A few years ago, a successful Tool & Engineering company with 120 employees located in Michigan’s Thumb region, discovered that 30 large parts, each costing the company approximately $1000, were out of tolerance in a specific measurement and had to be scrapped. The company needed to prove to their customer that the parts were in tolerance. They felt that the tolerancing of the parts, as indicated on the drawings, were the key to saving these parts. They decided to contract Inspec, Inc. to present a course in G D & T, and selected 11 members from their engineering, quality, machining and management teams to take our training class

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The course was an intensive 5 day course with training manuals and a lot of “hands on” experience for all 14 individual tolerance types. Originally, a hole position which had a Positional Tolerance of 0.004 inches was found to be out of position by 0.006 inches relative to a Datum hole. The Positional Tolerance Call-out was relative to the Datum hole at MMC (Maximum Material Condition). Having not taken a G D & T course, the employees did not understand how an MMC modifier could affect the Allowable Tolerance. In the class, they discovered that the size tolerance of the Datum hole was ± 0.004 inches. Since the Datum hole was slightly above nominal in diameter (0.002 inches above nominal) the Bonus Tolerance allowed for the Positional Tolerance was + 0.006. This Bonus Tolerance being applied to the hole which was “Out of Tolerance” gave the total Positional Tolerance of 0.004 + 0.006 inches for a total of 0.010 inches

 

 

 

 

 

 

 

 

 

Since the hole position was out of position by 0.006 inches, the customer found that these parts were actually in tolerance by correctly applying the MMC incorporated within the G D & T call-out. This saved the customer approximately $30,000 in what would have been scrap parts, had the customer not taken the course in G D & T.