L2 Certification Build -- Five Finned Folly

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jlabrasca

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I've been fabricating parts for this for a few weeks. I'll start glue and paint this weekend, so I thought I'd get in line for some kibitzing and criticism.

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sketch with dimensions

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5 fins (plus a spare)

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Centering rings/motor mount and parts for the baffle.
 
Different. I like it. I applaud the decision to scratch build your cert rocket too.
 
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Sanded the tool marks off the bevels on the fins, while the CNC cut some 64.9 mm to 75.8mm centering rings for the BT80 coupler to which the tail-cone will attach.
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I am experimenting with finishes on the spare fin. I shot two coats of Rustoleum 2X high gloss black. After that dries and cures, I'll try to top it with Spastix black and Spastix color changing paint -- to show off the bevels.

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Dry fit some parts. The U-bolt is a little to snug in the top baffle plate. Can't quite get it through. I'll open it up a bit with a Dremel. The centering rings are a little loose on the motor tube, but I am going to attach all three rings before sliding the tube into the airframe so its less of a construction concern than a what-did-I-mismeasure issue.

Next I have to cut some 1/16" plywood strips for the motor mount and for the sides of the frustum of a hexagonal pyramid that will go between the baffle plates.
 

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I haven't put a baffle in a rocket above a 24mm MMT, but I do like them. The ones I have handled the small red cap on AT Hobbyline motors quite well. Have you considered how the baffle will handle the larger caps used on the AT HPR motors with ~1.5g of BP? I've never had an issue with them, but I suspect they come off the motor quite energetically.

Of course that is assuming you will even be flying AT motors with motor eject.
 
Didn't get nearly as much done today as I'd anticipated. Too cold and wet to paint, and we got an unexpected invitation to some kind of pagan fertility celebration to do with eggs and rabbits and also a selection of spirits from a local distillery; so no power tools today.

I broke what brain cells weren't pickled trying to fit the remaining pieces to be milled -- plus a few parts for another project -- onto a single 12 x 24 sheet of 1/16" plywood.

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Work holding for thin stock is a challenge, so I try to cut as many pieces as I can in one job. I need to rebuild the X10 Xpress that disappeared into the suburban neighborhood around the club's low power field last fall -- so I am trying to fit the L2 rocket parts around those cool retro fins.

I haven't put a baffle in a rocket above a 24mm MMT, but I do like them. The ones I have handled the small red cap on AT Hobbyline motors quite well. Have you considered how the baffle will handle the larger caps used on the AT HPR motors with ~1.5g of BP? I've never had an issue with them, but I suspect they come off the motor quite energetically.

Of course that is assuming you will even be flying AT motors with motor eject.

No, you got it. I am planning to send it up on a 38mm AT DMS J motor, and to bring it down with a JLCR.

I have considered that the baffle is going to take some punishment, which is why I cut the top and bottom plates from 3/16" aircraft plywood and will epoxy the baffle to the airframe. There's weight penalty, but this design isn't intended to set an altitude record.

If I got the geometry right, the width of holes at the bottom end of the baffle will be just about 80% of the diameter of the ejection cap. I am going to face the inside and outside of that hexagonal cone with aluminum tape.
 
No, you got it. I am planning to send it up on a 38mm AT DMS J motor, and to bring it down with a JLCR.

I have considered that the baffle is going to take some punishment, which is why I cut the top and bottom plates from 3/16" aircraft plywood and will epoxy the baffle to the airframe. There's weight penalty, but this design isn't intended to set an altitude record.

If I got the geometry right, the width of holes at the bottom end of the baffle will be just about 80% of the diameter of the ejection cap. I am going to face the inside and outside of that hexagonal cone with aluminum tape.

I've never flown a DMS motor, but I know the RMS have a large plastic cap for the forward closure that holds the powder in. I imagine they could do more damage to a baffle then the hot ejection gases. I was just wondering if you were going to fly the AT RMS and had considered those caps.
 
I've never flown a DMS motor, but I know the RMS have a large plastic cap for the forward closure that holds the powder in. I imagine they could do more damage to a baffle then the hot ejection gases. I was just wondering if you were going to fly the AT RMS and had considered those caps.

I understood your question -- I just did a poor job of answering it.

I think the cap on the DMS motor is the same as the cap on the RMS.

It has an ejection cap, anyway, and I have considered the impact of that cap on the baffle. I am overbuilding the baffle (and the rest of the rocket) by a wide margin (I hope).

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I understood your question -- I just did a poor job of answering it.

I think the cap on the DMS motor is the same as the cap on the RMS.

It has an ejection cap, anyway, and I have considered the impact of that cap on the baffle. I am overbuilding the baffle (and the rest of the rocket) by a wide margin (I hope).

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The DMS cap is an “innie”, a small red rubbery plug, that inserts into the ejection charge well after drilling the delay, inserting the washer, and pouring in the black powder. It’s about 3/8 inch diameter.
The RMS cap is an “outie”, red plastic, sized to fit on the outside of the forward closure, so they are much larger. There wouldn’t be room inside the RMS ejection charge well for the powder and some kind of plug.
I wouldn’t worry about baffles being struck by a cap so much as being plugged by the cap, preventing ejection gases from escaping. Your tapered design looks like it could be susceptible to trapping the cap, but your design does not look like it could be plugged that way. If you’re flying an RMS motor you could just forego using the cap and wrap electrical tape around the forward closure to hold the powder in place. That has worked for many years and I’ve never had problems with it plugging baffles.
Edit: I do wonder about that 1.5 mm ply though; it’s pretty thin.
I would do the perforated disk out of 1/4 inch plywood and use it to anchor my shock cord as well.
 
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The DMS cap is an “innie”, a small red rubbery plug, that inserts into the ejection charge well after drilling the delay, inserting the washer, and pouring in the black powder. It’s about 3/8 inch diameter. ]The RMS cap is an “outie”, red plastic, sized to fit on the outside of the forward closure, so they are much larger. There wouldn’t be room inside the RMS ejection charge well for the powder and some kind of plug.
I wouldn’t worry about baffles being struck by a cap so much as being plugged by the cap, preventing ejection gases from escaping. Your tapered design looks like it could be susceptible to trapping the cap, but your design does not look like it could be plugged that way. If you’re flying an RMS motor you could just forego using the cap and wrap electrical tape around the forward closure to hold the powder in place. That has worked for many years and I’ve never had problems with it plugging baffles.

Thanks. I cracked open a DMS motor and measured the cap. About 13mm tall by about 10 mm in diameter. As you say, much smaller than than the RMS cap but still too large to fit --easily -- through the holes in the sides of the baffle. Even if it did ricochet off the cone moving towards one of those holes, there isn't anywhere for it to go. The there is only about 8 mm of clearance between the top of the hole and the airframe.

Edit: I do wonder about that 1.5 mm ply though; it’s pretty thin.
I would do the perforated disk out of 1/4 inch plywood and use it to anchor my shock cord as well.

The top and bottom plates are cut from 3/16 plywood. I am using 1/16 plywood for the sides because I will need to flex the pieces to assemble the cone.

I cut the sides of the baffle and did a dry fit this morning. It is going to take considerable dexterity, and some rubber bands, to get this thing put together. I don't think I'll be able to get a continuous layer of aluminum tape on the inside of the cone. I'll face the inside of each panel with aluminum tape, then wrap all the way around the the outside circumference of the cone with long pieces of aluminum tape.

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And yes, the recovery harness will attach to that U-bolt. I there is going to be an arrestor/shock absorber attached between the bolt and the parachute.

I am going to attach the bottom of the tail cone to a piece of BT80 coupler, so I cut a pair of centering rings. Having shot more than a few small-surface-area pieces off the spoiler and across the shop when the adhesive failed and the tabs broke, I decided to onionskin these. I set up the tool path to leave about 0.1 mm at the bottom of the cut. Breaking the rings out of the stock made a mess of the bottom ply, but the excess cut away easily and the rings are a good fit.

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This is what it looks like when a 20th century brain gets tired of pushing pixels around and reverts to old habits.

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And this is how I keep the robot from climbing off the bench and trying to kill me when my back is turned.

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The coolest part of the rocket that will never be seen again (knock wood)

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after putting foil on the inside faces of the panels for the cone I decided to use the scraps of aluminum tape to foil the bottom faces of the baffle plates. Won't do anything for strength or durability, but it kept a few pieces of tape out of the landfill.

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It wasn't quite as hard to put the cone together as I feared, but it was still a test of patience and muscular coordination. Handling the assembly it feels surprisingly rigid, but I'm still going to foil the outside after the Titebond dries.

The pieces fit together much more precisely than there was any reason to expect from my scribbled trigonometry. Hope my math on sizing the holes was as sound.

I dry fit the motor mount and fin can, and ran into the first real problem

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The threaded inserts I was planning to use to hold the motor retainer won't work. Trying to get them in, the plywood starts to split. Need another solution. Maybe t-nuts? I'll have to see what they have at the hardware store.

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Cutting and slotting the bluetube next.

I was planning a zipperless separation, but looking at the motor mount and baffle next to the airframe it occurs to me how little volume there will be between the baffle and the nose-cone -- so I am reconsidering. I wanted a payload bay, to mount a camera inside the airframe -- but maybe I can put that in the nosecone instead.
 

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The tee-nuts in stock at the hardware store nearest my house will not fit.

I found something that might work on Amazon, but I don't want to pay for a many-piece assortment or to wait for shipment -- so I grabbed a piece of scrap and cut a new aft centering ring. The mounting holes are sized now so that the threaded inserts can be pressed in. I'll epoxy them into place.

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The new ring fits everything else pretty much exactly like the first one, except that I cut the hole for the motor tube 0.2mm smaller. It is a very snug fit now.

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Thats a shot looking down a length of BT80 coupler, just to make sure I didn't move the mounting holes too far out. The coupler will sit aft of the bottom centering ring, to provide some place to attach the bottom of the paper tailcone.

It is cold and wet here today, so no paint or glue. I blew my morning off on the new CR, so slotting the tube will wait until the weekend. I still have to figure out where I am going to make the separation for deployment.

Five fins are cooler than any number of fins fewer than five.

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Still too wet here to paint. Found my first design error, The return on the fin that is supposed to make contact with the tailcone was cut at the wrong angle. It was supposed to be 12° w.r.t the long axis of the rocket. At 8.3° the diameter of the open end of the tailcone will be about 67mm -- leaving a gap of 2mm around the coupler that is meant to support it. If I make the cone out of a piece of of U-line mailer tube, instead of poster board, the wall will be about 1.1mm thick. Leaving less than a 1mm gap. I can deal with that -- except that it will be a PITA to make the cone out of thick-walled tubing.

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Going back through the history in Fusion 360 I see exactly where I went wrong. I fat-fingered the x-axis scale one the many times I resized the fins. "Measure twice, cut once..." except in this case it was fiddle and tweak and measure the virtual thing 300+ times, then screw up the validation before sending the g-code to the robot to make 6 copies of the slightly wrong part. Somebody in QA is going to get chewed out over this. ^_^

Decided its not too wet for glue, and started assembling the motor mount. Epoxied in the threaded inserts and attached the longitudinal pieces to the radial pieces with wood glue. The fillet of epoxy on the top ring is really just to use up what was left in the cup after the threaded inserts. The fins aren't glued in yet, just inserted to keep the centering rings square to the ...troughs...rails... is there a name for those longitudinal planks between which the fin tabs fit?

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For my next trick, I will ruin two perfectly good nose cones. Anybody have a good technique for cutting an ogive nosecone at at a specific diameter? I need to cut off the tip to leave a truncated cone with a top diameter that matches the 57.3mm diameter of the base of that balsa Little Joe Apollo short conical n.c.

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Very nice build! I like the charcoal avoidance shielding on your ejection baffle.

I’ve found cutting ogive nose cones to be a bit difficult. The only way I’ve managed to get good results is to make a hole the desired diameter in a piece of scrap wood. Then I use it as a template, with a framing square, to mark the cut line on the nose cone. It looks like you’ve got the saw to do the job hanging on the wall in your picture. (6 ½” Shark Saw)

Good luck! Let us know how it turns out.

Paul B.
 
Very nice build! I like the charcoal avoidance shielding on your ejection baffle.

I’ve found cutting ogive nose cones to be a bit difficult. The only way I’ve managed to get good results is to make a hole the desired diameter in a piece of scrap wood. Then I use it as a template, with a framing square, to mark the cut line on the nose cone. It looks like you’ve got the saw to do the job hanging on the wall in your picture. (6 ½” Shark Saw)

Yeah, I guess that is how I will do it.

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If I do it like this, with the template sized to the diameter of the body tube, I can use the bench top to get the template square to the long axis of the nosecone. I was kind of hoping there was some easier way to do it.

I am going to try to salvage the tip for use as a BT70 or 2 inch nosecone.

I had to go down and look at that dowel saw -- well spotted. I wouldn't have been able to tell you the brand.

Didn't get much else done today. STILL raining here.

Put beads of toughened CA on the motor mount, as dams to keep the epoxy from leaking under the rails (I am going to call the rails) and started the fillets. I am WAY overbuilding this.

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I set up the tool paths to cut motor retainer , This will be the 4th time I try to mill steel on the Shapeoko. So far, I am 1 for 3, with two endmills in the trash. I am going to cut some furniture components and other non-rocket stuff before put the sheetmetal down on the spoiler -- just in case I end up crashing something hard enough to break the CNC.

I also messed around some more with the tail-cone geometry, trying to decide between making the tail-cone longer, or making the walls of the cone thicker, to account for my mistake on the shape of the fins.

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The problem with having a robot that can do precise and repeatable cuts, is that EVERYTHING starts to look like a job for the precise and repeatable robot.

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Also finished the fillets on the motor mount and got some paint on my paint-test fin (weather was really nice here today).
 
I shot two coats of Rustoleum 2X high gloss black. After that dries and cures, I'll try to top it with Spastix black and Spastix color changing paint -- to show off the bevels.


From my experience, I found it easier and better looking to shoot Rustoleum Colour shift over rustoleum flat black.

It makes the contrast better if you have colour shift sections next to black: I did colour shift roll patterns over gloss black, and in the sun, the contrast between the gloss black and colour shift isn’t great in the shine.

Since then I tested the flat black and it worked better.

It’s also easier to see how much you’re laying down over flat black than it is on gloss, so I was less likely to put down too much in each pass. (Which I did the first time... ☹️)
 
The problem with having a robot that can do precise and repeatable cuts, is that EVERYTHING starts to look like a job for the precise and repeatable robot.

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Also finished the fillets on the motor mount and got some paint on my paint-test fin (weather was really nice here today).

I'm looking forward to having my robot to do my bidding. Soon, very soon. Nice cutting jig. Looking forward to seeing how it comes out.
 
I'm looking forward to having my robot to do my bidding. Soon, very soon. Nice cutting jig. Looking forward to seeing how it comes out.

Well here is a cautionary tale about feature creep and not seeing the forest for the trees -- and maybe something to do with blind men groping an elephant (I don't know. I am not good with parables). Anyway, maybe somebody can learn from my experience.

The original plan was as you see it in post #16. A square with sides equal to the diameter of the blue-tube airframe with a cut-out for the tip of the nosecone. I was going to square up the template and the nose cone (in a piece of airframe) against a straight edge.

Then I decided that the dowel saw was too flexible and needed support on both sides to make a straight cut -- and I have my ethical slave with router to do the cutting while I sip Veuve Clicquot on the veranda -- so why not make two copies of the template and put together a cigar-cutter?

But that meant making the templates bigger -- so that there would be clearance for the screws holding the two pieces together... which now means the template is wider than the airframe so I can't square the template and the cone together in two dimensions.

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I can fix this pretty easily (I am probably not going to do it on the CNC, because its two saw cuts and a few whacks with a chisel). Good news is, it seems like I got the size of the hole right, and slope of cone is shallow enough above the shoulder that I won't need to put it in a piece of airframe to get it parallel to the bench top.

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From my experience, I found it easier and better looking to shoot Rustoleum Colour shift over rustoleum flat black.

It makes the contrast better if you have colour shift sections next to black: I did colour shift roll patterns over gloss black, and in the sun, the contrast between the gloss black and colour shift isn’t great in the shine.

Since then I tested the flat black and it worked better.

It’s also easier to see how much you’re laying down over flat black than it is on gloss, so I was less likely to put down too much in each pass. (Which I did the first time... ☹️)

Thanks. I am trying out Spaz Stix paint -- which you are not supposed to use on wood. It is a system, with a black base coat and a color changing top coat. I am trying to use the Rustoleum to trick the Spaz Stix base coat into thinking I am painting the chassis of an RC dune buggy or something.
 
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Loving this thread and all the different techniques.. really looking forward to possibly getting to see and fondle this beauty at one of our launches :)
 
Loving this thread and all the different techniques.. really looking forward to possibly getting to see and fondle this beauty at one of our launches :)

Yeah, I was hoping to debut and try for certification at AP Showers, but family and job...you know, the usual excuses for not giving proper attention to a hobby. Fingers crossed for Spring Thunder.

Somebody emailed to ask if I'd ordered the blue-tube pre-slotted for 5 fins ... which I would have done if I'd had any idea I was going to do this when I ordered the blue tube 6+ months ago.

I think I can do it with the CNC -- the work area for the Shapeoko 3 extends in front of the frame by about 2 inches. The tricky part will be getting the slots spaced at 72° intervals around the circumference of the airframe.

I tried it out this afternoon, using a piece of 3" mailing tube. It worked pretty well. I am going to make some more tests, and there is going to be a lot of belt-and-suspenders marking and scribing on the blue-tube to match the assembled dimensions of the motor mount, before I commit to this method. I've left myself very little margin for misalignments.

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Cut the tip off the plastic nosecone. Decided to keep it simple and tack the cigar-cutter jig to the work bench with the same blue-tape and CA work holding technique I use for small pieces on the CNC. I ended up putting the cone into the airframe, to get more clamping surface.

It worked pretty well. The plastic clogged the teeth of the saw and the cut pieces adhered to each other pretty strongly. I was going too slowly for it to have been friction welding, I think the plastic is just that cohesive. I had to pop the tip off after the saw bottomed-out in the jig.

The tip will make a good secant nosecone for a BT70-based rocket.

The base of the cone will need some more work before I can get the conical balsa tip in place -- the plastic is a little thicker than I'd anticipated. I'll sand down the shoulder of the balsa cone a little, and file away at the inner lip of the plastic cone, until they go together.

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It's interesting to see how far off I was in my original estimate -- the purple smear on the cone -- made by jamming the cone into a piece of BT70 that I'd split and taped to the outside of another piece of BT70 (trying to make an inner diameter that matched the outer diameter of the base of the balsa cone. I'd've been sanding for days, and the residual tip wouldn't have been good for much.
 
The original idea for the nosecone

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no plan survives contact with reality:

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It took a lot of filing on the balsa cone, and some dremel-tooling on the variably thick plastic of the transition, to get it this far.

I think I can make this work

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a little more -- careful -- work with the bastard file to get rid of that last millimeter of shoulder on the cone, then some careful adjustment to the lip of the plastic transition with the disc sander.

Weather doesn't look good for painting this weekend. I still need to cut and slot the tube, and fabricate the tailcone. Also, have to decide whether its going to be rail buttons or glued-on rail guides?
 
Cut the tube.

The original hare-brained scheme didn't work (on account of being hare-brained).
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Those fins were meant to help me align this ring on the tube as a saw guide -- the plywood was too flexible, and the fit of the ring too snug -- the fins popped off as soon as I started sliding the ring along the tube.

I got over myself, and decided to do it like any sensible person would do it

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I also got two coats of sandable primer on the fins and almost go them inside before the rain started again <g>

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I started marking up the aft tube section for fin slots and rail buttons and quickly realized that the thinner wall of the blue tube means that the slotting fixture that worked well for the test on the ULINE mailing tube -- up in post #23 -- won't be adequate by itself. When I clamp the blue tube to the CNC the fixture holds the back end of the airframe too far out from the CNC frame -- so that the tube is not parallel to the x-axis of the cutter.

I am going to make another decagonal fixture for the forward end of the tube, and wrap tape around the middle of the tube to make up the thickness so that there will be three points of contact between the frame and the tube when it is clamped.

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Upon a momen't's reflection, the tape ring on the body tube was a bad idea. I was trying make up the difference between the radius of the airframe (the apothem of the decagon) and the radius of the decagon Since I was going to have to cut another decagon for the forward end of the tube, I decided to make two, and to dimension them so that this distance would be something easier to match with a piece of filler taped to the front of the frame.

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This worked pretty well, but work holding is a problem
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I did not know the speeds and feeds for blue tube -- and I still don't. To my best guess of the speeds for the Makita router, I used the values for vulcanized fiber I found here. The surface finish and edges on the slots are terrible. It will take lots of sanding and filing to clean them up. Despite all the clamps, the tube moved during two of the cuts. Not a complete failure, but I will need to adjust one or two of the slots a little and I am going to have at least one messy seam to fill.

I should have used an up-cut endmill -- the burrs on the inside of the tube are significant and VERY hard to reach with a file or sandpaper.

I did a quick test fit for length and position. I put the motor mount in upside down -- so that I would be able to pull it out -- and I can get a fin into the motor mount in all five positions, just not all at once.

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There is definitely something to this technique, and I can certainly come up with more secure work holding, and a some easier and more repeatable way to set up the cuts, but I am done fighting with the robot for a while.

I also fished the first iteration of the aft centering ring out of the trash, and cut a piece to make a backer for the aft rail button.

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I like my hand saws better than I like the CNC. Next rocket, I am not using anything but an Xacto and white glue.
 
Found my half-round file and scraped the burrs out of the inside of the tube and got the motor mount in the right-way-around. My original assessment was unduly pessimistic -- my janky and kluged up work-holding worked better than I feared. The troughs line up with the slots and all five slots are reasonably tight. Only one slot seems to have been compromised by the router dragging the tube back and forth -- but the gap along the root of the fin in that slot is less than a millimeter wide.

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Blue Tube is interesting stuff, but the claim that you can you can cut, ... and shape the material like you would an ordinary paper tube. seems like it might be a bit of an overstatement. The back saw goes through it pretty easily, and I got a clean edge with the dozuki (the teeth of which have a pretty narrow set) I wouldn't want to try to slot this stuff with a knife or a laminate saw.
 
Looks like it's comming along great, and a nice solid fin can. Are the socket head screws shown going through the tail cone for motor retention?
Paul B.

A quick sketch -- not to scale

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The very long screws that are sticking out now are there to give me something to hold on to while I push and pull and twist on the motor mount. I will use shorter screws when I put a motor in.

I still need to make the retainer. I have a couple different materials that will work. I want to try milling it from steel, but I will have to wait until I have the house to myself because it will be really loud.
 
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