When I begun with the framebuilding learning process, I found that was not that easy to find documentation on how to Tig weld steel frames, being most of it related with steel/oxy-acetylene or titanium/tig.
So, even if I'm for sure not the best to teach anything, at least I hope this small "manual" would be helpful to anyone who's interested in knowing about Tig welded steel frames, wich should not be too different to fillet brazing, for example, but there're always small details...
Trying to be as detailed as possible, it will take quite long so most likely I'll have to do it by chapters, will try my best not to take too long time between them ;)
I'll just get into the actual process, going over the frame design as I think this is another chapter wich is pretty well documented in mahy other posts/places/books and not necessarily specific to TIG process.
So, here we go with Chapter I: [Preparing/Mitering Seat tube vs BB shell]
-First of all, (and assuming BB shell is already faced) we need to "scratch" or mark the two centerlines of the tube (I prefer scratching as will not fade away after some work on the tubes). This is big key as will allows as to check phasing and proper alignment and also will be a good help for placing braze-ons. First you mark one line, for example I do it by resting the tube on the alignment table and then sliding the marker all the way top-bottom. Once this first reference centerline is marked, you've to locate it's opposite, to do so I made small plastic square papers measuring half the circunference of the tube, one for each diameter of tube I use (28,6-31,7-35-38-42). So, you place this paper aligned with the already marked centerline and then mark the opposite centerline. They should be at same distance one each other (damn, I'm pretty bad explaining things!). And you should also mark a centerline on the BB Shell, this is much easier as you just need to find out the center (normally 34mm) and do a full circle around, again, I use the alignment table for so, resting the bb shell on the table and locating the marker at 34mm height.
Here is a picture of one of the tube centerlines:
Once centerlines are done, it's time to place the tube on the vise for some saw/file action. Using a nice tube block makes a difference and I say this because I've used some v-blocks and when you tight the vise, then the round shape of the tube will ovalize, so when you're checking miter tightness during the process, this will not be accurate, as once you relief the tube from the vise, it's shape will be different to the one you checked on the vise... does it make any sense? I hope so... Anyway, having a nice and proper tube block will be helpful so you can tight properly the tube on the vise without any loss on the round shape. It will look something like that:
So, after a couple of saw cuts and some insistence with the file, you'll begin having the fishmouth shape you're looking for, something like:
To know when you should end this process, you need to check the join is in phase and square. To know if it's in phase you just need to check that both centerlines on the tube are matching the centerline of the BB Shell, at both sides! First times will be kind of try and go, but after some practice it's pretty straight forward and easy to manage. Here is a picture of one of the sides with centerlines aligned (the other side should be at the very same situation):
The other part on the deal, is to check BB and ST are in good angle, this case is 90º, so just need a precision square so to check how it looks. Depending on where the line goes you'll need to file one side more than the other, till you reach someting like this:
So, pretty much miter is done for the Seat tube, just need a bit of preparation before tacking. So first of all we sandpaper all the area, both outside and inside, and edges as well, to create the smoothest and cleanest possible scenario (specially important if some paint or oil is inside the tube). Important to keep the centerlines visible enough (do not sandpaper too hard!). Once we've sandpaper all, we can proceed to "broom" the inside of the tube so to take away any metal rest of the saw/file/sandpaper process:
Now that we have the tube nice and tidy, we clean it with some alcohol and make sure not to touch it from now with bare hands, just some cotton gloves and not letting the tube touch any dirt areas.:
Now we're ready for some fire! Just time to place the Seat tube and bottom bracket shell on the Jig for tacking. Make sure that Seat tube and BB shell are located according to the centerlines and without any "strange" forces, I mean that both pieces should rest one each other nicely and naturally, if miter is properly done then no gaps or the like would be around, and they'll match easily. If centerlines are phased, then seat tube will be dying straight on the bb shell. All those details will make a huge difference both on welding easiness and future alignment. So, we have all ready for tacking:
Before getting "hot", good to make sure all the related parts are as clean as possible. So time for cleaning the welding wire (I do use 1mm stainless steel 316L wire), specially if it's been resting for a while (dust ,etc). Also good time for sharpening and cleaning the tugnsten and put a brand new cotton glove cover to use on the left hand (well, the hand you'll use to feed the wire). Here is a picture of the dust cleaned from the wire:
And this will be end of chapter 1 as by the time I get the TIG machine ready, photo camera's baterries went dead :o
Man, that was a long text for such a small work! :o