Getting motor mount centering rings perfectly straight

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kavel_r

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What are your favorite methods for getting centering rings perfectly aligned with the motor mount tube?
 
I think you mean level on the motor tube? Honestly I usually just eyeball it and keep an eye until the epoxy / glue gets a hold and then leave it be.
 
I think you mean level on the motor tube? Honestly I usually just eyeball it and keep an eye until the epoxy / glue gets a hold and then leave it be.
Yep that's what I meant. Do you usually get good results? When I do that it's sometimes barely tilted, but enough to make me cringe.
 
Normally it comes out alright but I'll be curious to see what other more experienced people do. I imagine as you climb in impulse it becomes more crucial.
 
Normally it comes out alright but I'll be curious to see what other more experienced people do. I imagine as you climb in impulse it becomes more crucial.

For higher impulse rockets, many other factors and materials come into play: fiber glass materials, super tight fitments, interlocking fin tabs, etc, etc. Therefore, I put a drop of CA to temporarily attach the centering ring (CR) to the motor mount (MMT), then dry fit inside the airframe. If everything fits perfectly, I epoxy it in place permanently. If not, I remove the CA, and start again.

For lower impulse rockets, I also eye ball things. With either epoxy or wood glue, you always have time to go back and "tweak" things if they don't look just right.
It also helps to do both CRs at the same time. This way you can roll the MMT back and forth while the glue is setting, and see if the CRs (wheels) are wobbling. If yes, you have time to adjust them. After the glue sets in, go back and add the fillets as appropriate.

YMMV,
a
 
Epoxying the centering rings to motor mount tube so they are perpendicular I just eyeball it. Think it’s more important to make sure fins are on “straight”.
 
For higher impulse rockets, many other factors and materials come into play: fiber glass materials, super tight fitments, interlocking fin tabs, etc, etc. Therefore, I put a drop of CA to temporarily attach the centering ring (CR) to the motor mount (MMT), then dry fit inside the airframe. If everything fits perfectly, I epoxy it in place permanently. If not, I remove the CA, and start again.

For lower impulse rockets, I also eye ball things. With either epoxy or wood glue, you always have time to go back and "tweak" things if they don't look just right.
It also helps to do both CRs at the same time. This way you can roll the MMT back and forth while the glue is setting, and see if the CRs (wheels) are wobbling. If yes, you have time to adjust them. After the glue sets in, go back and add the fillets as appropriate.

YMMV,
a
The rolling wheels method seems like it would make it much easier to eyeball accurately. Thanks!
 
I measure a series of marks from the end of the mmt tube all the way around then slide the ring to the marks, or if I have a short section of the mmt tube I split the short piece and use it like the green rings Estes includes with some of the PSII kits.
 
I measure a series of marks from the end of the mmt tube all the way around then slide the ring to the marks, or if I have a short section of the mmt tube I split the short piece and use it like the green rings Estes includes with some of the PSII kits.
Marking the tube is a good idea, thanks.
 
I build my aft end around the retainer, using the retainer to mark concentric lines around the motor mount that the aft centering ring and forward centering rings line up with.
I put a ring of epoxy into the body tube at the forward end of the slots.
Then I push the forward centering ring up to the forward end of the fin slots with a coupler and stand it up so the epoxy forms a fillet ahead of the ring.
Then I build the fins and MMT using the marks to keep things square.
When I’m done the retainer is snug against the aft centering ring and the assembly is very strong.
 
If by aligned to you mean square then it's easy:
If you have a BT with a known square end you can use that to true your ring, have another ring already on the MMT further along (doesn't need to be glued) and slide it inside the body tube till the one you are gluing is flush with the end. Make sure its flush all the way around and tack in place with CA or just do the fillet for one side. Just be careful not to get any glue on the BT!
 
I build my aft end around the retainer, using the retainer to mark concentric lines around the motor mount that the aft centering ring and forward centering rings line up with.
I put a ring of epoxy into the body tube at the forward end of the slots.
Then I push the forward centering ring up to the forward end of the fin slots with a coupler and stand it up so the epoxy forms a fillet ahead of the ring.
Then I build the fins and MMT using the marks to keep things square.
When I’m done the retainer is snug against the aft centering ring and the assembly is very strong.
Do you mean you put the fore ring into the body tube before attaching to the motor mount?
 
If by aligned to you mean square then it's easy:
If you have a BT with a known square end you can use that to true your ring, have another ring already on the MMT further along (doesn't need to be glued) and slide it inside the body tube till the one you are gluing is flush with the end. Make sure its flush all the way around and tack in place with CA or just do the fillet for one side. Just be careful not to get any glue on the BT!
That's an interesting idea. Does the non-glued ring keep the mmt straight inside the body tube?
 
Do you mean you put the fore ring into the body tube before attaching to the motor mount?

Yes. Let the epoxy on it cure.
Then I epoxy in the motor mount tube, using the aft centering ring to temporarily line it up. I make sure the aft centering ring is square either by referring to the concentric lines I drew using the retainer or the aft end of the body tube.
Once the motor mount tube epoxy has cured I remove the aft centering ring and epoxy in the fins. Test fit first in case epoxy interferes with the fin tabs. Then I use the double butter method to stick the fins in place. By removing the aft centering ring I’m able to reach in and internally fillet the fin tabs.
My last three operations are
1) to butter the aft end of the fin tabs and inside of the body tube and outside of the motor mount tube and slide the aft centering ring into place. Stand it upright again so the epoxy goes where it’s needed.
2) to fillet the outside joint between the body tube and centering ring and finally to JB Weld the retainer to the short length of motor mount tube that extends through the aft centering ring. If you measured everything off the retainer in the first place it should snug right up against the aft centering ring perfectly with a small ring of JB Weld squeezing out around it. I form that ring into a small fillet. All the internal fillets are small.
3) to fillet the outside joints between the fins and the body tube.

Is this overbuilding? I don’t think so. Everything that adds weight here adds some measure of strength. The test for overbuilding is always a larger motor.


Steve Shannon
 
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I measure where I want it. Then I make a line around the motor mount tube to align the ring to. Align the ring and tack in place with a couple drops of super glue. Apply epoxy.


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I use a spare hunk of mmt tubing as a sleeve, cut square in a chop saw, then split on one side. I slip it on the mmt tube, use a small straight edge to align the ends, and tape in place. Bring the c/r down against the end of it, and apply adhesive on top. Once that's cured, remove the sleeve, and finish applying the adhesive. Bingo. Centering ring, perpendicular to the tube.
 
I use a spare hunk of mmt tubing as a sleeve, cut square in a chop saw, then split on one side. I slip it on the mmt tube, use a small straight edge to align the ends, and tape in place. Bring the c/r down against the end of it, and apply adhesive on top. Once that's cured, remove the sleeve, and finish applying the adhesive. Bingo. Centering ring, perpendicular to the tube.

I do something similar, but I just paper wrapped around the tube to ensure the ring is perpendicular, same premise.
 
For those who 'tack' CA the CR to the MMT, then add Epoxy, do you not worry that you have little to no epoxy in between the CR & MMT? only relying on epoxy fillets (top & bottom) to hold it in place?

Knowing I have a square end, I typically draw a line around the MMT (top end) at the 1/2" mark. (Wrap paper around tube, draw line on paper's edge) I then apply epoxy just under this line, about a 1/4". I then slide on the CR, and place the CR on 1/4" stand-offs (I have some 1/4" machined blocks. But have used balsa scraps or ply scraps, or even the 1/4" fins that came with the kit!) with the 'top end' of the MNMT on the table. 3 'spacer' points preferred, but 2 can be used if placed correctly. (yes upside down. I'll swish & swirl the CR on eth tube to ensure I have epoxy in the joint. I then place it for it's rest while cure' phase. I've made my 1/2" mark heavy, so I can just make it out thru the epoxy, and know it's fairly 'normal' the axis of the tube.

(and yes, you can see the eye bolt in upside down, you'll also notice it isn't tight ether!)

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dx3_0511.JPG
 
For those who 'tack' CA the CR to the MMT, then add Epoxy, do you not worry that you have little to no epoxy in between the CR & MMT? only relying on epoxy fillets (top & bottom) to hold it in place?

Knowing I have a square end, I typically draw a line around the MMT (top end) at the 1/2" mark. (Wrap paper around tube, draw line on paper's edge) I then apply epoxy just under this line, about a 1/4". I then slide on the CR, and place the CR on 1/4" stand-offs (I have some 1/4" machined blocks. But have used balsa scraps or ply scraps, or even the 1/4" fins that came with the kit!) with the 'top end' of the MNMT on the table. 3 'spacer' points preferred, but 2 can be used if placed correctly. (yes upside down. I'll swish & swirl the CR on eth tube to ensure I have epoxy in the joint. I then place it for it's rest while cure' phase. I've made my 1/2" mark heavy, so I can just make it out thru the epoxy, and know it's fairly 'normal' the axis of the tube.

(and yes, you can see the eye bolt in upside down, you'll also notice it isn't tight ether!)

I do not worry at all. If the centering ring fits properly there should be almost no epoxy that can fit there anyways. It comes down to bonding area, and if the fillets are sufficiently sized the area of the centering ring/motor mount tube is a fairly small percentage of the total bonding area.




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I do something similar, but I just paper wrapped around the tube to ensure the ring is perpendicular, same premise.
I do the same but with blue masking tape. I make several wraps after I've marked the tube and the tape acts as a shelf for the ring to sit on or for me to push it up against and keep it square. I epoxy the top side and then flip it over, remove the tape, and do the other side. (Assuming it's not the top ring.) I suppose if you had badly misaligned rings you could have off-axis thrust but I bet if someone did some measurements, a tilted centering ring probably does not impact the 'straightness' of the motor mount much unless all rings are off in the same direction.


Tony
 
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For those who 'tack' CA the CR to the MMT, then add Epoxy, do you not worry that you have little to no epoxy in between the CR & MMT? only relying on epoxy fillets (top & bottom) to hold it in place?

Knowing I have a square end, I typically draw a line around the MMT (top end) at the 1/2" mark. (Wrap paper around tube, draw line on paper's edge) I then apply epoxy just under this line, about a 1/4". I then slide on the CR, and place the CR on 1/4" stand-offs (I have some 1/4" machined blocks. But have used balsa scraps or ply scraps, or even the 1/4" fins that came with the kit!) with the 'top end' of the MNMT on the table. 3 'spacer' points preferred, but 2 can be used if placed correctly. (yes upside down. I'll swish & swirl the CR on eth tube to ensure I have epoxy in the joint. I then place it for it's rest while cure' phase. I've made my 1/2" mark heavy, so I can just make it out thru the epoxy, and know it's fairly 'normal' the axis of the tube.

(and yes, you can see the eye bolt in upside down, you'll also notice it isn't tight ether!)
I used this method today, supporting the ring with the 1/4" plywood fins. The epoxy hasn't dried fully yet, but it seems to be perfect!
 
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