Any Wing Bolt Fitting Experience...?

NoShopSkills

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Well now I thought this was a pretty cool little project. Nonetheless I thought I'd still run it by some of you experts here.

A buddy called me asking for some help with the finer details of re-machining a "bathtub" wing bolt fitting on a Kingair. Actually he asked if I owned the tooling for the job. Just a hobbyist, once I saw the specific and complex tooling, I laughed! More to that story in a while.

There is an inspection requirement every few years to make sure that the bolts (several) which hold the outboard section of wing on, don't wear into these bathtub fittings beyond the limits established by the manufacture. If and when they do, the manufacturer also has engineers to determine if your re-machined fittings are within tolerances, or if they need to be replaced. Replacement requires removal of the wing, then the skin and of course the commensurate down time for the aircraft, until it's all put back together. Something to be avoided if at all possible. Here's a photo of the two fittings. The fittings are riveted to the spars at the 8 connection points. Wing bolts joins them together in a blind tension affair.

endview.JPG Bathtub.JPG
 
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So... to further the story, the manufacturer loans/rents specialty tools to re-surface the recess(es) in any damaged or worn fittings. Since there's no way to run rotary motor driven equipment in the throat of the fitting, while maintaining the perfect 90 degree angle and generating the force needed to resurface the fittings internal face, several parts make up the tooling set.

A threaded tensioning bolt is used. A non-threaded engagement paul turns the cutting tool. The tensioning nut provides the cutting compresion force, but the actual cutting torque force is provided by hand!!!!!

BT2.JPG

cutter1.JPG

asemb1.JPG

cutter2.jpg
 
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That radius on the cutting tool is very important. The bolt load is centered by that radius, keeping the washer in the middle. There really isn't a non radiused surface in the connection. Everything is rounded and needs to be finished out with smooth inside corners. The wing bolts themselves are an interesting piece of engineering too.

wingbolt.JPG

There are just no square shoulders and the internal mating surfaces of the washers marry perfectly to the bolt shoulders. Interestingly, we are yet to determine if this tool (chosen from a box of many) will cut the face any deeper, should that become necessary. It's hard to get a photo of the cutter's working angles, but it looks to be about 85 degrees with enough relief that the face where the washer rides should get re-dressed, but the technician from the manufacturer doesn't "think" it will.
 
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Here's the loaner tools which cover all of the fitting sizes the manufacturer ever made.

loaners.PNG
 
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and lastly, here's the use of the tool. The wing bolt is removed. Then the tool bolt is inserted instead. The cutter head is placed in the fitting that needs resurfacing, followed by the driving/engagement paul. Last, the tensioning nut is installed on the tool bolt. Tension is maintained with one hand, while the cutter is rotated using a steel rod, 1/6th of a turn at a time BY HAND!!!!!

The cutter is a left (anti-clockwise) rotation tool, one can only suppose - to eliminate over tensioning of the torsion nut, caused by friction between the rotating drive paul and the nut's opposing face.

useage.PNG
 
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My advice to my buddy was to use lots of lubricant and to stop quite frequently to remove the whole tooling assembly and clean out any chips that may accumulate in the tool head, since there's no where for those chips to go.

I am not sure if the fittings are forged or not, but I suspect they are. Not surprisingly they are significantly softer than the cutter tool and obviously softer than the wing attachment bolts and washers too, given 1) the application and 2) the fact that the bathtub fittings get scored and worn routinely due to cyclic lift and turbulence loads. The determining factor for return to service, will be the thickness of various dimensions of the fitting after the resurfacing.

Get this. The manufacturer will not disclose the tolerance/specification limitations. They'll only give a thumbs up or thumbs down AFTER you've done your resurfacing and send them your finished measurements!

How would you handle this task?
 
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BTW, here's the best photo of the face of the cutter that I seem to have taken. If it does cut it will be a very shallow one or two degrees at a time. Cutting at 2 or 3 RMPs, it could take many man hours to machine down below any bolt engagement face damage to re-establish a fresh new surface! Sorry for the poor photo quality :(
Facer.JPG
 
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Oh I forgot to mention that there are several engineering support companies around the world that you can hire to assist in this process. For a small fee, they'll help determine if you need any work done on your fittings, or better prove that you don't! in the first place. They employ eddy current, dye penetrant and vast knowledge to argue with the manufacture, that your fittings meet all saftey driven specs and are therefore good until the next inspection.
 
Those look like a standard reverse spotfacers that you can buy off the shelf. Just been modified to do the required job you have there.
Personally, I wouldn't touch that with a ten foot pole. I would leave it to the licensed certified aircraft people that do that stuff, let them do it so it gets fixed to specifications. Yeah, It'll cost, but I would feel more at ease to ride in that airplane knowing it was fixed by the proper people out there rather than from one of us. I'm very surprised they have loaner tools available for do it your selfers. Or was that borrowed from a buddy who is certified to that kind of repair? Are you certified to do that kind of repair?
 
Well yeah, all of the above...Of course my buddy is an A&P IA (Airframe and Power-plant licensed aircraft mechanic, with federally recognized Inspection Authority), His assistant/employee is an A&P too. We have a long relationship as he has also maintained all of my aircraft on the side, but I'm the one with a bit of a machinist's background. This is out of the scope for the average work day for most aircraft mechanics. 30+ years of working on airplanes and my buddy has never run a lathe nor a mill. That's why he called me and one of those engineering firms over to take a look before he started any work. The loaner tools are actually rentals and no... they're not really for the DIY crowd. Without disclosing this aircraft's ownership and/or it's mission, you'd be perfectly legal, although I can't guarantee you'd be 100% comfortable or at ease riding along :) once it's returned to service.
 
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