Pat Sparks BOMARC Build

[LSP_Kevin]

I appreciate that the angle of this photo is not directly atop the parts, but rest assured that the reprinted BOMARC launch tower parts are indeed STRAIGHT!

 

 

There's a bit of clean-up work to do, which will happen while I await the arrival of the brass angle stock I've ordered for reinforcement purposes. Getting it all back together will be fun...

 

Kev

[dutik]

Brass angles should teach the whole thing to keep upright...

 

BTW, striking paint job

Would have been a shame to put this in the bin. Wish you good luck with your efforts!

 

Regards

- dutik

[LSP_Kevin]

OK, time for an update! I had to abandon my plan to use the brass angles to reinforce the structure, as they were large enough to partially block the lightening holes, and were not only easily visible, but looked terrible. Plus, I would have had to all but remove the mounting lip in the lower section to get the brass flush between the two sections, and it all started getting too hard at that point. It turns out I had a bigger problem anyway...

 

I drilled out all the mounting holes to accept some 3mm brass tubing, and did a quick test assembly:

 

 

It all fits nicely, but still doesn't look straight, right?

 

That's because, despite the hole in the upright only being 0.15mm larger than the brass tube, there turned out to be a tremendous amount of play involved:

 

 

This won't do at all! If I leave it like that, I'll be back to square one in no time.

 

At first I tried shimming the hole with some styrene tubing, attempting to drill and ream it out just enough to tighten the fit:

 

 

Despite a couple of attempts, this didn't work. The main issue is that I don't actually have a 3mm drill bit - only 2.8 and 3.18. I've been using the latter, which has been fine for the other holes involved, but introduces too much slop on this knuckle part. So using the 2.8mm bit and then trying to open the hole to about 3mm using a reamer just didn't work, and I ended up pushing the remaining styrene out of the hole with the brass tube while trying to force it in.

 

The other thing I discovered - but didn't take any photos of - is that somewhere along the way the hole has become slight slanted with respect to the perpendicular, forcing the upright to lean slightly to one side (as can be seen in the first photo above). My only recourse was to plug the hole complete with styrene, and try to re-drill it straight using the Dremel drill press:

 

 

I've attempted to do this once already, but faced the same issue of not having the correct size of drill bit. So this is on hold until I can duck out tomorrow and grab one. There's still no guarantee of success, much less that it will fix the problem, but it's worth one last go I think.

 

Stay tuned!

 

Kev

 

 

[Landrotten Highlander]

Just a thought, but what if you put a washer on one side of the axis?  To keep it upright and 'tight-ish'.

The washer would be hidden by the rocket anyway. and you can keep your drill size as before, as it would be the washer that prevents the slanting of the boom.

[LSP_Kevin]

8 minutes ago, Landrotten Highlander said:

Just a thought, but what if you put a washer on one side of the axis?  To keep it upright and 'tight-ish'.

The washer would be hidden by the rocket anyway. and you can keep your drill size as before, as it would be the washer that prevents the slanting of the boom.

 

It's not a bad idea, and probably worth a try. It would be visible though, as the missile is mounted on the other side. I'd have to go to the same hardware store to get the washers though, so I'll still be picking up a 3mm drill bit all the same.

 

Kev

[Landrotten Highlander]

In real life there would have been what I call 'spacers' which is basically another tube through which the axle runs.  I call them 'washers' on smaller scales. but you can  very easily put a carfully measured length of small tube in between the bottom part and the boom IF your hole sizes leave very little play.

Whith that much play I would simply cut some squares of plastic with the appropriate hole, then thin to suit the space leaving just enough 'play' for the boom to rotate.

[LSP_Kevin]

57 minutes ago, Landrotten Highlander said:

In real life there would have been what I call 'spacers' which is basically another tube through which the axle runs.  I call them 'washers' on smaller scales. but you can  very easily put a carfully measured length of small tube in between the bottom part and the boom IF your hole sizes leave very little play.

Whith that much play I would simply cut some squares of plastic with the appropriate hole, then thin to suit the space leaving just enough 'play' for the boom to rotate.

 

I'm not sure I follow, to be honest! I did think about drilling out the hole and 'replacing' it with a piece of brass tubing through which the existing one could slide, but there's really not enough resin material in that area to cope with removing that much of it. And I'm not sure I could find anything with a 3mm ID to suit. If the 3mm drill bit doesn't work out, my fall back will be to obtain some 1/8" brass tubing (which is 3.18mm) to match the larger holes I've already drilled. That's the theory, anyway!

 

Kev

[Derek B]

5 hours ago, Landrotten Highlander said:

In real life there would have been what I call 'spacers' which is basically another tube through which the axle runs.  I call them 'washers' on smaller scales. but you can  very easily put a carfully measured length of small tube in between the bottom part and the boom IF your hole sizes leave very little play.

Whith that much play I would simply cut some squares of plastic with the appropriate hole, then thin to suit the space leaving just enough 'play' for the boom to rotate.

 

4 hours ago, LSP_Kevin said:

 

I'm not sure I follow, to be honest! I did think about drilling out the hole and 'replacing' it with a piece of brass tubing through which the existing one could slide, but there's really not enough resin material in that area to cope with removing that much of it. And I'm not sure I could find anything with a 3mm ID to suit. If the 3mm drill bit doesn't work out, my fall back will be to obtain some 1/8" brass tubing (which is 3.18mm) to match the larger holes I've already drilled. That's the theory, anyway!

 

Kev

 

Hi Kev,

 

I think that he may be alluding to some form of stiffener on the outsides of the hole? The area does look a little fragile, so perhaps, following Landgrotten Highlander's thoughts - if the resin tolerances and fit allow after rework - reducing the 'fork' thickness of the hinged fitting (with the two holes) and facing them (both sides) with brass sheet? These can then be drilled and bushed with your desired thickness of tubing and the whole assembly in this area should then be very rigid and resistant to angular movement?

 

Regards

 

Derek

Edited December 22, 2023 by Derek B

[chrish]

7 hours ago, LSP_Kevin said:

OK, time for an update! I had to abandon my plan to use the brass angles to reinforce the structure, as they were large enough to partially block the lightening holes, and were not only easily visible, but looked terrible. Plus, I would have had to all but remove the mounting lip in the lower section to get the brass flush between the two sections, and it all started getting too hard at that point. It turns out I had a bigger problem anyway...

 

I drilled out all the mounting holes to accept some 3mm brass tubing, and did a quick test assembly:

 

 

It all fits nicely, but still doesn't look straight, right?

 

That's because, despite the hole in the upright only being 0.15mm larger than the brass tube, there turned out to be a tremendous amount of play involved:

 

 

This won't do at all! If I leave it like that, I'll be back to square one in no time.

 

At first I tried shimming the hole with some styrene tubing, attempting to drill and ream it out just enough to tighten the fit:

 

 

Despite a couple of attempts, this didn't work. The main issue is that I don't actually have a 3mm drill bit - only 2.8 and 3.18. I've been using the latter, which has been fine for the other holes involved, but introduces too much slop on this knuckle part. So using the 2.8mm bit and then trying to open the hole to about 3mm using a reamer just didn't work, and I ended up pushing the remaining styrene out of the hole with the brass tube while trying to force it in.

 

The other thing I discovered - but didn't take any photos of - is that somewhere along the way the hole has become slight slanted with respect to the perpendicular, forcing the upright to lean slightly to one side (as can be seen in the first photo above). My only recourse was to plug the hole complete with styrene, and try to re-drill it straight using the Dremel drill press:

 

 

I've attempted to do this once already, but faced the same issue of not having the correct size of drill bit. So this is on hold until I can duck out tomorrow and grab one. There's still no guarantee of success, much less that it will fix the problem, but it's worth one last go I think.

 

Stay tuned!

 

Kev

 

Plugging the original hole with a styrene plug then drilling the hole to match the pin size is good thinking….when the pin is too big for the hole introduces a new problem is it possible to chuck the brass rod “pin” in a Dremel and run a file against it to turn or “machine” the pin to fit the hole?

just tossing ideas…..

 

[Landrotten Highlander]

2 hours ago, LSP_Kevin said:

 

I'm not sure I follow, to be honest! I did think about drilling out the hole and 'replacing' it with a piece of brass tubing through which the existing one could slide, but there's really not enough resin material in that area to cope with removing that much of it. And I'm not sure I could find anything with a 3mm ID to suit. If the 3mm drill bit doesn't work out, my fall back will be to obtain some 1/8" brass tubing (which is 3.18mm) to match the larger holes I've already drilled. That's the theory, anyway!

 

Kev

Kev, I'll try to make my thinking a bit clearer (and don't fret about not understanding me, sometimes I don't understand me)

You have your parts printed and painted.  Using plastic card cut out some squared/rounds/whatever form you think right).  then thin this card so it fits between the bottom (thick) parts and the boom (thin) part without cutting away material from either part --> only thin the plastic card forms you cut.

Then sqeeze everything in place and put your rod throught.

IF your plastic card bits are too thick it will cause stress on your printed parts and not work, If too thin you made no difference.  BUT, if just the right thickness it will hold the boom vertical while allowing it to rotate (if this is what you want).

 

Hope I have confused you some more - not.

[LSP_Kevin]

11 hours ago, chrish said:

Plugging the original hole with a styrene plug then drilling the hole to match the pin size is good thinking….when the pin is too big for the hole introduces a new problem is it possible to chuck the brass rod “pin” in a Dremel and run a file against it to turn or “machine” the pin to fit the hole?

just tossing ideas…..

 

It's probably too long for that, as it goes all the way through the assembly, from left to right, and has to pass through 5 individual holes along the way. Otherwise, I think that would have been the easiest solution!

 

Kev

[LSP_Kevin]

10 hours ago, Landrotten Highlander said:

Kev, I'll try to make my thinking a bit clearer (and don't fret about not understanding me, sometimes I don't understand me)

You have your parts printed and painted.  Using plastic card cut out some squared/rounds/whatever form you think right).  then thin this card so it fits between the bottom (thick) parts and the boom (thin) part without cutting away material from either part --> only thin the plastic card forms you cut.

Then sqeeze everything in place and put your rod throught.

IF your plastic card bits are too thick it will cause stress on your printed parts and not work, If too thin you made no difference.  BUT, if just the right thickness it will hold the boom vertical while allowing it to rotate (if this is what you want).

 

Hope I have confused you some more - not.

 

I think I understand, and was thinking along those lines anyway. The only problem is that there's a lot of flex in the side parts, as they're not particularly rigid, and won't necessarily provide any resistance to lateral movement from the rail. But certainly tightening up the tolerances in that area can't hurt!

 

Kev

[LSP_Kevin]

OK, time for another update. Finally got hold of the size of drill bit I needed, and proceeded with the plan I outlined in earlier posts. I also shimmed the knuckle to try to inhibit as much lateral movement as possible. Testing showed one side needed more than the other:

 

 

 

When assembled in isolation, this did indeed appear to take most of the wobble out, though the new hole still seemed to be too large for some reason. I suspect my Dremel drill press has its own level of play, resulting in a lack of precision.

 

Of course, all was not what it seemed! When I tried to do a full test fit of all the components, it proved nearly impossible to get the brass tube to thread cleanly through all 5 of the holes involved, suggesting that there are other alignment issues contributing to the overall problem. I nearly broke the main assembly twice while trying, and though I eventually got it in, the struggle alone didn't bode well in terms of doing it for real. But the real kicker was that it ended up being just as wobbly as before!

 

 

It turns out the the cradle piece has its own issues with alignment and fidelity, and you can just make out in the photo above that the arms straddling the main knuckle are splayed slightly - probably caused by the pressure of the leaning rail assembly.

 

After examining my options, including adding more shimming (and just popping the whole thing in the bin), I've come up with Plan C. I managed to find a length of brass tubing that exactly fits over the one I'm currently trying to use as the main axle for the assembly. I'm going to use that as a central axle, through which I'll pass the existing brass tubing. This should give it the strength, precision and fidelity required.  I just need - yet again! - to obtain a drill bit of the necessary size. This will have to wait until after Christmas now. I think that'll be my last go round with this project, so let's hope it works!

 

Kev

 

 

[dutik]

If nothing solves the issue there is just one last solution: Skip the "moving" option, align it in the straight position and glue everything in place

 

Good luck, and Merry Xmas to you!

 

Regards

- dutik

[LSP_Kevin]

2 hours ago, dutik said:

If nothing solves the issue there is just one last solution: Skip the "moving" option, align it in the straight position and glue everything in place

 

Good luck, and Merry Xmas to you!

 

Regards

- dutik

 

Thanks, dutik - but it was already glued in place before! All the bonds became unstuck due to the stresses applied by the gradually worsening lean. This is not about trying make everything move, but just to get it all straight again, so that it doesn't fall apart again. So far, I'm not having much luck!

 

Kev