Marking and slotting airframe tubes

Chris_H

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I am building a slotting jig right now, for slotting fiberglass tubes with a router, and wondering about different methods that people use for scribing precise lines on airframes to help in fin alignment?



Also, unrelated, what is a good density of 2 part expanding foam for filling cavities around the motor mount/ fins?
 

JohnCoker

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Well, your slots will align your fins along the axis of the tube. To keep them straight out from the center, I like to use a flat jig with slots for the fins.

booster-fins-mounted.jpg
 

Chris_H

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If the slots are not cut straight, the fins will not be, either. I ordered a piece of slotted tubing a few months ago, and the slots were out by almost 0.010" from top to bottom over a 6" length. Maybe this is insignificant, but I am pretty sure I can set up to cut them more accurately.

I have heard about people marking tubes with a drawer edge, or angle iron, but I want something that can be used to scribe a line more accurately, without having to re-establish planes and elevations everytime.


I have some of those fin alignment jigs, and they seem to work pretty well.
 

JohnCoker

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Trying to achieve better than 1/100th inch accuracy with hand tools is pretty ambitious. I.e., this is measuring to 1/128" (or 0.25mm).

As you say, the traditional method is using a piece of aluminum L channel, which naturally lines up with the tube axis for marking. But I doubt that's going to be more accurate than what you state.

I'm not sure it matters for most rockets. As long as the error is randomly distributed, it shouldn't cause excessive roll. (And roll only really matters for camera rockets anyway.)

I have been experimenting with using a CNC router with a 4th axis for slotting tubes. It'll be interesting to see if this produces a better result. (My concern is that the error will not be random, even though smaller, and may actually induce more roll.)
 

Chris_H

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Trying to achieve better than 1/100th inch accuracy with hand tools is pretty ambitious. I.e., this is measuring to 1/128" (or 0.25mm).

As you say, the traditional method is using a piece of aluminum L channel, which naturally lines up with the tube axis for marking. But I doubt that's going to be more accurate than what you state.

I'm not sure it matters for most rockets. As long as the error is randomly distributed, it shouldn't cause excessive roll. (And roll only really matters for camera rockets anyway.)

I have been experimenting with using a CNC router with a 4th axis for slotting tubes. It'll be interesting to see if this produces a better result. (My concern is that the error will not be random, even though smaller, and may actually induce more roll.)


For most people, I would agree, however, I am quite comfortable in working at a very high level of precision, and have been for many years. My shop is well equipped with precision ground granite plates, calipers, micrometers, height gauges, depth gauges, dial indicators, bore gauges, etc. On a wood product that I have made well over 1000 of at this point, I routinely expect some tolerances to be under 0.0015", and will work the piece until I am there, but often see tolerances within 0.0005"of what I spec for with minimal tuning. Other tolerances on the same piece, have to be held under 0.005" or the piece is a 'second'. This is in terms of 'absolute precision' where the work interfaces with a metal piece of known and consistent dimension, as opposed to 'relative precision' where asometimes a looser set of tolerances will allow for 'precision relative to non critical neighboring components. I recognize that this is an anomaly in woodworking, and it requires some highly specialized processes.

In the machine shop, I do a lot of high precision bearing work, and am used to measuring in 'tenths'.

With a router in a properly built jig, holding a tolerance of 0.001" should be no problem, I do it all the time. Challenges that I see in routing a precise slot in a fiberglass tube are: holding the tube so that vibrations from the cutting action of the bit cause zero rotation (solved), and establishing all the planes in setting up the guides so that the router travels purely paralell to the axis of the tube. The plunge is less than 0.100", so there may be some very slight 'excess cut' at the bit entry point, but once the bit is plunged, it should be no problem to cut straight. Certainly straighter than 0.010'. In 5" that is a lot! That would be a lot for a plywood panel cut on a table saw.

As for the question of 'if it matters', I answer that everything matters. Straight slots will not cover for warped or otherwise imprecise fins, or problems elsewhere, but there is probably no harm in straight slots. In most everything, better is better. And, for me, creating things is a form of self refinement. If I am not doing my best work, why not? Most often, it takes little more time to 'do your best'.




I wish I had a CNC router in my shop! I have access to one, but am still learning Fusion 360 and some other software.
 

JohnCoker

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I wish I had a CNC router in my shop! I have access to one, but am still learning Fusion 360 and some other software.
Well, if you have access to a CNC router, then you can achieve much more accurate slotting.

Note that you don't really need to draw anything complex for this. You are just cutting a rectangular slot in the tube, turning the 4th axis and doing it again. I cut fin slots on a CNC router one at a time by rotating and zeroing the 4th axis and then by making a simple rectangular slot along the X axis. That doesn't require any complex CAD drawing, and in fact I do it all in the CAM software. (You don't even need to rotate the 4th axis during the cut because you want the side walls of the slot to be parallel anyway.)

finslots.jpg
 

Chris_H

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That is actually an interesting idea that I had not thought of. Thank you.


I have assembled a few CNC cut plywood projects in the past, and it feels like with plywood, <0.010" might be pushing it.

I know that I can hold a pretty accurate line with a (my) shop designed router sled, and it will be relatively small. It is a multi purpose jig for working with rocket parts, including slotting.
 

Nytrunner

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A club member made a Jig for me to slot a boat tail that came out perfectly. (He has more equipment than I can fit in my apartment)

It uses two tapered plugs made on his lathe that fit on each end. The big one had two lines at 90deg crossed at the center, and index holes were drilled on the lines. As long as the two lines are perpendicular, the holes will index to 90 degrees every time. (Now this as for a 4 fin boattail, so other #s require different angles. Got a protractor?)

A threaded rod ran through the end plugs and the whole thing is sandwiched between a fixed end of the jig, and a sliding end to accommodate variable length workpieces. We just stuck a pin through a matching hole in the sliding end to the index hole, dropped a plunge router bit through the skin of the boattail (ex-nosecone), and away we went! I'll try and track down pictures.
 

Nathan

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I do it the old fashioned way. Mark slot locations on end of tube by standing it on a 360° protractor, then extend the lines up the tube with an L channel. Then cut the slots by hand with a Dremel. As long as the slots are cut straight slight imperfection don't matter because they will be covered by epoxy fillets.
 

Chris_H

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A club member made a Jig for me to slot a boat tail that came out perfectly. (He has more equipment than I can fit in my apartment)

It uses two tapered plugs made on his lathe that fit on each end. The big one had two lines at 90deg crossed at the center, and index holes were drilled on the lines. As long as the two lines are perpendicular, the holes will index to 90 degrees every time. (Now this as for a 4 fin boattail, so other #s require different angles. Got a protractor?)

A threaded rod ran through the end plugs and the whole thing is sandwiched between a fixed end of the jig, and a sliding end to accommodate variable length workpieces. We just stuck a pin through a matching hole in the sliding end to the index hole, dropped a plunge router bit through the skin of the boattail (ex-nosecone), and away we went! I'll try and track down pictures.


Thank you, Nytrunner. I am not sure I follow that completely. If you have photos, I would enjoy seeing any that you share.
 

Chris_H

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If you already have a granite plate then just use a surface gage on your granite plate to draw precise parallel lines.
https://www.mini-lathe.com/Mini_mill/Accessories/Layout/surface_gage_.jpg


Not sure why I had not thought of that one, but yes I have the plate, and a height gage that would do the trick nicely. Only need to make a clamping jig for that idea, to secure the tube to the plate. And maybe a degree wheel to reference location around the diameter. Cool idea. Thank you.
 

Chris_H

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I guess a 'verified' piece of channel might be a good tool to have around. I was thinking more along the lines of a smaller tool, like a nice combination square, except for round tubing.

Do you run the dremel up the slots completely handheld? No Guide? That is bold, if so! I have done a bit of inlay work on instruments, but cutting a straight line on a round tube by hand, I can imagine some imperfections.

Thank you, Nathan.

I cut the inlay in this headstock completely freehand, but the thought of freehand cutting a slot in a tube gives me butterflies!

 

Screaminhelo

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I guess a 'verified' piece of channel might be a good tool to have around. I was thinking more along the lines of a smaller tool, like a nice combination square, except for round tubing.

Do you run the dremel up the slots completely handheld? No Guide? That is bold, if so! I have done a bit of inlay work on instruments, but cutting a straight line on a round tube by hand, I can imagine some imperfections.

Thank you, Nathan.

I cut the inlay in this headstock completely freehand, but the thought of freehand cutting a slot in a tube gives me butterflies!


My friend, that is gorgeous work!!

If you can cut that inlay by hand, I am confident that you could slot a tube to 0.001" with a crayon and a butter knife.
 

Chris_H

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My friend, that is gorgeous work!!

If you can cut that inlay by hand, I am confident that you could slot a tube to 0.001" with a crayon and a butter knife.

Hah! I am not sure about the 'crayon and butter knife' bit, but I think that with some care and thought, I will be able to design a tool for quick and repeatable, accurate slots.

Mainly, this thread was about hearing ideas for working with round tubes.
 
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