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Grinding hard chromium plating

Hard Chrome  
Plating pointer 

       
(2000)

Q. I am searching for information about machine grinding hard chrome. How many number of revolution/minute and how much shall I grind/minute. I like also know which type of grindstone is best.

Regards,

Anders Sundman
    surface finishing engineer
- Malmo, Sweden



.

A. Dear Anders,

The most important thing about grinding hard chrome is not to develop any heat.

Use a grinding wheel with an open structure to make good cooling possible. Keep your grinding wheel sharp by dressing it more often than you normally would do.

The grinding speed should preferably be about 35 mtrs/sec.

Consequently, the number of r.p.m. depends on the diameter of the grinding wheel.

We have more detailed information on this subject available.

Kind regards,

Roel Jaarsma
- Netherlands



.

Q. Re: hard chrome plating/grinding: We get "rejected" parts from our grinding shop that must be stripped and re-plated due to "cracked chrome." We have read that when the base metal gets too hot during grinding, the chrome can crack and look like a dried up lake bed - which is referred to as mud-flat cracking.

Our question is - at what temperature this occurs? What is the "danger range" for how hot the basis metal needs to get for this to occur? Typical basis metals are high strength steels and stainless steels.

Nick Cortese
airline - Atlanta, Georgia USA


.

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:

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)

James Watts
- Navarre, Florida

misc. plating stuff
For Sale cheap




.

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

Frederick Diekman
- Streamwood, Illinois, USA


++++

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.

Gennady Gurinovich
abrasive wheel supplier - St. Peterburg


June 16, 2008

Q. I want to know some more about grinding hard chrome.

greg Alexander
- six mile, South Carolina


October 28, 2011

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

Jos van Langh
- Helmond, Noord Brabant, Netherlands

December 14, 2012

Q. Anybody consider it can be done by benchwork? Polishing using additive (Autosol) can cause peel off too?

Endro
- Bandung, Indonesia


June 10, 2013

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.

Phill Morris
landing gears - Bolton, England



August 6, 2013

Q. Dear All,
I have a question in regards to BAC5032.

"The grinding wheel shall be dressed frequently with a finish dress when the part surface is a maximum of 0.003 inch from the finish dimension."

Does this mean a maximum grinding depth of 0.003" per pass? Any insight would be greatly appreciated.

Thanks.

Tiger Wu
Landing Gears - Xiamen, Fujian, China
  ^- Privately contact this inquirer -^


First of three simultaneous responses -- August 8, 2013

A. You absolutely cannot take 0.003 per pass on chrome, especially on aircraft parts.
What it is telling you is that when the grind dimension is within 0.003 of the final dimension that you must dress the wheel.
The correct grit and resin of the wheel makes a major difference in the finish. The wrong wheel and too aggressive grinding is begging for failed plating and it is not the plater's fault.

James Watts
- Navarre, Florida

misc. plating stuff
For Sale cheap



Second of three simultaneous responses -- August 9, 2013

A. I haven't read the Specification and the context of that paragraph could shed more light but to my understanding it does not imply any specific amount of material removal but to treat the last 3 mils gently. Good luck,
G. Marrufo-Mexico

Guillermo Marrufo
Monterrey, NL, Mexico


Third of three simultaneous responses -- August 9, 2013

A. Hi Tiger,

I am not going to second-guess Boeing's meaning, you need to talk to your Boeing technical representative to get a technical decision on interpretation.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom


August 12, 2013

thumbsup2Thank you all for your response.

Tiger Wu
Landing Gear - Xiamen, Fujian, China


August 28, 2013

A. The spec is saying that the wheel must be dressed .003" max above finished size. My company grinds chromed landing gear pistons and we dress .001" max above finished size, and we use .0002" max per cut.

Phill Morris
landing gear manufacturer - Runcorn England

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