File to Fit

It’s been a long time since I’ve ranted about framebuilding here (I do it every day in my head but rarely have time or want to spend it writing thoughts that few really care about). But hell, what good are ideas if you don’t put them out there to bounce off the interwebs?! That’s what blogging is after all, banter that may or may not be interesting to a small clique of the world.

So anyways, I have finished my 17th frame and it was a doozy.  I can’t say I enjoyed the entire build, but seeing it done is pretty satifying.  It’s almost the frame that hate built, to use the Over-Opinionated Framebuilder’s phrase.  En sum, I went through one extra top tube (cut too short, stupid SOHCAHTOA), one extra wishbone seatstay crown (burned a hole), two extra seatstay legs (slotted too short), one Ti coated drill bit (snapped at the tip), and a lot of brain cells.  The bending of tubes and the resultant geometry changes to the miter lengths and miter angles are stifling me. I am trying to use BikeCAD to get the miter angles and lengths but for bent tubes I now am just going to draw it all out life-size.  You can see some of the differences in the mitering numbers from what I had modeled in BikeCAD to what I drew out on paper in the picture below.  I’m sure many others can correctly use BikeCAD to input the right radius numbers…but I can’t.  Like i’ve said before here, I haven’t a clue how to get a given radius from the Harbor Freight tube roller unless I buy a expensive set of gauges that only help after I’ve rolled the tubes.  Ok, so I guess I could roll/check/roll/check to approach the radius I’m looking for but I haven’t tried that yet.

So what I do instead is file to fit. I am pretty sure most builders that have milling machines and expensive fixtures still file to fit a little bit but for their sakes I’m hoping they don’t do it quite as much as I do.  There was only one miter on a curved tube I got spot on and didn’t touch after the mill – the wishbone seatstay miter to the seat tube (17 degrees).  I’ll remember that number for some stupid reason for a long time. It’s stored in the same part of the brain that remembers the lyrics to those 80’s songs you really truly hate.  Of course I won’t remember how I finally figured out the right way to measure and miter the curved top tube angles.BikeCAD-notes

In the photo of the drawing only the circled numbers stayed constant. Really those are the most important since they are what control the geometry of the frame.  They’re what I set the fixture at and try not to move it more than 1mm (if i cut a tube too short).  They’re what define how the bike handles. Oops, just noticed I forgot to circle the head tube angle (but maybe that’s irrelevant as some don’t think that is an important parameter atmo).

I know over time I’ll learn to use my machinery and fixtures better, and learn to trust that ruler and my use of it so I don’t have to file to fit (as much as least).  I would like to be able to measure & mark the tube, load it in the mitering fixture on the mill, cut the tube, deburr, and load in fixture and be done with it. Paper tight miters without filing to finish.  No problem, right?

So this is what I’m going to do. No more bent tubes until I get the process for straight tubes dialed better, and when I start selling frames bent tubes will cost more because they do take a lot more time and effort.  Same goes for internal cable routing.  I do love how the frame looks without cable stops but not sure it’s worth the extra weight and effort.  Luckily for my friends I’m a non-profit framebuilder right now and willing to try anything even if it is above my pay grade. More details on this frame in post-to-come!

F17 outside

the finished product

One Response

  1. File to fit is all I ever knew for my first 100 or so frames – if you do it enough, it’s actually faster than doing it on the machine, IMO. Of course, I use a mill anyway because I’m lazy… bike looks rad!

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