03
Sep
12

Eight Tubes, Part 8: Cutting, Brazing, And Screwing Up

My buddy Jon stopped by today; he was finishing up a ride and he saw the garage door open. Luckily, I was able to give the impression that I was actually working on stuff.

Well, I was working on stuff.

Jon also reminded me that I had not updated this blog in a little while. Thanks, buddy!

Seriously, I’ve made progress. Metal chips have started to fly, and the torch has been busy. Oh, and I have to redo a chunk of what I did because of some, well, innovative production techniques.

Take a look:

You start building a frame by putting together the front triangle first. And you start building the front triangle with the top tube. You need to figure out how long you want it, and the angles of the seat tube and the head tube so you can miter the ends.

The bike will have a 55cm top tube (measured center to center), and the angles are 74 degrees (head tube) and 73 degrees (seat tube). The key measurement to figure out is the length of the top tube “miter to miter” along the top of the tube – the distance between the miters for the head and the seat tubes. This becomes important when you miter the tube ends.

For this frameset, the “miter to miter” measurement along the top of the top tube is 52mm.  How did I figure this out?  I cheated.  I have a 55cm frame that fits me perfectly, so I just measured the “miter to miter” distance on that one.  The other way to do it is to subtract half of the diamter of the head tube and the top tube from your target center-to-center length.  This gives you a rough approximation – it isn’t totally accurate given that the tubes are not at 90 degree angles to each other.  It is close enough, however.

Speaking of mitering tubes, it is now time to run off the paper patterns that will help you miter the tube accurately.  I mentioned these patterns a couple of installments back.   Nova Cycle Supply has a miter program that is pretty slick; you give it the the size of the tubes that are being joined and the angle, any it spits out a paper pattern that you cut out and tape on to the end of the tube.Get out the scissors! Art project!

Remember that “miter to miter” measurement that you figured out a couple of paragraphs earlier? Use that measurement to locate the paper patterns on the ends of the tube. The “miter to miter” distance will be the length between the “scallops” on the pattern. Take your time lining up these patterns on the tube so that when you finish the miters and you attach the head and seat tubes the top tube turns out to be (1) the proper length and (2) properly oriented to each other.  What I mean with #2 is that the head tube and the seat tube line up with each other when viewed from the front (0r back).  No twists.  Yes, this is a gross oversimplification of what goes on with this step, but you really don’t want to read about it in gory detail.

Confused? Look at the pretty girl until it starts to make sense.

she doesn't look nearly as insane as the pin up a couple of posts back, does she?

One last thing before you start cutting metal; you have to know which end of the tubing to trim when cutting it to length.  This can make a difference.  Some tubing makers (I’m looking at you, Columbus) design their butted top tubes and down tubes with a longer butted area on one end, with the intention that you cut on this end of the tubing.  Seat tubes are the same way.  If you cut on the wrong end, you can end up trimming away most of the thick part of the tubing that is supposed to go into your lug, ending up with a weak joint.  Most tubing makers will put a splash of paint on one end of the tubing to mark this out….so you have to know whether the splash of paint means “cut here” or “don’t cut here.”  The tubing maker will have this information on their website.  For Columbus tubes, for example, the paint generally means “don’t cut on this end.”

If the tube doesn’t have paint on either end, and you can’t get guidance from the folks who sold you the tubes, what do you do?  Me, I ended up taking metal off of both ends. Why? Simple.  I presume that the tube is butted the same amount on each end.

I really need to clean up my workbench

After trimming the tubes, I used a pneumatic die grinder ( aka “the dentist drill from hell”) to shape the ends of the tubes. You want the tubes that you are joining to fit together tightly, with no gaps.

See! No gaps!

You can double check your work by fitting the the lugs onto the tubes and seeing how well the contours match.

Damn I'm good...

I'd say those contours match...

Before we fire up the torch, there are a few other things that we have to do. First, and easiest, is that we have to drill a vent hole in the head tube. The reason for this is that when you heat up the tube – any tube – the air inside the tube will also heat up and expand. Unless you provide an escape hatch for that expanding air, it will try to make its exit through the joint that you are attempting to braze. That causes problems; the molten filler rod won’t flow into the joint when the expanding air is heading in the opposite direction out of the joint. I just drilled a 1/4 inch hole in the head tube, centering it in the joint with the top tube.

The second bit of preparation is to drill the holes for the bottle cage mount in the seat tube. You could probably do this later, but I found that it is much easier to set it all up on my drill press before the seat tube is irrevocably joined to the rest of the frame. I made a simple jig out of an old bottle cage to locate the bottle mounts. After measuring all of the bottle mounts on all of the bikes in my shop and finding a happy mean, I drilled the top boss about 24 cm from the bottom of the top tube joint.

Re-purposing useless crap that was cluttering up my workbench

After checking one last time to see if everything fitted together correctly, it was time to fire up the torch.

I brazed the seat tube/top tube joint first. The seat tube fit into the lug just loose enough that it would not stay in place by itself. So, I devised a rather Rube Goldberg-looking device involving a hose clamp and an old brake cable to hold it in place. That worked like a champ.

In fact it worked so well that I used my little invention to hold the head tube in place. And to make sure that things were nice and snug, I gave the cable a extra tug to make sure that everything stayed put.

Big mistake.

After I put away the torch, I broke out the angle finder to check my work. I did a great job lining up the head tube and the seat tube so that that the frame doesn’t have a twist to it (as viewed directly from the front). And the seat tube was close enough to my target of 73 degrees that I won’t worry about it being a little steeper than planned. I can tweak that with a little muscle.

The other good thing is that the lugs look pretty sweet. They are fairly thin, which means that they heat up quickly. Best of all, they have “windows” in the design that make them easy to braze. Just heat the joint, touch the rod inside the window, and direct the flow of the filler material by aiming your torch where you want the silver to go. Easy as pie. Once you remove the spent flux, very little clean up is necessary.

Black tassled loafers.  Tres' chic.

The place where I screwed up was on the head tube, thanks to my handy tube holding miracle device. When I put the cable low on the head tube and cinched it down that extra bit I ended up tweaking the angle; instead of the target of 74 degrees, the head tube ended up a rather sketchy 76+ degrees. Whoops. I should have checked things before I fired up the torch. Lesson learned.

Near-genius in action.

As for fixing the head tube angle, I’ll just have to get the torch out, sweat the joint apart, and try again. No big deal.

The other bit of collateral damage was to my long-suffering bike stand. I got the tubing hot enough to melt the rubber pads that line the jaws. They were torched anyway (THEY WERE “TORCHED”!! HAHAHAHA! I CRACK MYSELF UP!) so I’ll have to spend seven bucks to replace it.

Starting to look like...random tubes stuck together.

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1 Response to “Eight Tubes, Part 8: Cutting, Brazing, And Screwing Up”


  1. January 16, 2013 at 2:44 am

    Laser cutting is a quite recent developing procedure in comparison with some other developing procedures which may have their beginnings from the commercial revolution.


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