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Cutting

 

There are not many opportunities to cut steel in the bicycle business, but sometimes you may need to cut steel to make customized parts from larger salvaged parts, or may need cutting for other workshop jobs.  One bike situation in which cutting is ideal is a severely bent American ("Ashtabula") crank.  When the left side of these cranks get bent, you can no longer remove them from the bottom bracket, because the extra bend will not angle out of the bottom bracket shell. Normally, the bike mechanic cuts off the left side of the crank with a hacksaw. This can take quite a while.  A faster way is to use a cutting torch.  

 

 

Cutting off a one-piece American crank

(Notice the vise holding the downtube. Don't treat bikes you care about this way!)

A cutting torch is actually an attachment to your torch handle. Remove the cutting tip, and install the cutting attachment.

To cut steel: 

1. Turn the oxygen regulator up to 40 PSI.  Leave the acetylene regulator at 5 to 7 PSI.  Turn off the acetylene knob at the torch handle, and the oxygen knob on the cutting attachment.  Open the oxygen knob on the torch handle all the way.

2. Put on good leather gloves. Check your working area for flammables. Cutting steel throws off lots of sparks, and balls of molten steel.  These can catch in pants cuffs, or roll across the floor into piles of flammable materials.

3. Open the oxygen knob on the cutting attachment until you just barely hear oxygen coming out the tip. 

4. Open the acetylene valve until you hear the flow increase.  Light the torch.  Adjust the flames until they have blue inner cones without outer blue feathers, achieving neutral flames, just as you would with a welding tip.  If you have a typically large cutting tip, you may need to adjust the flames quite large.  If you hear any popping or the flames disappear, turn off the torch right away.  The flames are burning inside the tip, and will rapidly overheat it.  You need to relight the torch with the knobs open further to allow more gas flow. 

5. Hold the torch still, and very close to an edge of the item to be cut. The tips of the blue cones in the flames should just touch the item. If you are right handed, start at the right-hand edge of your piece, and hold the torch at about a 15 degree angle tipped to the left from perpendicular.

6. When the edge starts to melt, press the big oxygen valve lever on the back of the cutting attachment and start moving to the left (if you are right handed).   This will blow full pressure (40 PSI), pure oxygen at the molten metal, literally burning it, and pushing it out of the way.  The metal generates so much heat as it burns in the pure oxygen that it propagates the melting as you move. You'll find that you can move surprisingly quickly, even through remarkably thick metal. In fact, if you move too slow or irregularly, you'll lose the magical action, and have to repeat the process of preheating, and then pressing the oxygen lever to start burning again. Because you need to move smoothly, it is best to rest your hands on a comfortable surface and pivot to keep the torch moving steadily. However, be careful about balls of molten steel coming to rest against your gloved hands.

7. You won't have to test your work this time.  You'll know you've done a good job when the part you've cut off your workpiece falls. (Don't wear sneakers, and be aware of where the piece will actually fall.)

You can cut almost any metal, but metals other than steel are not as spectacular.  They don't burn, they just melt.  So, the technique varies.  To cut aluminum or brass, start in the same way, preheating the edge of the piece.  When molten, blow the molten section away with a puff of high-pressure oxygen.  Then preheat again, and blow away with another puff, and so on.

You can also cut steel without a cutting attachment.  In fact, if you are working with thin steel, such as bicycle tubing, you might have better results using an ordinary welding tip.  Preheat the edge of the area you want to cut.  When it melts, turn off the acetylene knob, while moving steadily to the left.  The cutting action will work in the same way, without any preheating flames.

 

 

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