Instructions for Teflon tube stretching

By Brittany Hernandez,2014-06-19 06:40
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Instructions for Teflon tube stretching

    Instructions for Teflon tube stretching/sizing and installation

General for sizing:

     Allow .003-6” push-rod clearance for music wire. Music wire benefits from minimal clearance, due to its flexibility it can use the extra support. Carbon rods should have .006”+ clearance as they are

    stiffer, requiring less support, and rougher surfaces and high spots will cause friction with tighter fits.

General for stretching:

     #16-22 can be gripped with the fingers, but clamping ends between popsicle sticks is best, with vice grips. As soon as the wood starts to crush, that’s plenty. Or grab the ends with good, serrated pliers, or Ca them between popsicle sticks or such. Only a few pounds force is exerted in stretching #16-24 tube, so if by yourself you can tape one end down to something stationary. #10-12 require more secure hold-down to stretch, especially beyond 2x (see, “getting full stretch”).

     Room temperature should be 68deg F minimum, but full stretch cannot always be achieved below 80deg F. If you typically keep your house cool, the easiest room to warm is of course the kitchen. Just turn the stove burners on for a few minutes. For safety, do not shut doors or leave the room, and shut off burners (for even heating also) before going to work on stretching the tubing.

     Pull at a slow, steady rate of about 1-4”/sec, though do not stop pulling until the desired length is

    reached. Otherwise you may find the tube unwilling or more difficult to stretch further. Hold for 30 seconds at the end of the stretch, then release tension. Rebound will be about 5%, so if you stretch a 36” piece 3x (110”), you will wind up about 6” short, or about 104”. Tubes usually break at the ends if overstretched. The sudden snap back will cause some waviness along the tube, but a light stretch will adequately correct this or any other kind of kink.

     Getting full stretch with #12 and larger diameters takes quite a bit of tension, and ends will have to be firmly secured. Clamping and gluing between popsicle sticks is best, and the only reliable way to

    exceed 2x. Use 1” pieces, clamp tube between with vice grips, then Ca.

     Resizing can be easily accomplished if ID was stretched too small, tube is out of round, or the tube as-is is just a little snug for your rod. Create a ball-draw using a rod or wire that will go through. Build

    up one end of it with a tiny bit of Ca, enough to form a tiny droplet, let it cure, a little kicker helps here (sand smooth if necessary). Wire requires some sanding first. When cured just pull it through, inch at a time. Alternatives include attaching a tapered bit of metal tube, .005-.010 over desired ID. Anything from telescoping tube, to syringe needles. Remove any burrs after cut-off of tubing. Also knotted string/thread stiffened with Ca. Ball bearings from a machine supply can be pushed through also.

General for mounting:

     Using a .002+ larger wire/rod dia for installation will create a straighter wall, lowering friction. This

    consideration is more important with cf rod for where there are significant bends. With cf rod, you

    can easily add a few thousandths to any section of your installation rod by coating with Ca (sand

    smooth if necessary). If the actual cf push-rod winds up fitting snug, it can be easily sanded in that area. Lightly tape Teflon in place where necessary to make contact with the pod and boom. With a fine micro tip or pin applicator, run a small amount of Ca along the entire length, allowing gravity to help. For neatness, you can stop short of the taped areas, and finish these last after removing the tape. Secure the stress areas at the ends, or any significant bends, with a glob of thick Ca, or little patches of glass, etc. Remove installation wire, and you now have an essentially slop and friction free push rod guide!

     INTERNAL guide applications have been nicely accomplished with steel installation wire and little rare earth magnets to pull this assembly against inside of boom then gravity feeding Ca. Tape tube ends up to prevent Ca from entering.

     You do not need to have one continuous length of tubing. You can string short lengths together if you miscalculate final lengths, just be careful not to let the Ca get into the junctions. Some tape applied separately before each junction end will tend to stop the flow if you get to close.


     Tubing is best kept in a relatively airtight enclosure to protect from moisture which can degrade bonding strength. But open storage for 1 year in the fairly dry environment of my house had no noticeable affect.

    #8 tube: Nominal id is .133. Stretch 2x for .07 cf rod; 3.25x for .06 rod. The .06 rod I have measures .062-.067. The .07 rod is also irregular, measuring from .070-.077. I don’t have any samples

    of .08 rod.

    #9 tube: Nominal id is .118. Stretch 2x for .07 rod; 2.5x is the max stretch with this tube, will be very adequate with .06 rod.

    #10 tube: Nominal id is .106. To get the minimum .064id, stretch 3.25x; stretch 2x (.075id) for .06 cf rod; stretching 1.75x will leave adequate clearance for .07 rod.

    #12 tube: Nominal id .086. Use as-is for .07 cf rod. stretch 2.1x for .05 rod; stretch 1.75x for .06 rod.

    #14 Nominal id .066. Stretch 3.25x for .04 cf rod (.046id); 1.5x or use as-is for .05 cf rod (.058id).

#16 tube: Nominal id .054. Stretch 3x for .034 min id (for .03” carbon rod, actually .026”); stretch

    1.66x for a final id of about .044”, for .04 (actually .036) carbon rods

    #17 tube: Nominal id .047. Use as-is for .04 cf rod; stretch 3.25x for .028+id; 2.25x for .034id.

#18 Nominal id .044. 3.5-4x for .023id; 3x for .025id; 2x for .028id. See Resizing if .04 rod fits snug.

    #20 tube: Nominal id .034, use as is for .031 wire and .03 cf rod. Stretch 3-3.25x for minimum .022id. See Resizing if .03 rod fits snug. First check to see if .03 rod is to spec, actual OD should be .026”.

    #22 tube: Nominal id .028. Stretching 3-3.25x for minimum .018id; stretching 2x gives .023id, fine for .018-.020 wire.

#23 tube: Nominal id .026. Stretching 2.4x brings you to .018id.

#24 tube: Nominal id .022. Use as is for .016 wire, see Resizing for.018 wire. Stretch 1.5x for .018id,

    for .014 and under wire. Stretching 2x gives .014id, how much smaller could you want to go?

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