Sanding G12 fiberglass to fit coupler to airframes

[billdz]

So I'm building my rocket, try to fit upper and lower airframes into coupler. Lower goes in smoothly, upper is too tight, sticks about 1/4 the way down (see attached pic). No problem, I think, a little sandpaper and it will slide all the way in. Twenty minutes of sanding later (80 grit paper), the upper airframe still will not go down fully into the coupler. Even worse, I removed the other side of the coupler from the lower airframe, and now the coupler won't even fit smoothly into the lower airframe, as it did a few hours ago, even though I did not touch anything on that side.

Do I need an electric sander or is there some technique to getting this done correctly?

Happy New Year,
Bill

 

[Worsaer]

80 grit is pretty rough, and likely worked well for the first cut, but left it rough. You probably need to smooth it out with something like 220, and then 400. That should make it slide much easier.

 

[Steve Shannon]

Bill’s correct. For fitting pieces together 80 grit is very rough. The fastest way to use sandpaper (for me anyway) is to mark the high areas (use a little dry erase marker on the piece that goes inside the other and then test fit them. You’ll be able to see where they rub. Just sand the high spots. Mark it again and sand the high spots again.
You must wipe off the sanding dust before testing. It’s worse than the original problem.

 

[billdz]

The fastest way to use sandpaper (for me anyway) is to mark the high areas (use a little dry erase marker on the piece that goes inside the other and then test fit them. You’ll be able to see where they rub. Just sand the high spots. Mark it again and sand the high spots again.

How do you see the high areas? I'm not seeing where they rub. The 2 pieces can be inserted into each other, but the fit gets tight about 1/4 of the way down.

 

[OverTheTop]

I use these flapper disks for opening out the ID and cleaning up the inside of tubes as required. You could probably use a finer grit as they remove material with not much effort (the advantage of power tools!).



Can use them on the outside of tubes as well if you are careful.

 

[manixFan]

Following up on Steve's post here's what I do:

* index the parts so they fit together the same way each time (mark on outside of body tube, inside of coupler)
* use a dry erase marker (sometimes I'll use a sharpie or even a pencil) to draw concentric circles all the way around the coupler, about 1/2" - 1" apart
* slide the coupler as straight in as possible using your index marks
* push to point of resistance, and then a bit further (but not so far you can't get them apart)
* inspect marks on coupler to find high areas and sand with 120 grit (I think 80 is too coarse and will raise grooves that impair fit)
*CLEAN OFF DUST! (needed to get an accurate fit)
* check fit and repeat as needed

Once you have it fitting on your index marks you can then rotate it 1/4 turn and see if it still goes. I try to index all my larger tubes so I'm not removing a bunch of material all the way around. But I make sure they don't really bind up if they do rotate to prevent any deployment issues.

Once I get a good fit I finish by wet sanding with 220 grit for a smooth fit. I've been really surprised at how much of a difference this can make with spiral wound tubes.

Listen to good music while you're doing it and it doesn't seem to take as long.


Tony

 

[Steve Shannon]

How do you see the high areas? I'm not seeing where they rub. The 2 pieces can be inserted into each other, but the fit gets tight about 1/4 of the way down.

That’s the purpose of the dry erase marker. You put it on one surface and slide the parts together. The marker ink will transfer to the other piece where rub.

 

[billdz]

Thanks for the replies, good suggestions. Looks like in my case for some reason this isn't going to be easy. As Steve and Tony suggested, I switched to 120 grit and marked with dry erase. On the very first try, I was able to get the airframe all the way down to the switch band. Removed and tried again, this time it stuck. Several more rounds of sanding, first with 120 then 400 for smoothing as Bill suggested, then clean out dust with a rag. Still sticking. Maybe I'll try a power tool as OverTheTop suggested, my arm's getting tired. Must be something simple that I'm just missing.

Is there any trick to the dust clean out? Dry cloth OK, or should it be damp? What about some silicon or WD-40?

 

[crossfire]

So I'm building my rocket, try to fit upper and lower airframes into coupler. Lower goes in smoothly, upper is too tight, sticks about 1/4 the way down (see attached pic). No problem, I think, a little sandpaper and it will slide all the way in. Twenty minutes of sanding later (80 grit paper), the upper airframe still will not go down fully into the coupler. Even worse, I removed the other side of the coupler from the lower airframe, and now the coupler won't even fit smoothly into the lower airframe, as it did a few hours ago, even though I did not touch anything on that side.

Do I need an electric sander or is there some technique to getting this done correctly?

Happy New Year,
Bill

Just from looking at your picture is the booster tube and upper tube the same FG tube, The upper tube looks whiter. I would open up the ID of the tubes a little.

 

[Steve Shannon]

Thanks for the replies, good suggestions. Looks like in my case for some reason this isn't going to be easy. As Steve and Tony suggested, I switched to 120 grit and marked with dry erase. On the very first try, I was able to get the airframe all the way down to the switch band. Removed and tried again, this time it stuck. Several more rounds of sanding, first with 120 then 400 for smoothing as Bill suggested, then clean out dust with a rag. Still sticking. Maybe I'll try a power tool as OverTheTop suggested, my arm's getting tired. Must be something simple that I'm just missing.

Is there any trick to the dust clean out? Dry cloth OK, or should it be damp? What about some silicon or WD-40?

I clean using a solvent such as denatured alcohol. I also use a dry lubricant (graphite powder or talcum powder).

 

[billdz]

Thanks to everyone's help, plus lots of elbow grease, sandpaper, alcohol, and talcum powder, it finally all fits together. Hard to believe such a small imperfection could require so much work to fix.

@crossfire - Yes, all the pieces are from the same kit. The lower airframe looks darker because the MM is inside. Also, this was the first (and probably last) time I used pigment with the epoxy, and it made a mess. There are still small spots of pigment and epoxy on the lower airframe, hope there is some way to remove those. I applied the pigmented epoxy 3 days ago and even today it still makes a black mark on my fingers if I touch it.

 

[Steve Shannon]

Following up on Steve's post here's what I do:

* index the parts so they fit together the same way each time (mark on outside of body tube, inside of coupler)
* use a dry erase marker (sometimes I'll use a sharpie or even a pencil) to draw concentric circles all the way around the coupler, about 1/2" - 1" apart
* slide the coupler as straight in as possible using your index marks
* push to point of resistance, and then a bit further (but not so far you can't get them apart)
* inspect marks on coupler to find high areas and sand with 120 grit (I think 80 is too coarse and will raise grooves that impair fit)
*CLEAN OFF DUST! (needed to get an accurate fit)
* check fit and repeat as needed

Once you have it fitting on your index marks you can then rotate it 1/4 turn and see if it still goes. I try to index all my larger tubes so I'm not removing a bunch of material all the way around. But I make sure they don't really bind up if they do rotate to prevent any deployment issues.

Once I get a good fit I finish by wet sanding with 220 grit for a smooth fit. I've been really surprised at how much of a difference this can make with spiral wound tubes.

Listen to good music while you're doing it and it doesn't seem to take as long.


Tony

Perfect!

 

[ChrisAttebery]

I mount the coupler on a threaded rod and pair of bulk plates in the drill press. I support the bottom of the rod with a hardwood block drilled to match the rod and clamped to the table. I use 120 grit emory cloth to evenly sand the coupler while it is turning in the drill press (~1000rpm). Every once in a while I remove the coupler, clean it with alcohol and then test fit it. Once it is very close I use 400 grit and then I put a light coat of Johnson's Paste Wax on the sliding surfaces. It makes the joint slide together much easier.

 

[Keisling]

I applied the pigmented epoxy 3 days ago and even today it still makes a black mark on my fingers if I touch it.

Even with the messy epoxy, that's a bad a$$ looking rocket!
Have you tried wiping down the cured epoxy with denatured alcohol? That usually removes the residue and leaves the epoxy with a nice matte finish.

 

[manixFan]

Perfect!

Thanks, Steve. A long time ago I learned to embrace the 'Zen of Sanding.' Good music is the key.


Tony

 

[Steve Shannon]

Thanks, Steve. A long time ago I learned to embrace the 'Zen of Sanding.' Good music is the key.


Tony

I actually enjoy sanding. I enjoy drywall work too, but do what I can to minimize sanding there. Like you, music makes it all more enjoyable.