Grinding problem for chrome part?

[3743735]

Hi,

Can anyone answer this question?
Couldn't aerospace part(Material 4340) after chrome plating be ground by CBN wheel? Is there any evidence to avoid using CBN wheel for grinding aerospace parts?Thank You!

Tony

 

[Goahead]

Hi,
from Metals Handbook Vol. 2, 8th edition on page 473 one reads:
"Because Chromium is hard and brittle, a soft grinding wheel is essential. [...] Good performance can be obtained with an aluminum oxide resin bonded wheel made with a grit of about 60 and of H-grade (hardness)".

From the Desk Edition:
"Cubic boron nitride has a high impact resistance and exhibits optimum results in the grinding of hard ferrous materials (>50 HRC)".

From the above one desumes that CBN is not recommended for
grinding hard chromium plating.

Hoping this helps,

http://www.welding-advisers.com/

 

[kenvlach]

Well, Metals Handbook Vol. 2, 8th edn. (1964) pre-dates CBN usage, and since chrome has an HRC of ~57, the newer Desk Edition would seem to recommend CBN.

Conventional wisdom for many years has been that because chromium plating is brittle and contains high residual stresses, aggressive cutting media and heat build-up must be avoided. Thus, relatively friable alumina accompanied by large amounts of coolant is used for slow material removal without glazing. That was the view in 1964, in MIL-STD-866B (1978), ASM Handbook, vol 5, Surface Engineering, (1994) and Machinery’s Handbook, 25th Edn., (1997). E.g.,

MIL-STD-866B GRINDING OF CHROME PLATED STEEL AND STEEL PARTS HEAT TREATED TO 180,000 PSI OR OVER (inactive) (1978), available at ASSIST-Quick Search:

http://assist1.daps.dla.mil/quicksearch/

“6.4.2 Grinding wheels (normally friable or semi-friable aluminum oxide) in a relatively soft grade normal to open structure, and vitrified bond are recommended in the largest grit that will produce the required finish. Hard, dense wheels in fine grit size are most likely to cause problems and their use should be avoided. This same philosophy also applies to coated abrasives.”
6.4.3 recommends copious amounts of coolant.

It is important to avoid glazing of the wheel (creates heat, which can crack the chrome and cause the formation of untempered martensite in high-strength steels). Lots of coolant is recommended.
-- ASM Handbook, vol 5, Surface Engineering, p. 189 (1994).
Note: the martensite transformation expands the underlying steel, leading to spalling of the chromium plating.

From Machinery’s Handbook, 25th Edn., p. 1112 (1997), wheel recommendations for cylindrical grinding of chromium plating are
Commercial finish SFA60-J8V [SFA = semi-friable alumina]
High finish A150-K5E
Reflective finish C500-I9E [fine SiC]

“…When you machine chrome you are actually relieveing some of the stresses and abusive machining can aggravate or increase the stress. Micro cracking that you see after grinding would give some indication of abussive grinding…”

http://www.finishing.com/52/87.html

[note: Cr plating is usually highly tensile stressed, which accounts for its microcracking, rather than compressively stressed as stated in the post]

Heat is definitely the enemy when grinding hard chrome.

http://www.finishing.com/4600-4799/4618.html

Now consider the use of CBN, as suggested in the preceding post.

“An important aspect of grinding with CBN and diamond wheels is reduced heating of the workpiece, thought to result from their superior thermal conductivity compared with aluminum oxide…The superior ability to absorb heat from the workpiece during the grinding process reduces formation of untempered martensite in the ground surface, caused by overheating followed by rapid quenching.”
-- Machinery’s Handbook, 25th Edn., p. 1086 (1997).
The greater thermal conductivity certainly is a plus for CBN. This same paragraph also mentions that CBN grinding may induce compressive stresses in the ground surface as opposed to tensile stresses produced by alumina. While this sounds beneficial (tensile stresses in Cr plating reduce fatigue resistance in plated steel), possibly the reversal of stress within the brittle Cr plating leads to distortion and failure at the point of attack [speculation on my part].

Some abrasive wheel vendors mention CBN wheels with porous structures to allow water or other coolant to penetrate more freely to the point of attack. However, in a brief Internet search, I didn’t note any CBN grinding wheels specifically recommended for Cr plating. Most hard chrome plating companies do grinding, but their websites don’t mention which abrasives are used.

To summarize: The conservative approach would be to continue using the types of alumina and carbide as described above. It may be possible to use certain CBN grinding wheels, perhaps with modified techniques, but I wouldn’t do it without testing or hearing from a qualified machinist.

 

[ornerynorsk]

Chrome plate is ground all the time in industry using aluminum oxide wheels, as goahead suggested.

 

[kenvlach]

Aluminum oxide is considered the "tried and true" method.
But, suppose your job depended upon finding a better method. Is there any technical reason not to use CBN? Cr plating solutions have changed within the past 20 years to yield lower residual stresses (among other things). This may affect the optimal grinding.

 

[EdDanzer]

It has been a while since we looked at CBN grinding, but it seems if you can grind it with one abrasive, a harder one should work better. We were considering doing creep feed grinding in 500 BNH plate in a HMC. Other than the initial wheel cost, experimentation is the only way to determine if it will work. Getting coolant to the abrasive point of contact is very important, as well as machine rigidity.