Mitering!

I just finished preparing the new tubes for my main triangle.  This time around, I marked the center lines a little differently than last time.  Instead of using a Sharpie to mark the center of the tube, I slathered it in layout fluid (a kind of easily removable paint) and dragged the height gauge along it.  That left a nice clean silver track through the coat of blue.  It was a much finer line than the sharpie left, quicker to place, and more accurate.

I also re-measured the butts on the tubes, as I'd swapped a couple of them out for new ones.  I'm glad I was careful; one of the tubes had the thinned section shifted about 30 mm towards one end.

I'm using some guidelines recommended to me by a talented builder (who coincidentally designed the slant-six lugs that I'm using) about the location of the butts and lugs.  He said that I should make sure that the lug tips don't come closer than 20mm from the end of the butt on the top tube and 40mm on the downtube.  There's not a ton of agreement about how much of a safety margin is necessary, but I think that it makes good sense for me to err on the side of caution for this first bike.

I marked the tubes at the end of the butt and at the beginning of the "safe area" to avoid confusion.

Mitering

Mitering is when you cut the "fishmouth" into one tube that lets it rest flush against another tube (the base tube) at a specific angle.  Figuring out where to cut can be tricky.  Luckily, Nova (where I buy my tubes) has a great little app on their site.

After you set the tube sizes and angles, the app gives you a printout that you can cut out and tape around the tube in question.  By lining up the center lines of the printouts with the center lines on the tubes, you ensure that everything is in the right plane.  Even better, you get a reference line that helps you space the patterns out.  In this case, the square end of the paper is 101mm from the center point of the base tube.  If you take the total distance from one intersection to another and subtract the offsets from both of the templates, it gives you how far apart they need to be.

I roughed out each miter with a Dremel, then knocked off the last few mm with a half-round file.  That kept me from going too far or heating the paper until it browned, which makes it hard to see what's going on.  Notice that the template has two cut-lines on it.  One represents the outside of the tube, the other represents the inside.  This helps you figure out which way the edge of the tube should be slanted, and how much.

Note: When you join two tubes of the same size, it gives you a pretty huge "outside" line.  This assumes that you can file the sides infinitely thinly so that they go all the way to the middle of the other tube.  You can't.  I just knocked mine down to a bit over the "inside" line.

I finished each miter by wrapping the corresponding base tube in sand paper and dragging it through, taking care of any remaining little issues.  Then I balanced the base tube on top of the miter and checked the angles of both tubes with my little Wixey Angle Gauge.  In one case I was within .1 degree of the correct angle on the first try.  With the others I had to do a bit more filing to touch it up.  

This weekend I should be able to finally start brazing my front triangle!

Cargo Rack, Part 2

In this series: Part 1, Part 2, Part 3, Part 4.

Yesterday I started brazing the rack, which was pretty exciting.  In this step, I just laid down a bit of bronze to hold things in place.  Later, I'll build up fillets.  A fillet is a meniscus of metal that joins pieces together.  When done properly, the metal creates a nice smooth transition between the two.  

Tabs

I cut the tabs out of some sample sheets of chromoly steel that I had lying around.  They're basically little metal guitar picks.  When the bike is done and all of the attachment points are installed, I'll drill them.  That'll let me level it out if something weird happens.  

I'm pretty sure, but not positive, that the sheets will be thick enough to resist bending when I'm carrying things.  If I was using it for loaded touring, I'd double them up.  I still may add a bit of reinforcement.  

Once the tabs were done, I slotted the vertical tubes of the rack, lined them up with the tabs on my basic sketch, and then squeezed them with a pair of pliers just enough to keep them from shifting (much).  

Brazing

I started brazing by securing the tabs.  I did each of the four connections one at a time.  The hard part was dealing with the last joint.  It kept wanting to spring free, as the rack widens slightly near the rear (for aerodynamic effect, or something).   I solved that with the sheet-metal vice grips that appear in one of the pictures below. 

Next, I attached the rails, where a bag will hang.  I clamped the first on one end, rotated it level, then tacked it on the other.  Then I was able to remove the clamp and tack the remaining end.  

The second rail was about 100 times harder, since it had to line up exactly with the first.  At one point I probably would have hurled it across the room if there weren't other folks around working.  Thankfully, after quite a bit of trial and error, it fell into place and I was able to tack it down before it slipped and hit me in the eye or something.

Here we go!  I need to finish the fillets and grind the ends a bit, but the look and geometry work.  

Next time: finishing fillets, adding tabs for the wooden deck, and making the strut to connect it to the seat cluster.

In this series: Part 1, Part 2, Part 3, Part 4.

Ignore This Boring Post About Butts

One quick bit of advice for anyone else embarking on this whole frame building thing.  Always ALWAYS measure your tubing.  Most of the tubes we use to make a custom bike are butted, meaning that they are thicker on the ends and thinner in the middle.  This lets the thicker section soak up the higher stresses near the joint, while the thinner section makes the finished bike a lot lighter.

This instructive daguerreotype, found in my great grandfather's memoirs, should help to illustrate what's going on inside the tube.  

You have to be careful though, because you can cut too much off of one of the ends and end up with some of the thinner tubing forming the joint... which may then fold like a coke can.  Even worse, tubing often comes with the butts shorter or longer than the spec sheets tell you they should be, so you really have to measure for yourself to be sure.

I held my tubes up to a bike light and stuck the depth measuring tool on a caliper into it, set for the point where the transition was SUPPOSED to start, then made sure that it lined up with the actual change in reflection indicating the start of the slope towards the thinner spot.  It can be hard to see what's going on in those things, so I had to bracket the measurement (-10mm is DEFINITELY falling short, +10mm is DEFINITELY going over that line).

I found two discrepancies: my seat tube started getting thinner about 1.5 cm lower than it should have and my chainstays (which are ovalized) are about 2.5 mm too wide and a bit too short.  The first won't be an issue provided that I leave the tube as long as possible when shaping the end to fit into the bottom bracket lug.  The chainstays will probably need to get just a kiss from the vise to be the right shape.  I'm a little nervous about that, but it shouldn't be a big deal.

Now I have everything marked, so I will know EXACTLY when I'm in trouble as I trim stuff to length and shape the ends.  Next post will likely be about some more practice welds, which will hopefully be more interesting than all of this measuring.

Center-lines

A few operations in the construction of a bicycle require you to have a clear center-line for the tubes.  The top tube, for instance, needs to be cut on both sides so that it will mate up with the head tube and down tube.  If those cuts aren't lined up with each other, you'll end up with a very twisty frame.  If you have a center-line, you just match it up to your templates (more on that when I post about mitering).  If you don't, it gets more complicated.

Before drawing on the tubes, I needed to figure out which way they bent, since it was pretty unlikely that they'd be perfectly straight. To do that I used an alignment table.  An alignment table is a big slab of stone or metal that is ground carefully until it's perfectly flat across the top.  It seems like any table would work, but it has to be pretty damned precise to make this work.  I rolled the tubes on the table, looking at the gaps underneath, until I found the flattest axis... then marked those as the "sides" of the tubes.  The center-line would be 90 degrees around the tubes from the "sides", because the tubes being slightly curved towards the front or back of the bike wouldn't make as much of a difference as having them lean slightly to the right or left.

Actually making that line is one of those things that seems easy, but ends up being really hard.  Lining up a straight edge with the tube is difficult.  Keeping it there while you mark things is pretty impossible without designing some kind of a fixture to hold everything.  I basically held two tubes together, with the point that I wanted the lines to start from deep in the "V" between them and dragged a marker down the center... again and again until the line started where I wanted it to. 

I then used the alignment table to check if they were straight with a specially made caliper.  If both ends of the line were the same distance off of the table, then I was good.  Otherwise, back to the drawingboard.  

When finished, I taped over them so that I wouldn't accidentally rub them off when working with the tubes. EDIT: NEVER DO THIS.  Taking off the tape is a full time job.  Next time I do this, I'll just get the miter templates and bottle cage mounts aligned right away.  

That's it for today, next time I'll share the diagram that I'll be working off of.