Curtiss Mohawk In RAF service

Started by Carl Smoot · 280 · 2 months ago
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    Carl Smoot said 3 months, 2 weeks ago:

    Thanks Spiros (@fiveten) and John (@johnb).

    John, 3D printing would be nice. Unfortunately, I do not have access to one right now (yet). I am considering options in this regard, but it will probably be awhile before I purchase my own. I did look for 3D models of them online in case I could find a way to print them, but nothing in pre-made models either. It's okay though. Old school is fun also.

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    John vd Biggelaar said 3 months, 2 weeks ago:

    @clipper, 3D printing brings a lot indeed, still looking to buy one myself. But as you said, to design and build one yourself is great as well.

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    Carl Smoot said 3 months, 2 weeks ago:

    I spent the morning working on casting the bomb pieces. The castings look a bit like short run model parts (i.e. need a lot of cleanup), but they should work out okay.

    I started out by making the master. Initially, I tried modifying an Airfix bomb from my Hampden kit, but that wasn't cutting it. So I changed my approach and found a piece of sprue rod that was near the correct diameter of the bomb at it's widest part. I kept the sprue longer than needed and then chucked it up in my motor tool. Running the tool on low RPM and taking my time to avoid overheating the part, I shaped it to match the template I drew out yesterday.

    Then I started working on the mold. I had originally thought about using thermoplastic which is ideal for a single pressing, but because it cools too fast, would not allow me to make multiple pressings of the master. I wanted to make a mold of five halves connected together by a runner in order to make it easier to handle and to flatten the bonding surfaces of the bombs by sanding. So I used some modeling clay I have and placed it in a small receptacle made from upside down Lego blocks.

    The reason for the Lego (and the clamps) was to give the clay some support and keep it at a consistent height when pressing the master into the mold. The Lego blocks also allowed me to add height adjustment runners so that I could press each mold to the same depth.

    To hold the master while pressing, I took a piece of acrylic rod and flattened one edge so that it would rest on the runners at a square angle. Then I drilled a small hole in the master and inserted a straight piece of stiff brass wire. A slightly larger hole was drilled through the acrylic rod and the wire was insert through this hole but left loose initially.

    I took some measurements of the master diameter and the distance between the bottom side of the acrylic rod and the top of the mold. Halving the diameter of the master gave me the measurement I needed to set the distance between the master and the acrylic rod. I used spacers set to half the height of the master to raise the acrylic rod and let it rest on the mold surface. The wire was then super glued to hold it at that position.

    Removing the spacers allowed me to start pressing the molds. First I used a piece of .040 inch plastic to press a runner in the mold, then used my master tool to press five pressings of the master, giving me 5 half molds.

    Once I was satisfied with the molds I took some low viscosity UV cure resin and tinted it using Tamiya clear blue (any color would have worked). I used a small pipette to add resin to the mold pieces (and the runner) and then cured it with the UV lamp.

    Removing the parts from the clay didn't work so well as the clay is quite sticky, but I did get the parts out without damaging the parts, but the molds were no good. I had to clean off clay form the molded parts using IPA.

    I remade the mold after fixing the clay and molded a second set. These were also cleaned up. I then sanded both castings on a flat sheet of sandpaper on the bonding surface. This flattens the bonding surface and made it easier to remove flash on the bomb part sides.

    At this point, I am ready to do the final clean up of the parts, remove them from the runners, assemble the halves and then work on the seams. Afterwards I can focus on the bomb fins and fuses. I'll post that when I get to it.

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    Carl Smoot said 3 months, 2 weeks ago:

    One down, nine to go. I will also need to make the shackles used to hold these. 🙂

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    Spiros Pendedekas said 3 months, 2 weeks ago:

    This is a really great technique, my friend @clipper! Kudos to your innovative thinking! The results look absolutely promising so far!

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    John vd Biggelaar said 3 months, 2 weeks ago:

    This looks wonderful, Carl @clipper
    What a great approach to finally get what you were aiming for.

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    Carl Smoot said 3 months, 2 weeks ago:

    Thanks Spiros (@fiveten) and John (@johnb). Getting these consistent has been difficult. The photo definitely shows some differences between them. I can rectify some of this through filing, but at their true size and at a normal viewing distance, they look better. Anyway, I have them assembled now, and can start working on the wing racks , or some other part of the plane.

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    John vd Biggelaar said 3 months, 2 weeks ago:

    To me they look pretty good, Carl @clipper
    Once painted they will look even more identical.
    You did a wonderful job on creating these tiny bombs.

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    Spiros Pendedekas said 3 months, 2 weeks ago:

    Looking great from here, my friend @clipper!

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    Carl Smoot said 3 months, 2 weeks ago:

    Thanks John (@johnb) and Spiros (@fiveten).

    Today was about catching up on some loose ends in the build. Cleaning up a few places where there was still some defects in the finish, drybrushing the engine to improve it's appearance, improving the fit of the brass landing gear struts (which were sticking down at odd angles), and starting on scratchbuilding some of the gear doors.

    The brass landing gear struts fit into tubes in the bottom of the wing but I didn't quite get the angles right on the tubes so the brass struts stick down at odd angles from one another. After considering options, I decided to simply reduce the diameter of the part of the brass that fits into the tubes so that they have a loose fit and I will rely upon epoxy and adjusting them manually to get the struts to extend at the correct angles.

    As you may recall, this fixed gear kit doesn't have any tail wheel doors so I first had to make something to go back there. I made it as a single piece which I will cut in half after I have finished detailing and painting it.

    The main gear doors (specifically the lower part of them) will be scratchbuilt copies of the Hobbycraft P-36 doors (so those originals can be retained for that kit). I'm making them from sheet and tube styrene and some Milliput.

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    Spiros Pendedekas said 3 months, 2 weeks ago:

    Excellent works so far, my friend @clipper! Nice scratch builds!

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    John vd Biggelaar said 3 months, 2 weeks ago:

    Very nice progress, Carl @clipper
    Good start on those scratch build doors.

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    Carl Smoot said 3 months, 2 weeks ago:

    Thanks once again Spiros (@fiveten) and John (@johnb) for following along. This phase of the build has been a lot of fun. I get to try out a bunch of new ideas and techniques.

    I'm continuing on with the landing gear. The gear on the Hawk is fairly simple, but the doors are a bit more complex. The main gear door and the strut need to fit together in a specific way. So after getting the main gear doors made I made sure that the struts I made will fit. Then I cleaned up the doors.

    Each set of doors consists of three separate pieces. The main door mentioned previously, a strut cover, and a scalloped door to close of the leading edge hump.

    If you recall, some time back I used my vinyl cutter to scribe shapes on thin plastic for the two smaller door pieces. The intention was to curve these parts when I was ready to start building the doors. To do that I came up with an approach to use hot water, metal tubing, paint brush handles , and of course the thin plastic parts.

    The parts are partially curved and then warmed up inside the metal tube. Then a paint brush handle of the correct diameter is inserted into the end of the tube to hold the plastic until it cools.

    The parts are done slightly oversize initially. Then test fitted, trimmed to final shape and once more inserted into the hot metal tubes for one more round of forming. This last step is because the parts flattened out somewhat during the final trimming process.

    Here are the final parts.

    This has been a lot of fun making these parts. I should be able to start painting them soon and getting them ready for future installation.

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    Spiros Pendedekas said 3 months, 2 weeks ago:

    This is fantastic scratch building, my friend @clipper!

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    John vd Biggelaar said 3 months, 1 week ago:

    This is some impressive scratch building with wonderful results, Carl @clipper
    Merry Christmas