Gas station without pumps

2017 November 11

Bending PVC failed

Filed under: Robotics — gasstationwithoutpumps @ 11:01
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In Thursday’s post Preliminary design review in mechatronics, I mentioned the idea of bending PVC tubing to make a ball ramp for storing the ping-pong ball ammunition. Yesterday I went to the hardware store (San Lorenzo Lumber, who may be calling themselves “ProBuild” these days—I can’t keep track of their name changes, and everyone locally still calls them San Lorenzo Lumber) and bought a length of 1 ½” Schedule 40 PVC pipe and a small bag of “play sand”.  I just noticed today that they charged me a lot for the PVC pipe—I think that they had stuck an incorrect barcode label on the 2′ length of pipe and charged me $4.49 for it (the correct price should have been around $2, as 1 ½” PVC pipe usually runs around 60¢ a foot).  It might have been cheaper for me to buy a 10′ piece and cut it up myself—that would have given me about 5 tries for the only 50% more cost.

I took a ping-pong ball with me and confirmed that a ping-pong ball does fit through the pipe, though it is a narrow fit—dropping a ping-pong ball through the 2′ section takes about 3 seconds, because of the air drag.

I made a form to bend the pipe around, by cutting a redwood scrap of 2×12 into a rough circle and turning it down to 7″ diameter on my wood lathe.  That’s the first time I’ve ever done faceplate turning, and it seems to have worked ok, given the soft, fibrous redwood and the knot in the wood.

The bending form is just a 1-7/8″ thick chunk of redwood turned to a 7″ diameter circle.

I tried the technique of heating sand in the oven to 450°F and using the residual heat of the sand to heat the pipe to bending temperature (170°F–220°F, or 75°C–105°C).  It didn’t work—the pipe never got hotter than about 130°F, nowhere near soft enough to bend.  I see two possible reasons for the failure:

  • The sand was not hot enough. An oven is an inefficient way to heat things, and the sand in the center of the can may not have gotten anywhere near the temperature at the surface of the sand.  I could try stirring the sand occasionally while heating it to get a more uniform temperature and raising the oven temperature.
  • The sand transferred the heat too slowly to the pipe.  The outside surface of the pipe is radiating heat as the heat is transferred from the sand.  If the transfer of heat from the sand to the pipe is slow, then the radiation from the pipe will keep the temperature from rising too high.  I don’t see any way to increase the flow of heat from the sand, other than raising the temperature of the sand.

It may be that using the residual heat of the sand only works for smaller diameter pipes, which are floppier to begin with.

I tried again today, heating the sand in a skillet instead of the oven, so that I can heat it faster and hotter.  I heated the sand to 250°C (about 480°F), as measured with an infrared thermometer. This got the sand hot enough to make the pipe flexible and I could bend it around the form.  The inner side of the pipe buckled a bit at the tight turning radius (3.5″ on the inside).

I had not made anything to hold the pipe in place as it cooled, and I did not want to hold it by hand (actually I was using my feet to hold the pipe in place) for the time it would take to cool, so I poured out the sand too soon and the pipe collapsed.

The collapsed pipe will be of no use for making a trough for ping-pong balls.

If I make a form with holes to insert pegs around the outside of the pipe after bending, I can probably avoid the collapse, as I can leave the sand in the pipe as it cools. But I don’t think that this will eliminate the ripples on the inner part of the curve, as they formed while the pipe was still filled with sand.  A better support mechanism (like a metal hose might help reduce the kinking and ovaling, but at some point the amount of stretch needed on the outside will limit the bending radius.

I looked for information about minimum bending radius for PVC pipe, and the best source I found was a video from BendIt, where they managed to bend 1 ½” PVC to a radius of about 8″ using internal metal hose support, with the pipe getting a bit oval. That is still a lot wider curve than what I need for my design, so I don’t think that this track design is going to be one I can make.

I’m going to have to rethink my ammunition storage for the robot, which will likely affect the delivery of the balls to the targets also.

I’ve been putting in so much time on the mechatronics lately that I’ve not made any progress on my book, so I think I’ll have to put the mechatronics to one side for a day or two, and work on my book instead.  Maybe when I come back to the mechatronics I’ll have a more doable idea on how to deliver the balls.

If I can’t put in the time that the mechatronics course takes, my fallback plan is just to make the base platform and sensing and do the coding, without the ball-delivery mechanisms (perhaps lighting LEDs when it thinks things are lined up well enough to fire).  It would not meet the requirements for passing the course, but I’d still have something to build on as a hobbyist.

1 Comment »

  1. […] today, I posted Bending PVC failed, in which I gave up on bending 1 ½” PVC pipe into a tight circle.  Even if I had been able […]

    Pingback by Cat-toy ball track | Gas station without pumps — 2017 November 11 @ 19:39 | Reply

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