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Making A Holding Fixture And Straightening A Buckled Frame Tube

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mike cates

Cruisin' on my Bluebird
First off I must make my disclaimer: I appreciate online repair methods by the pros with their use of hardwood and also metal blocks with precisely bored holes into them then splitting the block into halves to compress and reshape the tubing round again and this method works well for thin tubing. Also how brazing plays into this for repairing a dented frame.

I am not a professional frame builder/repair person nor a painter but merely a retired machinist and long-time bicycle collector that uses common sense and logic which works for me. I have overcome many problems by thinking it out, consulting others for advice (this is where you learn a lot and can trade experiences) and most of the time things can be fixed.

This is how I straightened a thicker walled frame tube on an antique cross framed bicycle that had a raised buckle back a few inches from the steering spindle on the underside of the main cross frame top tube probably caused from running into something that didn't move and the frame tube took the impact causing the buckeled area.

If you have a bent steel frame where there is a noticeable raised buckle in the tubing, you should remove any paint or rust at the buckled area and check this area by heating up the tubing with Oxy-Acetylene torch to see if any stress cracks are emitting from the upset metal area when it was cold bent by an impact. Use a soft flame and a small tip to slowly heat up the tubing.

The antique bicycle I am working on is a 1892 Star Lever Driven Hard Tire Safety made by the H. B. Smith Machine Co. Smithville, NJ.

I studied where the frame had to be unbent and came up with my method of anchoring the steering head spindle and also having a means of holding side-to-side alignment of the frame via clamping on some flat bar stock spaced so that it just cleared the diameter of the frame tubing diameter so it could slide within the flat bar clamped opening. This is a method that could be used for other styles of bike frames and it just takes just dreaming up a jig to solve your particular bend problem.

I made a jig fixture from scrap square steel tubing to hold the steering head spindle and hold the side-to-side alignment during heating of the tubing.

To inspect the raised buckled area, I heated up the raised buckled area to a dull red to orange color and could observe a stress crack that was longer than what was slightly visible before heating and could have overlooked this, prepped and painted the bike and found out later, the hard way, with a broken frame or possible accident on the road.

First I drilled holes at the ends of the crack that was visible to stop it from spreading further and mounted the frame in my home made jig.

I added some weight pulling down on the opposite end of the frame and started to heat up the tubing to a dull red to orange color to take out the buckle working the flame half way around the tubing diameter so the metal would expand in the direction I wanted it to go until the frame tubing became straight again. (Note: It is a good idea to have a limit stop block that you have calculated where the frame should end up when it becomes straight as the weight you added to help the frame bend under heat will just keep pushing it down if you didn't have a limit stop block). You may get some spring back after cooling meaning you have to reheat the tubing and make the stop block go further than where you feel the frame will relax to once it cools.

Once the frame hits the limit stop block LET IT TOTALLY COOL DOWN IN THE JIG. DO NOT USE WATER OR COMPRESSED AIR TO RAPIDLY COOL OFF THE HOT FRAME as it will pull the frame out of being straight and defeat the process you just went through.

After the frame was cooled off, I then saw the crack becoming longer and wider from the stretching of the tubing under heat and traveling around the side of the tube from the drilled holes. The drilled holes also elongated somewhat from the flame heating process showing the stretching of the metal in the area it needed to be stretched and becoming unbuckled.

Next step was to grind out the crack to prepare it for brazing it closed. Keeping the frame in the jig during brazing with the weight and limit stop block in place is best so the tubing cannot come out of alignment.

Once the braze joint has air cooled, remove the frame from the jig and rough grind down the brazed brass still leaving some extra to hand file down. Hand file at a steep acute angle nearly inline to the tubing using double cut then single cut files to blend to the surface of the undamaged tubing on either side of the brazed joint. The steep angle of filing will allow the file to guide itself off round tubing on either side of the joint to blend diameters.

Now it's a matter of using a straight edge tool (thick metal ruler or square ground flat bar you know is flat, or even the surface of a file) to check around the tubing for high and low spots. A body hammer can be helpful to massage the high spots which “may” raise the low spots and it's a touch and feel, hammering and lots of visual checks process from here. I use a white back ground to accent any high or low areas I spot check for light coming between the straight edge tool and the tubing. The best visual condition is if you can back light or be looking outside from a dark room or your garage for natural daylight as a light source so you will see the light coming through low spots or showing high spots. You just gotta keep further massaging the tubing to your liking. Remember the closer you can get the tubing to being round and straight, the less filler you will use and ultimately not notice the fixed area once the painting is finished which is your goal.

Everyone has their own filling and painting methods but the next step for me is sandblasting, priming with self etching primer, filling low areas with body putty, block sanding, applying a coat or two of high build automotive primer, sanding, finish wet sanding, applying a base primer coat and then top coat painting.

Mike Cates, CA.

PS: It is a good idea if there is any question about a possible crack in tubing, a bad braze joint, or other suspected damaged parts, use a 5X jewelers/machinists eye loupe with a bright inspection lamp,

in a well lit room or outside in daylight and totally check things out for flaws. You will be amazed at what you will see.

Treat your bike like it is an airplane to eliminate possible failures. You may not be in such a dire position as being up in an airplane when a failure happens but you still have a unsafe falling to the ground condition and needing something reliable supporting you.

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Whoops..... I forgot to include this photo of my jig fixture.
Here's my homemade jig fixture that I clamped into a bench vise.
The disassembled bicycle frame is turned upside down in it and the upper pointed steering spindle is put into a drilled hole that is centered side-to-side in the square tubing and the lower bearing cup end is clamped with a U-bolt strap clamp in alignment with the square tubing.
The two flat steel bars vise grip clamped onto the main square tubing length act an an alignment guide so the frame can barely slide free within them when heated and starts bending towards the jig fixture with a weight attached at the opposite end of the frame to stay aligned during this process.
The stop block I mentioned earlier is not shown but is just another piece of square tubing clamped at a height to where you want the tubing to end up to becoming straight. Again the stop block height may have to be altered in height for the frame to stop against since when the heat from the torch is removed the tubing may want to have some "bend back" somewhat and hopefully into a straight position when it finally starts to cool off.
Sorry I didn't take photos of the whole heating and bending process but the whole idea is to hold the spindle straight and support the frame in alignment while it moves being heated into the desired position.
Obviously in making a jig fixture for other frame designs, where the bend is will have to be thought out as to "where" you clamp your frame to the jig fixture..
Mike Cates, CA.

jig fixture.jpg
 
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