PithyBikes

Hello builders,
I'm new to building so please excuse my beginner question. I took a class and tig welded a steel MTB frame. In the class we fixtured up the seat tube to bottom bracket in a fixture just for that purpose and tack welded it. Is this step necessary when using a jig like the arctos jig for example? Or would one simply tack the seat tube and bottom bracket in the arctos jig? Thanks!

fietsbob

IDK the Jig in specifics ..

Picture would help.

can you reach all the way around to lay down the bead while it's in the jig?

PithyBikes

Here's a pic of the arctos jig
https://www.hcor.net/gallery/NemesisP...6702.sized.jpg
BTW, this is not my jig. I do not own a jig.

It can be tacked while in the jig. There is room to lay a bead but for tig welding, it is not recommended. Because the steel deforms a bit from the heat. So, for tig you're just tacking in the jig and then removing the frame from the jig for actual welds.

fietsbob

Seems you answered your own question.

unterhausen

the real issue is how you deal with the part of the seat tube that isn't attached. I don't like having the part of the seat tube inside the downtube loose to creak. And really, you are encouraging crack formation in an area where there is very likely to be cracking problems anyway. I think it's best practice to fully weld in the hidden area. However, there are plenty of successful builders that don't

Andrew R Stewart 

No reason why the jig couldn't serve as a tacking one for just the ST and shell. Although with a good miter and simple positioning/clamping this is one tacking step that can be done pretty much freehand. You'll be aligning the ST/shell after tacking won't you? Andy.

PithyBikes

Thanks guys. Andrew, your answer is exactly what I was hoping for. My plan is to build a jig kind of like the arctos. It's good to know I can do all the tacking in that.

Andrew R Stewart 

Here's a real simple tacking jig. Andy.

duanedr 

Here's my BB/ST jig. I'm using fillet brazing but that shouldn't matter. it's just a chunk of angle with a hole and bolt through it with some other bolts to adjust the angle/centering of the ST on the shell. I added a clamp that I brazed to the angle as well. it works really pretty well. Given what Andrew is using, this might be overcomplicated but, I've been having fun building the fixtures as much as the bikes!

ksisler

OP; The pix offered by Andy and Duane would be excellent jigs for the purposes queried. Of course a classic flat table jig would also do the job well as they have for many classic builders over the decades.

/K

PithyBikes

Very nice. Love seeing pics of fixtures. It's cool how you guys use a simple bolt to square things up. duanedr, I like your tube blocks. I need to make a set of those at some point soon. I think the jig I've got in mind may be overkill. When I'm learning something I tend to go overboard. Thanks for sharing guys.

duanedr 

Yes, fixtures are always fun. I think during my first build (Oct 2013 through June 2014) I spent as much time on fixtures as the frame! It's where real problem solving happens!



The blocks are from Paragon. For normal sized (std and OS) main tubes, the blocks are 2" squares so the center of the tubes are centered from all four sides. Easy to hold in a vice and easy to plop down onto the table to check for miter accuracy. The edges of the block allow me to use a digital protractor to check angles (you can see in the picture on the inside of the angle). I don't do a full scale drawing. I print out Rattle CAD PDF with all measures and check against that.

.

You can see, I use the tube blocks on a slab of marble. My process is still evolving but, installing them on all of the front triangle tubes is the first step in my framebuilding process. I install one on one end and then install the other on the other end loosely. I then put the tube with the blocks on my slab and press down while tightening the loose one- now I basically have a square tube that I can reference throughout front triangle assembly to ensure it's all square and in alignment. On the end of the blocks, I have marks that allow me to mark 3, 6, 9 and 12 o'clock positions on each tube in phase so I can make sure the center is matching up at the miters.

I then clamp the blocks in a vice with the tubes at the correct angle and tack braze the joint before going back to the slab to check alignment.

Here's a HT/DT/ST getting checked on the slab. If there's any rocking in the level between HT and ST or the blocks and the table, I can tweak it at this point. My last one came out perfect (atmo).


My next frame has some weird tube sizes 29.5mm TT and a 35mm DT and Paragon doesn't have blocks for these sizes. I'm talking to a machine shop next week to see what they'll charge me to make me a special set.

PithyBikes

Great setup duanedr. Looks fast and able to accommodate any geometry you throw at it.

I'm going to try my hand at making some blocks. I ordered some 1 x 2,1/2" aluminum flat bar. The standard for round cut outs I think is to notch a hole out of a single 2x2" block, like your Paragon blocks. But I decided to go with a 90 degree angle cut out. Only thing is one needs to be careful not to over tighten with blocks like this. Anyway, I have to machine one side at a time with an angle cut out.
The blocks would be something like this:
Pic is from 44bikes photostream.
https://www.flickr.com/photos/blackc...n/photostream/


I'll be machining it on a mini mill that I purchased. So far I have not used it! To be honest I don't even know if a mini mill is suitable for such things. I guess I'll find out. My vise jaws have 3" clearance sooo.. should be ok. But I'm worried about vise depth, only 1". It's all that will fit in my small work space (and in my budget). At some point maybe I can help you guys out with small machining needs when I'm no longer a beginner

This thread may be getting off topic. When I start building some stuff I'll be sure to create a new thread. I'll be making lots of beginner mistakes so I humbly ask for guidance from you guys.

duanedr 

No to the first and yes to the second.

With the v-blocks the center of the tube is not held the same distance above the plate for different size tubes. With center bored holes this is the case as long as the outside dimension of the blocks is the same. My process relies on this.

My most recent build had a .75in top tube and Paragon's .75in block has 1.5in outer dimension and this really screwed with my process! With my coming build with a 34.9mm DT and 29.5mm TT, I will need to have new block sets made of 2.5in outer dimension to make it work...

PithyBikes

If the V blocks are machined properly maybe they can be configured in this way...

duanedr 

Ah, got it. I don't have enough experience to consider options for tools or fixtures that aren't already part of my process (or were part of it and found to not work!).

I suppose if you make the V shallow enough you can use on a pretty wide range of tube sizes. It seems like that could work just fine. For them to work in my process, they would need to clamp solidly onto the tube otherwise, I worry the tube might slip out of the center of the V as I was doing the layout and checking miters. And clamping them tightly could dent the tube since there are only 4 contact points.

PithyBikes

duanedr, I agree with your concerns. I have never used any blocks before but I can just imagine what could happen if tightened too much or not enough. When I get to the stage where I actually have created and tried a set I'll post how it goes. What's the worst that could happen right? hehe.

JohnDThompson 

Trek's original fixtures used U-channel to hold the tubes:

unterhausen

those things worked very well except when you filled the head tube holder with flux. Dick Nolan was a smart guy

duanedr 

THANKS! You just gave me a great idea for how I'll hold odd sized tubes when I don't have the right sized tube blocks! a clamp with a sideways mounted U channel, centered 1 inch above my plate would line up a strange sized tube with a standard sized tube held in a 2 inch block.

York Alley

Man, I might be looking at this wrong, but...
I like your concept but is that down tube brazed to the wrong place on the head tube? Nothing but encouragement here because I can see me doing this, too! :-)

Andrew R Stewart 

York Alley- Perhaps but that would be a pretty bad mistake, not that with enough ego and distraction someone could miss set up their tubes this way...

More likely is that the frame is a small one and that the level hides the portion of HT extending above the DT joint. That the HT extends well below the DT only aids the tube holding during the set up. I let my HT extend both above and below the main frame joints until I'm ready to cut off and ream/face the HT.

One point about duanedr's process is that it seems to rely on the tube angles to be the determinates for configuration. As the tubes are often a bit bowed from the factory and adding braze ons further bend and bow the tubes I suggest (and many builders will agree) that it's the joint center points that need to be the determinates. In theory the tube could be a pretzel between the joint centers and the frame would still be straight and track well. And if duanedr's process takes this into consideration then I apologize in my assumption. Andy.

unterhausen

You'll note that it's sitting on top of a full size drawing, so it's unlikely to be in the wrong place. More likely is that he just left the head tube really long. It really does't matter if you cut off 1/4" or 6", and it sure is easier to line up a drawing if the tube is longer.

York Alley

Ah! Great points made by ARStewart and unterhausen! Thanks for the input. I'll just go back to my corner.... :-)

duanedr 

Yeah, what they said. The level is hiding the top part of the HT. This was a 24" MTB for my son.

To Andrew's point of the angles being the determinates, yes, they are. Obviously, i'm still on the VERY steep part of the learning curve so, you've opened up a few questions:

"it's the joint center points that need to be the determinates"

I'm assuming you mean the intersection between the two tubes if you draw an imaginary line between one joint and the next (if those two joints are in the right place). I'm not sure how i'd measure or build off of that without a jig given my current process.

Laying the tubes over a full sized drawing combats this, right? For example, wherever the DT lands between the BB and HT/DT intersection (joint center as you say) is fine as long as the HT/ST/BB are in the right places according to the drawing, right?

I'm just thinking of the jigs I've seen that hold the tubes in the middle but these work because the HT, ST and BB are set to the right places before the TT/DT are mitered, correct? Then the builder miters the tube they have in hand (curves and all) into the space allowed by the fixed points of the joint centers.

OK, I think I got that part.

Do the experienced builders that handle loads of tubes per year see a difference between thick walled tubes and thin walled tubes as far as bowing and such goes - assuming both are butted? I can imagine thin walled requires more working and would result in more unintended bowing from the factory and then would more likely deform from braze ons and such.

Thanks again everyone