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  #1261  
Old 05-08-2019, 04:14 AM
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Gojeep Gojeep is offline
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What a great idea Jack. Thank you for sharing that with us all.
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  #1262  
Old 05-08-2019, 12:48 PM
Marc Bourget Marc Bourget is offline
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Jack,


What about altering the clamp on the cable between bending intervals to "distribute" the pressure points from the mandrel "squares" to a slightly different contact point. This might shift the pressure points and lessen the "high spots" making the overall result smoother ???


FWIW
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  #1263  
Old 05-08-2019, 01:40 PM
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Jack 1957 Jack 1957 is offline
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Quote:
Originally Posted by Marc Bourget View Post
Jack,


What about altering the clamp on the cable between bending intervals to "distribute" the pressure points from the mandrel "squares" to a slightly different contact point. This might shift the pressure points and lessen the "high spots" making the overall result smoother ???


FWIW


Marc, The spacing between the squares seem to be pretty evenly spaced out as it is. I think that two improvements might help lessen the high spots; First is to use more squares with less spacing between them and second, adding a spring between the outter cable housing and the vice grip giving continuous pressure. Of the two, I think reducing the spacing would help the most
When I was making this thing, I was concerned about it getting jammed after the bend and not being able to get it out. I was thinking that fewer squares would give me a better chance at removing them once the pressure on the cable was released.. When I finished each of these bends, I had to tap the tubing with a wooden mallet to release a couple of the squares. It didn't take much to knock them loose but they seemed to be right at the verge of being jammed. Both bends acted the same way.

If the distance between each square was reduced, each dip between them would be a little less deep so that might make it easier, not tougher to release. That would take some time to experiment. I am getting consistent results as is and I can live with a little wobble in the inside and outside surfaces for this application.

If someone out there has some time to experiment with this, let us know what you come up with. Test 1/4" spacing vs 3/8" spacing. I don't think you want to go any less than 1/4" though. As the tube starts to bend, the spaces between the squares change from parallel to tapered like a pie slice. The edges near the inner wall of the bend will start moving closer together while the edges on the outter wall will spread apart. This is what you want. If there's not room to move closer together it could cause jamming but if you want to try 1/8" spacing do it last. If the rig gets hopelessly jammed you've already got your data. I used flat rubber washers but maybe the typical tapered faucet washers would improve results.

I would be interested in seeing what improvements could be made with this method, so if anyone wants to tinker with it, please post your results.
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Last edited by Jack 1957; 05-08-2019 at 07:36 PM.
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  #1264  
Old 05-08-2019, 05:32 PM
Charlie Myres Charlie Myres is offline
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Brilliant idea Jack; thank you for posting it!

Cheers Charlie
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  #1265  
Old 05-08-2019, 08:04 PM
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Jack 1957 Jack 1957 is offline
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RE: Marc Bourget's PM
Marc, empty out your PM inbox. It's full and you can not receive new messages. Here's a copy of your question and my response:


Quote:
Originally Posted by Marc Bourget
Jack,


By your answer, my comment was poorly expressed.


I was thinking of shifting location of the bend mandrel so the squares would relocate to a position between their former placement. You're stretching the outer radius, which causes the dips. If you bent, for example, 5 degrees at a time and moved the squares back and forth, it would "even out" the stretches, making the wall more consistent in thickness. It also might allow you to reduce the .035 clearance.


That being said, the bending mandrels for tubes I recall have a series of "balls" (not exactly) with spacers with convex ends. Wonder that, with a lot of effort, that can be repeated for square tubes. I'm thinking this would be an interesting project for my CNC Knee Mill!


As always, I view your work in admiration. You seem to have the same curiosity and intellect possessed by Kent White. I feel comfortable saying such as I know him personally.


Keep it up!

Marc, I understand what you're suggesting now. True, by moving the rig in by half the width of the gaps it would make the walls much smoother and possibly allow for a reduction of the clearance from .035" to maybe .015 but once the bend starts getting to roughly 35 or 45 degrees, if I released the tension on the cable, I think the rubber washers would push the squares back to parallel rather than tapered, right?

It seems to me that more squares per inch and smaller spacers would get the results you mentioned and do it in one shot without disturbing the locations of the squares.
Truth is, when I first tried this years ago, I was only looking for adequate results and got much better results than I expected so I never bothered tweeking it to see if I could improve it. Maybe someone out there will look into it and post their results.
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  #1266  
Old 05-09-2019, 11:36 AM
billfunk29 billfunk29 is offline
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Default Mandrel bending

When I have done this, I use 1/4"x12ga steel tube cut into spacers, maybe 1/4 long. I make my insert plates only .01" undersize. I use 1/8" steel cable. With a helper I use the same type bending press and have the friend pull the cable assembly out with a porta power while I am making the bend. Lots of grease inside. I was surprised you could get the mandrel out with a bike cable. But I usually do steel.



As posted elsewhere, if I am in a hurry, I cut the tube longwise, work the channels, then weld back together. No helper required.
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  #1267  
Old 05-09-2019, 11:13 PM
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Quote:
Originally Posted by billfunk29 View Post
I was surprised you could get the mandrel out with a bike cable. But I usually do steel.

Probably because of the extra clearance and thinner contact area of the 1/8" squares. They're thin and able to tip over when the pressure on the cable is released rather than have to be dragged out standing up. After I release the vice grip from the cable, about half of them slid out and the rest could be freed up by rattling them loose with a wooden mallet and light tugging on the cable.
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Last edited by Jack 1957; 05-09-2019 at 11:20 PM.
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  #1268  
Old 05-09-2019, 11:18 PM
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Progress. I have the first bow trimmed and installed temporarily.

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  #1269  
Old 05-12-2019, 08:43 PM
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I had originally intended to use three bows on the roof but after test fitting the second one and plotting out where the third would be, it didn't leave enough room for a rear window that would be proportionate to the car, or even useful.
I set the second bow back about 2 more inches and it looked right.
In the original renderings we left the roofline mostly rounded with the thought that I'd figure it out on the fly. I would prefer the rounded look, but it forces the rear window to be short and very low compared to the total height of the roof. It just looks wrong.
The last thing I ever want to do is incorporate something into the original build that appears to be a bandaid, a patch job to cover for a poor design.
I'm going to go ahead with only 2 bows and frame in the rear window. I'll lay some template cardboard over everything before welding and see how it looks.

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  #1270  
Old 05-12-2019, 10:15 PM
Don Papenburg Don Papenburg is offline
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Looking forward to see this car finished! WOW . Are you going to give the roof a thick padding ? Less than a Carson top but more than a factory top?
What does the fiber optic tube light up in your tail lights ?
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