DooMaw - building a STOL

Head in the clouds · August 2, 2015

these guys do the whole aircraft tubing kits, works out about US$2500-$3000 more expensive than just the materialhttp://vr3.ca/refdocs.html

have a look at the fuselage links at the bottom of the page, it has detailed part descriptions

Hmmm - at that price I'll just stick with the wrap templates, there's probably only about 20hrs total in templating and cutting the notches by hand with a 1mm disc and shaped 6mm grinding disc in a pair of angle grinders. That doesn't justify a AU$4K materials cost price increase unfortunately. I'm pretty sure I came across a different mob that were quoting about US$1500 and buying the tubing direct from US would save about that amount compared with buying it here, even allowing for the freight. I'll see if I can find the link again.

https://fbcdn-video-b-a.akamaihd.net/hvideo-ak-xtp1/v/t42.1790-2/11721705_954854061227322_1170893149_n.mp4?efg=eyJybHIiOjYyNCwicmxhIjo1MTJ9&rl=624&vabr=347&oh=280f7c0284085068bc7b33220edd6492&oe=55BB2342&__gda__=1438331377_e387fc9283b40ed1795e5164c124b85d
This link should work

Yes, nice - it's a technique called 'walking the cup' and produces very consistent welds. It works well when butt welding two tubes well prepped with a chamfer to their edges, or for a fillet weld, or for a lap joint. You can even use it to produce a fillet down the middle of a flat sheet by clamping a guide to the sheet. The point being that you need one or two edges as a guide to walk the cup along.

For those who don't TIG, the cup is the ceramic cup that acts as a nozzle for the Argon shielding gas. What is actually happening to produce such a regular weld is that as you swing the torch from side to side you are producing a zig-zig ladder in effect, and stepping the cup up the ladder as you progress. You just have to match the dipping of the filler rod with each swing of the torch. It takes a bit of practice but isn't as difficult as it might at first appear. I managed a reasonable weld the first time I tried it on a test piece but haven't had a need to use it on a real job yet. The hardest part is feeding the filler rod along in your hand as it gets used up, and I guess even harder would be changing for a new rod on very long welds like that one shown in the video.

Progress on DooMaw since the last update -

Wednesday and Thursday I got 7.5 hrs in after work all spent notching and fitting up the tubes for both sides of the aft fuselage.

I spent an hour and a half on Friday evening and got one side of one side tacked, a bit slow as I had some dramas with the welder which I later discovered was power brownouts in the area, electronic welders don't appreciate that very much!

On Saturday I tacked the first side of the second side, labelled and removed all the wood-blocks, tacked the second sides of both sides and fully welded out the first side. Today I welded out the second side, so as Mark predicted the aft sides are done by this weekend.

As I suspected the very thin tubes were quite demanding but with the welder turned right down and the pulse function disabled, and using the foot pedal to modulate it off and on as you might with a MIG, I was able to make quite reasonable welds though a bit tiring on the ankles. Unfortunately being a cheapie welder it doesn't go to really low settings like a Miller does and at the lowest setting the background current when using the pulse function is just way too high for very thin material. No complaints though, it got the job done and one day I'll invest in a Miller.

A few pictures show the aft fuselage tubes in the jig during tacking, one frame tacked and the second in the jig, the portable welding trolley that I set-up a couple of weeks ago, the jig blocks removed for welding out the frames, some of the welds on the thin tubing, the last one shows the tiny 1/4" (6.4mm) tube right at the rear -

That's another 25hrs in the project log making a total of 210hrs so far.

Next I was going to join the rear sides to the front sides but I've changed my mind, for easier manhandling and rotating it all for welding the internals of the clusters, I'm going to jig and join the two front sides with all the front cross-members and vertical and horizontal bracings. That will produce a very rigid cabin assembly and it will be much easier to join the aft fuselage into it once it's fully welded out.

So as spare time allows, it's back to the CAD work again, produce more wrap templates, cut more tubes, sand off the mill scale, paint them with primer, mark them for notching, notch them, make a wood-block jig for the fuselage sides, laser level them to be sure they're level and square, fit up the cross-members and diagonals ... rinse and repeat.

Kyle Communications · August 2, 2015

Very nice

Kyle Communications · August 2, 2015

You can see where the expense comes in..its not the tube or the notching its the man hours involved in the hand welding of all the joints that you are paying for in a premade fuselage that is made like this. It would be nice to have it CNC tig'd if it could be done.

Soleair · August 2, 2015

Beautiful welding, HIC.

Do you have to normalize or otherwise heat treat the material after welding? ISTR if you heat steel above its Upper Critical Temperature the grain reorganizes itself, which may affect ductility, etc? Or is it not necessary?

Which model welder do you have?

Bruce

Head in the clouds · August 5, 2015

You can see where the expense comes in..its not the tube or the notching its the man hours involved in the hand welding of all the joints that you are paying for in a premade fuselage that is made like this. It would be nice to have it CNC tig'd if it could be done.

I expect it could be done by CNC TIG but I don't think the production volume would justify the set-up costs. I haven't seen one in action and the ones shown on Youtube don't seem to have a provision for adding filler material, they seem to just meld the edges together. I'd imagine there'd have to be some that do add filler though, probably from a reel a bit like a MIG machine I guess.

Beautiful welding, HIC.Do you have to normalize or otherwise heat treat the material after welding? ISTR if you heat steel above its Upper Critical Temperature the grain reorganizes itself, which may affect ductility, etc? Or is it not necessary?

Which model welder do you have?

Bruce

You can bring the completed fuselage back to the N (normalised) condition by having it heat treated but of course it requires a large oven and special care due to the thin material. As far as I know it's not generally done even on earlier steel tube production airframes like the Taylorcraft, Piper Cubs/Pacers or the Austers. They were all welded using OA (oxy/acetylene) process and the technique there is to bathe the HAZ (heat affected zone - the edge of the weld which shows the colour) with the flame after completing the weld, and slowly moving the flame further away to provide a degree of stress relief to the weld.

The TIG process allows a good operator to complete the weld with a lot less heat than the OA process and so the stress relief is less important. Naturally not all welds go as well as you might like so if one does end up having been heated more than you're comfortable with you can go back and locally stress relieve it later with a propane torch. When doing that you just heat it gently and evenly around the area until the blue colour disappears and is replaced with a straw colour, the same as used for letting back (tempering) heat-treatable steel (which CRMO is) after hardening it. Some folks have thought this would leave it annealed (in its softest state) but that's not so, it's still harder than the N condition (as the tubing is supplied - hence the 4130N designation for CRMO tubing) and the N condition is harder than the fully annealed condition.

The really important thing when welding CRMO is to avoid there being any drafts in the workshop because the moving air cools the welded area too quickly and that can leave it brittle and more prone to cracking in service. My shop/garage has slatted walls so when it's windy I have to place shields around the area I'm working on.

My welder isn't a branded one, it has a stupid name "Super-200P" and was sold on ebay by a mob who called themselves Proweld industries and they promptly disappeared after selling a dozen or so of them. There are lots of similar ones on ebay, they have different cases and the switches and knobs in different places but they all have the same or similar electronics inside. They are so generic that mine even has some of the knobs labelled wrong which caused me quite some confusion at first, my "AC Balance" knob actually controls the Pulse Duty ratio ... and the "Clean Width" is actually the AC Balance, I eventually worked that out because most machines have either Clean Width or AC Balance controls but not both. It was a bit of a learning curve because the previous TIGs I've used were transformer types not inverters, so didn't have any of the fancy features.

These are really cheap, mine was about $800 delivered but if anyone considers buying one of these it's worth checking what features it does have (or doesn't). Mine claimed to have AC Frequency control but it doesn't, and that would be useful for fine welds on Aly. Mine also has a knob for initial slope but it doesn't do anything and the manual says it doesn't have it. The problem with all this is that the people selling them generally know nothing about them or about welding, they just buy and sell them.

This one also does stick welding and plasma cutting up to 16mm plate but I don't use those features although it does come with all the necessary accessories to do so.

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In gaps in real work I've been preparing the next stage of the build, printed the wrap templates for all the members that go between the two sides of the fuselage front and will form the cabin. There are a lot more members than at first meets the eye, more than 50 members which is nearly as many as the total I've done so far in building the front and aft sides, here's the Marking Plan -

Head in the clouds · August 9, 2015

Not much interesting stuff to report on but plenty of hours have been put in to get nearer to the next stage.

With the time spent on CAD and creating the templates described above (13hrs), correcting them to reduce the fit-up time later, cutting them out and then cutting the tubes to length (8hrs), descaling the tubes in the lathe, and descaling the larger tubes (spar and strut carry-throughs) by hand, then washing, masking and painting them and begin the wrap template stencilling (14hrs) has made for a busy week with another 35hrs on the project on top of normal work.

So the total hours on the project so far are 245hrs.

The pictures show the tubes during preparation -

Not too far off until I have the stencilling finished and the notching done, then I can start to jig the two fuselage sides together.

Head in the clouds · August 16, 2015

Nice progress this week, starting the assembly of the two sides.

It's good to see the shape coming together but the welding gets a lot harder, getting access to the insides of the frame to tack it all together when you can't rotate it into position is hard on the ageing back but probably good for personal mobility in the long run, certainly a lot better than sitting at a desk a lot of the time like my 'real' job.

The rest of the stencilling took a couple more hours then 15 hours to notch all the ends.

Next I had to make a very accurate jig for the top frame so that the carry-through members that the wings connect to are perfectly aligned each side. It involved a bit of machining, clamping, welding and gladly turned out very well. That allowed me to assemble and fully weld the top frame with its cross-bracing in the jig so that ensures that the wing rigging will be accurate and was also a lot easier way to weld it than as part of the main cabin frame.

Following that I checked and re-adjusted the levelling of the bench - the earth around here moves a fair bit according to moisture content and the laser showed it had moved 3mm across the feet of the bench which is magnified to 6mm across the full length of the bench. Easy to adjust with the threaded feet on the bench, then I could start to mark the positions for the wood blocking that holds the bottom of the side frames parallel.

Installed the blocks and packers to keep the bottom rails off the timber so that the larger cross-members could fit under them and then had to devise a means of holding the side frames vertical. I used small chains and turnbuckles a bit like rigging a yacht mast and had to add some chains under the bench as well to balance the load on the top edges of the bench.

With that in place I could set the laser up on top of the bench and set the side frames level and vertical, now it was nice and easy to adjust with the turnbuckles opposing each other.

Then I could drop the top frame on and was delighted to see it fitted almost perfectly, a light touch of the grinder and it dropped into place with barely a visible gap anywhere. That meant I could then remove the external jigging frame and add the side fore and aft members. None of those are welded in yet, I need to get all the vertical bracing in place before doing that.

With the top frame sorted I could turn to the front and rear cross-members, final fitted them and tacked them in and then added the vertical bracing. I used a couple of extra chains and turnbuckles to fine adjust the alignments where the complete welding of the sides had pulled them out of flat by a few millimetres.

Then I added the front horizontal bracing behind the instrument panel and the whole assembly is locked up and rigid so I could move the bench aside to put the car away for the night.

There was 41 hours in that lot, so the log shows 286 hours so far. Some pics -

[ATTACH]37355[/ATTACH] [ATTACH]37356[/ATTACH] [ATTACH]37357[/ATTACH] [ATTACH]37358[/ATTACH] [ATTACH]37359[/ATTACH] [ATTACH]37360[/ATTACH] [ATTACH]37361[/ATTACH]

Head in the clouds · August 16, 2015