Except for the balsa that makes the nose cone, everything I needed for this build was in the tube rack or parts pile. So the first step was to make the nose.
The balsa block was 3 1/4" square and 24" long. A 12" cone, 3" shoulder, and 1" base length gives a 16" block. I cut it 17 1/2" long with a chop saw, and immediately noticed...
...a crack or check or whatever it's called. The doggone thing was awfully close to the edge. I figured that it would be better if that was at the shoulder end rather than the tip. If it opened up during turning I could control the shaping of the nose better at the shoulder than if it opened up near the tip.
Next, drill a 1/2" hole for an aluminum shank. Rather little room to play with (3" diameter airframe!) so a brad-point bit was absolutely essential. Good thing I bought a nice set a few months back. Also a good thing I have a floor-mount drill press. The center was marked by slicing two diagonals on the cut end, with a razor knife.
Add epoxy to the hole and shove in a scrap of 1/2" aluminum rod. Now you can see the cuts that marked the center.
Carved off the corners to reduce the amount of work and make it roughly cylindrical.
Although I have a metal lathe, I wanted to use a drill press for this work, to see just how feasible it would be. Ran into a few issues.
A short piece of airframe tube was slid over the shank before chucking in the drill press. That was my gauge for turning the shoulder Once it's chucked in place DO NOT remove it till it's done. You may never get it centered quite the same.
In one of his books Peter Alway suggests the saw blade of a Swiss Army knife as a roughing tool. Following that idea I made a roughing tool by stacking half a dozen reciprocating saw blades (clearance, 50 cents each) and epoxying the end of the stack in a piece of PVC pipe for a handle. Those blades were extremely sharp with coarse, nastily pointy teeth, so in use I always cut on the side turning AWAY from me. If it got yanked out of my hand, I wanted it to go across the shop rather than into me. I like my insides and my blood right where they are.
I started out by making the block very roughly circular, with a taper. The block is going to wobble, and in the past I've learned that the more weight there is near the tip, the more pronounced the wobble/vibration. Lowest speed of the drill press at first. And worked on getting the shoulder diameter correct first. Once the shoulder is right, slide the piece of airframe tube onto it and you have a nice gauge for the base diameter; just bring it down to the o.d. of the tube. Unfortunately the shoulder wasn't perfectly circular, so I have a little hand work to do where the shoulder meets the base diameter.
If you simply press the cutting tool or sandpaper hard against the balsa, you end up with something that is decidedly non-circular. Rough cutting means holding the cutting tool as steady as possible, letting it hit the high spots. It takes a good while. As the high spots come down, use a finer tool. I used a coarse file for later cutting (still hadn't gotten all the high spots down). One way to hold the file fairly steady is shown below. The tip of the file is held against the column of the drill press. Much steadier than holding the tip in my other hand.
The highest speed I used was about 800 rpm. Work goes faster at higher speeds, but the vibration increases and once again, you get a non-circular result.
When the base was the correct diameter and the tip was about 1" diameter, I switched to sandpaper and sanding screen. I found that (in my hands) 120 grit sanding screen worked significantly faster than 50 grit sandpaper. Probably because the balsa dust could get out easily through the holes. However, sanding screen leaves its pattern of ridges if the screen is held at right angles to the axis. Hold the screen at a 30-45 degree angle and vary it a bit. I finished with 220 grit screen and 150 grit sandpaper.
And...finished....almost.
I still need to clean up the shoulder-base joint, give the tip (maybe the whole nose) a good soaking in thin CA. I left the tip about 1/4" diameter because it would surely break off if I took it to a point. It's a sport model anyway. And I need to cut off the rest of the shank. I could remove the shank entirely by cutting notches in the end of a piece of tubing, and use it as a trepanning bit to core out the shank. I've done that for other homemade nose cones. But I think I'll drill and tap the end for an eyebolt instead.
Unfortunately it's slightly over length (but it's a sport model anyway). And it's slightly oval in places, but you can't tell by looking at it.
If I were doing it over, I'd get a plane and carve down each corner with an equal number of strokes before turning. I think that would have made the thing vibrate less.
The balsa block was 3 1/4" square and 24" long. A 12" cone, 3" shoulder, and 1" base length gives a 16" block. I cut it 17 1/2" long with a chop saw, and immediately noticed...
...a crack or check or whatever it's called. The doggone thing was awfully close to the edge. I figured that it would be better if that was at the shoulder end rather than the tip. If it opened up during turning I could control the shaping of the nose better at the shoulder than if it opened up near the tip.
Next, drill a 1/2" hole for an aluminum shank. Rather little room to play with (3" diameter airframe!) so a brad-point bit was absolutely essential. Good thing I bought a nice set a few months back. Also a good thing I have a floor-mount drill press. The center was marked by slicing two diagonals on the cut end, with a razor knife.
Add epoxy to the hole and shove in a scrap of 1/2" aluminum rod. Now you can see the cuts that marked the center.
Carved off the corners to reduce the amount of work and make it roughly cylindrical.
Although I have a metal lathe, I wanted to use a drill press for this work, to see just how feasible it would be. Ran into a few issues.
A short piece of airframe tube was slid over the shank before chucking in the drill press. That was my gauge for turning the shoulder Once it's chucked in place DO NOT remove it till it's done. You may never get it centered quite the same.
In one of his books Peter Alway suggests the saw blade of a Swiss Army knife as a roughing tool. Following that idea I made a roughing tool by stacking half a dozen reciprocating saw blades (clearance, 50 cents each) and epoxying the end of the stack in a piece of PVC pipe for a handle. Those blades were extremely sharp with coarse, nastily pointy teeth, so in use I always cut on the side turning AWAY from me. If it got yanked out of my hand, I wanted it to go across the shop rather than into me. I like my insides and my blood right where they are.
I started out by making the block very roughly circular, with a taper. The block is going to wobble, and in the past I've learned that the more weight there is near the tip, the more pronounced the wobble/vibration. Lowest speed of the drill press at first. And worked on getting the shoulder diameter correct first. Once the shoulder is right, slide the piece of airframe tube onto it and you have a nice gauge for the base diameter; just bring it down to the o.d. of the tube. Unfortunately the shoulder wasn't perfectly circular, so I have a little hand work to do where the shoulder meets the base diameter.
If you simply press the cutting tool or sandpaper hard against the balsa, you end up with something that is decidedly non-circular. Rough cutting means holding the cutting tool as steady as possible, letting it hit the high spots. It takes a good while. As the high spots come down, use a finer tool. I used a coarse file for later cutting (still hadn't gotten all the high spots down). One way to hold the file fairly steady is shown below. The tip of the file is held against the column of the drill press. Much steadier than holding the tip in my other hand.
The highest speed I used was about 800 rpm. Work goes faster at higher speeds, but the vibration increases and once again, you get a non-circular result.
When the base was the correct diameter and the tip was about 1" diameter, I switched to sandpaper and sanding screen. I found that (in my hands) 120 grit sanding screen worked significantly faster than 50 grit sandpaper. Probably because the balsa dust could get out easily through the holes. However, sanding screen leaves its pattern of ridges if the screen is held at right angles to the axis. Hold the screen at a 30-45 degree angle and vary it a bit. I finished with 220 grit screen and 150 grit sandpaper.
And...finished....almost.
I still need to clean up the shoulder-base joint, give the tip (maybe the whole nose) a good soaking in thin CA. I left the tip about 1/4" diameter because it would surely break off if I took it to a point. It's a sport model anyway. And I need to cut off the rest of the shank. I could remove the shank entirely by cutting notches in the end of a piece of tubing, and use it as a trepanning bit to core out the shank. I've done that for other homemade nose cones. But I think I'll drill and tap the end for an eyebolt instead.
Unfortunately it's slightly over length (but it's a sport model anyway). And it's slightly oval in places, but you can't tell by looking at it.
If I were doing it over, I'd get a plane and carve down each corner with an equal number of strokes before turning. I think that would have made the thing vibrate less.
The tube has been sanded and wiped clean with 91% IPA, but it will be sanded and wiped again where the epoxy will go, just before gluing.
