grinding gears without coolant 1

[Tmoose]

http://www.geartechnology.com/email/star0916.htm

"The second finishing pass, carried out by a grinding wheel, removes the remaining stock without overheating the workpiece, therefore resulting in a completely dry process."

In my limited experience with parts ground using flooded coolant I've seen dozens of parts flunk wet method fluorescent mag particle inspection with "heat checks."
I'd be mighty scared of dry grinding for even quite modest amounts of stock removal.

 

[gearcutter]

Dry grinding goes a long way to eliminating the risk of developing un-tempered martensite. In a gear grinding process, Martensite can only form in the presence of a liquid that can act as a quenching fluid.

 

[mfgenggear]

great for soft parts but not for case harden parts. it would be useless for our type of work.

 

[tbuelna]

With carburized/ground precision aerospace gears, the amount of case removed during finish grind is carefully controlled. The gear teeth are often rough ground prior to carburizing to compensate for heat treat distortions. After grinding, the tooth root fillets are typically shot peened to improve fatigue performance. Then the tooth flanks are honed/lapped to smooth their surface texture.

After finish grinding, but prior to shot peen and honing/lapping, all ground surfaces are etch inspected to check for de-tempering caused by excessive heating from abusive grinding.

 

[gearcutter]

all ground surfaces are etch inspected to check for de-tempering

There's more to post grinding/finishing inspection than just looking for de-tempered areas.
If not managed well; a wet-grinding process can significantly increase the risk of developing areas of un-tempered martensite. This is what causes and is sometimes referred to as 'grinding cracks'.........hence the reason why there is a push to move away from wet-grinding processes.

From this page -

http://www.ndt.net/article/ecndt98/aero/041/041.htm

"A further temperature increase [from that which results in B class thermal damage, also called a temper burn], well into the austenite temperature range, above 720° C and subsequent immediate quenching by the coolant used in the [wet]grinding process, will cause a D class thermal damage, also called a rehardening burn. This type of surface defect is characterised by an unacceptably hard and brittle layer of untempered martensite. Spots of a D class burn are always surrounded by areas of B class burn. A residual stress profile across a rehardening burn is complex and exhibits both compressive as well as highly tensile stress zones. Since the level of the residual surface stresses after grinding has a detrimental effect on the fatigue strength it is important that any of the above-described defects are detected before the component is placed in service."

 

[mfgenggear]

Hi Gearcutter

The grinding process has been to my experience is down to a science. very rarely is there grinding cracks , or over temper. the NDT processes used up to date are very precise and well read.
There is more emphases to the metallurgical properties of aircraft parts. and is a big factor.

 

[gearcutter]

Hi mfgenggear,

Yes, I agree with you.

Industry is leaning towards phasing-out wet grinding processes.
In Europe there is a greater push to phase-out wet grinding due to the environmental issues all of the used-up coolant creates.

 

[tbuelna]

...There is more emphases to the metallurgical properties of aircraft parts. and is a big factor.

That is indeed the case. Currently, many AGMA class 3 aircraft power gears are still carburized/ground AMS 6265 (9310 double vac melt). The typical heat treat involves a final double temper at <375degF. So care must be taken to minimize heat generated at the surfaces being ground. With AGMA class 3 aircraft gears, the temper etch inspection acceptance classification is often FA/NB1 or FA/NB2. Which means no grinding temper indications are permitted on functional surfaces like gear flanks/roots, bearing journals, sprag clutch races, etc.