Flaring, Cutting and Bending Brake Lines by Jim Clark


Cutting, Bending and Flaring Tubing Article and Photos by Jim Clark

    Cutting, bending and flaring tubing is a skill that should be mastered by anyone building or modifying a hot rod. There are three types of tubing used in automotive applications: rigid, semi-rigid and flexible. Rigid tubing includes JIC steel, stainless steel, titanium, heavy-walled steel, hard drawn copper, or similar types. Semi-rigid tubing includes soft copper, brass, aluminum, thin-wall steel or special alloy. Flexible tubing is usually called hose. In automotive applications the cutting, bending and flaring will usually be restricted to rigid stainless or semi-rigid thin-wall steel used in brake lines. Aluminum, soft copper or thin-wall steel for fuel, cooler, A/C and power steering plumbing lines.   Rigid tubing is usually supplied in straight lengths, semi-rigid tubing in coils.

 

Uncoiling Tubing: Care should be taken when uncoiling tubing. Determine how much tubing is needed, then place the coil on a bench or on the floor and hold the end with one hand. Unroll the coil straight back without pulling it out sideways from the coil as this will put a twist in it and tend to throw it out of round.


Straightening Tubing: Careful uncoiling still usually results in some kinking of the tubing requiring straightening. One method is to place the tubing on a flat bench and place a flat board on top of it. Then strike the board gently to flatten down the high spots. Care must be taken not to flatten the tubing out of round or collapse it.

Another method of straightening long lengths is to slap the tubing against the floor, turning the tubing as you progress. Two people are usually needed to do this, but one can do it if they secure the other end.

Cutting Tubing: A tubing cutter is usually the safest and best way to cut thin-wall tubing. Other methods have a number of disadvantages, the primary one being the difficulty in getting a perpendicular cut. Exceptions to this rule are stainless and titanium tubing. These are work hardened by a tubing cutter making the flaring process more difficult. A hacksaw is the preferred tool for cutting this type of tubing. Getting a perpendicular cut is difficult without the use of a sawing vise. This is especially true for heavy wall or large diameter tubing.

 

 

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: A sawing vise is the preferred way to get a clean perpendicular cut, especially on high strength, heavy wall or large diameter tubing.

 
 

When a tubing cutter is used properly it will provide a clean right-angle cut with very little burr inside the tube. To accomplish this the cutter should be fed slowly in small increments while revolving the tool around the tube.

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Tubing cutters are available in a variety of sizes. Mini cutter on the right is useful for cutting tubing within confined spaces

 
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Some cutters have interchangeable cutting wheels to allow for the cutting of harder tubing like stainless.

 
 

Reaming Tubing: It is desirable to remove the burr created in the end of the tubing after it is cut. The softer the tube the more pronounced the burr will be. Feeding the cutter in very small amounts during each revolution of the cutter will help to minimize the burr but will not eliminate it. So the use of the reamer usually included on the cutter will usually remove the burr. Care must be taken not to over-ream the end of the tube because this can create a weak flare due to the thinning of the metal. Any tool used to ream the tube should be held square to the tube and pointed down to avoid getting chips in the tube.

 
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Most tubing cutters also have a reamer attached to remove the burrs created inside the tube when it is cut

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A tapered reamer can be used but care must be taken not to cut into the tubing reducing the wall thickness, which would weaken the flare

 

       

    Flaring Tubing: Two angles of flare most commonly formed are 45° and 37°. The 45° flare is the type required when tubing is to be connected with SAE 45° flare fittings. These fittings are widely used in automotive, instrumentation and refrigeration applications.

 
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This 45° flare and fitting are the most commonly used in hot rod applications.

 
 
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             Three types of flare are shown in this photo. The 37° and 45° are the most commonly used in hot rod       applications.  The 37° flare is widely used for JIC hydraulic, aviation and industrial applications. It has come into common usage in automotive applications with the wide usage of AN (Army Navy) type connections created for the military.        

 
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              AN (Army Navy) fittings were created to satisfy a military  spec. They have 37° flare  rather than the   45° flares common in most other applications

 

Flares can be either single, one thickness of the tubing flared out to a specific angle or a double flare. A double flare is two thicknesses of tubing created by folding the end of the tubing back onto itself, forming a double thickness flare. The single flare is the most commonly used but the double flare is desirable in some applications where splitting of the tubing while flaring is a possibility. Applications like thin-wall steel tubing are places where this would apply.

Preparing the Tubing: End of the tubing should be cut off squarely and not out of round. Any burrs in the end should be carefully removed with care taken not to create a thinner wall at the end of the tube. Cutting with a hacksaw will leave a burr on the outside end of the tubing and this must be removed also before flaring.

Making the Flare: Standard bar and yoke type flaring tools are equipped with either a fixed or swivel cone. Swivel cones remain stationary when the advance screw is turned; fixed cones rotate when the screw is advanced. Either will create a smooth flare when the burrs have been properly removed.

A good, smooth, narrow-flared joint is much more desirable than a rough-looking, uneven, wide-flared joint. A proper flare will have a uniform wall thickness from the base to the lip of the flare and will be unscored at the base.
Two types of flaring tool are commonly used. The older style is the compression-type flaring tool. It flares the cone spreading the tube end until its expanded outer diameter makes contact with the countersunk flare profile in the die. Over tightening the flare cone can produce a thinning out of the flare and can cause score marks around the base.

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    Tubing is clamped in the flaring bar projecting above it equal to the depth of the recess in the bar       

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  The compression type-flaring tool forces the cone into the tube creating the flare against the countersunk recess in the flaring bar      

                

 

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The newer design is the generating-type flaring tool. The flaring bar holds the tube end above the surface of the bar and the flare is formed in the air instead of against the countersunk form in the bar. The base of the flare is above the die so there is no danger of thinning the flared tube wall by scoring or applying too much torque.

A generating-type flaring tool creates the flare above the die, leaving uniform thickness throughout the flare

 

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