same starting point for their job functions. They were also impressed with the MMC concept (bonus tolerance), which allows more manufacturing tolerance. We designed and manufactured functional gages to be used by operators at their machines. This has reduced inspection time and improved quality.

The hardware problems have essentially been eliminated on these parts, thanks to GD&T.

Name and company withheld by request.

Excerpt from The GD&T Rag, Number 38;

Application of Geometric Dimensioning and Tolerancing is not universal across industry, as it should be. We have several types of personnel dealing with GD&T, those that do not know and do not want to know, those who think they know and do not, and those who do understand. I make this statement out of experience over a period of time and over several manufacturing houses.

Geometric tolerances are a benefit when properly applied. Recently we had a situation where four expensive parts were rejected because some bored holes were out of position and deemed as scrap based on data recorded by an inspector. When I say expensive, I am referring to parts selling for about $25,000 each, once finished. Prior to physically scrapping the parts, several questions were raised as to the data recorded by the inspector. One question was. "Was the data recorded on this inspection report measured and recorded as bilateral information or was it converted and recorded in GD&T terms?" The inspector did not understand the question or the differences. Another question was; "Were all the allowable tolerance applied prior to the documentation of the defect and how much could the datum feature be opened up to gain additional tolerances?"

This investigation revealed that the primary locating datum inside diameter was on the very low end of the allowable diametrical tolerance and could be re-machined to gain about .010 inch of additional positional tolerance. Once this was accomplished, the out of position features could be re-bored to maximum size while locating the datum feature with a minimum diameter pin and clocking the part in the direction required to minimize the positional discrepancy. Three of the four parts were acceptable for use and were shipped. A net savings of $75,000 for a few questions asked and for applying the allowable tolerances with in ASME Y14.5M. Not a bad day's work, but the real issue is that understanding ASME Y14.5M would have reduced the entire issue to normal machine shop practices.

Jim Clausen
Senior Flexonics, Inc.
Ketema Division
El Cajon, California

Excerpt from The GD&T Rag, Number 39;

Our company received a call from a new supplier of molded products that was full of bad news. It seemed the production order they just finished for us did not meet the drawing specifications. The mold would have to be modified and the parts remade to meet the original order.

While this was bad news for the supplier, it was worse for us because it would delay the launch of a new implant system. Events like this spell DISASTER for an orthopedic company.

After working through the tedious details, it was obvious that proper interpretation of the geometric dimensioning and tolerancing specified on the drawing would prove the parts did meet the design criteria and were 100% functional. The supplier did not understand GD&T.

For successful events like this to occur, GD&T must be spoken by all involved. I have forwarded TDC's name to the Vice President of the molding company and recommended that several people in his organization take your workshop. They can't afford not to enroll!

Vince Guida
Plastics Operations Manager
Howmedica, Inc.
Rutherford, New Jersey

Excerpt from The GD&T Rag, Number 40;

Recently one of our major customers requested we work with them from a design standpoint to reduce cost. We identified five high cost parts we produce for them and formed a three person team to review these products. The team was comprised of representatives from two manufacturing cells and myself representing Quality Assurance.

The first thing I noticed was that several critical features were tightly toleranced using the coordinate dimensioning system (typical X, Y dimensions). We recommended and explained the benefits of positional tolerancing (a gain of 57% tolerance area), plus we suggested specifying material condition modifiers for additional tolerances.

Our customer accepted these recommendations along with some others that resulted in an increase in throughput, and an overall cost reduction for the five parts in excess of $50,000.00 per year.

John Manning
Senior Quality Specialist
Carpenter Advanced Ceramics, Inc.
Auburn, California