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Thread: Welding Airframe Sequence

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    Aeronut's Avatar
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    Welding Airframe Sequence

    First, I want say how impressed I am with the PA-18 airframe pieces I ordered from VR3 in Canada. The customer service was awesome, the quality top notch (pun intended), the price great, and it was only a couple of weeks from order to delivery. Id do business with them again in a heartbeat.

    Im preparing to start welding my airframe together within the next couple of weeks. For those with experience doing this, how do you recommend setting up the jigging and sequence of welding. If I had to do it now I would start from the pieces under the pilots seat and work out and up from there. Basically using the logic of starting from a good foundation. However, Id like to avoid painting myself into a corner.

    What do you recommend? Is there a best practice?

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    RVBottomly's Avatar
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    I don't think there is a particular dogma, especially if you have a full jig. But people often start by laying out and jigging the lower longerons, cross braces, and diagonals. Fit them together before starting to weld and then make sure they are secure.

    Then do the same for the upper longerons. After that you set up a jig to connect the upper and lower, and so on.

    But some do it differently. I've heard of people welding the sides first, joining top and bottom longerons and then bending it together to join them.

    Tony Bingelis recommended starting from the rear if you are new to welding so you would get practice on less critical sections. I'm not sure the rear is less critical, considering the stress on the tail in bush flying, but that was the suggestion in the 70s-80s.

    Most important thing is to lay everything out before committing to weld. That helps avoid the inevitable "oops, I forgot the cross-piece that I can't get to fit now" problem (been there myself).

    The other most important thing is to double or triple check measurements when you jig. It's probably not as important with the precut tubes you have, but I often found myself astounded before welding at how far off my assembly was. Misplaced reference line, etc.
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    Aeronut's Avatar
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    Thank you for the solid advice, RV! I feel like a lot of time spent verifying measurements will be time well spent.

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    First suggestion is check out the “Building a Scratch Built Cub” thread on this forum. Good stuff there.

    Also, try some communication with Dan Dufault, he’s in Lincolnville ME, Supercubsnorth.com
    If anyone can guide you on an assembly/welding sequence it’s probably him. He’s built 35+Supercubs and other rebuilds and builds.
    Google him up, his contact info is on the home page of his website. Really a helpful and nice individual, easy to talk to.

    With a different thought , what about VR3? They offer welding services and from the reviews and testimonials seem to have their act together. Their customer service does sound top notch (couldn’t resist) so I would think they could definitely point you in the right direction to get started right.

    Happy building!
    Oz

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    Aeronut's Avatar
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    Thanks for the leads, Oz! I‘ve seen some of the scratch building stuff and I‘ll have to go back through those threads. I didn‘t know about Dan just 2.5 hrs North of me. I‘ll check out his website. I actually don‘t recall seeing where VR3 welds also. Since I‘m already in possession of the pieces I believe the combined logistics and costs wouldn‘t be desirable. I‘m also pretty set on tig welding this thing together for the experience.

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    Once you get the fuselage built and proceed to the cowling work. Hang a motor or box of weight off the front, the fuselage will flex a bit with the weight. It sucks to build a nice 3 piece boot cowl and not have the holes line up once you hang a motor. Been there done that.
    DENNY
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    Aeronut's Avatar
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    That‘s solid advice, Denny, and I will heed it. I also assume it‘s just one more check on the integrity of the welds and will give me a good opportunity to re-inspect everything.
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    If you are going to TIG weld it, be sure to clean the mil scale off all the joints and clean each joint with acetone prior to welding. Not quite as important if gas welding, but makes a huge difference if TIG welding.


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    Aeronut's Avatar
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    Absolutely, dgapilot! I‘ve been practicing with thin-walled 4130 and I‘ve been removing mill scale to at least 1“ away from the joint and cleaning with acetone. Thorough prep work has been one of the habits I‘m incorporating.

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    Something I like to do before your final weld is to sweat the material with a propane torch right before you weld. It makes for a cleaner weld, and it softens the line at the edge of the heat affected zone, thus making it less likely to crack. It's surprising how much moisture comes out of a piece of light 4130, especially when the shop temp is a bit cool.

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    Aeronut's Avatar
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    1934A, do you prefer propane over mapp or is it just a personal choice?
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    Would you mind expanding on what the pre-heat does with the TIG. I’m not quite up to speed with it! Thanks
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    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by dgapilot View Post
    If you are going to TIG weld it, be sure to clean the mil scale off all the joints and clean each joint with acetone prior to welding. Not quite as important if gas welding, but makes a huge difference if TIG welding.


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    Never have...... but I might give it a try


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    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by 1934A View Post
    Something I like to do before your final weld is to sweat the material with a propane torch right before you weld. It makes for a cleaner weld, and it softens the line at the edge of the heat affected zone, thus making it less likely to crack. It's surprising how much moisture comes out of a piece of light 4130, especially when the shop temp is a bit cool.
    Never have...... but I might give it a try


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    Kid Durango, preheating the material allows the material to heat, expand, and contract at controlled rates. If the material were to heat up too quickly or cool too quickly cracks may form. Allowing it to cool too quickly may also „lock in“ stresses resulting from the variation in heating and therefore material expansion. For this imagine bending a pipe. One side will be in tension and the other in compression. If you heat that material to certain temperature for the proper amount of time, the molecular structure „relaxes“ and relieves the stresses from bending. The same mechanisms exist when welding.

    From what I understand material under 0.120“ wall thickness does not require pre and post-heat. Some people also use pre-heat to remove moisture that may be hanging around.

    That‘s my understanding but perhaps some more experienced with welding can correct my errors or add to my information.
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    Quote Originally Posted by Aeronut View Post
    1934A, do you prefer propane over mapp or is it just a personal choice?
    Either one works, propane is cheaper, use what you have on hand.

    As far as preheating, I do it to remove moisture from my weld, and also to relieve any stress that is in the metal. A few years ago, I was welding on some material that had to pass x-ray testing. Just put of curiosity, we purposefully didn't preheat one piece. The difference in weld cleanliness, and temperature zone lines was very obvious. When Oxy-acetylene welding, you're already preheating the material as soon as you touch your flame to the weld area, because it takes a few seconds to build your puddle. When MIG and TIG welding, your metal rises in temperature much quicker, therefore, the metal is sort of 'jolted' up to temp very quickly, making it more likely to trap stress in your weld zone. Preheating will slowly bring it up to temp, and just like Aerobic said, relax the material to be joined. With TIG, especially, since your heat affected zone is much smaller, I think preheating is important to sort of 'blend' the temperature zones so as not to cause an area that a crack could possibly get started. Tale this for what it's worth, obviously there are lots of chro-moly planes flying that weren't preheated before welding, but it's a pretty simple process, and it gives me a little piece of mind knowing I did it. Plus, it only takes 10 or 12 seconds per weld, so it doesn't really add much time. TIG welding is sort of my relaxing for me, so I'm usually not in a hurry!
    Last edited by 1934A; 10-18-2020 at 07:04 PM.
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    1934A, how are you checking the temp of the preheat? Are you using a temp pencil? If so what temp are using and how far away from the weld?

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    Quote Originally Posted by Aeronut View Post
    1934A, how are you checking the temp of the preheat? Are you using a temp pencil? If so what temp are using and how far away from the weld?
    No, I don't use a temp pencil, I just heat until I see the moisture evaporate, 'and then some'. Right at the weld area, it's probably around 175 degrees, as I move away from the weld area, I taper off, so at 3" or so, all I'm doing is getting the moisture out of the metal. The purpose is to taper the heat affected zone out, along that 3" area.
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    Quote Originally Posted by Aeronut View Post
    Thanks for the leads, Oz! I‘ve seen some of the scratch building stuff and I‘ll have to go back through those threads. I didn‘t know about Dan just 2.5 hrs North of me. I‘ll check out his website. I actually don‘t recall seeing where VR3 welds also. Since I‘m already in possession of the pieces I believe the combined logistics and costs wouldn‘t be desirable. I‘m also pretty set on tig welding this thing together for the experience.
    Aero, I wasn’t suggesting that you have VR3 do the welding even though they do offer those services. Just thought they might be able to give you some pointers on where to start and how to set it up from their experience and POV then you can fire up the TIG and have a ball. What TIG machine do you prefer?

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    My misunderstanding, Oz. I'll reach out to them and see what they say. I'm using a Lincoln Electric TIG 200 that I bought earlier in the summer. I have some experience with some MIG welders and one other TIG welder so I don't have much to compare this TIG welder to. I purchased this one based on it's size and brand reputation. I've been using it weekly and I've been happy with it. Most of the appreciable gains in quality have been from my skill increasing, changing cup sizes, and using a finger button on the torch. The latter piece allows me to practice from left and right hand as well as nearly any position
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  21. #21
    Fat Kid's Avatar
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    There is a 4130 steel tube airframe construction dvd thats very informative at black hills airports.com. He goes through the whole process in the dvd.


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    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by Aeronut View Post
    My misunderstanding, Oz. I'll reach out to them and see what they say. I'm using a Lincoln Electric TIG 200 that I bought earlier in the summer. I have some experience with some MIG welders and one other TIG welder so I don't have much to compare this TIG welder to. I purchased this one based on it's size and brand reputation. I've been using it weekly and I've been happy with it. Most of the appreciable gains in quality have been from my skill increasing, changing cup sizes, and using a finger button on the torch. The latter piece allows me to practice from left and right hand as well as nearly any position
    Get some clear Pyrex cup kit from https://usaweld.com or other companies


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  23. #23
    Crash, Jr.'s Avatar
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    The key with TIG is a really large cup (Furick cup, or similar) and a fair amount of tungsten stick out with 1.5 to 2x the normal amount of argon. The commercial welders damn near max out the argon flow gauge and run 1/2" to 3/4" of stick out. Allows you to really get into those clusters.
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    Quote Originally Posted by Crash, Jr. View Post
    The key with TIG is a really large cup (Furick cup, or similar) and a fair amount of tungsten stick out with 1.5 to 2x the normal amount of argon. The commercial welders damn near max out the argon flow gauge and run 1/2" to 3/4" of stick out. Allows you to really get into those clusters.
    Ive been using the Furick Jazzy 10 with gas lens. Big enough to get good shielding when welding clusters and almost 1/2 stick out, but not so big that you need to waste a lot of argon. The Fupa 12 would be my next choice, clear so you can see better, able to get more stick out.


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    I just received a #12 pyrex cup. I prepped some 1“x0.049“ 4130 with a 60 degree notch. I‘ll play around with some you all‘s suggested tips tomorrow afternoon. What do you all recommend for amps? I‘ve been having the best results with 45-49A for the aforementioned thickness.

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    Rule of thumb to get in the ball park, 1 amp per .001 thickness. If you run pulse, a little more. That generally works with steel.


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    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by Crash, Jr. View Post
    The key with TIG is a really large cup (Furick cup, or similar) and a fair amount of tungsten stick out with 1.5 to 2x the normal amount of argon. The commercial welders damn near max out the argon flow gauge and run 1/2" to 3/4" of stick out. Allows you to really get into those clusters.
    too much argon flow will just MIX with the ambient air..... MORE is not always better, as I learned long ago.....

  28. #28
    Aeronut's Avatar
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    I‘ve been having good results with about 17 CFM Argon flow. I‘ll see if that needs to go up a bit with more stick out.

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    Aeronut's Avatar
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    I got an email yesterday from VR3 and, in keeping with their customer service, Don VanRaay sent me a detailed document laying out how an assembler should approach welding the fuselage together.
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    Quote Originally Posted by Aeronut View Post
    I got an email yesterday from VR3 and, in keeping with their customer service, Don VanRaay sent me a detailed document laying out how an assembler should approach welding the fuselage together.
    Please share any tips or information that he provided. Is there any info in his instructions that adds to, or contradicts any advice given in this thread?
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    1934A, the overall idea is to start with a flat table and then build it in the upright position from there. They recommend beginning with the firewall sub assembly and then drawing and using reference lines on the flat table to assemble from there. The key is to have a fixed reference lines and work from there. Also, assembly may be done in halves.

    Cleaning - Wipe off any excess oil or protective coating from the outside and inside of the tube. Use a Scotch-Brite pad to remove and expose bare metal. Wipe with acetone. (I'll have to give this a try. I've had the best finish with a 120 grit flap disk but haven't tried Scoth-Brite.)

    Welding - VR3 is pretty clear, their tubes are notched to fit well. A gap indicates a mistake with assembly and should be addressed. Tack welds to hold as much structure as possible together. This allows you to confirm assembly and dimensions prior to final welding.

    Like with all things, I get the impression that time spent on preparation and ensuring accurate measurements from the start on the table will be time very well spent. I imagine I'm going to spend a fair amount of time just setting up my assembly welding table. Since my work space is used for other things I'll probably build it on casters so that I can move it to the side. Fundamentally it must be rigid.
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    Aeronut, Did they tell you to work from the bottom up? Or was there any mention of making two identical sides and then joining them together?

    I've never done one from scratch and likely won't. But I have thought that making the two identical sides first would give more accurate results.

    It seems to me, working from the bottom up would provide more possibilities of a twisted result.
    N1PA

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    Aeronut's Avatar
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    Skywagon, they explicitly state not to. "Do not build the sides as flat sub-assemblies because the tube kits have the profiles, pre-formed longerons and other tubes based on the final 3D geometry." They seem to take pride in their notching which is designed to all fit snuggly together and provide additional weld support. I interpret their literature to mean that they designed it with a specific assembly method in mind. Perhaps it wouldn't mattter on another assembly, but on this companies it looks like it does.

    Edit to add more info from their literature:

    "2. Build the kit “vertical up” in it’s normal flying position:a. Helps avoid confusion about left hand/ right hand orientations
    b. It may be tempting to build the structure relative to an alternate ‘flat’ surface or
    upside down. Do not be fooled as this will lead to confusion as the assembly progresses."
    Last edited by Aeronut; 10-20-2020 at 02:00 PM.
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    There is a guy that built a Skyote using a VR3 kit. Lots of pictures how he built his jigs on his website. Granted, lots smaller than a Cub, but same idea.

    Skyoteblog.blogspot.com


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    Quote Originally Posted by Aeronut View Post
    I got an email yesterday from VR3 and, in keeping with their customer service, Don VanRaay sent me a detailed document laying out how an assembler should approach welding the fuselage together.
    Nice! Glad that worked out. Very much interested in their document, always learning.
    Also...That Lincoln TIG 200 is a great little machine, portable, light, handy. Glad you’re happy with it.

    Thanks for for the additional info they provided. Clarifies some things.

    Cheers, Oz
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  36. #36
    skywagon8a's Avatar
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    Aeronut, Thank you, it sounds like an erector set. Must be very nice.
    N1PA
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  37. #37
    Aeronut's Avatar
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    I'm looking at it like a big piece of Ikea furniture that requires welding.

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    Take lots of pictures, and keep us posted on your progress! This sounds like a great kit, their workmanship sounds top-notch! I'm excited to watch it come together.

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    Will do, 1934A!

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    I'll be following too. I love watching others' projects while I slog along on my own.
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