How to obtain position-agnostic alignment

I'm sure there's a proper term for this, but whenever I drill holes in anything rocket-related, no matter how careful I am, I need to index or otherwise key the location, so that the holes line up again. As examples:
I made a nose AVBay for my L2 Frenzy so my Eggfinder can fly free of any signal-attenuating threaded rod. I still have to align it in a specific way to get it to work though.

Same for the sheer-pinned nose- I added some low-class decal tags to the surface so I can align more easily.
I built a Nuke Pro Max as a D/D workhorse- it can fly on so many 24mm, 29mm and 38mm that I can enjoy it any time, and with dual deploy configuration, I can get some much-needed practice. I fiberglassed the tubes and did T2T on the fins. I had to add this yellow tag so I could secure my avbay to my payload tube.
Even my newest build, a 3" MacPerformance Rayzor, is heavily indexed- the booster is sheer pinned to the av-bay to eliminate drag sep. The av bay bolts onto the payload tube with 6-32 screws, and the nose sheer pins to the top. I used a sweeping paint design to ensure that all the holes line up as needed.
This shows almost the full stack- the central hole is one of three static ports- above are the 6-32 holes and below are the 2-56

I'm NOT a machinist-A) my attention to detail could use improvement, and B) my equipment is bargain basement. Are there simple steps that can be employed to increase consistency- to the point that any hole drilled 120° away from its neighbor will mate with any other hole in the airframe (this is my holy (er, holey) grail!

I have pondered this question. In the interest of keeping making things, rather than overthinking, I just use the keying method. KISS. There always seems to be something that needs to be facing in a particular direction anyway.

Without access to a CNC machine, this will be pretty hard to achieve. Not only are you trying to line up the holes every 120°, but also each the same distance from the edge of the tube vertically (which also assumes the edge of the tube is perfectly square to the body). I agree with OTT, keep it simple. Drill a hole and insert a pin before drilling the next hole. I always use a index mark at the joint, plus one hole is usually a 1/2" lower than the others to give me a starting point to line things up with out a whole lot of body twisting.

I think Adrian hit the nail on the head. The short answer to this is 'precision'. And it needs to be precision on a level that is difficult to achieve with the kind of tools most of us use.

I use index marks of some kind to ensure I always line things up the same. In spite of my darndest efforts it's really hard to, for example, make exactly equidistant holes around a tube. I get it "close enough" to do the job and solve the possible reassembly problem with index marks.

On FWFG tubing I usually make a small notch (with a Dremel cutoff wheel) that straddles the tubing splice. That way I don't have to worry about a Sharpie marking fading or anything.

cvanc said:

I think Adrian hit the nail on the head. The short answer to this is 'precision'. And it needs to be precision on a level that is difficult to achieve with the kind of tools most of us use.

I use index marks of some kind to ensure I always line things up the same. In spite of my darndest efforts it's really hard to, for example, make exactly equidistant holes around a tube. I get it "close enough" to do the job and solve the possible reassembly problem with index marks.

On FWFG tubing I usually make a small notch (with a Dremel cutoff wheel) that straddles the tubing splice. That way I don't have to worry about a Sharpie marking fading or anything.

Click to expand...

I do this on ANY tubing type. Multiple separation points get multiple notches as I get towards the bottom. I tried drill marks, but I think I prefer cut notches. It's simpler across a tube splice. To use a drill mark on a separation point, you have to apply good pressure to keep the drill tip from moving one of the pieces.

FWIW, Wildman was playing around with shear pin holes using his fin cutting CNC. Not sure how that ever turned out. You would still have to drill the coupler (or nose cone or what have you underneath), but, it is a start towards better precision. The problem here is that he has to know in advance what size shear pins you'll be installing. Not huge, but still...

Oh, hi Carl!

I prefer to use a pin at the interface point. I originally started using steel dowel pins but have recently changed to using carbon fibre rod.

Parts are mated. Hole drilled at the interface line. Pin glued in place on the part that has the coupler. Here is one on my 8.25" Nike Smoke.


This really simplifies lining the sections up.

jd2cylman said:

Without access to a CNC machine, this will be pretty hard to achieve. Not only are you trying to line up the holes every 120°, but also each the same distance from the edge of the tube vertically (which also assumes the edge of the tube is perfectly square to the body). I agree with OTT, keep it simple. Drill a hole and insert a pin before drilling the next hole. I always use a index mark at the joint, plus one hole is usually a 1/2" lower than the others to give me a starting point to line things up with out a whole lot of body twisting.

Click to expand...

This is really elegant- I like the idea of deliberately (I'm assuming it's deliberate, anyway, if not, sorry!) placing one of the holes in a completely different plane- When I was painting the rayzor (orange and green thing in my photos), I lost my orientation during the prime, so I had to rotate the tube through 360deg to find the matching holes (since they were close, but not exact). Will consider this for my Adventurer 3 from RW that I haven't painted yet (am not going to, most likely- just enjoy the way it looks). Prior to my maiden flight, I used a dremel wheel to whirl away a line across both joints- that's mentioned below, and it worked well, once- now that I've had to replace the av-bay and the payload, I may try your idea.

cvanc said:

I think Adrian hit the nail on the head. The short answer to this is 'precision'. And it needs to be precision on a level that is difficult to achieve with the kind of tools most of us use.

I use index marks of some kind to ensure I always line things up the same. In spite of my darndest efforts it's really hard to, for example, make exactly equidistant holes around a tube. I get it "close enough" to do the job and solve the possible reassembly problem with index marks.

On FWFG tubing I usually make a small notch (with a Dremel cutoff wheel) that straddles the tubing splice. That way I don't have to worry about a Sharpie marking fading or anything.

Click to expand...

This is a good idea, and I did it this way on my Adventurer 3 that remains unpainted, as above. No reason to not continue, just shopping for other ideas!

OverTheTop said:

I prefer to use a pin at the interface point. I originally started using steel dowel pins but have recently changed to using carbon fibre rod.

Parts are mated. Hole drilled at the interface line. Pin glued in place on the part that has the coupler. Here is one on my 8.25" Nike Smoke.
View attachment 313061

This really simplifies lining the sections up.

Click to expand...

This is great! I had seen a build where the tube was cut in a triangle, and the resultant piece was glued to the coupler on the opposing face, so it was keyed that way, but yours is much easier and low profile! Won't detract from a nice paint scheme, either...may have a winner!

may have a winner!

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Thanks. It works for me

Boeing went to enormous expense a while back to eliminate slotted holes for rivets in one of their wing assemblies (might have been 777). To cut the holes they used a milling machine with an 8m bed. The head position was measured with laser interferometers which were corrected for the refractive index of the air under different temperature/humidity conditions. I think the hole positions were worked out based on the workpiece temperature also, to compensate for Cte. Final accuracy was about 3um (0.12 thou) at the extremes of the bed IIRC. Millions of dollars spent and I think the result was a success. Precision costs.

OverTheTop said:

Thanks. It works for me

Boeing went to enormous expense a while back to eliminate slotted holes for rivets in one of their wing assemblies (might have been 777). To cut the holes they used a milling machine with an 8m bed. The head position was measured with laser interferometers which were corrected for the refractive index of the air under different temperature/humidity conditions. I think the hole positions were worked out based on the workpiece temperature also, to compensate for Cte. Final accuracy was about 3um (0.12 thou) at the extremes of the bed IIRC. Millions of dollars spent and I think the result was a success. Precision costs.

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Jeez-louise...that is incredible! I spent the early part of my career cutting human tissue at 2-4um for histopathologic evaluation, so I can appreciate just how minute those variations are, but across 8m...WOW. That's impressive. I guess I can fish around with my screw a few times until I find its home!

Typically like others I use a locating pin to verify proper postioning of bulkeads like found in your first picture, usually its a piece of 1/8" brazing rod cleaned of flux and epoxied into the mounting CR.

If you want to drill holes at precise angles (precise enough for any of our hobby needs) you don't need a CNC machine. You can index your tubing using a spin indexer:

https://www.grizzly.com/products/5-C-Spin-Index/G5649



Steve Shannon

Steve Shannon said:

If you want to drill holes at precise angles (precise enough for any of our hobby needs) you don't need a CNC machine. You can index your tubing using a spin indexer:

https://www.grizzly.com/products/5-C-Spin-Index/G5649



Steve Shannon

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That is intriguing, but I know so little that it actually seems a little intimidating. What, exactly, is that?

rharshberger said:

Typically like others I use a locating pin to verify proper postioning of bulkeads like found in your first picture, usually its a piece of 1/8" brazing rod cleaned of flux and epoxied into the mounting CR.

Click to expand...

I get OverTheTop's idea since it's about radial alignment, but how do you leverage the same when it comes to axial alignment (a la the bulkhead fitting in my first photo?) Do you sink a hole, epoxy a pin, and use that as a reference? Now that I write it out it doesn't seem nearly so complicated...

soopirV said:

That is intriguing, but I know so little that it actually seems a little intimidating. What, exactly, is that?

Click to expand...

It's just an indexing head that allows you to turn a workpiece, such as a body tube, to any degree setting. For three equally spaced holes you would turn it to 0, 120, and 240 degrees.

Similarly there are dividing heads that you can use to evenly divide a workpiece into integer divisions, such as when cutting a gear.
https://www.grizzly.com/products/Dividing-Head-BS-1/G1054

Here's a video that helps explain how to use a spindexer:
https://youtu.be/M66Zf8wHxBU


Steve Shannon

soopirV said:

I get OverTheTop's idea since it's about radial alignment, but how do you leverage the same when it comes to axial alignment (a la the bulkhead fitting in my first photo?) Do you sink a hole, epoxy a pin, and use that as a reference? Now that I write it out it doesn't seem nearly so complicated...

Click to expand...

Its actually pretty easy. I was having similar issues lining up nose cone Av-Bay lids.

You can index your tubing using a spin indexer:

Click to expand...

Not a bad gadget.

I have a Universal Dividing Head on my mill, and I still use the keying method. Keeps things located easily and precisely. It really simplifies stacking the rocket. Prevents things like a rotational shift preventing the insertion of RBF pins because the guide rail is in the way.

Here I am milling some fin slots in a 54mm tube:

OverTheTop said:

Not a bad gadget.

I have a Universal Dividing Head on my mill, and I still use the keying method. Keeps things located easily and precisely. It really simplifies stacking the rocket. Prevents things like a rotational shift preventing the insertion of RBF pins because the guide rail is in the way.

Here I am milling some fin slots in a 54mm tube:
View attachment 313114

Click to expand...

Nice.
Yes, I would key it also. And honestly I don't try too hard to make my three holes perfectly spaced because there's really no point as long as I've keyed the assembly together. I always want the switches for electronics to be facing away from the rail, so I'm never going to assemble the rocket in one of the other two directions at least between the aft section and the center section.
For the center section to the nosecone either keying or setting it up to be assembled in any of three different directions are just two ways to do it. I key it there also, simply because it's easier. I don't have a spindexer or a dividing head yet. I only put the spindexer out there as a fairly inexpensive answer to the original question.


Steve Shannon