Mitering for tapered head tubes

[Joe Roggenbuck]

Hello list!

I'm considering building a frame in the coming months with a tapered head tube (the Columbus 1.5 - 1.25" available from Nova) and was wondering what kind of solutions listers have found/thought of for the DT-HT miter.  Non-round tubing is quite common, but usually these tubes intersect with cylindrical tubing.

It seems that a DT-HT "hockey stick" sub-assembly would probably be the easiest way (for me) to build a frame with such a tube, making the project seem more feasible, however the miter still seems like a tedious process.  At least if you botched it you'd be minimally invested.

The best I can think of would be to machine miter the DT for the smaller OD of the HT, and then tediously hand file that miter to actually fit the tapering profile of the HT.  Or possibly notch the taper with several cutters, methodically increasing in diameter, careful not to undercut the miter, and finish by hand.

I know that there have to be means of machine mitering such a cope because production bikes -- especially alumimum frames -- have had to accommodate for such tapers for years.  They maybe have special cutting tools made for each unique cope, or use specially profiled grinding tools?  Possibly cnc laser cut like in that popular Cannondale factory tour video.

It seems like an interesting hurdle.  I wonder if there are any practical/affordable solutions for small-scale custom builders to machine miter, or if time is best spent with the ol' file.

Cheers,
Joe Roggenbuck

Syracuse, NY

[Brent Curry]

BikeCAD Pro gives you the option to specify the profile of your tapered head tube and will accurately account for that shape in the miter templates: http://www.bikecad.ca/miter_templates

As of BikeCAD version 10, the additional complexity of an elliptical top tube or down tube can also be accounted for: http://www.bikecad.ca/ovalized

Regards,

Brent Curry

http://www.bikecad.ca

https://www.facebook.com/BikeCAD

https://twitter.com/BikeCAD

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[Hubert d'Autremont]

Hi Joe, 

I would think the best thing to do would be to use and abrasive mitering set up. Then you could make a mandrel that fit the taper. 

The challenge I would thing would be incorporating different angles for different length headtubes, all having different tapers.

-Hubert

[Jona Aal]

Joe,

If you are good with maths you can read the attached pdf which describes the geometry of the problem you have. It also has the advantage of being written from the point of view of sheet metal work which has common geometric issues to frame building.

If I remember, there is a piece of software, maybe even a spreadsheet that would take the dimensions of your tubes and the angle between them and then draw you a nice curve which you can print out, wrap around your frame tube and use a grinder and file to achieve the mitre. Brent mentions BikeCAD pro, but I bet this isn't free! I bet someone on this list can remember what it is and where to get it.

The other issue with tapered head tubes is that they are actually flared slightly at the top and bottom (at least the ones I have seen are though there are several different ones). I wonder if setting up the tubes at the correct angles and then scribing the joint would not be the easiest way to make one frame. You could grind most of the material away and then move on t a file which would take all of an hour at the very most. Just setting up all the cutters in the mill would take at least that long!

Also, I have to poke a little fun, filing is a discipline to be enjoyed under the auspices of craft skills, not a tedious process! I bet that watching the custom cutter milling all the Trek top and down tubes would be tedious, loading the machine would be even worse!

Sounds like a nice job though, post some pictures of your solution.

Jona

Northumberland, UK

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Jona Aal

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[Brent Curry]

Since Jona suggested it, here is the link to BikeCAD Pro: http://www.bikecad.ca/bikecadpro . Jona is correct; BikeCAD Pro costs $350 Canadian. However, that is a one time fee. As new versions of the program are released, all owners of BikeCAD receive the new version for free. I'm proud to say that Joe is already a customer so it sounds like he's on his way to downloading the latest version. Something I'd like to invite all BikeCAD customers to do according to the instructions at: http://www.bikecad.ca/downloading_bikecadpro .

Brent Curry
http://www.bikecad.ca
https://www.facebook.com/BikeCAD

[Hubert d'Autremont]

Hi Joe, 

A solution I have been doing on the last few road frames is to machine a custom headtube in house that is spec'd for Chris King's 1.25"  Devolution Headset. Then by swapping the baseplate you can run an 1.25"-1.125" steerer. Not as big as 1.5" Taper but plenty stiff, lighter than 44mm and it gives you a better stack height. Then you are dealing with round tubes which is obviously way easier to deal with. Paragon is now making a tapered 1.25-1.125" headtube but I think the problem is still the mitering set up. On top of that it doesn't look silly big. 

-Hubert

[Mike Mcdermid]

hubert thats what we have come up with ,alternately a universal mill with a tapered porcupine cutter or radially abrasive mitering ,something quite clever in its own right

in fact for one offs its as easy to file or use a pipemaster

its easier to build a machine if your doing production and then each cutter will only work for a specific taper or go as far as a 4 axis mill

[Jim Merz]

I know most small frame builders would not have a CNC mill, or the CAD/CAM software to program complex surface machining, but this would be a quick and easy way to do the task. Not really worth it for one frame though.  Specialized has just recently patented a alloy frame joining method using hydroformed head tube. The weld joint becomes a simple butt weld. Almost no cleanup and very smooth looking joint. Otherwise, I would use the small contact wheel option on a Square Wheel belt grinder and just eye ball the miter when the tubes are all set in the fixture. You could do the cut with a normal hole saw then grind the area where the taper on the head tube is larger. The Anvil frame fixture is good for doing top and down tubes in this way as these tubes are not clamped at all.

Jim Merz

Big Sur CA 

[Darren Larkin]

I've done this a couple times for 1.5" tapered head tubes

Pick a cutter that is close to the average of the two diameters that the downtube will actually interface with.

Adjust your miter a few degrees (measure it) to make up for the taper angle of the head tube.

I still had to file this in, but just by a whisker.

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