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!

Blueprints

Today I put together the blueprints for the new design.  I use Rattlecad, an awesome free open source program that does a lot of the calculations for you.  You give it important information, like the slope of the seat tube and the drop of the bottom bracket, and it calculates the less exciting stuff.

I started with the geometry of a bike that I knew I liked, then lengthened the head tube about 4mm, bringing the imaginary bars up and back a little.  I entered the sloping top tube (6 degrees to match the lugs), and the offsets for where the tubes enter the lugs (how far one tube extends beyond the intersection with another).  I also tweaked the water bottle locations.

I'll print this full scale so that I can line the tubes up against it.  It'll let me quickly double check pieces and dimensions without TOO much 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.

A Bicycle is a Slightly Different Series of Tubes

The new tubes, lugs, and small parts have come in!  

Here's the entire bicycle, laid out.  The two tubes with an obvious taper are the chain stays (they connect the rear wheel to the bottom bracket.  The four thicker tubes make up the front "triangle" of the bicycle.  We call it a triangle because people get confused when scruffy bike guys use words like "quadrilateral".  

These tubes, with the exception of the head tube, are butted.  They get significantly thinner at the center to save on weight.  Since most of the strain on the tubes is near the joints, you don't lose a lot of strength.  

The small tube near the center with a drilling in the middle will hold the rear brake.

A lot of my favorite goodies are in that little plastic bag.  Reinforcement plates, classy adjustment barrels, and rack mounts.  I'm looking forward to dressing them up a bit this time around.  

This set of lugs are a little smoother than the last set.  They cover more of the tubes and sweep pretty gradually between them.  Unfortunately, this eliminates some of the elegance of the lugs I was originally using, which have a crease that better defines the tubes.  The flip side of that is that there's a fair amount of material to play with.  I might grind some detail work into the shorelines.

Tomorrow I'm going to prep a bunch of tubes (and the spare, cheap set of lugs I included in the order) for practice brazing.