Groover: Fundamentals of Modern Manufacturing, 5e

Case Study by Daniel Waldorf, California Polytechnic State University

Grinding Case Study: Ryan’s Internship on a Surface Grinder

Ryan, an engineering student at Peoria Technical Institute (PTI), decided to accept a summer internship with a company that designs and produces small machined components for silicon wafer processing equipment. In addition to participating in part design teams and learning the various stages of design, he spends 3 hours a day operating one of the surface grinders that produces flat, parallel sides on many of the components. Although some of the parts get additional honing and polishing operations (from an outside contractor), many of the parts are complete after Ryan’s grinding process.

One high value part made by the company is the pivot table. The pivot table is made of annealed 5050 steel (approximately 190 BHN). The 8-inch diameter grinding wheel set up on Ryan’s grinder has an ANSI designation of C-18-S-15-V. As shown in Table 24.3 in the text, this means the grinding grits are relatively coarse and made of silicon carbide. The vitrified bond strength is graded as medium-hard, and the grain structure is considered very open.

When Ryan sets up the grinder to run the pivot table part, he knows to select a relatively slow cutting speed for steel so he tries a surface wheel speed of 3500 fpm. The work is set to move (work speed) at 20 fpm, the infeed (depth of cut) is .002 inch per pass, and the width of cut (crossfeed) is .25 inch.

The resulting part quality on the pivot table is less than what Ryan and his boss were hoping for. The surface finish is definitely rougher than what is called for on the part print. Although no burn marks or thermal discoloration appear on the part (Ryan used plenty of grinding fluid to reduce temperatures), Ryan finds it hard to control part dimensions due to wheel wear. He even has to replace the wheel a couple of times.

After running the pivot table, the part is changed to an aluminum spacer. Ryan changes the wheel speed to 6000 fpm for aluminum. To achieve a higher productivity rate, he changes the workspeed to 35 fpm and the infeed to .004 inch per pass. The process seems to go better. The aluminum parts end up coming out quite nicely, with decent finish and dimensional control, though these parts are scheduled for additional outside polishing.

WATCH THE THREE VIDEOS on Basics of Grinding

1.  How does a cracked grinding wheel sound when it is struck with a non-metal object?

2.  An option on Ryan’s grinder allows for “automatic dressing and geometry compensation” as part of the process. What problem of Ryan’s could be helped if he chose to employ this option?

3.  How does dressing a grinding wheel with a diamond tool help to prevent burn and chatter marks on the work piece?

4.  What single aspect of a grinding operation helps to reduce friction, remove heat/maintain temperatures, reduce power requirements, maintain quality/dimensional control, and prevent heat damage?

GO TO THE TEXT: Chapter 24

5.  How can Ryan improve the surface finish on the pivot table part by changing the speed/motion control settings on the grinder? See Section 24.1.

6.  Should Ryan have expected a short wheel life when using his setup to grind steel? Explain. See Section 24.1.

7.  If Ryan were to select a new grinding wheel for the steel parts, what characteristics should the new wheel possess? See Section 24.1.

8.  The energy it takes to grind away a unit volume of material is generally much more than it would take in a turning, milling, or drilling operation. Give three reasons why grinding is so inefficient from an energy standpoint. See Section 24.1.

9.  Surface grinders produce flat surfaces. What types of grinding operations produce cylindrical or axi-symmetrical surfaces? See Section 24.1.

SOLVE

10.  Compute the material removal rate (RMR) for Ryan’s setup for grinding the aluminum spacer part. Assume no change in the crossfeed.

11.  If the cutting force in grinding the aluminum part is measured to be just 2.63 lb, compute the specific energy for the cut, U (in-lb/in3). Be careful of the units in Equation 24.7. Compare your answer to the value of specific energy for turning aluminum in Table 20.2