Warped Centering Ring Advice . . .

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markfsanderson

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Greetings!
I'm slowly but surely putting together my LOC/Precision 7.5" Doorknob. I have found that the centering ring forward fin attach point has warped. Flattening one side against the table, I've about 4 or 5mm warpage on the opposite side. Is there a known good method for flattening large centering rings? My thought process only is taking me to 'put heavy weight on it for a couple of days' and then immediately glue it to the motor can before it can 'find' its memory. Perhaps I could glue one 120 degrees or so, let it dry then glue the rest? Soaking in warm water? Any ideas would be greatly appreciated - thanks!

Mark!
 

ksaves2

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You can dink around with it all you want but is it worth your time?
Might be a better idea to buy a centering ring that isn't warped.
You could try writing LOC, provide postage and they might send you a new one? I wouldn't do that if the kit was sitting around for years as the warp might have occurred after you acquired the kit.
In that case, just buy a new centering ring and perhaps another kit or two from LOC.

My deceased prefect had one heck of a shop and could turn out a replacement in minutes. I'd just show up with the material and it would be done.

I and I suspect you aren't in that position though with such a shop.
(Would be nice though. :))

Oh BTW, beautiful standard poodle in the avatar. We had a miniature one when I was a kid and he was a wonderful, feisty son of a gun. My parents didn't have him neutered and he had one heck of a bark (and smelly pee!). Was a good watch dog. If some stranger showed up, Pepe would bark like a banshee and indeed his bark was bigger than his bite. Very deep tone and sounded bigger than he was. Mom would keep the curtains closed in the house so if some potential burglar showed up, he or she would have to decide whether or not they want to deal with the dog on the other side of the door who was barking like he was ready to kill. (Even though he was a pipsqueak in size! Hence the reason for drawing the curtains. )

Kurt Savegnago
 
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markfsanderson

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Kurt,
Great Poodle Story! That one pictured is my Show Dog (AKC CH. @ 7 years!), Service Dog (hand and voice commands), Guard, Scoundrel, Ladies Man, Party Boy, Baby Sitter Extraordinaire and etc. If we didn't take him places, he'd hold a grudge and lock the door. He had a special bark (high pitched yip) to tell my wife our then baby Charles needed changing, would make passes at my wifes (only the good looking ones) girlfriends, discriminating and accurate guard dog, first figured out that my wife was pregnant with our son and became *very* protective - the list goes on and on. Passed away 4 years ago not quite reaching an age of 8. Freak of nature - I doubt we'll ever see one like him again. "... Louis, go check on Chares" he'd hop up go to Charles room and come back and lie down. Yes, he was named after the 'Sun King', and has his own Facebook page naming him as royalty . . . So far the FB folks haven't challenged me on that!

Well enough of that . . . Yeah I could probably buy a CR from LOC - but the shipping cost @ a minimum of 12.00 is annoying. Honestly, I don't believe there is really much that can be done to prevent this except glue the part quicker than I did! The warpage most likely comes from the fact that it is nearly 7.5" in diameter on this outside and about 6.5" diameter on the inside - just a small band of plywood about 1 or 1.5 inches wide (it's not in front of me to measure). I could probably buy 3 of these and if I wasn't fast enough all 3 would eventually warp!

My hope is that weighing it down with a 5 pound sledge under a flat 10"x10"x2" piece of wood will at least temporarily straighten it. I was hoping someone had experience with this issue . . . Which probably only occurs with plywood CR on large diameter rockets - but I'm not sure.

Mark!
 

manixFan

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When I had a large (also 7.5") warped CR I applied a layer of fiberglass and epoxy on each side and then covered it with the appropriate material and put it between pieces of laminate shelving. I then stacked about 85 lbs on it (two bags of cat litter) and let it get to the green stage, then trimmed it and let it finish curing under weight. It worked well and the change in weight was fairly minor since I only used one layer. I put a u-bolt though it so the increased strength was a plus.

Just something to consider.


Tony
 

markfsanderson

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When I had a large (also 7.5") warped CR I applied a layer of fiberglass and epoxy on each side and then covered it with the appropriate material and put it between pieces of laminate shelving. I then stacked about 85 lbs on it (two bags of cat litter) and let it get to the green stage, then trimmed it and let it finish curing under weight. It worked well and the change in weight was fairly minor since I only used one layer. I put a u-bolt though it so the increased strength was a plus.

Just something to consider.


Tony
Tony - great idea! This certainly will keep it from warping . . .

Mark!
 

dr wogz

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I do have to ask if the warp really causes any structural concern. It's a CR, and as long as the parts fit over and inside it, what is really the issue? And, unless teh fins are expected to be an exact fit between them (even then, slight shorten one or two fin tabs to fit.) The transferred forced from the MMMT tube thru the CR will transfer. And, with not 90° between the tubes & this CR, you have less chance of buckling (ever so slight with either 92° / 88°...) that a curved CR might actually be a bit of a benefit..

just my thinking..
 

markfsanderson

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I do have to ask if the warp really causes any structural concern. It's a CR, and as long as the parts fit over and inside it, what is really the issue? And, unless teh fins are expected to be an exact fit between them (even then, slight shorten one or two fin tabs to fit.) The transferred forced from the MMMT tube thru the CR will transfer. And, with not 90° between the tubes & this CR, you have less chance of buckling (ever so slight with either 92° / 88°...) that a curved CR might actually be a bit of a benefit..

just my thinking..
Paul,
Perhaps you are right . . . It does offend my perfectionist sensibilities having a CR that is not quite perfect - haha! I can situate the CR such that there is a 2mm deviation 180 degrees apart ... This shouldn't have any negative effect - or will it?
 

dr wogz

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well, that's what I'm thinking. a square or rectangle with 90° sides will "parallelogram" with loads applied. But a trapezoid offers more resistance to this 'parallelogramming' tendency.

And, me also being the perfectionist, but lazy & not wanting to wait & teh hassle involved getting a new one, would use it.. And only I would know..

Fins on the other hand.. or AV bay bulk plates. that's a different story!
 

mtnmanak

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You can flatten the CR (Tony's advice above is excellent) and should have no problem with this build.

If you plan to stick with HPR and get into scratch building, at some point, it becomes easier to cut your own parts. When I moved in that direction, the first tool I bought was an adjustable hole saw cutter:


It is kind of expensive, but the ability to walk into your garage and walk out 10 minutes later with a CR/bulkhead cut exactly to your specs was worth it to me. I found the saw to be great for plywood - cuts through 1/4" birch ply easily. Fiberglass is a lot tougher and eats up the blades quickly. I was even (somewhat) successful in cutting a stepped bulkhead with this tool, although that was more of an experiment to see if I could do it. It took to long to make it something I would want to do regularly.

So, when I decided to get serious, I next moved to a desktop CNC, which has become very useful for things beyond rocketry. I bought a Shapeoko 3 last year and still find myself cutting something with it almost weekly. Fiberglass is still a royal pain to cut with a CNC (I snapped a lot of bits in half before I dialed in the right "speeds and feeds"), but you add the capability to cut aluminum, which is very useful. I realize this is beyond the scope of your question, but don't be surprised when you end up walking down the path. The allure of new tools is sometimes overwhelming... :)
 

Sandy H.

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I haven't built a Doorknob before, but if the centering rings aren't required to be in contact with the fin tabs by design, I'd just let it ride. It may have warped due to storage or it may have just been a piece of wood that once the plies found their home turned out to not be flat.

If you 'force flatten' the ring, it will have stresses on the glue joints. Might not matter at all, but if the glue is what is holding it flat, then the glue is taking that stress. Similarly, if you use tricks with water or steam to flatten, once it dries out it will want to warp and stress the glue.

Fiberglassing it is a good thought as well, as the stresses would be carried in the fiberglass, not the airframe.

I'd either leave it as-is or do the fiberglass thing. Even having a laser cutter and CNC, I wouldn't remake the ring, unless it is integral to the fin attachment scheme.

Sandy.
 

AHansom

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Is it a newer Doorknob with the LOC-N-FIN technology?
 
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Flyfalcons

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Unless your fins interlock with it, it really doesn't matter. Spray it with water, place it on a flat surface with weights on it for a day or two, then build on. Or just skip that part and build on.
 

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