Grinding hard chromium plating

A. Not that simple. It is the tiny area under the grinding wheel where the chrome reaches a high temperature and then immediately cools off as soon as the wheel moves.

Causes:

• The grinders are taking too deep a cut per pass
• They are using the wrong resin in the wheel
• They are using the wrong grit size
• They are using too high a RPM for the size of the wheel or are using too slow a part speed.
• Or all of the above.

My experience is reason one and two, 99% of the time.

The machine shop gets paid for getting the parts out and rush grinding. The fact that it recycles is the platers union problem, not the machinist union.

There is about 1 in a thousand chip cutter that will listen to a plater, even when presented articles stating fact. After all, machinists are highly talented and trained people. If you do not think so, just ask one.

Been there and done that. Talk with straight tongue. Every time we had a problem, I would check the grinders and sure enough, they for got to order wheels and were using unsuitable substitutes. They also thought that a 0.005 grinding cut was fine, because it "worked". (for about 100 revolutions)

A. Excellent reply Mr. Watts! You've hit the nail squarely on the head. Improper grinding costs this country billions of dollars a year in excess tool wear and component failure, but woe to the engineer who tells the grinder operator that there is a problem.

A little additional information: I have personally measured temperature under a grinding wheel of up to 1200 F, and suspect that my temp measurements are low. These high temperatures can cause the metal to expand so much that it plastically deforms because it kept from expanding by the cool metal around it. Then when it cools, it tries to contract but cannot, and a tensile residual stress is the result. Since chrome is more brittle than most steels, it stands to reason that the chrome will crack long before the steel.

In the aircraft industry, great effort is often made to achieve low stress grinding. I know of one company that will deeply investigate for heat damage if they see a spark while grinding. Yes, that is A spark.

It might be worth your time to investigate CBN wheels in addition to taking Mr. Watts advice to heart very seriously. Good Luck

A. Mr. Diekman,

You mentioned CBN wheels and it just rings the bell with me. We have had a very positive experience grinding Molybdenum plasma sprayed component by a vitrified CBN wheel. We used to apply a set of two wheel for this operation - an electroplated to remove the bulk of the material and alumina wheels for finishing. WE have been able to replace them both by a single vitrified CBN wheel with induced porosity. It grinds very cool and puts mush less stress on the component. It does not inflict metal burns, that were common before.

A. Chromium is a very good conductor of heat and therefore it is possible to create grinding burns, Tempered or Untempered Martensie (white layer) creates cracks in base material and also in the chromium layer.
Regards,
Jos

A. We use i to k grade wheels with 45 to 60 grit and grind 0.0002" max cut per pass, and only feed on at one end usually at tail stock. We also dress at 0.001" above final size and have [ed. note: haven't?] any problems with etch inspection.