M31 Bridgelayer conversion

[Beekster]

Well, now, this is getting just a wee bit complicated, isn't it? Priming & painting this one will be fun...








The filler caps have chains, the big boxes are on, and so are the back porch and bridge mount. The porch still needs some bolt head & nut details added to all four of the mounting points. The bridge mount is on, but I have some concerns about it. Right now, the top supports coming down from the crane boom are just pinned in place to locate the thing while the pins in the tow lug mounts dry. I will leave this alone for another day while that happens. I'm concerned about them being strong enough to hold the mount by themselves, but I don't want to glue the turret down and permanently secure the pins through the boom support legs and the bridge mount supports. One, it would make painting easier to have the turret off. Two, it will be much more challenging to thread the winch cable through the turret front and onto the winch drum if the turret is permanently installed. Tomorrow, after the glue work I've just done has set up, I will thread the cable and see how hard it will be to work with it if I do glue the turret down. At least I can leave the hatches off until the cable is attached, and a proper length established to support the bridge. I won't be able to photograph the whole thing without setting up my larger light box on a card table, so I'll avoid that for as long as I can since it's a chore to do that and rearrange the lighting.

[dogfish7 (R.I.P.)]

A lot of hours of work there. Sure is looking good Beekster.

[Beekster]

Thank you! A lot of hours, and a lot more to come. Haven't selected nor built tracks yet; haven't completed the crossbar supports for the bridge and whatever lugs & shackles will connect it to the main winch cable; haven't done any mud.

If I seem oddly productive right now, I suppose that I am. With the stay-home orders, my standing twice weekly wargame session is suspended, so we won't start the Battle of Cedar Creek, 1864, for some weeks yet. Likewise, a Star Fleet Battles campaign that I am involved in up in Seattle is on hiatus, so the several long conversations I have with my teammate each week regarding strategy and options aren't happening, either. I ought to be exercising more, but have curtailed that too just to keep from being out where I could be exposed or expose others if I'm one of those asymptomatic types. Without my other major hobby available to me, more hours are being spent at the model bench.

[dogfish7 (R.I.P.)]

I'm certainly enjoying it.

[JCON]

Me too Bruce!!!

[Beekster]

Good news on this one today. The bridge support stays put when the crane supports & turret are removed, though it is flexible. Extreme care will have to be taken when working around it going forward, but I think I can manage that. I've added some concentric tubing to the winch drum and drilled it so that the winch cable can be glued into the hole during final assembly. The plan is to thread the cable through the turret front and superglue it in place in the hole. Then I can attach the bridge assembly to the support frame, and attach the other end of the winch cable to whatever solution I come up with to mate it to the bridge assembly...probably a lug and a shackle, maybe with some chain. I'll figure it out... Anyway, once that is done I can set the angle I want for the bridge, support it there, and reach a finger in through the hatch opening on the turret to wind the cable on the drum until it is taut, then glue the drum in place.

[Beekster]

And we have a bridge! Four sections of M2 Treadway bridge, which are 3D printed items from Shapeways. Each pair is pinned together with brass rod & styrene tube, with the brass providing some strength. The crossbars and wedges are Evergreen stock, with punched washers & bolt heads. The forward crossbar has a simple lifting lug fabricated from Evergreen stock, and you can see how I intend to suspend it with the main winch cable and a large shackle. I don't know if this was how it was done; some images imply something this simple while others had more complicated designs. It probably depended on the unit, since these were all field-expedient lash-ups.

[Beekster]

And then the disaster happened. Just the wrong sort of bump, and the bridge mount snapped off the front of the tank. The resin standoffs snapped in two just behind the carrier frame. No gluing them back together while on the tank; way too much awkward motion involved. Had to snap off the tow lugs from the diff cover to remove the long broken ends of the standoffs, drill out the pins to restore the holes, and glue those back on the diff...job done. The shorter remains of the resin standoffs were sliced from the bridge carrier frame and the frame cleaned up, another job done. Resin standoffs are now glued back together, but the joints are messy and the parts clearly aren't strong enough...but they will serve as patterns for new parts made from styrene I-beam, just as the Resicast bridge mount served as a pattern for my own. New standoffs have been cut to length and sanded square, but cutting and matching the necessary angle on them is a job for another day. Once that's done, finishing the standoffs with new mount plates & holes for mounting to the tow lugs will be straightforward. I probably should have just scratchbuilt the entire bridge mount structure, as I am now forced to do, right from the start. For a really pretty nice resin kit, there won't be much of it left when I'm done.

[JCON]

Oh my!!!

[dogfish7 (R.I.P.)]

That is very sad and reminds me how brittle resin parts are.

[Beekster]

May 10, 2020 6:22:53 GMT -6 dogfish7 (R.I.P.) said:

That is very sad and reminds me how brittle resin parts are.

Indeed...especially thin profiles like these I-beams.  This is a setback to be sure, but it can be fixed.  The final product will be stronger than before, since the whole of the mounting to the tow lugs will now be a standard plastic-to-plastic join, and there's quite a bit of surface area for the glue to work on:  Both sides of the tow lug, both inside surfaces of the standoffs, and the pins running through the tow lugs and standoff together.  It is noteworthy that the break points were near the bridge frame, but not right at it.  I had added styrene stock to the front of the standoffs, so that a standard plastic join could be used where the standoffs met the bridge frame.  Those joints held perfectly well; the resin snapped about an eighth of an inch down from there. The  new weak point will be where the tow lugs join the differential, but if those break the whole assembly will likely come off as one piece.  Now that I'm thinking about it, I should use this opportunity to strengthen those like Madill did when they built yarders.  I have no proof that something like this was done during WW2, but there's no reason reinforcement of the tow lugs in some fashion couldn't have been part of engineering the bridge apparatus in the first place.

[Beekster]

Repairs are in progress:

At the top of the image, you can see how the resin parts broke (and how I more or less put them back together to serve as templates). In the middle are the new standoffs in process. At the bottom is one of the four new plates which will sandwich the tow lug between them. The three pieces of strip glued down are temporary; they will serve as guides for the drill bit when I drill the through holes in the plate. I won't bother cleaning them up much once I pop these fixture strips off since these sides will face the tow lug and be glued to it. I will use that .039" drill bit, and then wiggle it around to enlarge the holes for .047" rod (which I already know will fit through the tow lug holes).

And I have reinforced the tow lugs with strips of .015" x .030" strip along the base, roughly equivalent to strips of half inch thick bar stock an inch wide, bent to match the curve of the diff and welded on. Looks reasonable to me, and will probably be nearly invisible in the end after mud application.

[Beekster]

No new photos yet, but some progress and a lot of pondering going on. The replacement styrene standoffs are on for the mounting frame, and it fits over the tow lugs. It's just pinned there for now; it still has to come off again for detail work like weld seams and I may add some reinforcement plates where the standoffs meet the mounting frame, and might even use strip stock to plate over most of one side of each of them as a reinforcing doubler plate. Haven't seen those in the few images I have, but it isn't an insane idea for structural integrity so field engineers might have at least considered it seventy-five years ago.

And now the pondering: When the standoffs broke, so too did the double-angled support beam coming down from the crane boom...the little plate that is pinned to the boom snapped off. I've got it back on, but it also broke off at an earlier stage of construction. I'm not sanguine about it holding together now. It's becoming clear that taking the turret on and off for fitting various things is a hazard, and the resin support beams are very vulnerable. I am reconsidering my original intent to paint the turret and hull as separate pieces and assemble everything at the end; I fear that things are just too fragile to survive that. Perhaps I should just lock the turret down, and the bridge support beam, crane stiff-legs, and upper beam supports along with it to make one more or less rigid structure and deal with paint as best I can. With this in mind, and with my uneasiness about that broken tab on the support beam, I have started on styrene replacements for both support beams. If I have to replace one, I might as well do the other too so that they match. While I'm pretty sure the pattern maker used the same dimension materials that I am, the finished resin products are smaller due to mold shrinkage. Fitting replacement beams to the crane boom is easy; I just have to pry loose one piece for each and glue it back after the hinge tab is in place.

In other news: Fresh RTV silicone arrives today and my Achilles pair tomorrow or Wednesday. And the Ram Badger is in the paint shop for base coats, so that thread will see an update soon.

[dogfish7 (R.I.P.)]

Makes me wonder why Model Manufacturers even attempt to engineer some of these kits and also realize the structural integrity of the original 1:1 vehicles.

[Beekster]

May 11, 2020 14:29:29 GMT -6 dogfish7 (R.I.P.) said:

Makes me wonder why Model Manufacturers even attempt to engineer some of these kits and also realize the structural integrity of the original 1:1 vehicles.

Well, there's some degree of demand for odd and complex conversions like this, that justifies the production of kits like this.  What sometimes gets lost, I think, is that structural integrity issue you bring up.  I'm pretty sure that the Evergreen stock we use is relatively less stressed than the original steel; the 3D printed + styrene + bits of brass tube M2 Treadway bridge as built here has to be far less than 1/35 the weight of the original, so either styrene or resin ought to be strong enough for carrying that.  The structural integrity problem really has to do with the flexibility (or not) of the material.  Resin is quite brittle, and will fail when subjected to a load far less than the scaled-down stresses of the original steel.  Styrene is more flexible, indeed far more flexible than steel, so it can deflect under load in some circumstances and not break.  I'm pretty sure the plastic master patterns for this conversion kit are more stout than the resin copies, and my experiences with this build would tend to support that assumption.

[Robbo]

Wow, this is a modelling masterpiece

[Beekster]

Deep breaths...deep breaths...

The bridge mount is back on, as are the new supports which connect it to the crane boom. Everything is just pinned in place for now, but the thing feels pretty solid so I am leaning towards flowing glue into all of the relevant joints, then trimming and finishing the pins. The bridge mount sits a bit higher than dead level, but that in no way affects it's function so I have to think that if the dimensions on the real one were a bit off and it sat like this, the engineers wouldn't care and would proceed with securing the apparatus as it sits.








And this is what it looks like with the bridge posed on the mounting frame. There's not really enough room in the small light box for this, but I made it work...once. I won't tempt fate by trying it again; in future I will drag out the card table and the bigger light box.






Not installed here is the main winch cable and the shackle. I have determined that I can rig the cable through the turret front with ease, and with some finagling I can get the end into the hole I drilled for it on the winch drum. That's great, because it means I can leave that off for now. If I can do it at this stage, I can do it again after paint. And won't that be fun, with all these various hoonamockers hanging out in the breeze and crazy angles...

[ogrejohn]

That is so cool! 

[JCON]

Wow!!!

[dogfish7 (R.I.P.)]

No wonder it had so much stress!!