brentius
Well-Known Member
Hi, my L1 certification rocket has 4 fins, and I'm using an unslotted body tube. How can I go around and cut slots in the tube for the fins, and how can I make these slots as accurate as possible?
Fin alignment and cutting
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NTP2
Well-Known Member
Do you have access to a 3d printer? I or someone else can make a jig.
brentius
Well-Known Member
I do have a 3d printer, and I've already lasercut a jig. The thing is, what's difficult is cutting the slots for the fins.
Go to
payloadbay.com
You can make one from there.
payloadbay.com
You can make one from there.
brentius
Well-Known Member
Cardboard, it's just LOC BT-3.9.
Titan II
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Get a few feet of 1" to 1.5" angle aluminum 1/16" thick, or metric equavelant in your case. Then an Exacto knife (#11) with sharp blades. Use the alum as a guide and then light strokes on the slots. Works perfectly.
If you like reading, here is more....
https://www.rocketryforum.com/threads/any-tips-on-cutting-ttw-fin-slots.161877/
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I cut down the length of each side of a slot, alternating one pass for each side. Do not try to cut through with a single stroke. Multiple passes are necessary. Takes me 5-7 passes depending on BT thickness and the pressure. The first pass will guide the others to some extent, so be more careful on it.
When the sides are cut thru, I "punch" the point of the blade thru each end of the slot to finish. A little trimming, etc. may be needed.
brentius
Well-Known Member
So, should I put the fins in along with the motor mount, or should I put the motor mount in first then put the fins?
I actually cut the slots all the way down the tube to the end. I build the Fin can outside the tube, and paint it. I paint the tube another color. Then I drop the tube down over the fin can and adhere/seal it to the fin can.
LOC newer instructions are also showing how to do this alternate method to build. Like a sounding rocket build a fin can then put an aerospace body over the real works.
I'm too lazy to look for it...but someone very recently asked about how to do a fin can, such that internal fillets could be applied before adding the aft ring. Anyone out there who's not as lazy as I am?
I use an aluminum angle to mark the center of the fin on the tube. Then measure what half the width of the fin is and mark those places and using the angle to mark the width of the slot. Then I, and I might be the only one to do this, I use my Dremel with a cut off wheel to cut the slots. I can cut 4 slots in about a minute. For me the slots don't have to be perfect because the fillets will cover the cut. I put the mmt in first and attach the fins to the mmt.
Capt. Eric
Well-Known Member
I got nutin' to do, so here goes:I'm too lazy to look for it...but someone very recently asked about how to do a fin can, such that internal fillets could be applied before adding the aft ring. Anyone out there who's not as lazy as I am?
I do this often. Not too hard.
1. Glue the fwd ring to the mmt and do the fillets. Make sure it is straight and in it's final position.
2. With a stick, spread plenty of glue inside the body tube just before it's final position.
3. Run the mmt with fwd ring into the BT, through the glue (making it's own fwd fillets), and into position.
4. Slip the aft ring over the mmt and into the BT just enough to hold the mmt in the center position. MAKE DARN SURE there is NO GLUE on the aft ring, MMT or BT where it is going to sit while the other glue dries.
5. Stand the BT upright with the MMT down. This allows the glue fwd of the fwd ring to settle into it's fillet. Or you can rotate the rocket in the horizontal position until the glue stops flowing.
6. Once dry, remove the aft ring (see below). The MMT stays in position while you insert and fillet the fins & behind the fwd ring.
7. When ready, spread some glue around the BT and MMT then insert the aft ring. Again, stand upright so the glue settles into it's own internal fillet. Then fillet the outside of the ring if you like.
Key to this is being able to remove the after ring after the glue dries. To aid in this you have some options:
A. Make your ring with 1 or 2 holes in it. You can pass a string through the holes and use it to pull the ring out. You can always fill the holes later if you don't use them for motor retainer (Z clips or the like).
B. I have a ring with a couple of small screws in it that I use just for this step. Easy to grab the screws like handles and pull the ring out.
C. Pass some dental floss through the ring's center and around the outside in a couple of places. Do this in 1 or 2 places before inserting the ring. Use the string to pull the ring out. Make sure the ring isn't too tight so it comes out without too much effort, breaking the string.
As always, do a dry run before using the glue to make sure it all works as planned.
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Or if you do a lot of rockets with this size BT and MMT combination, make a “sacrificial” ring in the size you need with some robust handles like some long #8 or M4 screws passed through holes from the forward side and fastened with nuts on the aft side. Once your glue sets, pull the CR out with the handles, apply fins and fillets, and then apply glue and seat the actual CR. If you are concerned about robustness or longevity of the sacrificial ring, use plywood or fiberglass if you have the bucks. You should be able to use this over and over on other projects.
And I second what @Capt. Eric said: absolutely do a dry run.
And I second what @Capt. Eric said: absolutely do a dry run.
Someone that has no idea how to mark a body tube or cut fin slots has no business flying a HPR rocket.
Simple answer.
( let the bashing begin)
Simple answer.
( let the bashing begin)
Ouch
, but you do have a point.
First, it is a L1 cert. Second, maybe he has only built rockets that have the slots all ready cut. Third, you gotta start sometime.
I've done this since I was a little boy, as many have, and now I'm old. Kind of proud I have actually made it this far. I am L3, and I still ask how to do stuff
I've done this since I was a little boy, as many have, and now I'm old. Kind of proud I have actually made it this far. I am L3, and I still ask how to do stuff
Everyone has to learn, when people ask questions on how to do, it is because they respect and want knowledge you have.
You are not incorrect.
Assuming the OP is just looking for the lazy way out... there isn't a real easy way out unless you can buy a kit with everything precut. It isn't that difficult to do it all yourself.
Make your own marking pattern by wrapping a piece of paper around the tube, doing measurements, etc. Put 2 marks on the tube at each slot. If you are using cardboard of phenolic then buy a razor saw and use that to cut out the slots. The slots don't have to be perfect, make them a bit oversize then after the fins are glued in you can fill the gaps with the first coat of fillets.
On tubes that had not been pre-slotted, when I built the Outside of the tube fin can, and put a centering ring on the top of the motor tube; I could slide the body tube down to the top of the Fincan and use a fine Sharpie pen to mark both side of the fin.
Then use angle iron to extend the marks and cut with with many soft strokes of a #11 Blade Knife.
Then use angle iron to extend the marks and cut with with many soft strokes of a #11 Blade Knife.
Yes. Paper wrap works for any arbitrary BT outside diameter. Easy to fold evenly in thirds or quarters , as desired.
I use a length of 90 degree angle (3/4" or 1.5" depending on by diameter) aluminum to mark straight lines along the BT.
For cardboard, I use xacto #11 freehand. occasionally with the angle as a fence.
Razor saw is great if cutting from the BT end.
For phenolic (blue tube) or fiberglass, I have a jig for the table saw, but that's hard to use because the fin mark is hidden on the bottom against the table. May be better to use the Visa card and get the mfr to cut the slots.
I measure the marks on my paper in mm with a metric ruler, then it is easy for me to divide by 3 or 4 or whatever and measure out the additional marks with the ruler. I've done a lot of cutting of LPR tubes recently and used XActo for that. A few years ago I cut slots in a phenolic tube for MPR using the razor saw. Even if the slot doesn't extend all the way to the end, if I can get a cut started I can continue cutting from there. (One time many years ago I watched a carpenter cut a rectangular opening in a house door using a standard carpenter hand saw so I learned from that how to start a cut.)
I do that if I'm building a fin can outside of the rocket. I'll build the fin can then slide it partway into the rocket so I can mark where the slots need to be the I cut the slots. If I'm gluing the fins on through the slots the I cut the slots first.
That is a Great Idea!!! - Thanks!!!
I did this on a recent build. I used a long MMT to act as a stuffer tube too, with 3 centering rings. The front ring and rear ring were both removable so I could do internal fillets. I had a piece of string attached to each ring so I could pull them out after the MMT first MMT centering ring dried.
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