reply to post by Zaphod58
This issue might not be an engineering issue, it very well could be a manufacturing issue.
Several well published episodes of airframe component failures, that in media were attributed to "fatigue" and or "out of spec" manufacturing, have
actually been attributed the finish machining processes, for the most part in milling.
The issue shows itself mostly in high strength Al alloys such as 2024,7075 and to a lesser degree in 6061 and Ti alloys .
As machine tools and cnc controls evolved the ability to remove metal advanced exponentially in the 60's thru the 80's. The possible volume of metal
removal increased to a degree that master aerospace machinist from the sixties would scarcely recognise the processes in use by the nineties.
In order to make more money , you have to remove more metal per minute.
In aeroepace parts a lot of metal is removed, in the case of the F35 the bulkhead that attaches the cockpit assembly to the main airframe, starts as
11,000 lb billet of forged 6Al/4V Ti, and spends nearly 30 days 24 hrs a day to machine down to a roughly 400lb piece.
So what was discovered in the late nineties, by a supplier looking back at failed part, is that even in a high pressure flood cooled cnc , with
through the tool coolant, the temperature gradient between the cutting edge and work surface is so great that submicroscopic surface cracks form,
that eventually will propogate into larger cracks.
This effect is very pronounced in heavily pocketed parts with small corner radii.
What happened in the machining world is that ultra high speed machining came about.
In UHS machining depth of cut and %age of tool engagment are substantially decreased and spindle speed and speed at which the tool moves across the
work piece are increased.
An example is, lets say when the SR 71 was built, Ti was machined at a cutting speed of 120 ft/min. That means the the edge of the cutting tool is
moving at a rotary rate of 120 feet per minute., with a tool engagement of 70% and a depth of cut of .5-1.00 x dia.
For a 3/4" endmill , that equates to 1/2" width of cut x 3/4" of depth of cut. This is a spindle speed of 611rpm and a table speed of 12.25 inches
In the modern world , I would program the same part at 1500 surface ft min, with a depth of cut of .06" and a tool engament 60% and chipload of .02
per tooth compard to .005 for the previous example.
This gives a spindle speed of 7,640rpm and a table speed of 611inches per minute. What happens is that the extreme speed of the spindle generates so
much heat that the base metal softens beyond it's yield point and the tool is moving so fast across the surface that all the heat is ejected with the
chip instead of being absorbed by the work piece.
So this brings me back to the F35 mention from before, in light of the advances in metal removal, I was surprised to see Lockheed using 50 yr old
techniques to machine the bulkhead.
edit on 26-2-2014 by punkinworks10 because: (no reason given)