Machining a forging

And the folks in AMC use at least two dozen different
systems, not just SW.
Not only that, many well know the related fields, such as
product & forging design.

 

Ah, now I see what I'm missing in this thread. Your talking about the
design stage as well as through manufacture. By all means, one could
associate the forging model to update with the part model. Might be a
bit trickier than the other way around but I've done. The beauty of it
is that the finished part model is what's key. One other issue with
ordering forgings is they can and will grow or shrink as time goes by.
You'd want to be able to account for forging changes as they come from
different vendors. I've seen this lots with old forgings for FAA type
parts.

I now see what your talking about. Many time the forging is very much
the finished part but for some locating surfaces and a few other
things. In that case, the forging is so close to the finished part. Why
not just model the forging first to use as the basis of the finished
part. In other cases the forging is basicly a "blob" shape around the
finished part where everything is machined. I'm no designer but in the
case of the former, I model the part then, create the forging based on
it. Since most of my work has been programming this type, that's where
I'm coming from.

Along this topic, it seems that nowadays everyone just goes for hogouts
from solid forged square blocks as forging lead times can be long or
not available. This creates a real mess as we must sometime provide all
of the existing draft angles even though it's no longer from a forging.
Very common with gov't contracts for old aircraft spares.

 

Bill,

You mean a part that was once made from a near-net forging but is now
machined 100%, and they want all of the draft angles from the original
near-net shape machined in ???

Seems kinda dumb to me, unless there's a structural reason for these
features. But then, your dealing with a big bureaucracy. It'd probably cost
5 million bucks just to change one component.


Regards

Mark

 

It's dumber than dumb! Problem is these are usually older parts from
the 70's. They are being made for the Dept of Defense as spares. They
supply the origional Douglass drawings. There is very little they will
do when it comes to changes.

 

Yep. It usually contains the most information. Almost
everything else is directly related to it to.
It *controls*.

 

Bill wrote:
.... it seems that nowadays everyone just goes for hogouts

I find this amusing since they aren't likely to get decent
grain flow that way, which is one major reason to forge
in the first place. I guess the it does compact the metal
and close up voids, though.

As a forge die designer I would be very happy if some of our
customers would take draft into account when designing parts,
but then I'd be a lot less busy and have to bring a pillow to
work...

Cordially yours,
Gerard P.
Lakeview Forge Co.

 

Many current rolled materials may be better than the
forged alloys of yore.

Probably the same AMS specs apply either way.

 

Excellent. An on-topic discussion, and a guy who's brain I need to pick a
little.

I do a lot of 3D mechanical design and modeling;

I realize this is like asking "How far is up?" but here goes;

What draft angles do you guys like to see?

I assume it varies with length and width of feature and such.

Radii?

 

It's an aerospace part. Deviating from the print is the biggest no-no
you can imagine. And how much would it cost to re-engineer the print
and get FAA aproval for the new part? I wonder how close you got with
your guess?

Later,

Charlie

 

Scott:

Our standard draft angle ranges from 5 to 7 degrees. We use 7
the most. If a part has a fairly deep, narrow cavity (deeper than it
is wide) we may go to 10 to prevent sticking. The draft angles
don't really vary with the part size.

Fillets & corner radii vary a lot more. Our standard here is to
have .094" corners and .125" fillets, but .063" for both is also fairly
common. Some of our smaller parts have corners as small as .031"
where necessary (clevis legs, for instance, or the flats on a hydraulic
fitting) but this is best kept to shallow features where the stress
concentrations in the die aren't going to be a huge problem, or
where a thin rib leaves room only for that.

Fillets are usually more critical because they become corners
in the die, which metal must flow around. Picture a rib standing
from a flat part, with fillets at the base. As the dies close, metal
flows around these fillets into the rib. If the fillet is too sharp,
it
won't quite make the corner, but will leave a gap, and as the rib
fills, the metal may turn back on itself, leaving a crack at the rib
base. Yuck.

Tougher material also calls for larger radii, because it flows less
readily.

This has not been the most orderly post, unfortunately, but I
hope I have answered your questions.

Cordially yours:
Gerard Pawlowski
Lakeview Forge Co.

P.S. Cliff: True enough that we have some good alloys nowadays.
Unfortunately they aren't always easy to get, but that wasn't my point,
really. I have on my desk a forged brake part that has been cut in
half and etched to bring out the grain patterns, which follow the
part's contour quite closely. With a hogged-out part, you can't get
this; the grain follows the original block. This isn't so good for
fatigue
resistance. Of course the parts were probably heinously overdesigned
to begin with, if they are government parts...

 

Thanks very much.

This answers my questions perfectly.

Scott

 

IIRC Most aerospace parts are designed to a similar safety factor.
They don't add weight just because they can.
A replacement part with a lesser strength would be a point source of
failure in the overall system.

Usually, they'd like it to all fail at once <G>.
(At something like 5 X maximum possible loads or somesuch.)

You may wish to look at the various standards for materials,
such as the AMS ones, that might apply in the case suggested.

 

Dear Sir:
I was being a little facetious in my post; you're right, of course,
but (for the record) I was perfectly serious about the grain flow
comment. Call it AMS-S-5000 or 4340 steel; the part will still perform
better if attention is paid to the material's grain.

Cordially yours:
Gerard P.
Lakeview Forge Co.

 

That's what we do also. Start out with both forge and finished parts and
create the forging model. Then each of the machining operations gets
it's own configuration. That way, when designing the machining fixtures,
we know exactly how the partially finished part is going to look in the
fixture.

 

But how do you design the forging?
In this case it seemed like they were first designing the forging
and THEN trying to design the finished part ....

 

Often major landing gear components have a "fuse groove" machined in
them to ensure that any failure will cause the gear to collapse in a
direction that will reduce damage elsewhere. (hope that reasures any
frequent flyers here :^)

Pete

 

Which raises an intersting question.
How many have worked with planned failure mechanisms?
Controlling the mode of failure can be more important that
trying to raise it's limits, right?

That groove, as an example, would weaken the parts
allowing them to fail earlier with less applied stress
(or whatever)..... but is clearly worth doing.