Dave Moulton

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Tuesday
Oct172017

Frame Jig.

In the mid-1980s I commissioned Photographer Mike Graves to take some photographs of my frameshop. Above is a picture of my frame jig used to assemble frames. My work area was not as messy as it appears here, but Mike took some artistic license and added some extra tools and fixtures to compose a more interesting picture.

The picture does however give me an opportunity to explain what was going on before Mike took over to create this picture. Frames were made in three separate parts, the main triangle, which is not actually a triangle as it is made up of four tubes. Top and down tube, seat tube and head tube.

The second part is the rear triangle, made up of chainstays, seatstays, connected by the rear dropouts. The third part is the front fork that is assembled on a separate jig not shown here.

The main triangle was assembled here then fully brazed out of the jig to allow the metal to expand and contract freely. There is less distortion and built in stresses that way.

The main triangle is fully brazed at this stage. The lugs have yet to be filled and polished, but the surplus head tube has been cut off and the head tube has been machined ready to take a head bearing. The excess seat tube has also been cut off and filed to the shape of the seat lug.

At this stage the rear triangle is being assembled, so that it matches up with the main triangle.

This is evident by the seat stays left long at this point, to extend beyond the seat lug. (Pictured right.)

The seat stays at the rear dropout will be tacked in the jig, then fully brazed out of the jig.

The seatstays will be later cut to length and a machined seatstay cap will be brazed into the seatstay top end.

The main triangle and the rear triangle will both be finished filed and polished separately, because they are easier to handle and manipulate in a bench vise as separate smaller parts.

When the two parts are ready, they are assembled and brazed out of the jig to accommodate the distortion due to heat. Because the main and rear triangle were initially assembled in the jig, they will fit accurately out of the jig for the final brazing.

This modified vise-grip (Above.) has two pieces of angle iron brazed to the jaws, these clamp onto the seat tube. A short piece of round tube is welded to one side, at right angles to the seat tube.

The seatstays are then clamped to this tube on the fixture with two engineers clamps. One shown here. (Left.)

The chainstays are held in place at the bottom bracket with a vise grip on the chainstay sockets.

Alignment tools are used to make sure a wheel will sit central in the frame.   

Frames like the Fuso were made in batches of five frames all the same. So the set up in this picture will be used to make five rear triangles, for example. Each stage of the assembly process is repeated five times. With one exception.

Front fork blades, and chainstays were identical no matter what the frame size, so these were prepared ahead of time in batches of 20 or 30 pairs. Fork blades were bent (Raked.) slotted, tips brazed in, then fully filed and polished and cut to length.

Chainstays were cut to length, slotted, and the rear dropout brazed in. Followed by filing and polishing. One extra item, the right chainstay is flattened on the inside to clear the sprockets when the rear wheel is removed.

This is where this little tool comes into play.

The thin end of the chainstay was placed in one of the slots, heated to a red heat, and hammered flat with a small hammer.

This was done prior to slotting the tube to receive the rear dropout. 

 

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Reader Comments (7)

As a beginning frame builder I really appreciate posts like this one. Thanks!

October 19, 2017 | Unregistered CommenterJohn C.

Dave, when you first learned to build frames were they pinned and brazed the 'old school' way?

October 19, 2017 | Unregistered Commenteredstainless

ed,
Yes, the first framebilders were blacksmiths, and I learned from Albert "Pop" Hodge who had been building since 1907. He pinned the frames with a Penny nail, which was a size of nail reference. He laid the frame out on the brick floor of his shop, using the lines between the bricks and marks he had made, as reference points.
When the frame was fully assembled in this fashion (Main and rear triangle.) he placed the frame in a hearth of hot coals the heat it, then fed the brass into the joint.
He brazed one section at a time in this fashion, BB, head lugs, and seat lug including the seat stays.
He had a hand held torch that ran off the town gas supply, (Coal gas.) Boosted with compressed air. He used this to add braze-ons, brake bridges etc.
Dave.

October 19, 2017 | Registered CommenterDave Moulton

Hi Dave!
Any idea why the chain-stays remains constant for all frame sizes? This happens in general with all bikes out there. In my view they should increase too, just as the front of the bike does - this way the overall balance between the wheels remains close to constant throughout frame size spectrum.
Thank you!
Great blog!
Mircea

October 24, 2017 | Unregistered CommenterMircea Andrei Ghinea

Mircia,
No not all frames, of course, but in the case of the FUSO which was a limited production frame, it was convenient for economic reasons to have them all the same. I could do this because the wheels are all the same diameter for all sizes.
Dave

October 24, 2017 | Registered CommenterDave Moulton

Thank you, Dave!
I was talking more in general. 99% of the bikes out there have the same chain-stays length for all frame sizes. I don't understand why this is happening. In my view that's not ok, because the overall balance is compromised. What do you think?
Thank you!
Best regards,
Mircea

October 25, 2017 | Unregistered CommenterMircea Andrei Ghinea

Hey Dave! Great blog here! i read some of your blog posts but this one is better. It has relevant information. Your blog is so interesting and very informative. Keep it up.

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