Centering Rings

[jadebox]

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]

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]

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]

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]

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.

 

[heada]

It's not a place I normally look to for rocketry supplies but these might work for LPR.

https://www.woodysrocks.com/store/p205/1/8"_Chipboard_Discs_.html#/
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]

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]

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]

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.

 

[Hobie1dog]

has anyone used Poplar wood for centering rings? It's a fairly light and very strong, tight-grained wood.

 

[lakeroadster]

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.

What are you using for a cutter? Hole Saw, or a Fly Cutter, or ?

 

[RocketTree]

What are you using for a cutter? Hole Saw, or a Fly Cutter, or ?

Using hole saw for plywood rings.

 

[rharshberger]

My new method, 3D printed router patterns. Tack the ring stock two or 3 layers thick, thrim close with a bandsaw on OD, drill out as large as possible on ID, and finish up on router table with a flush trim bearing router bit, little or no sanding required to fit.



Edit: Took a bit to get the file uploaded, for some reason my phone and the forum weren't seeing eye to eye. I have a newer version of the pattern, that has knobs that attach where the screws are, the pattern pictured is for T300 with a 38mmt, I also have them for the same tubing in 29mmt and 4x24mmt, T80 in 29 and 38mm, Loc 3.90 in 54, 38, 29mmt's, Loc 3.1 in 38 and 29mmt, Loc 2.63 in 29mm and 38mmt. Now its just printing all the various patterns and enough knobs.

No more fly cutters unless needed and or sanding jigs like in the past.

 

[Hobie1dog]

and my question is ignored

 

[Zeta]

and my question is ignored

Hobie1dog, more likely it has not been tried yet.... i am guessing here but when I think about the advantages of thin birch plywood ..it's hard to beat.... however I am guilty of trying to craft the ultimate junk/found parts/free stuff rocket just to say I did it...for small rings you can probably get away with a lot of things ... for large diameter rings I would want biaxial material but that's just me

 

[Zeta]

Show me how you make them. What tools do you use? I have a laser engraver, but it is not powerful enough to cut.

David, the White Bear Makers Space has a CNC router and the software to design parts is free with a membership.... not sure if that helps or how far you live from it but it's capable of cutting rocket grade material ... unless you are planning a new 30 foot tall V2 ?

 

[lakeroadster]

has anyone used Poplar wood for centering rings? It's a fairly light and very strong, tight-grained wood.

and my question is ignored

ignored... perhaps because nobody has used it for model rocket centering rings?

I use Poplar for making wind vanes and other such projects. I've never seen it available in 1/32", 1/16", 1/8", etc. so I've never thought about using it for centering rings, since basswood and balsa are readily available in those thicknesses.

 

[heada]

Solid poplar(or any solid wood) has a weakness of short grain. On either side of the ring, if there is any flexing, the short grain wi fail easily. To get around this, you would need to layer the wood so that each layer is 90 degrees offset from the previous layer. This is how plywood gets its strength

 

[DaHabes]

I use a drill press and hole saws. I cut out aircraft plywood squares an inch bigger than the CRs (one per CR) then super glue them in a stack to a piece of cheap 3/16ths plywood. The bottom piece prevents splintering of the bottom CR when the bits cuts through. Then I clamp the stack to the drill press platform. If my eyebolt is a 1/4 and centered I use the 1/4 inch pilot bit, if not (3/16th eyebolt or off center) I don't use the pilot bit since the stack is clamped. I drill the inside hole first, outside hole second. As I drill the outside hole, I stop after each CR is cut and remove it from the bit since it will spin and create a lot of friction (and smoke) against the next lower CR if you don't. I use hole saws such that the OD is a bit big and the ID a bit small, then use a Dremel sanding drum to sand to the proper size.

 

[Kelly]

has anyone used Poplar wood for centering rings? It's a fairly light and very strong, tight-grained wood.

I don't like 'real' wood (as opposed to an engineered material, like plywood) for most rocket parts because it's not as stable. Wood will shrink or expand, and change shape, in response to changes in humidity and internal moisture content. You would probably be OK using it for a centering ring, but I think plywood is always going to be a better choice. Of course balsa wood has been used since the beginning of model rocketry due to its lightness, but if you're going to go with something as (relatively) heavy as poplar I'd just use ply.

 

[Hobie1dog]

thanks, plywood from now on for me.

 

[bjphoenix]

When I've made centering rings I used cardboard and a drafting circle template, then cut them out with normal hobby knife, sand to fit. This is for LPR. I have cut a few out of aircraft plywood in which case I did it the hard way- drilled, sawed, filed, sanded, etc. until they fit.

For MPR I extend fins to the motor tube so the transfer of thrust from motor tube to airframe is primarily through the fins and not the centering rings. The centering rings are there to be bulkheads to hold the ejection charge pressure. Actually my centering rings are still substantial and still glued in pretty well so it is a matter of belt and suspenders. And for MPR I put small T-nuts in the rear bulkhead to use with my homemade engine retainer.

 

[Hobie1dog]

1/8" aircraft plywood- 6 ply
https://www.ebay.com/itm/2824823934.../fIXd7x4xu64OVZnU469nhH1/9Kx|tkp:BFBM3uGP0Lpf

 

[jbr]

cnc them from baltic birch

 

[beeblebrox]

Waste Not... Want Not...

Yea, what he said... Cut centering rings from the fins and still use the fins! Although the least waste of material is to 3D print them if you can. 3D printed rings can also be made with alignment notches for TTW fin tabs, and perfectly aligned holes for clusters. The laser engraver is good also for marking. drill center hole to fit motor tube last. rough cut the outer edge of the ring with a sabre saw, put 1/4" hole in center attach a 14/20 left hand threaded bolt and nut thru it (keeps nut from loosening). Turn it on a drill press and use a belt sander to grind it down to the mark made with the laser engraver, testing diameter as you get close. you can stack several and sand all at once. when done tape them all together before removing screw. clamp them to drill press table then drill out the center with a hole saw. Note: the problems with that is the limited diameters available, and fly cutters are dangerous. These can still end up not perfectly centered.

Need rings, don't have 3d printer? Message me I can make a few things but am not set up to do a full blown production. I can make you onsey twoseys... Nominal fee for materials and shipping. Most likely less than $10 total for up to 4" Diameter. Your location will have a greater effect on fees due to shipping further costs more. (I am in Penna.)

 

[beeblebrox]

cnc them from baltic birch
View attachment 495713

I wish I had a CNC machine... There is a laser cutter available to add to my 3D printer however... Maybe next year.

 

[jqavins]

Show me how you make them. What tools do you use? I have a laser engraver, but it is not powerful enough to cut.

What's the power on your machine? I'm the farthest thing from an expert, but I have read of cutting thin wood with as little as a 40 W laser if one can take it slow, though 50 or 60 W seems to make it a lot easier. Again, all second hand information from unverified sources.

Also, a person or two has mentioned printing templates, and someone mentioned papering stock before cutting to avoid splintering. You can print templates and use that paper to avoid splintering, giving you the pattern to cut and sand to nicely secured to the stock.

Or, if you're not concerned about splintering, you can use your engraver to draw the template directly on the wood. Draw the OD, ID, and center.

For the love of all that you or anyone you've EVER met holds holy.........crank way way down on that flying head.

I thought I had, almost to the point of breaking it, when mine came loose, flew across the garage at warp 9, and smacked me in the mansack. Oddly enough, I remember it in ultra 4K detail in super slow motion flying through the air at me........but my son remembers it as hearing a loud crack and me dropping like a string cut marionette in near instant fashion.

It was worse than kidney stone pain. Way worse. Took a while to heal and walk 'normal' again, too.

Thankfully I was across the garage retrieving another piece of material at the time, otherwise, close up, it might have caused some serious damage.

For the love of all that was, is, or ever will be holy, never use a fly cutter at high speed. Place it a drill press and set the speed as low as it will go. Feed slowly so you're making repeated shallow cuts. Cuts. You're cutting, not drilling.

Okay, they say ignorance is bliss, and I am L-0, but I have wondered for a while about this.

I use foam board for centering rings for low power and some mid power birds. I understand it may not be strong enough for high power. But it seems like it would be easier to cut longitudinal rectangular “spokes” out of wood that would easily center the motor mount... There is probably a reason it won’t work or somebody would have already done it.

The only reason I can think of is that rings are easier and this sounds like more trouble than it's worth.

 

[Donnager]

Usually a combination of Credit Card, Hole Saw, Jigsaw, and sander. I do use the drill press to spin parts to make the exterior round.

I haven't bought a fly cutter. I may already have enough scars.

 

[Hobie1dog]

For the love of all that you or anyone you've EVER met holds holy.........crank way way down on that flying head.

I thought I had, almost to the point of breaking it, when mine came loose, flew across the garage at warp 9, and smacked me in the mansack. Oddly enough, I remember it in ultra 4K detail in super slow motion flying through the air at me........but my son remembers it as hearing a loud crack and me dropping like a string cut marionette in near instant fashion.

It was worse than kidney stone pain. Way worse. Took a while to heal and walk 'normal' again, too.

Thankfully I was across the garage retrieving another piece of material at the time, otherwise, close up, it might have caused some serious damage.

I'm still having nightmares reading about this.

 

[lakeroadster]

I'm still having nightmares reading about this.

Opinions Vary. My 1st paying job was working at a sawmill. You learn good practices and safety real quick when there's a 4 foot diameter blade, exposed flat belt pulleys and logging hooks.