# Centering Rings

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##### Roger Smith
Everyone who laughed at my post.......SEE, it's not just me!
What happened in my case, is that one of the cutters came loose and flew off. It bounced off a board I, fortunately, happened to be holding. It left a rather significant gouge in the board.

I have used the cutter since then, but am extra careful when I do.

#### dr wogz

##### Fly caster
A note on what Babar posted:

if the engineering is done right, TTW fin tabs can be used as supports for the MMT, so that CRs 'technically' aren't needed. There was some discussion a few months (years?) ago about the need for CRs (and a MMT tube even!) in our tubes. Rails or spacers to ensure the motor is centered in the tube is what we are really after. But, a cap or some sort is needed to prevent the ejection charge blowing out the back as opposed to what it is intended to do. Motor retention was an other issue, but somewhat easily fixed with a bracket or ..

So, with properly designed fin tabs, you only really need 1 CR in the MMT..

#### mtnmanak

##### TRA & NAR L2
TRF Supporter
A note on what Babar posted:

if the engineering is done right, TTW fin tabs can be used as supports for the MMT, so that CRs 'technically' aren't needed. There was some discussion a few months (years?) ago about the need for CRs (and a MMT tube even!) in our tubes. Rails or spacers to ensure the motor is centered in the tube is what we are really after. But, a cap or some sort is needed to prevent the ejection charge blowing out the back as opposed to what it is intended to do. Motor retention was an other issue, but somewhat easily fixed with a bracket or ..

So, with properly designed fin tabs, you only really need 1 CR in the MMT..
There quite a few other reasons you need to cut circles out for rockets besides stabilizing the MMT, though. Ebays and nosecones, for example. Nosecones in particular are difficult to get right from standard size bulkheads/rings, I always seem to need to cut something custom to get the nose cone assemblies right.

Point is that having a good method to cut precise circles out of normal rocket materials is fairly essential, especially if you are scratch building a bird.

#### BABAR

##### Builds Rockets for NASA
TRF Supporter
But, a cap or some sort is needed to prevent the ejection charge blowing out the back as opposed to what it is intended to do.
Absolutely. But a cap backed by three or four spokes, could be made of much thinner, lighter easier to cut material (I suspect would be easily 3D printed, basically a plastic washer.). It isn’t a load bearing structure anymore.

#### mooffle

##### Well-Known Member
TRF Supporter
I feel outclassed here...
I just find some decent cardboard in my recycling bin and take a small pair of scissors to it. Sand inside and out with a round file then 300 grit out until the circles are concentric. LPR only of course.

Non corrugated lego boxes are good, they're higher strength than cereal boxes and consistent quality. Otherwise my wife regularly buys off brand cheez-its that have good cardboard too.
I like the mini scissors on swiss army knives. The small blades allow for pretty good control.

##### Well-Known Member
It's not a place I normally look to for rocketry supplies but these might work for LPR.

They laser cut them and can do custom sizes.

100 1" with 1/2" holes for $15 ain't that bad.$0.15 per 1/8" thick laser cut ring.

#### BABAR

##### Builds Rockets for NASA
TRF Supporter
I feel outclassed here...
I just find some decent cardboard in my recycling bin and take a small pair of scissors to it. Sand inside and out with a round file then 300 grit out until the circles are concentric. LPR only of course.

Non corrugated lego boxes are good, they're higher strength than cereal boxes and consistent quality. Otherwise my wife regularly buys off brand cheez-its that have good cardboard too.
I like the mini scissors on swiss army knives. The small blades allow for pretty good control.
Not at all. For low power I have used foam board, I use a hole drill bit for cutting the inside hole (go about 2 mm smaller than you want, put sandpaper on a dowel and sand it up until it fits snug. Once you have the inside hole, use a template to draw the outside diameter..
Rough cut it out with scissors or a knife. Here’s the fun part, use a metal straight to make multiple straight tangential cuts along the outside, it takes surprisingly few cuts to get it pretty close to round, sand to fit.

I like foam board because it is a little thicker and tends to “waggle” less.

I have also used 1/2 masking tape, take a roll and made circumferential cuts (careful of your fingers!) so I have a roll with three strips each 1/6 inch wide. For small mount to motor differences (BT-5 to BT-20, for example) I just wrap the 1/6” tape strip round and round until it just fits. Soak in some thin CA, and Bob’s your Uncle!

#### mooffle

##### Well-Known Member
TRF Supporter
Here’s the fun part, use a metal straight to make multiple straight tangential cuts along the outside, it takes surprisingly few cuts to get it pretty close to round, sand to fit.
This is basically the same method I use for all my balsa nose cones, eight cuts to an octagon and you're just shy of a circle

#### RocketTree

##### Well-Known Member
I have been cutting plywood centering rings with a drill press.

Cut the outer diameter of the rings from the wood sheet.
Place all the rings into a piece of tubing.
Cut inner circles through the stack.