Need to straighten 2219 bar after gun-drilling and OD turn...

[Electrofusion]

Like the subject says...

I've got a component that needs to be straightened after machining and I'm looking for the best method.

Details: Initial material is Ø2" x 36"L 2219-T3511 bar stock. It is gun drilled to final ID of 1.575" +.005"/0" full length. Subsequently, it is machined to a final length of just under 34" and lathe turned to a general wall thickness of 0.055" - though there are a some slightly thicker details at the ends for later Electron Beam Welding operations.

The problem I'm encountering is that I am ending up with a large amount of bowing after machining. Setting up on v-blocks at the ends, I'm getting a TIR of about .030". This part is used in the construction of segment of a particle accelerator/collider beam line, so it's subject to thermal bakeouts as part of the standard vacuum conditioning processes in its final use. In the last assembly I built, the intial runout was around the 0.030" specified above, however, after a 12 hour bakeout at 250ºC (internal vacuum, external atmosphere), the tube seemed to spring even more - to almost .050".

Can one of you fine folks give me an idea how to relieve and/or straighten this type of tube? Is there a thermal treatment I should be subjecting the bar stock to prior to machining - full anneal or something? Can I place the bowed tube in a 'straightening fixture' and thermally relax it to the desired geometry?

Thanks in advance...

 

[racookpe1978]

Are you supporting the tube in the middle (at one spot, none, or several?) while machining?

How are you cooling the tube while drilling to avoid thermal growth

How straight are the tubes before drilling?

 

[Electrofusion]

The machining is done, as you can probably imagine, in several steps by two different shops.

The initial bar stock (not tube) is approximately 0.300" oversize in diameter. The ends are faced and the full length is 'skimmed' (~0.050" - 0.100") to true it up straight and centered for the gundrilling operation. The gundrilling is concentric to 0.003" full length. This operation is lubed and cooled on the drilling machine. I *believe* that the drill (a custom 48" unit) has an internal coolant passage to flood the chips out and up the flutes and that the workpiece also has additional external flooding for cooling. I say *believe* only because I have not done a site inspection, personally. I have not witnessed what type of fixturing scheme is used in this phase of the machining either. Upon receipt of the skimmed and drilled piece, we are in good shape dimensionally & geometrically (straightness). It may be worth noting also that the hole tends to only have a variation in diameter of .001" max for the full length. The uniformity of the diameter, as well as the quality of the surface finish down the bore leads me to believe that things are running very true and straight for this operation - no evidence of back drag/cut from the drill.

The OD machining operations are performed at another facility. In their operations, the 'tube' is supported on a precision steel mandrel on a lathe, with the support of a "steady rest" (?). The OD of the body of the tube is cut to final thickness, per print, with water soluble machining fluid. Finally, the tube is removed from the mandrel, and the ends are faced to length and receive the final weld joint details - also on the lathe.

Needless to say, going from a Ø2" solid bar to a tube with Ø1.575" ID x .055" wall requires the removal of a temendous amount of material (relatively speaking).

I was not surprised to find that the finished tube was not super straight. I was, however, surprised to find that a bakeout at 250º caused the straightness to deteriorate so significantly. Any ideas on the best way to straighten it and thermally treat it so that it doesn't 'spring' after bakeout again?

 

[MikeHalloran]

Any aluminum part is likely to deform just from gravity during bakeout at 250 degC.




Mike Halloran
Pembroke Pines, FL, USA

 

[racookpe1978]

Can you bake it out hanging vertically?

 

[TVP]

There is no way that you can machine this part to its final dimensions and then subject it to a 250 C baking operation without some type of distortion. What is the initial temper of the 2219 bar? What does the temper of the end product need to be (mechanical property requirements)? You will need to perform a 250 C stress relief prior to final machining if you want to reduce/eliminate the distortion.

 

[Electrofusion]

@ MikeHalloran:
I've considered that as a possibility. It may be worth noting that there is very little weight supported by the tube. The overall assembly is a beampipe measuring 8.4 feet in length. It is terminated with Ø2.75" ConFlat Flanges at each end. The pipe body is ConFlat to (approximately) 34" 2219 Alum tube to 32" Beryllium (S-200F, Ø1.575 ID x .030" wall) to 34" 2219 Alum tube to ConFlat. The baking position is - by customer specification - simply supported at the flanges, so it is a cantilevered loading. The mass of the Be center section is just 5.15 oz. - much less than the 15.47 oz of the Alum segment... I suppose it could still be enough, but I haven't seen this issue with larger (Ø60mm) assemblies of greater length.

@racookpe1978:
Unfortunately, a vertical bakeout is not permitted due to the customer specification mentioned above.

Does anyone have any recommendations for themally processing the machined aluminum component prior to assembly? For example, should I restrain it in a fixture and heat it to straighten it followed by some tempering or something similar to help it 'survive' (geometrically) the bakeout process?

 

[Electrofusion]

@TVP:
Intial temper is T3511. End temper is not specified or critical for strength of the end assembly. (Refer to post above regarding specifics of mass and loading.)

Would you recommend a 250ºC stress relieve before gundrilling and an additional intermediate 250ºC stress relieve prior to the OD work? A stress relieve after roughing the OD before final?

 

[metengr]

I would agree with what TVP mentioned above -

You will need to perform a 250 C stress relief prior to final machining if you want to reduce/eliminate the distortion.

.

 

[Electrofusion]

metengr,

Any recommendations on:

Would you recommend a 250ºC stress relieve before gundrilling and an additional intermediate 250ºC stress relieve prior to the OD work? A stress relieve after roughing the OD before final?

 

[metengr]

Electrofusion;
I would use only one stress relief operation and that would be before final machining.

 

[mfgenggear]

Electrofusion

In addition what has been said before.
which is very good advice.
for a part like this.

rough, face ends & od, finish Drill the hole , to true up the rough OD & faces leave approx .100 stock total on diameter on OD finish id,(to make it beefy)
stress relieve @ 250 deg F
( Note 2219-t3511 is annealed @ 1000 deg F, water quenched
it aged @ 375 deg F & final stretched to stress relieve)

finish turn faces
then finish turn OD.

the last vendor that did the turn is also part of the problem. inserting a tapered lathe arbor & machining the OD is also tweaking the part. the part OD needs to be turn in the free state. locate each end of the holes buy means of centers /or internal soft jaws. or similar or equivalent.

minimum pressure must be used to turn the shaft OD.
the part is distorting because it is stress relieving
during it's running cycle or what ever.

to fix you current condition, "warm straighten" find a heat treater in your area that can do it. should be no problem.

MfgEngGear

 

[MikeHalloran]

It sure seems like a truckload of work to make a piece of pipe. ... and from what is clearly the wrong material to boot.



Mike Halloran
Pembroke Pines, FL, USA

 

[Electrofusion]

@mfgenggear:
Any basics on how to warm straighten? I have both atmospheric and vacuum furnaces at my disposal in-house...

@MikeHalloran:
Definitely a truckload of work. Material selection in this case is driven by the ability to weld it to Beryllium and its behavior in high radiation environments - not structural/dimensional stability considerations.

 

[mfgenggear]

Electrofusion

It's mainly the ability of the operator/fabricator to straighten. by heating the part to 250 deg F then
using several large v blocks to support the part
then using a special tool shaped to the contour of the part
hit the high & low spots to straighten. he or she must know from experience how much press to push the part without
deforming it. it's tricky but doable.

if the operator is not experienced it can become worse.
so easy does it.

MfgEngGear

 

[Electrofusion]

I'm assuming we'll just do a furnace cool after straightening it. I'll take your word for the 250ºF - but it sounds kind of cool compared to the extended 250ºC (482ºF) bakeout it will see later.