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kelltym88

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What is a good material to use for cardstock CR's? You know, something simalar to what Estes uses.
 

flipper

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Also, what is best place to pick up single sheets (preferable 8.5 x 11) - should i try a hobby lobby kind of place?

Wanna try building Fliskits Midnight Express
Ok - maybe I will need a package instead of single sheet since ill probably mess it up several times :0)
 

powderburner

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There are many good candidates, including "SIG" model aircraft plywood in thicknesses ranging from paper thin up to (and beyond) 1/4 inch. You can go to your local home improvement store and find high-quality plywood (it's labeled 'Scandinavian' or something like that at my nearby Home Despot) and a sheet of that will last for *many* centering rings.
You can get a sheet of fiberglass G10 stuff, often for sale on ROL auctions and even on ebay. This material is most frequently used for fin stock, but you could cut CRs out of it too.
My all-time favorite material for CRs is scrap picture framing cardboard. It is high-quality cardboard, often with a very smooth white surface on one side (great for drawing the patterns), sometimes with a really pretty color on the other side, possible to cut (with a little work) with an Xacto blade, and most often IT IS FREE if you ask nicely at your local picture-framing shop. Try Michael's MJ Design, Hobby Lobby, Frames-R-Us, etc. They are always throwing out small pieces that are perfectly usable for low-power-sized-BTs.
If you need something a bit stronger, you can reinforce picture-frame cardboard by soaking a little CA into the edges. I most often use the mega-cheapo 'six little tubes on a card' CA from the dollar store because I can use up one of those tubes and not worry about re-sealing it.
If you want to use cardstock or thinner cardboards, remember that the CR is there mainly as a gas seal to keep the ejection gas going forward. You can use a fairly thin piece of cardboard (like Estes) and it will work quite well in most cases. If you have a really wide ring, or a really big motor, and you feel the need to reinforce the motor mount, don't forget that you can still use the thin cardboard for a gas seal and use radial shear webs to take the thrust loads. These shear webs span the gap between the outside of the MMT tube and the inside of the main BT. It is best if they also completely span the lengthwise space between the forward CR and the aft CR, and are glued around their entire perimeter. You do this by installing the MMT with the fwd CR in place and the shear webs in place, and adding glue internally on all joints. You glue and add the aft CR last.
When I get home later I will look for a picture to post to illustrate what I mean by these webs.
 

mike_bar

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Originally posted by kelltym88
What is a good material to use for cardstock CR's? You know, something simalar to what Estes uses.
110 lb. cardstock is good for centering rings.

Buy a single sheet at a copy center like at Staples, or peruse their paper selections. Small packs of 100 sheets should be a few dollars cost.

Bristol Board from an art supply store is best. This will cost you a few more dollars for a pad of 50 to 100 sheets.

Also check out this article. It's very good.

Apogee's Model Rocketry E-Zine
Issue #62 (09/20/01)
Rocket Origami
https://www.apogeerockets.com/education/newsletter62.asp
 

jflis

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I've done a bit of photo framing in the past and often save my scraps from matte board for centering rings. Good stuff :)
 

rbeckey

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You can get a circle cutter at Michaels. The circle cutter can be used for thinner cardboard and foam core board. Shirt boxes make decent LPR centering rings, as well as the previously mentioned matte board. I have cut CRs from shirt boxes with a circle cutter and laminated them together for additional strength.
 

powderburner

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Do you have one specific make/model that you recommend?

The last time I went looking at circle cutters, they cost like $20 (and you can order a WHOLE lot of real nice, laser-cut CRs for that). They looked like rather flimsy plastic contraptions and I was not sure how accurate (round?) they would really be (it was explained to me that these cutters are really for 'scrap-bookers' to cut pretty circles out of paper sheets). They did not cut diameters below one inch, which leaves out BT-50, BT-20, and BT-5 sized CRs----kinda defeats the whole purpose?

I am a little surprised that one of our vendors has not come up with some sort of tool that would allow a few of the basic size holes to be cut or punched (in a moderately thick cardboard). I would be willing to pay maybe $10-15 for a tool that works.
 

Missileman

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Circle cutter for cardstock CRs.
The elcheapo way to go , and works quite well.
Put your hobby knife in a compass (sp?)
in place of the pencil. Don't try to cut all the way through in one pass. Let the knife do the work.
Circle cutters for plywood, for use with a drill press are about $6 to about $12 all day long on ebay.
 

wwattles

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I got a single-bladed, high quality fly cutter for my drill press from Craftsman (Sears brand) for around $20. 2-bladed, lower quality from Harbor Freight for about $14.

WW
 

BobH48

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Originally posted by flipper
Wanna try building Fliskits Midnight Express
Ok - maybe I will need a package instead of single sheet since ill probably mess it up several times :0)
For paper rockets you want to get 110# cardstock. You can pick up a package of 100 sheets at Walmart. I'm not sure but I think it was about $6 or $7.

If your printer won't handle 110# stock, you can use 65# coverstock. It will work, but it just won't be as sturdy.


Originally posted by kelltym88
What is a good material to use for cardstock CR's? You know, something simalar to what Estes uses.
I have used the same 110# cardstock and cut centering rings and then laminated about 3 thicknesses together. Glue them together and put them between wax paper and put a heavy book on them to keep them flat and let dry over night.

I use the knife blade in the compass for these also.
 

powderburner

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I found a picture of a recent build that shows the shear webs I was talking about in my earlier post.
In this instance I began to build a MMT with Estes-style paper CRs and thought it would be enough. After reconsidering, I decided to beef up the MMT a bit and add the webs. Since I already had the aft CR glued in place, that left me with the problem of how to make a good glue joint between the outer edge of the web and the inside of the BT.
Well, you can see the pencil marks that indicate where I cut holes. With the MMT dry-fitted in place, I dropped some glue through the holes (one side at a time) and tilted the rocket to get the glue to run back and forth and cover the length of the joint. Not the best way to do it, but it worked in the end.
The shear web material could have been heavy cardboard but in this case was balsa. Note the grain direction runs inboard-outboard.
 

mike_bar

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Originally posted by powderburner
I found a picture of a recent build that shows the shear webs I was talking about in my earlier post. (snip)
Powderburner,

I like your motor mount construction with paper centering rings (CR’s). I have a pet peeve about over-built rockets, so I like using paper. In addition, I prefer the “elegance” of lightweight construction techniques. I hope this does not come off as stuck-up.

The shear webs are a great structure; you may consider using gussets as well. Gussets are triangular shaped structures. In like fashion, gussets will handle the shear stress placed on the motor mount tube (MMT). Listed below is a good article discussing paper CR’s.

I have one question regarding the balsa shear webs. Should the wood grain be vertical, instead of horizontal, like in your attached photo (dscn7463cutuprmmt.jpg)? Would the horizontal grain possibly buckle under high stress, sort of compress and bend? Would vertical grain strengthen the vertical shear stress applied to the MMT during powered flight? It’s just a thought I had. Also, for gussets the grain would be along the leading edge like in balsa fin construction.

By the way, that’s a nice looking MMT, and I still love your signature!
Regards,
Mike (not any ol’ weenie, a proud C-D-E model rocket weenie)

Apogee's Model Rocketry E-Zine
Issue #63 (09/26/01)
Paper Centering Rings for HPR.
https://www.apogeerockets.com/education/newsletter63.asp

Quote from Apogee E-Zine:
… vertically mounted gussets. These are radially spaced around the ring, and glued directly to the ring to stiffen it up. But they do far more than stiffen the ring. They transfer nearly all the forces from the engine mount tube to the walls of the rocket.

The amount of force that they can take is incredible. I haven't done a stress analysis of the arrangement, but my guess is they add 100 times to the strength of the ring.
 

Stymye

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avoid the scrapbooking "circle cutters" , instead look for a "compass cutter"

heres a good cheap compass cutter for centering rings, I've used it on .058 mattboard..you can find them in most craft stores
it will cut a wide range of sizes down to 1/2 inch dia

https://www.artstuff.net/olfa_compass_cutter.htm
 

wwattles

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Originally posted by stymye
heres a good cheap compass cutter for centering rings, I've used it on .058 mattboard..you can find them in most craft stores
it will cut a wide range of sizes

https://www.artstuff.net/olfa_compass_cutter.htm
I've got a knockoff of one of those, and it does work well on paper, cardstock, and matting. It doesn't, however, work well on foam core boards. By the time the blade gets far enough into the board to cut the back side of it, the entire thing is angled considerably inwards.

Just a simple limitation on an otherwise useful design.

WW
 

powderburner

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stymye, that little gem looks like exactly what I need, I'll have to go shopping this afternoon

Mike BAR, thanks for the nice comments. As far as grain orientation, I suppose the absolute strongest would be to orient it diagonally/outboard, since balsa is strongest in the direction of the grain. I orient mine inbd/outbd because I feel it resists handling and crushing loads on the local BT a bit better. Truth be known, it probably doesn't matter very much when you recognize that low-power motors just don't produce internal structural loads that are very high.

A better and simpler material to use would be notecard stock or cardstock (and we come full circle back to the thread?). As long as I am putting in a piece, I usually go ahead and do a full web and not just a gusset. Again, it probably doesn't matter much, I just like the idea of using an 'elegant' structure (versus the Mother-of-all-Centering-Rings).

My signature/comment is a reference to a recent thread where someone was sort of looking down his nose at low-power stuff. We don't need those threads around here. I basically don't fly high-power, but I sure don't want to denounce anyone else's rights to pursue it. It's probably time to move on and change the signature to something else.
 
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