This stretching thing is turning out to be harder than I thought it would be.....
1st mistake is, I didn't leave any overhang on the table ends to tape underneath it. But that is a somewhat easy problem to overcome....
2nd problem I am having is, once I do have it taped down securely, it seems to be stretching the mylar more on the ends than in the middle.
I mark lines 10" apart in the center of the mylar. I move the end piece of my jig about 1" and the distance between the 10" grows to 10 3/8"..... I release the tension and the center goes back to 10" apart but the mylar is now much, much looser and doesn't fit the table back at all.
This tells me I've stretched it beyond its elastic limits, SOMEWHERE on the surface of the mylar.
I had the same problem when I tried just using my wire stretcher as a stretching jig but figured the jig was the problem and not flat enough..... so I built the wooden jig you see in the picture above.
For reference, my total table length ("closed") is 54 5/8". I am trying to get a 1.6% stretch so I need to stretch the mylar 7/8".
Am I missing something?
1st mistake is, I didn't leave any overhang on the table ends to tape underneath it. But that is a somewhat easy problem to overcome....
2nd problem I am having is, once I do have it taped down securely, it seems to be stretching the mylar more on the ends than in the middle.
I mark lines 10" apart in the center of the mylar. I move the end piece of my jig about 1" and the distance between the 10" grows to 10 3/8"..... I release the tension and the center goes back to 10" apart but the mylar is now much, much looser and doesn't fit the table back at all.
This tells me I've stretched it beyond its elastic limits, SOMEWHERE on the surface of the mylar.
I had the same problem when I tried just using my wire stretcher as a stretching jig but figured the jig was the problem and not flat enough..... so I built the wooden jig you see in the picture above.
For reference, my total table length ("closed") is 54 5/8". I am trying to get a 1.6% stretch so I need to stretch the mylar 7/8".
Am I missing something?
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I see that your stretching jig is configured to stretch the film in the vertical/length direction (only). That being the case I assume you are going to use horizontal support spacers? In any case, I think you will need to also apply at least some tension in the width direction.
When I built my segmented welding rod panels I used horizontal spacers and a mechanical stretching jig much like yours that stretched the film in the vertical direction only. I had a lot of difficulty getting even tension on the film and I ended up having to also apply horizontal tension using the tape pull method.
Given that your stators have two vertical center supports (rather like my own new wire panels) it might have been better in hindsight to to use vertical spacers and tension the film equally in both directions with a bike tube jig, which gives very uniform tension.
I think you will be OK though, using the tape pull method, as recommended by Golfnut.
It's good to see you're so close to completing your project !
Crap, you know, I didn't even think about that.....
. My support spacers are vertical just like yours. Well, I guess I will be making a bike tube jig now
.
Crap, you know, I didn't even think about that.....
Well, I guess I will be making a bike tube jig now
If you do decide to build a bike tube jig, it will need to be two inches longer and wider than your panel and all edges should be rounded over and smoothed so as not to snag and tear the fragile Mylar film. You might want to dust a bit of baby powder on the jig and rub it in along the edges before wrapping the film over it.
Also, my jig was 14 x 50 inches and I used a 700 x 35 cm bike tube with a Shraeder type spring loaded valve stem purchased from Walmart... you don't want to use a Presta type valve or it will leak down before you can turn it closed.
14 x 50 inches was quite a stretch for a 700cm tube... don't know you panel dimensions but if you have to use a significantly larger jig, you might not be able to stretch a tube that far.
700cm is I think the largest available diameter-- standard road bike wheel diameter. It should be possible to splice two tubes together using tube repair patch glue.
I think the tape pull method would be easier. You could set the tension with one of those spring scales that fishermen use to weigh fish. Just hook a 2 inch wide piece of metal on the end of it to lap the tape onto and apply uniform scale tension all around-- starting with the four corners, as Golfnut suggested.
Would you mind posting a couple of pics of your bicycle tube stretching jig?
The largest tubes I can find are for 29" rims.... so I have 146" of jig circumference with 91" tube circumference to stretch over.... not sure I like my chances there....
But according to this guy, you could possibly go up to 2X.... so maybe there is hope....
http://www.diyaudio.com/forums/plan...m-adhesive-material-choices-2.html#post636808
The largest tubes I can find are for 29" rims.... so I have 146" of jig circumference with 91" tube circumference to stretch over.... not sure I like my chances there
....But according to this guy, you could possibly go up to 2X.... so maybe there is hope....
http://www.diyaudio.com/forums/plan...m-adhesive-material-choices-2.html#post636808
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CharlieM stretcher pics...
http://www.diyaudio.com/forums/planars-exotics/164489-help-needed-esl-2.html#post2143672
Although the tape and fish scale method sounds a bit fiddly, I found it works rather well for large diaphragms.
Thanks for that... I figured they had to be on the site somewhere here. I tried in vein searching around for it .
I can see another mistake I made... if the lines are spaced 10" apart on the center then they will grow to 10.16" when stretched to 1.6%. I was expecting it to be 10 7/8" but realize now that doesn't make sense.
I'm planning to pick up a couple of 29" x 2.10-2.25" inner tubes and see if stretching them around my existing table is even possible. If so, I will modify it to be a tube stretching jig....
Guess this is something else to consider when doing vertical spacers
. I can see another mistake I made... if the lines are spaced 10" apart on the center then they will grow to 10.16" when stretched to 1.6%. I was expecting it to be 10 7/8" but realize now that doesn't make sense.
I'm planning to pick up a couple of 29" x 2.10-2.25" inner tubes and see if stretching them around my existing table is even possible. If so, I will modify it to be a tube stretching jig....
Guess this is something else to consider when doing vertical spacers
Would this do for scales: Portable Digital Balance Electronic Fish Weigh Hanging Hook Scale 25kg/5g OO55 | eBay
Since the mylar will be rectangular does that mean the tension needs to be different from width to length eg to get even stretch per square inch of mylar with a panel dimensions 10" x 30" the tension on the long axis will need to be 3x that across.
Heres good advice from:The Time Has Come (the Walrus said) Quad ESL-63 Repair (Complete)
Since the mylar will be rectangular does that mean the tension needs to be different from width to length eg to get even stretch per square inch of mylar with a panel dimensions 10" x 30" the tension on the long axis will need to be 3x that across.
Heres good advice from:The Time Has Come (the Walrus said) Quad ESL-63 Repair (Complete)
An externally hosted image should be here but it was not working when we last tested it.
That looks like a PITA
They make 32" if this is advertized correctly, and if it is, that's what I'll be using.
Link: https://www.walmart.com/ip/21635143...75035&wl11=online&wl12=21635143&wl13=&veh=sem
Oh, and glad to see you're back from the dead..
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They make 32" if this is advertized correctly, and if it is, that's what I'll be using.
Link: https://www.walmart.com/ip/21635143...75035&wl11=online&wl12=21635143&wl13=&veh=sem
Oh, and glad to see you're back from the dead..
Bike tube size is based on max O.D. of the tire inflated on a wheel. For example; a 29" tube and a 32" tube have the same I.D. and fit the same rim diameter but a 32" tube will fill a fatter tire volume.
That was from a quick net search, but I'm going to go to wally world tomorrow and take both their 29" and 32" tubes out of the package and see if the same.
Here's how it's listed at Amazon "Fits 32" wheel" https://www.amazon.com/Kent-Bicycle-Inner-Wheel-91044/dp/B00FQT3NPE
That brings up another question though, if it's as you say, that it will inflate a fatter tire, I wonder if that type of tube could be stretched further just from there being more material there...
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That was from a quick net search, but I'm going to go to wally world tomorrow and take both their 29" and 32" tubes out of the package and see if the same.
Here's how it's listed at Amazon "Fits 32" wheel" https://www.amazon.com/Kent-Bicycle-Inner-Wheel-91044/dp/B00FQT3NPE
That brings up another question though, if it's as you say, that it will inflate a fatter tire, I wonder if that type of tube could be stretched further just from there being more material there...
Well, I suppose that much of a fatter tube's area would reside closer to the larger O.D. so maybe it will stretch further.
......
That brings up another question though, if it's as you say, that it will inflate a fatter tire, I wonder if that type of tube could be stretched further just from there being more material there...
Good find on the fat 32" tube as it should both stretch further around a frame for fitting large panel dimensions and expand more evenly . It might stretch really nicely if pre-warmed in very hot water - even warming is required so a long soak to equalise temps is a good idea.
Its really important the tube is high quality with even wall thickness for even expansion to allow uniform tension on the mylar. Id suggest inflating it on a smooth floor to check for even dilation. Ive seen bicycle inner tubes that bulged in places when pumped up searching for slow punctures. If there is a thin wall it expands more, gets even thinner, has more expansive force, bulges more etc in a positive feedback style runaway style.
I still wonder if the tape and scale method is much better able to produce precise tensioning.
Standard bike tube glue will have no problem holding two spliced tubes together. Don't let that stop you.
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